Andy Dewhurst
12-2-08
dewhurstaj@aol.com

The effects of Ignition timing on a 1966 427 390 hp Corvette Engine.
Includes notes for HEI distributor installation.
History Lesson:
I purchased my 1966 Corvette Convertible 427 390 HP in March of 1977.
The car came with an engine that was recently rebuilt to factory specifications & the entire car was stock except the distributor which came from of a 1966 327 340 HP Corvette. Both big block & small block Chevy engines shared common distributor hardware with only the ignition curve being different between the two engines.
This is a dual point no vacuum advance distributor that was designed for a racing engine. Racing engines are often run a wide open throttle “WOT” when the engine vacuum is measured to be to near 0 in of Hg. Thus, there is no need for a vacuum advance unit because there is little or no engine vacuum to operate it properly.
See Dual Point spec below.
Delco Remy Distributor: 1110985
327 / 340 HP CE D CL/NV/SO/TD
CE= Cast Iron External Adjustable Distributor
D= Dual Point
CL= Cam lubricator mounted on breaker plate
NV=No Vacuum
SO= Stovepipe Oiler
TD=Standard Transmission
Made: 1 K 12 = 10/12/1961
Proper engine ignition timing is critical for maximum engine performance & fuel economy. Until I struggled to keep this engine cool, I was mostly concerned with a distributor curve that allowed for maximum power from 5 - 80 MPH @ WOT. Gas mileage & 3,000 RPM cruise speeds were not a priority. I am now seeking an ignition timing curve that performs well for the following criteria:
1. 5 - 80 MPH @ WOT acceleration - measured at 3,000 RPM
2. 3,000 RPM cruise speed (70 MPH using 336 rear end gears) I’m trying to keep the big motor cool
3. Improved gas mileage. I only averaged 11 MPG on my recent 2,200 trip.

Total Ignition Timing = Initial Timing + Centrifugal Timing + Vacuum Advance timing
* Initial Timing – set @ idle by rotating the distributor – measured in crankshaft degrees
* Centrifugal advance – weights spin in the distributor - measured in distributor degrees
* Vacuum Advance – unit only adds timing during part throttle condition because during WOT engine vacuum is zero – measured in distributor degrees
The distributor turn ˝ the speed of the crankshaft
Distributor Degrees * 2 = Crankshaft Degree
Examples:
3,000 RPM Cruising: 12 Initial degrees + 12 (* 2) centrifugal degrees + 8 (*2) vacuum = 52 Total Ignition Timing
3,000 RPM WOT: 12 Initial degrees + 12 (* 2) centrifugal degrees + 0 vacuum = 36 Total Ignition Timing


The following Five Ignition timing curves will be discussed:
1. Corvette Forum Lars Recommended Dist Curve
2. Dual Point Distributor Curve - when purchased in 1977
3. Dual Point Distributor Curved for 93 Octane gas 10-2008
4. ProComp HEI out of box curve 11-20-2008
5. ProComp HEI curved for maximum power, cruise performance & mileage


Let take a look at Corvette Forum – Lars Recommend Dist Curve
Corvette Forum - Lars Recommended Dist curve
1000 Min Dist RRM
3000 Max Dist RPM
12 Dist Degree -
2000 Max - Min RPM
167 RPM / Dist Degree
12 Initial or lead timing
8 Vacuum Advance - NA
3000 70 MPH Cruise Speed RPM
36 WOT Timing @ 3000 RPM
52 Timing @ 3000 RPM 70 MPH Cruise Speed

This curve should give me the best chance of meeting the criteria below:
1. 5 - 80 MPH @ WOT acceleration
This curve gets all the centrifugal advance “12 Deg dist” in < 3000 RPM
Perfect Score: Measured 36 degree – Spec = 36-38 degrees
2. 3,000 RPM 70 MPH cruise speed
This curve utilizes a vacuum advance unit that adds the correct amount additional ignition timing to optimize cruise performance & hopefully keep the big engine cool!
Perfect Score: Measured 52 degree – Spec = 52 degrees
3. Improved gas mileage
This curve utilizes a vacuum advance unit that adds the correct amount additional ignition timing to optimize fuel economy.
Perfect Score: Measured 52 degree – Spec = 52 degrees





