hot1corvette1

Hi all,

I have been working an overheat problem on my 427/425 L72 motor. Good progress, so far, with a new aluminum hi flow water pump, new thermostat, new Aluminum radiator. I have replaced the old distributor (mechanical advance only) with a new distributor.

Problem: We have a fairly high-advance vacuum can on there, and I believe that we are getting way too much vacuum advance which on top of the base advance of 8* and the mechanical of about 28* is causing a bit of a problem both at idle and at rpm above 3000. I think we are getting more than 20* advance off of the vac. advance can at idle. The idle is way to fast and a slight roughness at 3000 rpm and above. However, with the vac. advance working, the engine runs way cooler both at idle and at speed. I would like to replace the vac. can with one that has lower advance so as to keep the benefits of the cooling and yet eliminate the idle and rpm symptoms.

Question: Is there a place to find info on the various vac. cans available so I can determine which one to purchase? Also, is it possible to replace the vac. can without removing/disturbing the distributor???

btw, I will be at the Monterey Del Oro shows, the NCRS meeting in Monterey, and the Monterey Historics next week. Maybe we can connect while there.

Thanks,
Mark

TheOman

Mark

You should go back to some of the posts from SWCDUKE. He has a number for an Echlin vac can that he swears is correct for most of these high perf engines from the 60's that are operating with todays rotten gas but with the higher compressions of the past. . I don't recall the part number but it is in the post. It might be that you were even part of the thread that I am suggesting you look up, but FWIW I think it might help.

I have been meaning to ask Duke if he has or could possibly write a quick discertaion on why this can or that can makes sense and how the vac advance map actually effects temp. Perhaps Duke will see this and respond.

Enjoy Monterey. I visited there often when I was out on the coast with HP. I also had occasion to visit there in the late 60's when a friend got discharged from Fort Ord. He had orders for Nam in 68 /69 and he got pulled of a plane at Alameda for reassignemt to Fort Ord. He thought he died and went to heaven! When he separated form the Army I flew out and bummed around with him for a few months out there then we drove back to Connecticut together.

JohnZ

The one you want is Echlin #VC-1810 - provides 16* advance at 8" Hg, so it's fully-deployed at idle with a wild cam. I'm at Monterey this week too, and Duke will also be here today - don't know if he'll have a laptop with him or not. Do a search on "vacuum advance" or just on "SWCDuke" - Duke went through this in gory detail two or three times in the last week in multiple posts.

64BB

i tried the can duke suggested and it works fine. cost me $12 at NAPA. however, i also tried the can that came with my ac delco single point used distributor. it begins to open about 8" and is fully open about 17". it works better. the car was quicker off the line and had better mid range torque.
finally, i bought the accel variable advance can for $22, and now i can diddle to my heart's content. i would get the accel can because every motor and cam etc are a little different as is the gas we use and the carbs and valves and the ignition.
i am not sure that "one size fits all", and for the $10 extra, the accel can gives me lot's of flexibility and allows me to experiment more. IMHO

jerrybramlett

Yes, there is a place to find specifications on the various vacuum canisters available.

Most aftermarket vacuum canisters are made by Standard Ignition Products. These are marketed under several names now other than Standard, including CarQuest, Echlin, and Delco. The Standard cansiters sold by Delco are even repackaged in Delco boxes! Standard canisters can be recognized by the code stamping on the mounting bracket. They all have the letter B followed by a number. Although unique for each canister design, this stamped code isn't the Standard part number.

CarQuest stores have an illustrated parts guide for "Engine Controls". Within this 1 inch thick book are photos of the different vacuum advance canister designs available, along with a sheet summarizing the specifications for them. The specifications are organized according to the Standard part number system. If you have a canister labeled B28, you must know that this is Standard part #VC-171 before you can look up the specifications for it.

You can replace the canister without removing or rotating the distributor. It is much easier if you use a vacuum gun to hold the canister at full advance while you remove the screws.



[Modified by jerrybramlett, 2:52 PM 8/9/2002]

SWCDuke

Hello! Does anybody read my posts? Like John says, I've written a good thousand words on this subject in the last month alone, but the basic rule of thumb is to have a vacuum can with about 16* of advance at a vacuum level that is a couple of inches less than what the engine generates at idle, so if you have a SHP engine or an equivlanet cam that idles at 10-12" the V1810 can is the way to go because the full 16* is in at 8" with initial timing in the range of 8-16 depending on how much centrifugal in in the distributor has and how much total WOT timing you want to run.

My Cosworth Vega is loaded and cocked for an 0500 departure tomorrow morning for Monterey, and I'll be taking some back roads to give the DOT legal racing tires, suspension, and Spax shocks a workout, so who know when or even IF I'll even get there. Hey, it's a Vega, right? ;)

Duke

achapman

You say Vega.... I say Viagra.... :lol: :lol: :lol:

Good Luck DUKE.... even though its not a Vette.... It is a Chevy..... You know were rootin for ya.... :cool: :cheers: :flag

TheOman

John and Duke

What you guys wrote is what I needed. Thanks a lot. A simple short concise paragraph like each of you so kindly gave is perfect. . All the theories and ideas interlaced amongst your earlier words of wisdom in other posts just clouded the issue. The "Well my cam does this" and "My buddy says' and "It rained the day I tested the car and and da da da da just had things so clouded in my mind that I lost track of where this all started.

