C4 Technical Summary and FAQ - Please Read on a Regular Basis.
11-25-2004, 08:27 PM
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C4 Technical Summary and FAQ - Please Read on a Regular Basis.
The first and most important, where do new users go for the answers to the simple questions? Right here!
This is a living document. As suggestions are made, they will be updated within this page, so users don't have to surf. I would like to see posts kept to a minimum, or, at the very least, deleted every so often.
Within the text will appear words in bold print, that means it is a topic covered in a section within the FAQ.
This is NOT the place for questions, just comments and suggestions to improve the FAQ! Thanks!
Your C4 Corvette and E85
This has been added due to the increase in questions about using E85.
Q: What is E85?
A: It's a newish fuel blend designed to lower our dependancy on foreign oil. Will it work? I donno. E stands for Ethanol, the 85 is the percentage of ethanol, the remaining 15% is regular gas. It's designed to give the fuel colour, odor and flame.
Q: Can I use it in my Vette?
A: In a word, no. More accurately, not without work. The ECU must be tuned to deliver enough of the new fuel, because it requires much more E85 to burn than it does the gas. Also, ethanol is very costic, so the fuel delivery system needs to be able to handle this.
Q: What about MPG?
A: As stated above, it requires more fuel... so you can expect to see a 30% or more drop in economy.
Q: Where can I get it?
A: Mostly in the mid-west. There are, at current writing, only about 600 stations selling it. I have been informed that there are several stations around Ft. Meade, MD, that sell E85. This is due to military/governmental usage of the fuel. Perhaps that means more stations will appear around military bases across the country?
I hope this clarifies the point!
Model year changed and milestones
1984 - First year of the C4, introduced in March, 1983, as a 1984 model. This was done due to QC problems. USDOT gave GM permission to sell the car as a 1984, because all requirements for the model year had been met. The only engine available was the Crossfire, L83, 205hp.
1985 - Tuned Port Injection introduced. 230 hp.
1986 - Aluminum heads introduced mid way through the model year. Chassis modified to resist cracking problems at the firewall and b-pillar. ABS introduced. First year of 1-piece rear main seal. Aluminum heads introduced at roughly #18,000 on Coupes and all verts had it. If the car came with aluminum heads, it has 235hp.
1987 - Carry over of 1986. Beginning of the B2K, Callaway Twin-Turbo option. First year of the hydraulic roller cam. Introductrion of the Z52 option. Aluminum 128 heads were standard.
1988 - 35th Anniversary - two-toned white/black top/white wheels and interior. First year of the D-port 113 head, which carried over to 91. Two sets of wheels offered; a 16" wheel came with the base suspension and a 17" came with the Z51 and Z52 suspension. Five HP increase for coupes with 3:07 rear axle.
1989 - First year of the ZF sourced manual transmission. Last year of "Atari" digital dash.
1990 - Redesigned interior, drivers side airbag standard. Start of ZR1, 375hp, unique rear facia with squared tail lights and standard front end.
1991 - Redesigned exterior, ZR1 style back end, with new, rounded front end. Last year of B2K option. Stock L98 produced 250 hp.
1992 - LT1 introduced, 300 hp, 330 lb-ft torque. Interior revamped with new digital display in speedometer and finish changes (Dul-cote) on plastic surfaces. Introduction of ASR (traction control); standard on all Vettes.
1993 - 40th Anniversary, "ruby". Special Ruby coloured interior, exterior and on verts, top. ZR1 up to 405 hp. Cam change on LT1 added 10 lb-ft of torque. Midway through the year, ZF 6spd manufacturing move to USA.
1994 - Redesigned interior, passengers side airbag. New ECU, added sequencial fuel injection. GM changes the automatic transmission to become electronically controlled (4L60E).
1995 - Revised Optispark. J55 Heavy Duty Brakes made standard. No other major changes. Last year of ZR1. OBDII hardware installed, but not fuctional.
1996 - Last year of C4. Special editions: Collectors Edition, all silver with either grey, black or red interior; GrandSport, 1000 made, all came with Admiral Blue paint, white stripe, red hashmarks on front left fender, black ZR1 style wheels, rear fender flares (coupes only), interior black or red/black - All GrandSports were LT4/6spd cars. All manual cars came with LT4 (330hp), automatics came with LT1. ODBII fully functional.
Definitions and Abbreviations
F-Body - Referring to V8 equipped Camaro Z28 and Firebird Formula or TransAms. They used a version of the LT1, same heads, 2 bolt main.
ZF6 - Six speed manual transmission used from 1989-1996.
4+3 - Four speed manual transmission, with planetary overdrive, available from 1984-1988. Made by Doug Nash.
Frisbee - A metal chunck on the waterpump pulley on 1988-1991 L98 cars. It was designed to retain the belt better, sadly, it weighs a ton and robs the motor of 5hp! Remove it!
OptiSpark - This is the distributor used by LT1 and LT4 engines. It is mounted below the waterpump. It is prone to water damage, and the waterpump can leak on it, causing all sorts of damage. For more detail, see the OptiSpark section.
OBD - On Board Diagnostics. There are two levels of OBD - I & II. Level one is basically everything before level two came out! Level one changed many times over the years, as it refined and defined itself. Early cars have only one O2 Sensor, or no knock sensor, etc... As time went on, OBDI cars gained a second O2 and knock sensor. But OBDII is most sophisticated. I has the ability to compute values of unknowns based upon known values. For example, there is no "sensor" for the catalytic converter, but because there is an O2 sensor before and after the cat, the computer can determine if the cat is working ok.
Misc Specs & Cautions
What does this mean? Newer wire is better. It is better able to deal with heat and harsh chemicals found under the hood.
For those of us who own a pre 1991 Vette, we have a problem. The heat under the hood is slowly eating our wiring away. The insulation starts to get brittle and break, causing possible shorts and grounding conditions.
If you are having a problem with your early C4, and all other logical fixes ain't fixing it, get out a test light and a DVM and commense chasing the harness. It sure isn't pretty, but it might be your only fix.
At this time, I am unaware of aftermarket harnesses being made. Additionally, I do not know if any new parts from GM are NOS, or newly made, meaning they have new wire. If push comes to shove, a new GM harness will last 20 years or so!
Fluids
Reading and Documentation
Performance Mods that Don't Work
This is a short list of things that don't work... and could waste your hard earned money!
Performance Mods that Do Work
Wheel Offsets
This section has been enhanced, due to info from CentralCoaster.