Dual Point Dist Curve when purchased in March 1977
1000 Min Dist RRM
3500 Max Dist RRM
11 Dist Degree
2500 Max - Min RPM
227 RPM / Dist Degree
16 Initial timing
0 Vacuum Advance - NA
3000 70 MPH Cruise Speed RPM
38 WOT Timing @ 3500 RPM
34 Timing @ 3000 RPM 70 MPH Cruise Speed



2. Dual Point Distributor Curve – when purchased in 1977Again – this is a racing distributor that does not utilize a vacuum advance unit so its strength will only be @ WOT acceleration. All other engine igniting timing conditions will not be optimized.
1. 5 - 80 MPH @ WOT acceleration
This curve gets all the centrifugal advance “11 dist degrees” in < 3500 RPM
Perfect Score: Measured 38 degree – Spec = 36-38 degrees

2. 3,000 RPM 70 MPH cruise speed
Poor Score: Measured only 34 degree – Spec = 52 degrees

3. Improved gas mileage
This curve utilizes a vacuum advance unit that adds the correct amount additional ignition timing to optimize fuel economy.
Poor Score: Measured only 34 degree – Spec = 52 degrees

70 MPH @ 3000 RPM & the engine only had 34 degrees of total ignition advance.
Comments:
I was not concerned about engine detonation or engine cooling when I purchased the car in 1977. Premium leaded gas with an octane rating >105 was available. I removed the thermostat in the summer & only had over heating concerns on days when the outside temperature reached >100 degree & the car was in heavy traffic for over an hour. If the engine temperature hit 210 degrees, middle of the gauge, I would put the heater on high & the coolant temperature never went over 215 degrees. Once traffic started to move the engine temperature quickly decreased to normal ~180 degrees without a thermostat.

Fast forward to October 2008:
The engine was rebuilt to the specification below:
1. Block bored & stress honed .030” over size for stock 10.5 -1 compression ratio dome pistons
2. Balanced: Crankshaft, cast piston assemble, flywheel & pressure plate
3. Heads: Upgraded to: intake 2.19” & exhaust 1.88” valves, high performance valve springs, completion valve job
4. Camshaft: Stock - .461 intake .480 exhaust 268 / 274
Comp Cams .470 intake .507 exhaust 265 / 272
5. Mobile #1 15W50 oil & Oil filter
6. Cooling System all stock with newer Harrison Radiator in excellent condition & 160 degree high flow Mr. Gasket Thermostat.







7. Dual Point Distributor curved to run on 93 Octane Gas 10-08

Dual Point Dist Curved for 93 Octane Gas October 2008
1100 Min Dist RRM
3600 Max Dist RRM
10 Dist Degree
2500 Max - Min RPM
250 RPM / Dist Degree
12 Initial timing
0 Vacuum Advance - NA
3000 70 MPH Cruise Speed RPM
32 WOT Timing @ 3600 RPM
27 Timing @ 3000 RPM 70 MPH Cruise Speed

Again – this is a racing distributor that does not utilize a vacuum advance unit so its strength will only be @ WOT acceleration. All other engine igniting timing conditions will not be optimized.
The local speed shop recommended this curve to run the 427 10.5 -1 compression ratio on 93 octane gas.
1. 5 - 80 MPH @ WOT acceleration
This curve gets all the centrifugal advance “10 dist degrees” in < 3600 RPM
Avg Score: Measured 32 degree – Spec = 36-38 degrees
2. 3,000 RPM 70 MPH cruise speed
Poor Score: Measured only 27 degree – Spec = 52 degrees
3. Improved gas mileage
This curve utilizes a vacuum advance unit that adds the correct amount additional ignition timing to optimize fuel economy.
Poor Score: Measured only 27 degree – Spec = 52 degrees
70 MPH @ 3000 RPM & the engine only had 27 degrees of total ignition advance.
I feel, this maybe the cause of the overheating problems.

The Problem:
The cooling system appeared to be working well & temperatures never reached 210 degree with the current mis-calculated “high” gauge during the engine 500 mile break-in period.
I then took the car on a 2,200 mile trip & had to run the heater most of the time to keep the engine cool. The engine temp would rise slowly & after a few hours continue past mid gauge or 210 degrees. I would then put the heater on high to cool the big motor.
The stock temperature gauge & temperature sending unit are in need of calibration to verify the actual engine temp. I know the temperature gauge reads a minimum of 10 degrees & maybe 25 degrees high. Make no mistake about it, the temperature of the air coming from the heater was hot & the mis-calibrated gauge approached 230 degrees. Each morning on a cold engine & would tap off the radiator with fresh GM Dex Cool & when I shut off the car for the evening sometimes it would spit out excess coolant due to expansion.

