THE TRUTH About Vacuum Advance- What is it and WHY do you need it?? Ignition Timing 101

Why do you need vacuum advance?
Port or Manifold? I give my take on which I prefer. Also, do you use lock washers? You might throw them away after watching this!

[DavidVizard]

Regardless of what this video tells you - subscribe as it has good content.
David vizard

[hughobrien]

Man oh man! You’re explaining a lesson I wished that I had learned the easy way.
Close to 20 years ago I built a SBC for a 1969 SS Camaro convertible. Very nice restoration. Engine had to be built to look exactly like it was off of the show room. So no HEI conversion.
What the customer elected to do was a Pertronics conversion in a dual point housing.
The distributor was a “rebuilt” brand from a popular parts store here in the U.S.
The conversion kit was straight forward and looked very clean.
Where I got into trouble as the engine builder…..
When I built the engine, degreeing the cam off of top dead center from the positive stop method, I noticed that the timing reference on the balancer was off near 45 degrees from the factory timing pointer.
The harmonic balancer was brand new but some stock form of aftermarket. I did not purchase this for the build it was supplied with the rest of the parts.
I’ll further explain that it’s been my long term policy to lock the crankshaft down after the cam is degreed in and verified. Then I will close everything up around the front cover and install the balancer and verify timing reference. Just for this reason. Ensuring proper ignition timing.
So in this case. I taped the balancer to paint a mark in line with the timing pointer and painted a nice bright white stripe on the balancer to correct. Big mistake!!
So the engine is fired up. Brand new transmission from a reputable transmission shop that supplied everything surrounding.
When we filled the transmission with fluid. A very loud noise began. It was the flex plate being pushed into the transmission inspection cover and the shift linkage had issues to be worked out.
The car owner was standing right there and elected to take the car to the transmission shop right then and there.
We managed to set timing and check for leaks but the engine did not get much running time. Maybe 5 minutes and I was not comfortable with the situation either way.
A few days later I get a call at work from “the transmission guru”.
He just wanted to tell me that my timing mark was off about 15 degrees. So he fixed that for me.
I was busy with work and didn’t really have time to realize just what had been done.
When it gets out of the shop the car owner called me to inform me that the engine was running hot.
No problem, I’ll set the timing and check everything out.
When time comes to set timing and I’m looking for my painted timing mark, it’s gone. That transmission guy took it upon himself to strip the painted timing reference off of the balancer and repaint the balancer. He then hastily remarked the balancer with a piece of chalk you could barely see.
The explanation on how transmission guru determined top dead center??? Not worth mention the guy was screwed up in the head and just screwed me over big time.
So now I’m trying to set timing based roughly off of feeling for compression and doing my best to get a mark on the balancer. It was mess everything in the way. All culminated into uncertainty.
When I tried to set timing the initial was set around 8 degrees and when I looked at the total timing I was well passed 45 degrees. I thought the vacuum advance was still hooked up. It wasn’t.
Where is all of this advance coming from?
I was in a hand wringing situation. The car owner was very understanding. They could see and knew enough about the situation to realize that we were out in left field for referencing.
They took the car home with them with the plans to disassemble the front end of the engine and reestablish TDC reference later.
I took my problems to work and explained them to my boss. A very wise engine builder. He immediately asks me why I didn’t use the mill or something else to make a permanent mark on the balancer. I had the tools available to do so and didn’t.
But where was all of this extra advance coming from. It’s all brand new store bought rebuilt??
The answer came a few weeks later on the engine dyno at work.
Another engine. This time a 327. Same popular parts store “rebuild” dual point with an electronic conversion kit.
This time the timing marks were right and not tampered with. This engine was hitting near 52 degrees with no vacuum advance.
Mr. Boyd calls me into the dyno room and asks me if this is what I was seeing with my project??
Exactly. Same brand distributor from the same parts store doing the same thing.
So he has me pull this distributor and he goes at disassembly.
The mechanical advance plates!!! The stop pins had deep wear grooves. The plates the stop pins were mating to, worn out!
He calls popular parts store and explains. Their response….. No way!
For sure for certain. It was.
It was the same on the distributor that had plagued me with issues.
Moral of the story. Permanently mark your timing references. The uncertainty that little detail created was more than I want to ever deal with again.
For those that set timing by ear. It isn’t happening when your mechanical advance is in this condition. You’re flat lost with a slow boiler that will never cool down.
My apologies for the long winded this one I felt was just too important.

[kkyler]

TIMING AND VACUUM ADVANCE 101 by GM Engineer.

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.

[dudebro]

Great explanation! Thank you Mr. Gold for the walk-through on vacuum advance with the visuals (excellent graph.) