Modified 302 wants more timing at idle - Vintage Mustang Forums
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post #1 of 31 (permalink) Old 09-08-2019, 11:09 PM Thread Starter
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Modified 302 wants more timing at idle

Engine specs

306 cid 10:1 CR
AFR 165 Heads
Edelbrock RPM Air Gap Intake
Holley 570 Street Avenger
Lunati 218/218 Cam EFI compatible 351W firing order
Roller Rockers
LT headers
Ford Electronic Ignition
NGK 7373 Plugs
MSD 8mm wires

At 30* timing purs like a kitten and fires immediately with the first turn of the key. Back it off to 16* and it is kind of rough and choppy...doesn't start at first crank, but after a couple of secs it will.

I'm thinking 16-18* is where this setup wants to idle 750 rpms. Doesn't change whether I disconnect the vacuum advance or not so I no it's ok on the idle setting and not opening the port vacuum yet.

Climbs up to 30* at 200-3K rpms

Thoughts?

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post #2 of 31 (permalink) Old 09-08-2019, 11:37 PM
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Are the timing markers correct? Does the distributor have a vacuum advance and are you disconnecting the vacuum hose?

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post #3 of 31 (permalink) Old 09-09-2019, 01:15 AM Thread Starter
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Originally Posted by Huskinhano View Post
Are the timing markers correct? Does the distributor have a vacuum advance and are you disconnecting the vacuum hose?
So these are all questions that I tried to answer today and verified. I put #1 at TDC, and that looked to be about 10* BTDC on the timing mark when the rotor was closest to pointing at #1 on the distributor cap. I did notice that the timing marks on the Pioneer 289 balancer are different than the Pioneer 302 balancer I got too. I couldn't use the Pioneer 302 balancer that my engine builder supplied as I needed a 289 balancer. The 289 balancer is thinner and doesn't have a lip on the front of it like the 302. The OEM AC triple crank pulley wouldn't line up with the thicker 302 balancer...plus it has a lip on the front of the balancer so the triple crank pulley wouldn't even mate up to flush in the first place.

I also compared where the timing marks between the two balancers by looking at where the balancer keyway was with respect to the timing marks. The key way timing marks are definitely different with the 302 balancer vs. the 289.

With #1 at TDC, showing about 10* BTDC on the balancer and the rotor pointing at #1 on the dizzy (approximately 2 o'clock position) everything looks good. The vacuum advance is pointing about straight towards the Rad.

I did set the timing without the vacuum advance disconnected from the vacuum advance on the carb. Then connected it. I don't believe the port vacuum opens until the throttle opens further, so it didn't change at idle. My understanding is that the throttle has to open more for the ported vacuum advance to start working. It starts after turning and giving it a few pumps. Then idles around 700-750.

At 30* BTDC the engine really sounds good and fires up quickly with the distributor turned clockwise all the way over to where the vacuum advance touches the thermostat housing and revs quick too. But that shows 30* BDTC at idle with or without the vacuum line hooked up and I think it is right because the dizzy turned all the way clockwise is touching the thermostat housing. But there is no way in hell that I would keep it that way as it would end up being like 50* BTDC above 3K rpms (vacuum advanced).

I did question the pointer location too. I had the older style timing cover with the built in timing mark, but the engine builder put a newer style timing cover on it. I had to buy the Scott Drake timing pointer and install it on the driver side so I had a timing pointer. I'm pretty sure the timing mark is correct though as at 30* the idle and start up is fantastic, but the dizz vacuum advance should be closer to straight ahead vs. turned all the way clockwise.