Power Steering Trivia
This is from Rick93Z07:
C5 Brake Adaptors
Forum member Eike sent these links to me. The PDF contains the CAD drawing for C5 Brake Adaptors. Enjoy!
The files are a little slow to load, it's coming in from Germany! When I have a moment, I will save them locally. There is a pic, but it needs resized!
CAD Drawing as PDF file.
Fuel Injection Definitions
Tuned Port refers to the L98 setup. It alludes to the long runners of the intake manifold.
Sequential Injection fires an injector in time with the spark plug.
Batch fire is when all 4 injectors on one side fire at the same time, followed by the other side.
The advantage to batch is at high rpm operation - ie, racing.
The advantage to SFI is all other times... better MPG, better idle, better drivability, and better tunability.
Online Resources
Please note, as of this update (January 22, 2006) the Forum search function IS working!!!
Engine Conversions
These are the two threads that are important to doing LSx type conversions into a C4. At the same time, I am going to be adding any other major engine conversion thread in here as they appear.
Tools
A good set of tools is required for working on these cars. A nice 100+ piece Master Mechanics set from Sears (Craftsman) is a great start!
Also, you should have:
Differentials
This topic has been fodder for fights for a long time now! I hope we can clear the air on this subject once and for all.
In 1984, only the Dana D36 was available. It was used on both manual and automatic cars.
In 1985, the Dana D44 was released for use with the manual, ONLY. No automatics came from the factory with a D44 rear end.
The difference between the two is ring gear size, and the associated strength that comes with a larger ring gear.
The D36 has a 7.5" ring gear; the D44, an 8.5" ring gear.
The maximum ratio that will fit in a D36 is a nominal 3.73. After that, your taking your rear ends life into your own hands. The D36 has a power limit of about 450hp... but it won't last long on the strip, even then...
The maximum ratio for a D44 is a nominal 4:11. It will require some minor case machining to make it fit perfectly. The D44 is known to deal with over 650hp with no problems.
I use references to horsepower. This is because most folks can relate to horsepower better than torque. And since the SBC creates about equal amounts of both (neat trick, eh?), it's just as easy to use horsepower. Finally, the real killer to the D36 is drag slicks. They will flat out tear a D36 to hell. The more the power, the stickier the rubber, the shorter the lifespan. A D44 is NOT indistructible. They do have their limits, but they are high, and with care, they will last.
When drag racing, especially when the power levels get over about 600hp, the independant rear end becomes a handicap. At this point, it might be worth looking into a 4-link live axle setup, if this is the case, a Ford 9" is the best piece for the job.
At the time of this writing, a D44 is worth about $2,000.00, complete (differential, casing, driveshaft, C-beam and batwing).
The best ujoints are the Spicer units.
As mentioned above (see Fluids), use two bottles of Differential Additive before adding the gear oil. This recommendation comes from Gordon Killebrew.
ZF6 Transmissions
This from Joe (Anesthes)
ZF6 Information:
There are two ZF6 transmissions that were used in the C4.
A "Black Tag" ZF6 that was used from 1989 to sometime in late 1993.
It was produced in Germany and literally has a black service tag. The Black tag makes more gear noise due to the different
angle of the gear machining. The black tag also has a 1.375" diameter front collar, therefore requires a clutch disc
with an integrated 1.375" throw out bearing.
A "Blue Tag" ZF6 was used from sometime in 1994 until 1996. It was produced in the United States, and has
a Blue service tag. Re-angled gear machining lead to more quiet operation. The collar on the front has been reduced
to 1.311" so that a nylon insert could be pressed into the Throw out bearing. (To reduce wear, I'm told). Therefore,
your clutch disc will need to be specific to the transmission you have.
ZF6 Clutch Specifics:
In stock form, the ZF6 uses a "Pull style" clutch, similar to the LT1-T56 transmission. The only difference is,
a ZF6 does NOT use a sprung hun clutch disc, but rather uses a dual-mass flywheel with integrated springs between
the mounting surface and the friction surface. The OEM flywheel sells for nearly $700 US.
There are a few options for clutch replacement, other than stock. For example, you can change to a singlemass flywheel
and re-use a OEM type ZF6 clutch kit, OR use fbody clutch components, OR convert to a push style clutch.
1) If using a Singlemass T56 flywheel out of a 93-97 V8 Fbody (OEM# 10125379), you can use a Fbody sprung hub clutch disc (OEM #12551309),
but must use the propper ZF6 pressure plate for your transmission. Additionally, the flywheel must be resurfaced .090" for clearance
with the clutch fork to bell housing. This will only work on 1986+ motors with 1pc rear main seal!
2) If using a Singlemass 4+3 flywheel (OEM# 14088646), you can use an OEM type ZF6 clutch kit, but due to the lack of a dual mass flywheel
OR sprung clutch hub you will get some gear noise.
3) You can use a Singlemass 4+3 flywheel with an OEM type 4+3 PUSH clutch, but NOT with the ZF6 hydraulics!
You need to convert to a hydraulic throw out bearing (such as Howe 82876 for Black Tag ZF6 with a 1.375" TOB). LD85 has completed
this swap and has added the following information:
"Essentially, I use a 4+3 FW , PP and Disc, .100" spacer between the BH and Block, and a Hydraulic Howe Push Type slip on TOB,
a Wilwood 3/4" bore Master Cylinder, and a Clutch Pedal travel stop."
Expect to pay about $160 for the TOB, $160 for the master cyl, and another 40-50 in materials to make the spacer + pedal stop.
AGAIN, A DUAL MASS or A T56 flywheel will only work on a 1986 AND UP MOTOR WITH A 1PC REAR MAIN SEAL!
Automatic, OR 4+3 to ZF6 swap:
A few points to add:
1) You can re-use your 4+3 pedals, and thats IT.
2) You need a small-nose starter because of the bell housing, or a mini/race starter
3) Stay away from ZR1 bell housings (You need a 6 1/8" deep BH)
4) Some folks say you need a ZR1 pivot stud when using a singlemass flywheel. Centralcoaster
did NOT use one, so I'm going to assume this is not needed unless someone who HAS USED
ONE can respond.
Centralcoaster and myself have done the swap. This is the minimum parts list from him with
my edits:
Pedals:
For 4+3 owners, your pedals can be used for the ZF6
For Automatic owners, you need a set of pedals. It's a safe bet that
pedals for 84-89 cars will be the same, and 90-96 cars be the same due to
dash differences.