The Get Well Plan:
1. Replace the 46 years old stock GM distributor & coil with a 50,000 Volt HEI self contained until that utilizes a vacuum advance for improved ignition timing & supports a mechanical tach drive. Will the improved ignition timing @ 3000 RPM keep the engine cooler ~ 200 degree with a calibrated temperature gauge & sending unit?
2. Keep the wiring harness & all stock parts in “unmolested” original condition for possible future use.
3. Calibrate the temperature sending unit & temperature gauge using a non-contact thermometer & ohm meter. Part #2 due 12-31-08
http://www.harborfreight.com/cpi/cta...emnumber=93984
4. Measure the Vette cruise temperature @ 3000 RMP (70 MPH) every 15 minute for 3 hours & document the results. Part #2 – 12-31-08

http://www.ecklers.com/product.asp?p...5&dept_id=1285
Corvette HEI Distributor Conversion, 1962-1974

• Converts Dated Ignition Systems To Modern HEI Technology
• Easy To Install
• Improves Engine Performance
• Shown With Black Cap, Supplied With Red Cap
Now with a simple distributor swap you can get all the benefits of HEI technology plus performance enhancements on top of the original HEI. For example the original HEI had a 38,000 volt module, pickup and coil, this unit provides 50,000 power packed volts! The original system had bushings in the housing; this model has needle bearings for smoother operation and longer life. Plus, this one-piece distributor has a 4340 hardened steel cam gear for use with all roller cams and brass terminals in the distributor for greater conductivity than the stock aluminum terminals. So when you compare this tach drive distributor to what your car came with you're getting a tremendous hi-performance distributor that eliminates the points and coil and gives you all the firepower you need for your Corvette's ignition. This unit simply bolts into place; all you have to do is run a 12 volt power lead to the distributor.

Item# 47935
In Stock

Regular Price $249.99 ea.
ON SALE! $212.49 ea.

The Purchase:
1. HEI Distributor $212.49
Comments: This HEI distributor came in a ProComp box & is mostly likely mfg. off shore.
Non flexible mounting gasket looks different & feels slick. It maybe conductive to GND the HEI to the engine intake manifold. If the gasket is not conductive, the HEI will only be GND through the oil pump! Not too Cool!

Like: - big bang of the buck?
Dislikes: no installation instructions, no springs or weights included to modify curve, no distributor curve information, no Bat wire & terminal connector included, the vacuum advance unit was none adjustable & not labeled, no phone support for installation & setup questions, no black color option.
Options: I had the local speed shop measure & re-curve the HEI distributor before installation.
Cost $88 which included labor, springs, advance weights & vacuum advance unit.

Let take a look @ this HEI out of box curve:
ProComp HEI Distributor Curve out of Box 11-20-08
1600 Min Dist RRM
4200 Max Dist RRM
14 Dist Degree
2600 max - min RPM
186 RPM / Dist Degree
12 Initial timing
12 Vacuum Advance Dist Deg 5 Hg -15 Hg @ 12 Dist
3000 70 MPH Cruise Speed RPM
27 WOT Timing @ 3000 RPM
38 WOT Timing @ 4200 RPM
51 Timing @ 3000 RPM 70 MPH Cruise Speed

1. 5 - 80 MPH @ WOT acceleration
This curve gets 14 dist degrees in @ 4200 RPM – better late then never?
Poor Score: Measured 27 degree – Spec = 36-38 degrees @ 3,000 RPM
Perfect Score: Measured 38 degree – Spec = 36-38 degrees @ 4,200 RPM
2. 3,000 RPM 70 MPH cruise speed
Perfect Score: Measured 51 degree – Spec = 52 degrees

3. Improved gas mileage
This curve utilizes a vacuum advance unit that adds the correct amount additional ignition timing to optimize fuel economy.
Perfect Score: Measured 51 degree – Spec = 52 degrees