Car just doesn't want to start as easily at 12-14*, I got it started around 16* and then adjusted the idle...it is a little choppy but I put it in gear and the idle didn't even change in drive...stayed at 750 rpms.
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post #4 of 31 (permalink) Old 09-09-2019, 10:32 AM
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Is your vacuum advance connected to manifold vacuum or ported vacuum? It sounds like ported as the timing advances once the throttle is opened a little. If so, change to manifold vacuum and see how it runs.
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post #5 of 31 (permalink) Old 09-09-2019, 10:52 AM
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Mechanically verify your tdc reference marks, simplest way to see if your close is a screwdriver in the #1 spark plug hole at tdc. Move the crankshaft back and forth to see when the piston is all the way to the top and check your mark. Don't judge the amount of timing by your distributor housing position hitting an obstacle as that can easily mean the distributor is just indexed wrong.
You should be using manifold vacuum, set with the hose disconnected, then adjust idle speed after base timing is set and hose reconnected. Ported vacuum can be used but is typically emission related settings.
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Last edited by Matter; 09-09-2019 at 10:57 AM.
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post #6 of 31 (permalink) Old 09-09-2019, 01:51 PM
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Quote:
Originally Posted by Shinobi68 View Post
So these are all questions that I tried to answer today and verified. I put #1 at TDC, and that looked to be about 10* BTDC on the timing mark when the rotor was closest to pointing at #1 on the distributor cap. I did notice that the timing marks on the Pioneer 289 balancer are different than the Pioneer 302 balancer I got too. I couldn't use the Pioneer 302 balancer that my engine builder supplied as I needed a 289 balancer. The 289 balancer is thinner and doesn't have a lip on the front of it like the 302. The OEM AC triple crank pulley wouldn't line up with the thicker 302 balancer...plus it has a lip on the front of the balancer so the triple crank pulley wouldn't even mate up to flush in the first place.

I also compared where the timing marks between the two balancers by looking at where the balancer keyway was with respect to the timing marks. The key way timing marks are definitely different with the 302 balancer vs. the 289.

With #1 at TDC, showing about 10* BTDC on the balancer and the rotor pointing at #1 on the dizzy (approximately 2 o'clock position) everything looks good. The vacuum advance is pointing about straight towards the Rad.

I did set the timing without the vacuum advance disconnected from the vacuum advance on the carb. Then connected it. I don't believe the port vacuum opens until the throttle opens further, so it didn't change at idle. My understanding is that the throttle has to open more for the ported vacuum advance to start working. It starts after turning and giving it a few pumps. Then idles around 700-750.

At 30* BTDC the engine really sounds good and fires up quickly with the distributor turned clockwise all the way over to where the vacuum advance touches the thermostat housing and revs quick too. But that shows 30* BDTC at idle with or without the vacuum line hooked up and I think it is right because the dizzy turned all the way clockwise is touching the thermostat housing. But there is no way in hell that I would keep it that way as it would end up being like 50* BTDC above 3K rpms (vacuum advanced).

I did question the pointer location too. I had the older style timing cover with the built in timing mark, but the engine builder put a newer style timing cover on it. I had to buy the Scott Drake timing pointer and install it on the driver side so I had a timing pointer. I'm pretty sure the timing mark is correct though as at 30* the idle and start up is fantastic, but the dizz vacuum advance should be closer to straight ahead vs. turned all the way clockwise.

Car just doesn't want to start as easily at 12-14*, I got it started around 16* and then adjusted the idle...it is a little choppy but I put it in gear and the idle didn't even change in drive...stayed at 750 rpms.

I suspect you have the wrong timing marker. Up to and including 69, Ford used a cast in marker and 3 bolt balancer. From 1970 and up Ford relocated the lower hose, relocated the timing marker and used a 4 bolt balancer. While some of the brackets and pullies look the same, nothing will interchange with each other, nothing! So Ford redesigned the timing cover to be universal with bolt on timing markers. 1 for up to 69 and a 2nd for 1970. I think what you have is the 1970 timing marker for your 1969 and earlier balancer @Matter hit the nail on the head. His comments are excellent!

Now on balancers. I have a 97 GT40P dressed to look like a 289. I used a PRW balancer. These are really excellent. They have both sets of balancer bolt patterns, 3 & 4 bolt. They also have 3 sets of timing marks. In addition they use a bolt on balance weight. You can use this balancer on a zero internally balanced motor, 28 or 50 ounce unbalanced motors be leaving the weight off, or using the appropriate weight. Under $100 and very nicely made. If it doesn't work, you probably have a LS motor.