Transmission:
89-93 black-tag ZF6 and 89-93 clutch set
or
94-96 blue-tag trans and 94-96 clutch (Remember the 1.375" vs 1.311" difference!)
Flywheel:
Stock flywheel from manual trans vette of your year (Cheapest route)
or
ZF6 Dual mass flywheel asssuming you have an 86+ motor with a 1pc rear
main seal.
or
T56 Flywheel with propper machining, and a Fbody clutch disc (OEM#12551309)
assuming you have an 86+ motor with 1pc rear main seal.
Starter:
88-91 starter, or ministarter, or LT1 starter.
Driveshaft:
Driveshafts depend on your rear end, so if you have a Dana36 or Dana44
you need the respective driveshaft. Your 700R4 or 4+3 yoke however will NOT
fit the ZF6.
Everything else:
91-96 ZF6 bellhousing (Stay away from ZR1 bell housings)
89-96 ZF6 clutch fork and pivot stud
89-96 ZF6 clutch master cylinder
89-90 ZF6 clutch slave cylinder
89-90 ZF6 clutch slave hose
89-96 ZF6 Yoke
TH400 Rear seal, Timken #9449 if using a used ZF6
backup switch connector, a.k.a. horn connector. (AcDelco PT1357 or Napa ECHEC11)
(5) metric M12 x 45mm transmission bolts
(6) standard 3/8 x 1-5/8" pressure plate bolts ($2.50 each from GM)
3 bottles of GM 1052931 or Pennzoil Synchromesh
Sheetmetal and rivets for patching the trans tunnel, and some extra carpet insulation.
Speedo sensor from my 85 4+3 fit the ZF6. I bought a red 37 tooth driven gear from GM for it to match my 3.07 rear end ratio.
Getting the 37 tooth driven gear to mesh requires installing the speedo sensor upside down and tweaking the hold down clamp a bit.
The 3.07 rear makes 6th gear unusable under 85mph. I recommend upgrading to a 3.45 or 3.55 ratio.
Some Generic ZF6 myths:
1) You do NOT have to have your flywheel "match balanced". That is bogus information. You need to have the correct flywheel for your motor.
1985 and earlier used a 2pc rear main seal flywheel. Any 153 tooth 2pc rear flywheel for a 350 is balanced correctly if it's an OEM replacement.
Any 1986 and up 153 tooth 1pc rear flywheel for OEM replacement is balanced correctly, including a T56 flywheel. *
Aftermarket flywheels for "race use" may need balancing. Make sure the flywheel is for OEM replacement! Some flywheels will specify
things such as "internal balance 383". This is not what you want. You want an OEM direct replacement 153 tooth flywheel for your year
motor.
2) You cannot "re-use" a 4+3 clutch setup with the ZF6 hydraulics. No way.
3) You cannot use 4+3 hydraulics with the ZF6. Not happening.
OptiSpark
I touched on the downfall of the OptiSpark up top under Definitions, this is more about how to remedy the OptiSpark.
History
The OptiSpark (Opti) was introduced in 1991, with the LT1 as part of the 1992 model year. Why did they do the Opti? Even Gordon Killebrew doesn't have a clue.
The Opti is made up of two sides, one side, the side against the engine, is an optical pickup. The pickup is made by Mitsubishi, and is a very reliable component. The optical system reads a disc that has marks in it. On the outer edge are 360 marks, inside that, are four marks. The outer marks create the so called "High Resolution" signal to control timing, the inner marks, or "Low Resolution" are used to signal fuel delivery.
A common shaft joins the optical side with the high voltage side. The high voltage side is similar to any distributor, except it is much thinner. The unique part, the sides of the cap are sequenced, so all the odds are on one side, evens on the other!
In 1995, the second generation of the Opti was released. It has a venting hose and is better constructed.
Problems
Codes can present themselves if an Opti is failing. These codes very by year, but for the earlier versions, codes 16 and 36 are bad(Note: these codes only apply to 1992 - 1995, 1996 uses ODBII and has different codes). This means the optical portion has a problem. Most likely, the error is caused not so much a failure in the optical portion, but the bearings having worn out, causing a wobble in the drive shaft.
A common problem is when coolant from a bad waterpump leaks in. Coolant will eat the grease inside the sealed bearings. This causes the common shaft to wobble.
If there is a wondering idle or a high RPM miss, you can blame a bad cap/rotor. The cap/rotor fails due to carbon tracing, caused by moisture getting into the Opti.
Factory Options
New Optis are available from the dealer. Also available is the Venting Hose Kit, that can be retrofitted to the original cap design.
You can also update an earlier engine to the later Opti, by changing the timing cover and various internal parts related to the timing gear.
I cannot advise a cap/rotor change. The labour is the same... and there is no guarentee that a rotor change will result in a long term repair.
If your car is a 1995 or newer, then a cap/rotor change might just be worth investigating.
Regardless of the vintage, removing the cap and sealing with RTV is a good idea. It might not be a bad idea to install a drain hose on the waterpump, to direct the weap holes output to the side, instead of on the Opti.
I do not recommend the Ecklers or Mid-America cap venting system. It is a single hose, and as such does not create positive pressure. It might suck some stuff out, but I just don't see how it can without creating a complete loop.
Aftermarket Options
There are now a few options available:
Generic replacement Optis are not recommended. They don't include the cap/rotor and are of questionable reliability.
Consumer Alerts
This is a new section where I will list blatently bad customer service problems or known problems with a product. Sometimes, it's rather broad, sometimes, very specific.
Reman'd ECUs
Kittmaster brought this one to the forum's attention on Sept. 2, 2006. His reman'd ECU was freaking out. His analysis showed that the seal on the bottom, where the chip access panel is, was bent, allowing water into the ECU. This is very bad.
For more details, check out the thread: Reman'd ECM Thread. There are some detailed pictures in there, too.
The bottom line is thus: Do not take a reman'd ECU out of the store until this seal has been checked. Warrantee's on electronics can be measured in seconds.
ZF6 Clutch Slaves from GM
This has been an on going problem for several years. Recently, CentralCoaster took a slave apart and found that a seal inside has been installed BACKWARDS. The concensus is that new GM slaves should be checked to be sure the seal is on correctly.
I cannot yet speak on the quality of the aftermarket parts. I await empirical data.