Let’s take a look @ this HEI distributor when curved to Lars Spec
Re-curved HEI Distributor 11-25-08
1000 Min Dist RRM
2800 Max Dist RRM
11 Dist Degree
1800 max - min RPM
164 RPM / Dist Degree
14 Initial timing
5 Vacuum Advance Dist Deg 8 Hg -14 Hg @ 5 Dist
Speed Shop was afraid of engine detonation
been using the same VA for 25 years! no need to change!
3000 70 MPH Cruise Speed RPM
36 WOT Timing @ 3000 RPM
36 WOT Timing @ 4000 RPM
46 Timing @ 3000 RPM 70 MPH Cruise Speed

1. 5 - 80 MPH @ WOT acceleration
This curve gets 11 dist degrees ” in < 2800 RPM -
Perfect Score: Measured 36 degree – Spec = 36 degrees
2. 3,000 RPM 70 MPH cruise speed
Good Score: Measured 46 degree – Spec = 52 degrees
3. Improved gas mileage
This curve utilizes a vacuum advance unit that adds the correct amount additional ignition timing to optimize fuel economy.
Good Score: Measured 46 degree – Spec = 52 degrees
I’m going to try the 5 dist degree vacuum advance unit & see how it works. However, I realize that it can swapped it out for a 7 dist degree vacuum advance unit that will provide 50 degrees of total ignition timing @ 3,000 RPM cruise

2. ACDelco Spark Plug Wires 628H ~$73.00 local Auto Parts
Comments: This is a good quality 8mm set with 90 degree angle boots @ the cap & straight or 0 angle @ the spark plug.
Like: ACDelco name on the wire – keep it stock & keep it cool!
Dislikes: 4 out the 8 spark plug wires are too short for his over the valve cover application. The #2 cylinder wire has almost zero slack & runs from the plug to the #1 cylinder on the cap in a straight line. I’m going to have to purchase 4 larger spark plug wires to match the above set or build a custom set with high performance after market spark plug wires. This set may work fine for a SBC. Thus, I recommend you measure or bring your old set with you so you can compare spark plug wire lengths before you make your purchase.

3. ACDelco RapidFire #4 Spark Plug ~$28.00 local Auto part
Comments: I gapped the plug to .055” & hope it will run better that the stock .045”
Like: Plug resists fouling – remember I did not have a vacuum advance & other plugs would foul easily.
Dislikes: - none

4. GM HEI Terminal Wire mfg Big End ~$7.5 local Speed Shop
Comments: I recommend using a purchased wire & terminal to keep the quality high & look original.
Like: - This is a high quality #12 wire that feeds 12 Volts to the HEI.
Dislikes: no installation instruction! Is a relay required or not? Can I use the stock wire for this upgrade?