Sorry about the content of my post that you quoted. As I read it I was asking for a complete redo of all your work to date and that is not what I wanted. What you and John Z offered is what I needed and wanted.

Thanks again.

Oman


[Modified by TheOman, 3:35 PM 8/9/2002]

SWCDuke

Thanks, I am really looking forward to this trip as it will be the my first opportunity to meet John Z in person. Bringing the CV is important because John Z was the production engineer in charge to getting the CV into production at Lordstown!!!

We'll be having some serious discussion about all the DESIGN CHANGES I've had to make on this car over the past twenty years - like the oil cooler to keep from frying the engine at the track, the bigger brakes that actually made it stop properly without rear lockup - the retuned suspension that eliminated the limit oversteer, the VACUUM ADVANCE I had to add because it had none originally, and the reindexed cams so it has some low end torque.

Of course I know what John will say - "Hey I was the manufacturing guy. You'll have to take up all your gripes with the design engineers". ;)

Duke

hot1corvette1

OK, OK....yes I have read and reread all of the earlier posts. I am, perhaps, very dense on this topic. I very much like the idea of getting an adjustable vacuum can. I would try to start with the numbers that SWCDuke is suggesting and then diddle from there.

But, noone answered my second question: Is it possible to swap vac. cans without removing the distributor?

And lastly, I am very much looking forward to meeting SWCDuke and JohnZ in person.

Thanks for all your help.
Mark

jerrybramlett

There is no need to remove the distributor from the car.

After removing the two screws that hold the vacuum canister bracket to the distributor housing, I wiggle the actuating rod out of the points baseplate. Sometimes I have to use a small tool to push the tip of the rod below the baseplate. The vacuum gun makes it easier because it will hold the baseplate in a position that allows straight access to the screw heads. It also helps access to remove the rotor while you are changing canisters.


[Modified by jerrybramlett, 7:37 AM 8/10/2002]

SWCDuke

:U Hey, don't mind me. I'm just getting antsy as I approach departure for a thousand mile solo road trip in a car that only three other guys in the world can fix besides me. My obsessive compulsive side is taking over. ;)

I have heard that on a TI dist. the vacuum can can be swapped without disassembling the distributor. I've swapped cans on a TI and I think I took it out and apart to do it, which is really not much of a job, but I can't remember for sure. On a single point, I'm sure you have to pull and disassemble the distributor, and on an HEI I'm equally sure that the can can be swapped with the dist. installed.

Duke

JL66REDCPE

I recently swapped cans on my 427/425 with TI. I pulled the dist and took it apart to change the can. Biggest challenge was the shielding - as usual.

lars

hot1 -
Here is a listing of a bunch of the vacuum advance units with their specs in my vacuum advance tech paper. A clean, properly-formatted version of this paper is available if you e-mail me a request to:
V8FastCars@msn.com

Distributor Vacuum Advance Control units
Specs and facts for GM Point-Style Distributors

by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. New 4-19-01


I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few facts and definitions, and provide a complete part number and specification listing for all vacuum advance control units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much time to research each part number by application across each of the GM Motor Divisions – it took me long enough to compile just the Chevy stuff…!). I have not included the HEI listings at this time (the HEI distributors use a longer control unit, so these part numbers CANNOT be used on an HEI) in the interest of time, but I can certainly compile those specs, too, if there’s interest.

As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in this information, please let me know so I can correct it.

Background
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.

ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

The following listing is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for most performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations.

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).

P/N............ID#......Application....... .....Starts @ “Hg..Max Adv (Distr. Degrees @ “Hg.)

VC680........B1.......59–63 All Chev......8-11...................8 @ 16-18
..............................1964 Corvette exc. FI
..............................1964 Impala, Chevy II
..............................1965 396 High Perf.
..............................1965-67 283, 409
..............................1966-68 327 exc. Powerglide
..............................1967-68 All 396
..............................1969 Corvette 427 High Perf.
..............................1969 396 Exc. High Perf.
..............................1969 Corvette 350 TI
..............................1969-70 302 Camaro
..............................1970 400 4-bbl
..............................1970 396 High Perf.
..............................1970 Corvette 350 High Perf.
..............................1973-74 454 Exc. HEI

VC1810......B28......65 409 High Perf...3-5....................8 @ 5.75-8
..............................1965 327 High Perf.
..............................1966 327 High Perf.
..............................1964-67 Corvette High Perf. FI

VC1605......B9........1965 impala 396....7-9...................10.3 @ 16-18
..............................1965 327 All Exc. FI
..............................1969 327 Camaro, Chevelle, Impala
..............................1969-70 Corvette 350 Exc. High Perf.
..............................1969-70 350 4-bbl Premium Fuel
..............................1970 350 Camaro, Chevelle, Impala High Perf.
..............................1971-72 350 2-bbl AT
..............................1971-72 307 All

VC1675......B13......68 327 Camaro A/T..9-11...............8 @ 16-18
..............................1968 327 Impala AT
..............................1968 307 AT
..............................1968 302, 307, 327, 350 Camaro, Chevy II
..............................1970 350 Camaro, Chevelle Exc. High Perf.