======================================== =============
Final Thoughts
It is important to understand that the LT1 still needed the harmonic balancer (essentially the crank pulley) to be balanced and "keyed" to the right place on the crank. If you ever remove the harmonic balancer, mark the location on the crank CORRECTLY or it will be a problem. Sadly, the crank is not keyed, this means the pulley can go on anyway it wants to. <I copied this from the clutch section, but with good reason>
Before you condemn the Opti, make sure the coil, ignition module and coil wire are in good shape.
======================================== =============
As I said, this is a living document. Please comment on errors/omissions.
Credits and Thanks!
A lot of people are making this happen! Thanks to you all! The support is GREAT! In alphabetical order:
aj1988
Atok
BadjerJim
CentralCoaster
kalister1
kittmaster
lite-blue
Mr Mojo - for making this a sticky!
Nathan Plemons
pettvette
Rick93Z07
ShawnZR-1
Strick
ToniH
tonymax2
Vader86
Bruce Witherspoon
This is a living document. As suggestions are made, they will be updated within this page, so users don't have to surf. I would like to see posts kept to a minimum, or, at the very least, deleted every so often.
Within the text will appear words in bold print, that means it is a topic covered in a section within the FAQ.
This is NOT the place for questions, just comments and suggestions to improve the FAQ! Thanks!
Your C4 Corvette and E85
This has been added due to the increase in questions about using E85.
Q: What is E85?
A: It's a newish fuel blend designed to lower our dependancy on foreign oil. Will it work? I donno. E stands for Ethanol, the 85 is the percentage of ethanol, the remaining 15% is regular gas. It's designed to give the fuel colour, odor and flame.
Q: Can I use it in my Vette?
A: In a word, no. More accurately, not without work. The ECU must be tuned to deliver enough of the new fuel, because it requires much more E85 to burn than it does the gas. Also, ethanol is very costic, so the fuel delivery system needs to be able to handle this.
Q: What about MPG?
A: As stated above, it requires more fuel... so you can expect to see a 30% or more drop in economy.
Q: Where can I get it?
A: Mostly in the mid-west. There are, at current writing, only about 600 stations selling it. I have been informed that there are several stations around Ft. Meade, MD, that sell E85. This is due to military/governmental usage of the fuel. Perhaps that means more stations will appear around military bases across the country?
I hope this clarifies the point!
Model year changed and milestones
1984 - First year of the C4, introduced in March, 1983, as a 1984 model. This was done due to QC problems. USDOT gave GM permission to sell the car as a 1984, because all requirements for the model year had been met. The only engine available was the Crossfire, L83, 205hp.
1985 - Tuned Port Injection introduced. 230 hp.
1986 - Aluminum heads introduced mid way through the model year. Chassis modified to resist cracking problems at the firewall and b-pillar. ABS introduced. First year of 1-piece rear main seal. Aluminum heads introduced at roughly #18,000 on Coupes and all verts had it. If the car came with aluminum heads, it has 235hp.
1987 - Carry over of 1986. Beginning of the B2K, Callaway Twin-Turbo option. First year of the hydraulic roller cam. Introductrion of the Z52 option. Aluminum 128 heads were standard.
1988 - 35th Anniversary - two-toned white/black top/white wheels and interior. First year of the D-port 113 head, which carried over to 91. Two sets of wheels offered; a 16" wheel came with the base suspension and a 17" came with the Z51 and Z52 suspension. Five HP increase for coupes with 3:07 rear axle.
1989 - First year of the ZF sourced manual transmission. Last year of "Atari" digital dash.
1990 - Redesigned interior, drivers side airbag standard. Start of ZR1, 375hp, unique rear facia with squared tail lights and standard front end.
1991 - Redesigned exterior, ZR1 style back end, with new, rounded front end. Last year of B2K option. Stock L98 produced 250 hp.
1992 - LT1 introduced, 300 hp, 330 lb-ft torque. Interior revamped with new digital display in speedometer and finish changes (Dul-cote) on plastic surfaces. Introduction of ASR (traction control); standard on all Vettes.
1993 - 40th Anniversary, "ruby". Special Ruby coloured interior, exterior and on verts, top. ZR1 up to 405 hp. Cam change on LT1 added 10 lb-ft of torque. Midway through the year, ZF 6spd manufacturing move to USA.
1994 - Redesigned interior, passengers side airbag. New ECU, added sequencial fuel injection. GM changes the automatic transmission to become electronically controlled (4L60E).
1995 - Revised Optispark. J55 Heavy Duty Brakes made standard. No other major changes. Last year of ZR1. OBDII hardware installed, but not fuctional.
1996 - Last year of C4. Special editions: Collectors Edition, all silver with either grey, black or red interior; GrandSport, 1000 made, all came with Admiral Blue paint, white stripe, red hashmarks on front left fender, black ZR1 style wheels, rear fender flares (coupes only), interior black or red/black - All GrandSports were LT4/6spd cars. All manual cars came with LT4 (330hp), automatics came with LT1. ODBII fully functional.
Definitions and Abbreviations
F-Body - Referring to V8 equipped Camaro Z28 and Firebird Formula or TransAms. They used a version of the LT1, same heads, 2 bolt main.
ZF6 - Six speed manual transmission used from 1989-1996.
4+3 - Four speed manual transmission, with planetary overdrive, available from 1984-1988. Made by Doug Nash.
Frisbee - A metal chunck on the waterpump pulley on 1988-1991 L98 cars. It was designed to retain the belt better, sadly, it weighs a ton and robs the motor of 5hp! Remove it!
OptiSpark - This is the distributor used by LT1 and LT4 engines. It is mounted below the waterpump. It is prone to water damage, and the waterpump can leak on it, causing all sorts of damage. For more detail, see the OptiSpark section.
OBD - On Board Diagnostics. There are two levels of OBD - I & II. Level one is basically everything before level two came out! Level one changed many times over the years, as it refined and defined itself. Early cars have only one O2 Sensor, or no knock sensor, etc... As time went on, OBDI cars gained a second O2 and knock sensor. But OBDII is most sophisticated. I has the ability to compute values of unknowns based upon known values. For example, there is no "sensor" for the catalytic converter, but because there is an O2 sensor before and after the cat, the computer can determine if the cat is working ok.
Misc Specs & Cautions
What does this mean? Newer wire is better. It is better able to deal with heat and harsh chemicals found under the hood.
For those of us who own a pre 1991 Vette, we have a problem. The heat under the hood is slowly eating our wiring away. The insulation starts to get brittle and break, causing possible shorts and grounding conditions.