5. Valve Cover Gasket – Fel Pro $43.95 local speed shop
http://www.edwardsracingengines.com/gaskets_seals.html
Comments: I recommend adjusting the valves & performing a compression test whenever the spark plugs are removed.
Like: - This is a thick high quality valve gasket set. Don’t cut corners on quality here! You may need to use longer valve cover bolts.
Dislikes: a little pricy
The Preparation:
Clean up the harmonic balancer & timing makers on the timing cover. I recommend a damp rag with paint thinner to clean the area & apply white paint using a fine tip brush.
Apply new wire markers to the new HEI cap using the old cap as a guild.
Apply new wire markers to the new HEI spark plug wires using the old wire set as a guild.
Gap the new Rapidfire Spark plugs ~.045” +/- .005 (I pushed it a little & gapped mine to .055”)
I also recommend that the valves be adjusted & an engine compression test be performed while the plugs are out of the motor. Record your results for future use.
These are easy steps that will pay dividends during routine maintenance may time over in the next few years.
The Removal – Original Dual Point Distributor.
1. Disconnect all spark plug wires from spark plugs & remove all spark plugs from the engine. This will allow the engine to turn easy. Keep all the spark plug wires on the original distributor cap for future reference.
2. Remove the original GM distributor cap & turn engine until the #1 plug is ready to fire @ ~ 14 degrees BTDC. The engine can be turned by hand using the bolt on the vibration damper or a remote starter switch can also be used.
The remote starter switch has 2 wires that need to be connected.
The large remote starter wire goes to the largest bolt on the starter from the Bat.
The smaller wire connects to the starter solenoid marked “R”
3. Label all wires connected to the coil & ballast resistor before removal. Remove the stock coil wiring, coil & coil mounting brackets
Note: Once the original distributor is removed the engine crankshaft should not be rotated until the new HEI is properly installed.
Note: the location of the distributor base, vacuum advance or tach drive location & rotor position before & after removal.
4. Remove the original distributor hold down bolt & clamp. Pull the original distributor straight up & note the current location of the rotor it will turn “counter-clockwise”.
The HEI has to be installed with its distributor base , vacuum advance or tach drive location & rotor in this “counter-clockwise” position to enable maintain the #1 cylinder ~14 degree timing BTDC.
5. Prep for HEI Installation:
Clean the engine intake manifold surface thoroughly. This is also an excellent time to clean up around the wiper motor & firewall while the distributor is removed for the car.
By-pass the ballast resistor, remove all wires from the ballast resistor & use the terminal closest to the distributor as a terminal block. The terminal block should now have 4 wires:
1. GM HEI Terminal– you have to purchase this wire & connector. It was not include with the distributor. I got mine form the local speed shop it is mfg by Big End GM HEI Terminal 54105. This is a #12 wire, much thicker then stock, & I still not 100% sure if it is required to connected via relays for maximum performance.
2. A pink wire connector contains two pink wires
3 A pink wire connector with a single wire
4. Larger black wire that goes to dash.
6. HEI Installation:
Make sure the distributor gasket is on the distributor shaft & the intake in properly clean. Apply engine assemble lubrication to the HEI drive gear before installation. Align the distributor base, make note of the rotation of the vacuum advance or tach drive & turn the rotor to the “counter clockwise” position & install the HEI. If all goes well it should just drop in engage the oil pump. Make the required adjustments, slight turn of the rotor or adjust oil pump position to get the HEI properly installed. Install the distributor clamp & tighten bolt until it holds the HEI during timing. Connect the distributor cap to the HEI. Connect the GM HEI Terminal wire to the HEI distributor.

7. Manual Engine Timing:
I recommend timing the engine manually before use the timing light.
Connect a new HEI spark plug wire to the #1 HEI cap terminal & the other end to a GND spark plug. Now turn the key on & turn the distributor quickly left & then right until you see the spark jump the plug gap. Snug the distributor bolt where the distributor fires the plug. In our example this should closed to the ~10 BTDC. If you do not have spark verify you are getting 12 Volts @ the HEI terminal & all connections are secure. During my installation I found a loose wire connection inside the distributor.

8. Install the Rapidfire spark plugs & labeled spark plug wire on the HEI Cap.
9. Using the Timing light & Voltmeter
Connected the timing light leads to the battery
Clip the timing light pick-up to the #1 cylinder spark plug wire.
Turn the key on & using a voltmeter measure voltage a:
Battery – should be ~12.3 Volts
HEI Bat terminal - should be ~12.3 Volt
Make sure all is clear before try to start the engine:
Crank the engine over using the key or remote start & measure the battery voltage during cranking it should be ~8.9 Volts
10. Start the Engine & Test Drive
If all the voltages check out, then point the timing light @ the timing chain cover & measure the ignition timing while cranking the engine. Rotate the distributor to obtain ~14 degrees BTDC. Give the engine some gas & try to start the engine. My engine started right up & idled very smooth. Re-check the distributor hold down bolt. It must be tight or it will leak, remember it that special gasket. I found that out the hard way! Connect the HEI vacuum advance to full manifold vacuum & go for a test drive.
My engine responded well to the HEI! It starts up easier, seemed to run quieter, “most likely that Ľ turn past lash point valve adjustment”, idle smoother & accelerated quickly from 5 – 80 MPH. However, I cannot say that I noticed a major improvement in acceleration when compared to the 42 year old stock system. Remember, old system was fully functioning & in-tune system, it was swap out for improved ignition timing @ 3,000 RPM cruise speed to resolve over heating concerns. If your stock ignition system is in need of a tune up & you upgrade to a HEI system you can expect to see much larger benefits.

One last note, I’m currently using the stock vacuum advance tube that connects from the base of the stock Holley carburetor & the HEI. However, I’m thinking about going straight from the intake manifold barb fitting to the vacuum advance for possible improved performance.

11. The Follow up
I plan to share the measured results of the temperature gauge & sending until calibration & HEI ignition timing as it relates to the engine cooling temperatures before the end of the 2008 year.

Keep your motor running!
Andy