VC1760.......B19.....69 350 Camaro4-bbl..5.5-8............12 @ 14-18
..............................1969-70 350 2-bbl

VC1765.......B20.....65 396 Impala Hi Per..5-7...............8 @ 11-13
..............................1966-67 Corvette Exc. High Perf.
..............................1966-67 Impala 427 Exc. High Perf.
..............................1966-68 327 Powerglide Exc. High Perf.
..............................1969 307 All
..............................1969-70 396, 427 Camaro, Chevelle High Perf.
..............................1970 400 2-bbl
..............................1970 307 MT
..............................1973 Camaro 350 High Perf.

VC1801......B21......1971 350 2-bbl.....7-9.....................10 @ 16-18
..............................1971-72 400, 402
..............................1971-72 307 AT

VC1802......B22......1971-72 350 4-bbl..7-9...................8 @ 14-16


Other Part Numbers & Specs:

VC700.........B3............8-10............11.5 @ 19-21
VC1415.......M1......................... ......10 @ 13-15
VC1420.......M2............5-7..............11 @ 16-17
VC1650.......B12..........8-10.............10 @ 15-17
VC1725.......B18..........8-10.............12 @ 13-16
VC1740.......A5............6-8...............12 @ 15-17.5
VC1755.......A7............8-10.............12.5 @ 18-20.5
VC1804.......B24..........6.5-8.5.........10 @ 12-14
VC1805.......M13.........6-8................12 @ 14.5-15.5
VC1807.......B25..........5-7................8 @ 13-15
VC1808.......B26........................ .......8 @ 11-13
VC1809.......B27..........5-7................9 @ 10-12
VC1812.......B30..........5-7................12 @ 11.75-14

64BB

christ lars, that was a wondrful post. thanks.

Subfixer

Oh Lars, god of the vacuum can numbers...... Excellent post.

Here's one for ya.. While trying to determine my timing problems, I located the vacuum can number stamped where you indicated it would be. Here's what mine had: MS 201 (the "201" is under the letters). What the heck is this??

lars

Sub -
That number is not a Dana-built control unit, so I can't cross-reference that one - sorry! I've seen numbers like the one you reference, but I'm not sure on the manufacturer on those series numbers.

magicv8

Why dink around with specs and can switching when Accel makes a cheap adjustable can that you can set to your heart's and engine's content?

SWCDuke

201 were the last three digitsof the GM part number of the can typically used on medium performance engines (300HP), and the "B1" is the equivalent replaceent.

Contrary to lars recommendation, the B28 probably belongs on any engine that has an aftermarket high performance cam. It was used on L76,79,84 engines from early '64 and up. When you replace an OEM 300 HP cam with an aftermarket high performance cam, idle vacuum drops from 16" or more to 12" or less. Consequently, the vacuum advance is not fully deployed at idle, and the plunger dithers around due to varying advance and manifold vacuum, which contributes to idle instabililty because of the varying advance and idle overheating because there is not enough total idle advance.

If you "upgrade" a 300 HP engine with the L-79 or similar aftermarket high performance cam, you need the B28 can and you can verify by checking idle vacuum. If the vacuum can doesn't provide full vacuum advance AT LEAST 2" below what the engine pulls at idle, you don't have a properly matchedvacuum can, and you will have both idle stability problems and idle overheating problems, particularly if you have an old radiator.

Duke

lars

magic -
The reason you select a fixed unit and don't use one of the adjustable units is because the adjustable units only have ONE adjustable parameter: the length of the curve - you adjust the allen screw inside the unit to adjust how much advance it pulls in. The units are not adjustable for how much vacuum is needed to start the curve and to terminate the curve. So if you need the curve to come in sooner or later, you need to measure your manifold vacuum and select a unit that brings the timing in when you need it to occur.

SW -
Good points, and I agree. The quick unit (B28), as I noted, is a great tuning aid for engines that need the rapid advance - engines with lumpy cams. Your notes also specifically apply to guys who are running straight manifold vacuum to their vacuum advance - typical of the early cars. If you're running straight manifold vacuum, you need to select a unit that pulls all of the advance in at idle, otherwise the unit will fluctuate and alter the timing erratically as you noted. Later cars used ported vacuum for the vacuum advance, so this does not become an issue, and you can select a unit that brings all of the advance in at the vacuum present at cruise rpm. For this reason, I've noticed that the B1 can seems to work best for these later applications. But early cars with big cams and manifold vacuum applied to the advance can make excellent use of the B28.