If you are having a problem with your early C4, and all other logical fixes ain't fixing it, get out a test light and a DVM and commense chasing the harness. It sure isn't pretty, but it might be your only fix.
At this time, I am unaware of aftermarket harnesses being made. Additionally, I do not know if any new parts from GM are NOS, or newly made, meaning they have new wire. If push comes to shove, a new GM harness will last 20 years or so!
Fluids
Reading and Documentation
Performance Mods that Don't Work
This is a short list of things that don't work... and could waste your hard earned money!
Performance Mods that Do Work
Wheel Offsets
This section has been enhanced, due to info from CentralCoaster.
Code:
Just because a wheel fits without rubbing doesn't mean it'll look right. The wheel dimension that effects it's appearance on the car is what I call "front spacing". It measures from the hub to the outer bead and determines the placement of the wheel relative to the wheel well. Once you exceed a certain frontspacing, the tire sticks out past the fender. You calculate it by subtracting the offset from half of the wheel width. For a zero offset wheel, the front spacing distance is simply half of the wheel width, got it? So check the chart below for what wheels your car has on it. Then compare it's frontspacing with any other wheel to see if you need adapters to make up the difference. (Also, backspacing measures the distance from the hub to the inner bead, plus a little extra for the wheel lip. If you exceed a certain backspacing, the tires will rub in the fenders. I haven't calculated backspacing yet.) (WIDTH / 2) - (OFFSET / 25.4) = FRONT SPACING OEM CORVETTE WHEEL FRONT SPACING: 84-87 base 8.5"/2 - 32mm/25.4 = 2.99" 84R-87 Z51 9.5"/2 - 38mm/25.4 = 3.25" 88-96 base 8.5"/2 - 56mm/25.4 = 2.05" 9.5"/2 - 56mm/25.4 = 2.55" (also used for fronts on some models) ZR1 rear 11"/2 - 36mm/25.4 = 4.08" Grand Sport rear 11"/2 - 50mm/25.4 = 3.53" C5 base 8.5"/2 - 56mm/25.4 = 2.05" 9.5"/2 - 65mm/25.4 = 2.19" C5 Z06 9.5"/2 - 54mm/25.4 = 2.62" 10.5"/2 - 58mm/25.4 = 2.97" C6 base 8.5"/2 - 56mm/25.4 = 2.05" 10"/2 - 79mm/25.4 = 1.89" C6 Z06 9.5"/2 - 40mm/25.4 = 3.18" 12"/2 - 59mm/25.4 = 3.68"
This is from Rick93Z07:
Code:
I would like to add a "Tech Factoid" to this thread relating to a secret running change made to LT1 cars. The LT1 Corvette had reliability issues with power steering pressure hose & reports of engine bay fires resulted in recalls. Several running changes were made to o-rings. These efforts were largely unsuccessful because the real culprit was engine vibration and the steel line was fracturing under the fittings. Sometime in 1995, GM re-engineered the P/S pressure hose, snaking it with a 90 degree turn under the idler pulley toward the right side of the car and then downward with a counterclockwise twist into the rack. This special hose #26042088 was the proper fix for all LT1/4. Anyone with a '92-'96 car needs this hose. Sometime in very late 1995 or early 1996, GM made another modification. They added a threaded restraint boss to the large aluminum accessory bracket that holds the P/S. This threaded boss points leftward and it's positioned just behind the idler pulley. The newer style #26042088 hose will clamp to this new boss using a restraint clamp. This improves durability of the upper steel section of the hose under high RPM use. The newer style P/S hose is a great upgrade and it's even more durable if you swap out your old accessory braket for a newer one.
Forum member Eike sent these links to me. The PDF contains the CAD drawing for C5 Brake Adaptors. Enjoy!
The files are a little slow to load, it's coming in from Germany! When I have a moment, I will save them locally. There is a pic, but it needs resized!
CAD Drawing as PDF file.
Fuel Injection Definitions
Tuned Port refers to the L98 setup. It alludes to the long runners of the intake manifold.
Sequential Injection fires an injector in time with the spark plug.
Batch fire is when all 4 injectors on one side fire at the same time, followed by the other side.
The advantage to batch is at high rpm operation - ie, racing.
The advantage to SFI is all other times... better MPG, better idle, better drivability, and better tunability.
Online Resources
Please note, as of this update (January 22, 2006) the Forum search function IS working!!!
Engine Conversions
These are the two threads that are important to doing LSx type conversions into a C4. At the same time, I am going to be adding any other major engine conversion thread in here as they appear.
Tools
A good set of tools is required for working on these cars. A nice 100+ piece Master Mechanics set from Sears (Craftsman) is a great start!
Also, you should have:
Differentials
This topic has been fodder for fights for a long time now! I hope we can clear the air on this subject once and for all.
In 1984, only the Dana D36 was available. It was used on both manual and automatic cars.
In 1985, the Dana D44 was released for use with the manual, ONLY. No automatics came from the factory with a D44 rear end.
The difference between the two is ring gear size, and the associated strength that comes with a larger ring gear.
The D36 has a 7.5" ring gear; the D44, an 8.5" ring gear.
The maximum ratio that will fit in a D36 is a nominal 3.73. After that, your taking your rear ends life into your own hands. The D36 has a power limit of about 450hp... but it won't last long on the strip, even then...
The maximum ratio for a D44 is a nominal 4:11. It will require some minor case machining to make it fit perfectly. The D44 is known to deal with over 650hp with no problems.
I use references to horsepower. This is because most folks can relate to horsepower better than torque. And since the SBC creates about equal amounts of both (neat trick, eh?), it's just as easy to use horsepower. Finally, the real killer to the D36 is drag slicks. They will flat out tear a D36 to hell. The more the power, the stickier the rubber, the shorter the lifespan. A D44 is NOT indistructible. They do have their limits, but they are high, and with care, they will last.
When drag racing, especially when the power levels get over about 600hp, the independant rear end becomes a handicap. At this point, it might be worth looking into a 4-link live axle setup, if this is the case, a Ford 9" is the best piece for the job.
At the time of this writing, a D44 is worth about $2,000.00, complete (differential, casing, driveshaft, C-beam and batwing).
The best ujoints are the Spicer units.
As mentioned above (see Fluids), use two bottles of Differential Additive before adding the gear oil. This recommendation comes from Gordon Killebrew.