Follow Matters advice on finding TDC and verify timing marks.

Tom

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post #7 of 31 (permalink) Old 09-09-2019, 02:54 PM
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Is your vacuum advance working at all? If it is, 30 initial would be too much for initial timing if it is really at 30 which is another question. It isn't unusual for even a mildly modified engine to like 16 or 18 degrees initial timing.


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post #8 of 31 (permalink) Old 09-09-2019, 11:00 PM Thread Starter
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Ok so I rechecked everything
#1 Piston TDC, both valve shut on compression.
Harmonic Balancer 0* TDC
Rotor pointing at 1, and re-indexed.
Dist. Vacuum connected to Holley Timed Vacuum Source to Distributor Advance.

Here is what I did incorrectly. I was timing it looking straight down at the balancer from the top, because of the angle I found that timing light hits the marker making it appear that it has 2* more advance than it does. I used the timing light from the angle of the AC Compressor looking straight onto the timing pointed and I could actually see the numbers much better. I was having problems seeing the numbers from top down and actually thought I had a dim bulb on my timing light but it was me being the dim bulb. I went back and forth with the timing light and it definitively appears to be 2* looking from the top down and is difficult to see.

Anyway now it is set to 14-15* with the vacuum advance on or off as it doesn't activate until the throttle is cracked open more. The timing advance goes up towards 30* when I bring up the RPMS. Idle is set to 800 rpms. Engine fires right up at first turn of the key and sounds bad *** with a slight lope.

I also included a pic from the passenger side of FPA (Stan's) LT Headers. The fitment is superb and I have power steering. The only thing I had to do on both moto mounts is turn the rear bolts so that the bolt head was on top and the nut was on the bottom, the threads were making contact with the threads pointed up. My understanding is that the pro thane motor mounts are little larger based on a 73' motor mount than the stockers. Anyway hats off to Stan for having a perfectly fitting LT Header.

Thanks for the help and recommendations on what to check. Sorry the pics won't post straight up and down.
Attached Images
File Type: jpg Harmonic Balancer.jpg (344.1 KB, 14 views)
File Type: jpg Top Down.jpg (516.3 KB, 15 views)
File Type: jpg FPA Passenger Side Header.jpg (388.5 KB, 14 views)
File Type: jpg Dist Rotor off Top View.jpg (488.7 KB, 11 views)

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Last edited by Shinobi68; 09-09-2019 at 11:13 PM.
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post #9 of 31 (permalink) Old 09-10-2019, 12:16 AM
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Sounds like you are still on ported vacuum? You will probably be happier if you get it dialed in on manifold vacuum. Check your total advance also if you can, not sure what number you should shoot for total but I would guess around 36-38 if you are around 9.5 to 10.5 compression. Make sure you are not over advancing. Not spending the time with your timing numbers and curve profile is leaving a lot of power on the table.
Nice looking engine btw!

Last edited by Matter; 09-10-2019 at 12:19 AM.
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post #10 of 31 (permalink) Old 09-10-2019, 01:06 AM
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Forgot to mention your total advance should be checked with vacuum hose unhooked, otherwise with no load on the engine it would likely show way over advanced. The vacuum advance can be adjusted later if needed. Try both manifold and ported vacuum and see which works better for your application. Neither is right or wrong as you may possibly need ported, or more accuratly "timed" vacuum. Either one is pretty much done when you drop below around 6" of vacuum anyway so it's all about the day to day drivability.

Last edited by Matter; 09-10-2019 at 01:33 AM.
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post #11 of 31 (permalink) Old 09-10-2019, 10:29 AM Thread Starter
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Quote:
Originally Posted by Matter View Post
Sounds like you are still on ported vacuum? You will probably be happier if you get it dialed in on manifold vacuum. Check your total advance also if you can, not sure what number you should shoot for total but I would guess around 36-38 if you are around 9.5 to 10.5 compression. Make sure you are not over advancing. Not spending the time with your timing numbers and curve profile is leaving a lot of power on the table.
Nice looking engine btw!
I hooked up the vacuum per this pic in the manual and have the tranny vacuum hooked up to the front of the carb near the PCV connection. I will check the total advance with the vacuum line off as you suggested as I didn't think to do that and only checked at idle. I definitely do no want to leave any power on the table after doing this build. My static compression ratio is 10:1.