ZF6 Transmissions
This from Joe (Anesthes)
ZF6 Information:
There are two ZF6 transmissions that were used in the C4.
A "Black Tag" ZF6 that was used from 1989 to sometime in late 1993.
It was produced in Germany and literally has a black service tag. The Black tag makes more gear noise due to the different
angle of the gear machining. The black tag also has a 1.375" diameter front collar, therefore requires a clutch disc
with an integrated 1.375" throw out bearing.
A "Blue Tag" ZF6 was used from sometime in 1994 until 1996. It was produced in the United States, and has
a Blue service tag. Re-angled gear machining lead to more quiet operation. The collar on the front has been reduced
to 1.311" so that a nylon insert could be pressed into the Throw out bearing. (To reduce wear, I'm told). Therefore,
your clutch disc will need to be specific to the transmission you have.
ZF6 Clutch Specifics:
In stock form, the ZF6 uses a "Pull style" clutch, similar to the LT1-T56 transmission. The only difference is,
a ZF6 does NOT use a sprung hun clutch disc, but rather uses a dual-mass flywheel with integrated springs between
the mounting surface and the friction surface. The OEM flywheel sells for nearly $700 US.
There are a few options for clutch replacement, other than stock. For example, you can change to a singlemass flywheel
and re-use a OEM type ZF6 clutch kit, OR use fbody clutch components, OR convert to a push style clutch.
1) If using a Singlemass T56 flywheel out of a 93-97 V8 Fbody (OEM# 10125379), you can use a Fbody sprung hub clutch disc (OEM #12551309),
but must use the propper ZF6 pressure plate for your transmission. Additionally, the flywheel must be resurfaced .090" for clearance
with the clutch fork to bell housing. This will only work on 1986+ motors with 1pc rear main seal!
2) If using a Singlemass 4+3 flywheel (OEM# 14088646), you can use an OEM type ZF6 clutch kit, but due to the lack of a dual mass flywheel
OR sprung clutch hub you will get some gear noise.
3) You can use a Singlemass 4+3 flywheel with an OEM type 4+3 PUSH clutch, but NOT with the ZF6 hydraulics!
You need to convert to a hydraulic throw out bearing (such as Howe 82876 for Black Tag ZF6 with a 1.375" TOB). LD85 has completed
this swap and has added the following information:
"Essentially, I use a 4+3 FW , PP and Disc, .100" spacer between the BH and Block, and a Hydraulic Howe Push Type slip on TOB,
a Wilwood 3/4" bore Master Cylinder, and a Clutch Pedal travel stop."
Expect to pay about $160 for the TOB, $160 for the master cyl, and another 40-50 in materials to make the spacer + pedal stop.
AGAIN, A DUAL MASS or A T56 flywheel will only work on a 1986 AND UP MOTOR WITH A 1PC REAR MAIN SEAL!
Automatic, OR 4+3 to ZF6 swap:
A few points to add:
1) You can re-use your 4+3 pedals, and thats IT.
2) You need a small-nose starter because of the bell housing, or a mini/race starter
3) Stay away from ZR1 bell housings (You need a 6 1/8" deep BH)
4) Some folks say you need a ZR1 pivot stud when using a singlemass flywheel. Centralcoaster
did NOT use one, so I'm going to assume this is not needed unless someone who HAS USED
ONE can respond.
Centralcoaster and myself have done the swap. This is the minimum parts list from him with
my edits:
Pedals:
For 4+3 owners, your pedals can be used for the ZF6
For Automatic owners, you need a set of pedals. It's a safe bet that
pedals for 84-89 cars will be the same, and 90-96 cars be the same due to
dash differences.
Transmission:
89-93 black-tag ZF6 and 89-93 clutch set
or
94-96 blue-tag trans and 94-96 clutch (Remember the 1.375" vs 1.311" difference!)
Flywheel:
Stock flywheel from manual trans vette of your year (Cheapest route)
or
ZF6 Dual mass flywheel asssuming you have an 86+ motor with a 1pc rear
main seal.
or
T56 Flywheel with propper machining, and a Fbody clutch disc (OEM#12551309)
assuming you have an 86+ motor with 1pc rear main seal.
Starter:
88-91 starter, or ministarter, or LT1 starter.
Driveshaft:
Driveshafts depend on your rear end, so if you have a Dana36 or Dana44
you need the respective driveshaft. Your 700R4 or 4+3 yoke however will NOT
fit the ZF6.
Everything else:
91-96 ZF6 bellhousing (Stay away from ZR1 bell housings)
89-96 ZF6 clutch fork and pivot stud
89-96 ZF6 clutch master cylinder
89-90 ZF6 clutch slave cylinder
89-90 ZF6 clutch slave hose
89-96 ZF6 Yoke
TH400 Rear seal, Timken #9449 if using a used ZF6
backup switch connector, a.k.a. horn connector. (AcDelco PT1357 or Napa ECHEC11)
(5) metric M12 x 45mm transmission bolts
(6) standard 3/8 x 1-5/8" pressure plate bolts ($2.50 each from GM)
3 bottles of GM 1052931 or Pennzoil Synchromesh
Sheetmetal and rivets for patching the trans tunnel, and some extra carpet insulation.
Speedo sensor from my 85 4+3 fit the ZF6. I bought a red 37 tooth driven gear from GM for it to match my 3.07 rear end ratio.
Getting the 37 tooth driven gear to mesh requires installing the speedo sensor upside down and tweaking the hold down clamp a bit.
The 3.07 rear makes 6th gear unusable under 85mph. I recommend upgrading to a 3.45 or 3.55 ratio.
Some Generic ZF6 myths:
1) You do NOT have to have your flywheel "match balanced". That is bogus information. You need to have the correct flywheel for your motor.
1985 and earlier used a 2pc rear main seal flywheel. Any 153 tooth 2pc rear flywheel for a 350 is balanced correctly if it's an OEM replacement.
Any 1986 and up 153 tooth 1pc rear flywheel for OEM replacement is balanced correctly, including a T56 flywheel. *
Aftermarket flywheels for "race use" may need balancing. Make sure the flywheel is for OEM replacement! Some flywheels will specify
things such as "internal balance 383". This is not what you want. You want an OEM direct replacement 153 tooth flywheel for your year
motor.
2) You cannot "re-use" a 4+3 clutch setup with the ZF6 hydraulics. No way.
3) You cannot use 4+3 hydraulics with the ZF6. Not happening.
OptiSpark
I touched on the downfall of the OptiSpark up top under Definitions, this is more about how to remedy the OptiSpark.
History
The OptiSpark (Opti) was introduced in 1991, with the LT1 as part of the 1992 model year. Why did they do the Opti? Even Gordon Killebrew doesn't have a clue.
The Opti is made up of two sides, one side, the side against the engine, is an optical pickup. The pickup is made by Mitsubishi, and is a very reliable component. The optical system reads a disc that has marks in it. On the outer edge are 360 marks, inside that, are four marks. The outer marks create the so called "High Resolution" signal to control timing, the inner marks, or "Low Resolution" are used to signal fuel delivery.
A common shaft joins the optical side with the high voltage side. The high voltage side is similar to any distributor, except it is much thinner. The unique part, the sides of the cap are sequenced, so all the odds are on one side, evens on the other!
In 1995, the second generation of the Opti was released. It has a venting hose and is better constructed.
Problems
Codes can present themselves if an Opti is failing. These codes very by year, but for the earlier versions, codes 16 and 36 are bad(Note: these codes only apply to 1992 - 1995, 1996 uses ODBII and has different codes). This means the optical portion has a problem. Most likely, the error is caused not so much a failure in the optical portion, but the bearings having worn out, causing a wobble in the drive shaft.
A common problem is when coolant from a bad waterpump leaks in. Coolant will eat the grease inside the sealed bearings. This causes the common shaft to wobble.
If there is a wondering idle or a high RPM miss, you can blame a bad cap/rotor. The cap/rotor fails due to carbon tracing, caused by moisture getting into the Opti.
Factory Options
New Optis are available from the dealer. Also available is the Venting Hose Kit, that can be retrofitted to the original cap design.
You can also update an earlier engine to the later Opti, by changing the timing cover and various internal parts related to the timing gear.
I cannot advise a cap/rotor change. The labour is the same... and there is no guarentee that a rotor change will result in a long term repair.
If your car is a 1995 or newer, then a cap/rotor change might just be worth investigating.
Regardless of the vintage, removing the cap and sealing with RTV is a good idea. It might not be a bad idea to install a drain hose on the waterpump, to direct the weap holes output to the side, instead of on the Opti.
I do not recommend the Ecklers or Mid-America cap venting system. It is a single hose, and as such does not create positive pressure. It might suck some stuff out, but I just don't see how it can without creating a complete loop.
Aftermarket Options
There are now a few options available:
Generic replacement Optis are not recommended. They don't include the cap/rotor and are of questionable reliability.
Consumer Alerts
This is a new section where I will list blatently bad customer service problems or known problems with a product. Sometimes, it's rather broad, sometimes, very specific.
Reman'd ECUs
Kittmaster brought this one to the forum's attention on Sept. 2, 2006. His reman'd ECU was freaking out. His analysis showed that the seal on the bottom, where the chip access panel is, was bent, allowing water into the ECU. This is very bad.
For more details, check out the thread: Reman'd ECM Thread. There are some detailed pictures in there, too.
The bottom line is thus: Do not take a reman'd ECU out of the store until this seal has been checked. Warrantee's on electronics can be measured in seconds.
ZF6 Clutch Slaves from GM
This has been an on going problem for several years. Recently, CentralCoaster took a slave apart and found that a seal inside has been installed BACKWARDS. The concensus is that new GM slaves should be checked to be sure the seal is on correctly.
I cannot yet speak on the quality of the aftermarket parts. I await empirical data.
======================================== =============
Final Thoughts
It is important to understand that the LT1 still needed the harmonic balancer (essentially the crank pulley) to be balanced and "keyed" to the right place on the crank. If you ever remove the harmonic balancer, mark the location on the crank CORRECTLY or it will be a problem. Sadly, the crank is not keyed, this means the pulley can go on anyway it wants to. <I copied this from the clutch section, but with good reason>
Before you condemn the Opti, make sure the coil, ignition module and coil wire are in good shape.
======================================== =============
As I said, this is a living document. Please comment on errors/omissions.
Credits and Thanks!
A lot of people are making this happen! Thanks to you all! The support is GREAT! In alphabetical order:
aj1988
Atok
BadjerJim
CentralCoaster
kalister1
kittmaster
lite-blue
Mr Mojo - for making this a sticky!
Nathan Plemons
pettvette
Rick93Z07
ShawnZR-1
Strick
ToniH
tonymax2
Vader86
Bruce Witherspoon
Last edited by savewave; 01-16-2008 at 07:44 PM. Reason: Enhanced Offset and PS Info & General Editing
The following 2 users liked this post by bogus:
C4_Downunder (01-03-2022),
Itoen (07-22-2016)
05-22-2005, 03:39 PM
#2
Burning Brakes
Items to add to do work performance tips
Great idea, one thing id add to the do work preformance tips
are
1.cut air lid
2.low temp stat
3.K&N FILTER
4.REAR GEAR CHANGE
5.INTAKE CHANGE (UPPER AND LOWER) (TPIS- BIGMOUTH)
6. ROYAL PURPLE OIL
7. INJECTOR AND FUEL REG
THANKS
rad
are
1.cut air lid
2.low temp stat
3.K&N FILTER
4.REAR GEAR CHANGE
5.INTAKE CHANGE (UPPER AND LOWER) (TPIS- BIGMOUTH)
6. ROYAL PURPLE OIL
7. INJECTOR AND FUEL REG
THANKS
rad
06-01-2005, 06:42 PM
#3
Burning Brakes
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hey,I just looked at your data for the rearaxle ratio's available for C4's,
3.31's were on 84's with Z51 4+3's
3.33's wer on 90 6spd's
not 100% sure,but I think 2.73's was the performance axle on some of the early LT1 convertibles.
3.31's were on 84's with Z51 4+3's
3.33's wer on 90 6spd's
not 100% sure,but I think 2.73's was the performance axle on some of the early LT1 convertibles.
10-13-2005, 08:12 PM
#6
Melting Slicks
Another Good Tranny/Gear Fluid
Another good gear fluid is the Lubrication Engineers LE607.
It's the slickest stuff I've seen.I use it in my 96 GST ( Heavy Modified) and My 03 Honda S2000( Mildly Enhanced)!
\db
It's the slickest stuff I've seen.I use it in my 96 GST ( Heavy Modified) and My 03 Honda S2000( Mildly Enhanced)!