There are 4 vacuum lines on the Holley Street Avenger.

1. Brake Booster in back
2. Distributor timing advance
3. Full Manifold vacuum front near PCV (I have tranny hooked up to it)
4. PCV

Thanks!
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Last edited by Shinobi68; 09-10-2019 at 10:38 AM.
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post #12 of 31 (permalink) Old 09-12-2019, 01:38 AM
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Try hooking your distributor to the same port with a tee that you have going to your transmission for full manifold vacuum, and see how you like that. You will have to readjust your idle speed as it will raise a bit.
The timed (ported) vacuum originally was a way to retard the timing at idle speeds to help raise exhaust temps to assist burning hydrocarbons with the thermactor and other exhaust burn systems. All about emissions, not performance. The distributors on those typically had more mechanical advance built in to compensate.

Last edited by Matter; 09-12-2019 at 01:48 AM.
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post #13 of 31 (permalink) Old 09-12-2019, 03:23 PM Thread Starter
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Quote:
Originally Posted by Matter View Post
Try hooking your distributor to the same port with a tee that you have going to your transmission for full manifold vacuum, and see how you like that. You will have to readjust your idle speed as it will raise a bit.
The timed (ported) vacuum originally was a way to retard the timing at idle speeds to help raise exhaust temps to assist burning hydrocarbons with the thermactor and other exhaust burn systems. All about emissions, not performance. The distributors on those typically had more mechanical advance built in to compensate.
Interesting I did not know that and will definitely try it. Question on the intake manifold vacuum. There is a port that I plugged that is actually at the back of the intake manifold, in some pics I have seen it looks like this is or where there would be a T. Should I hook my tranny manifold vacuum up to it and then have my distributor vacuum go to the manifold vacuum at the front of the carb?

I guess it's the same except the tranny would not be going through the carb to get to intake it would just be directly connected to the intake near the back. Basically I'm just getting rid of the T it seems like. Unless there is a difference between the intake manifold vacuum on the intake and the one on the front of the Carb?

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post #14 of 31 (permalink) Old 09-12-2019, 05:26 PM
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I just went through your identical situation with my rebuilt 289, now 331 with Edelbrock cam. My engine wants way more timing at idle then before, it wants 16-18 degrees to idle smoothly. I tried to set timing with my Pertronix vacuum advance plugged, and quickly found out that the distributor was adding 30 degrees of mechanical advance. When I set the timing at 36 degrees total on the timing light, I only had 6 degrees at idle, and of course, the engine would not idle. Since I knew I had mechanical correct at a total of 36 degrees, I tried connecting the vacuum line to MANIFOLD vacuum at the carb, and fired the engine, after which it idled just awesome at 18 degrees. If I connect to PORT vacuum the motor dies immediately with 6 degrees at idle.


I am concerned, as are other members, that you will have WAY too much mechanical total timing when you check it, since you are timing idle connected to port vacuum.
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post #15 of 31 (permalink) Old 09-12-2019, 06:11 PM
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Here is an article written by a GM engineer that has circulated on the forums. It has some good info.

I am in a similar timing situation https://forums.vintage-mustang.com/v...-question.html

Rusty


by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. B 8-19-02

Part 1 - TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

Part 2 - Distributor Vacuum Advance Control Units

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…!). This latest revision to this paper also includes the HEI listings (the HEI distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be interchanged).

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. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

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 driveability 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.
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1965 Coupe, Caspian blue, A code 4 speed. Started project fall 1993, hopefully back on the road Spring 2019!

Last edited by rusty1161; 09-12-2019 at 06:34 PM.
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