\db
01-22-2006, 03:31 PM
#9
Team Owner
Thread Starter
Mech - there are lotsa old school hotrod books for iron head engines. Another thought, and it's rather cheap, is to upgrade to later style (128 or 113) aluminum heads. They will save a TON of weight... and are a good value. It's not unheard of to buy used heads, complete, for about $300.00 a pair.
02-07-2006, 11:23 PM
#10
Burning Brakes
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Just something I thought of bougus... I always experienced a loss of power in my 84 when I started auto-xing or sliding the car. It would act like it wasn't getting fuel or cutting out. I figured out the the 84's "crapfire" injection had a fuel pump that didn't really met the demands of the engine while under WOT and hard cornering/etc. My observation appeared to be correct.
To fix th problem I put in an 85 vette fuel pump... did did the trick. I never had another "starvation" problem. The first throttle body on the crossfire system automatically regulates the fuel pressure to the correct level and the TPI fuel pump will not overwhelm it. Just a little FYI for those hardcore Stocker guys/gals or those that just love that damn Crossfire system.
To fix th problem I put in an 85 vette fuel pump... did did the trick. I never had another "starvation" problem. The first throttle body on the crossfire system automatically regulates the fuel pressure to the correct level and the TPI fuel pump will not overwhelm it. Just a little FYI for those hardcore Stocker guys/gals or those that just love that damn Crossfire system.
02-11-2006, 10:42 AM
#12
Racer
Member Since: Nov 2005
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Nice job!! That is very well organized. I learned things about my car I never knew. For example, I have a 1994 convert and my insurance company says that it doesn't have air bags. Unfortunately I have nothing to prove them wrong till now. Thanks again for your effort.
02-23-2006, 05:58 PM
#13
4th Gear
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Great area
This area is obviously for people like me...new. Thank you very much.
I would love to get suggestions on what performance applications work together for engine (cam, heads, intakes, etc) suspension parts, and other products that the readers have found works. I know there are many who have gone through a lot of good money to learn, and I would love one place to find it all.
I would love to get suggestions on what performance applications work together for engine (cam, heads, intakes, etc) suspension parts, and other products that the readers have found works. I know there are many who have gone through a lot of good money to learn, and I would love one place to find it all.
02-23-2006, 07:41 PM
#14
Team Owner
Thread Starter
go through the links. I suggest Nathan Plemons link. He has much data there that will help you make the right selections.
Glad WE could help! I sure didn't do all of this by myself!
Glad WE could help! I sure didn't do all of this by myself!
02-26-2006, 01:25 AM
#15
Team Owner
Member Since: Jun 2001
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St. Jude Donor '08-'09
Note on the "mods that don't work"
The throttle switch works very well actually. It just doesn't last too long and will go on the frits after about a year or so of use causing you to need to remove it.
It's good for the track though.
I remembered the first time I got it. I pulled out of the Ecklers parking lot at full throttle (without it) and then pulled back around and then installed the switch. I then pulled out the same way and was supprised that the tires broke loose with almost no effort this time. It seems to cause quicker accelleration, etc. I really noticed a seat of the pants change in the car's accelleration.
Does it add HP? No. Does it make it fly off the line faster? I think and felt that yes, it did. Would I replace the bad one I have now? I don't know. I had a LOT of fun with it while it worked...so maybe I will get another some day.
This was installed on an 89 with the ZF6.
The throttle switch works very well actually. It just doesn't last too long and will go on the frits after about a year or so of use causing you to need to remove it.
It's good for the track though.
I remembered the first time I got it. I pulled out of the Ecklers parking lot at full throttle (without it) and then pulled back around and then installed the switch. I then pulled out the same way and was supprised that the tires broke loose with almost no effort this time. It seems to cause quicker accelleration, etc. I really noticed a seat of the pants change in the car's accelleration.
Does it add HP? No. Does it make it fly off the line faster? I think and felt that yes, it did. Would I replace the bad one I have now? I don't know. I had a LOT of fun with it while it worked...so maybe I will get another some day.
This was installed on an 89 with the ZF6.
02-26-2006, 02:56 AM
#16
Team Owner
Thread Starter
Originally Posted by GIJoe
Note on the "mods that don't work"
The throttle switch works very well actually. It just doesn't last too long and will go on the frits after about a year or so of use causing you to need to remove it.
It's good for the track though.
I remembered the first time I got it. I pulled out of the Ecklers parking lot at full throttle (without it) and then pulled back around and then installed the switch. I then pulled out the same way and was supprised that the tires broke loose with almost no effort this time. It seems to cause quicker accelleration, etc. I really noticed a seat of the pants change in the car's accelleration.
Does it add HP? No. Does it make it fly off the line faster? I think and felt that yes, it did. Would I replace the bad one I have now? I don't know. I had a LOT of fun with it while it worked...so maybe I will get another some day.
This was installed on an 89 with the ZF6.
The throttle switch works very well actually. It just doesn't last too long and will go on the frits after about a year or so of use causing you to need to remove it.
It's good for the track though.
I remembered the first time I got it. I pulled out of the Ecklers parking lot at full throttle (without it) and then pulled back around and then installed the switch. I then pulled out the same way and was supprised that the tires broke loose with almost no effort this time. It seems to cause quicker accelleration, etc. I really noticed a seat of the pants change in the car's accelleration.
Does it add HP? No. Does it make it fly off the line faster? I think and felt that yes, it did. Would I replace the bad one I have now? I don't know. I had a LOT of fun with it while it worked...so maybe I will get another some day.
This was installed on an 89 with the ZF6.
02-26-2006, 10:34 PM
#17
Team Owner
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St. Jude Donor '08-'09
Originally Posted by bogus
the reason we added that as a "don't work" is that it adds no power and only tricks the computer into ramping up the throttle a little. I suspect that when installed, it would make tuning an ECM a real trick...
I just figured I'd make a comment on that one since I actually bought one and really noticed a difference in my launches.
No biggie, just didn't want anyone to get the wrong impression on that particular mod. It's a really cool and fun mod if you have a heavy foot.
Too bad they don't last very long. Mine died after a year and a half. A friend had one also and his lasted a year or so too before the car started acting up. When that happened, just remove it and the car is back to normal. I wish there was a way to make these last longer.
02-27-2006, 06:12 PM
#19
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St. Jude Donor '08-'09
Originally Posted by bogus
What I would like to see are some empirical numbers. 1/4 mile before/after... things like that.
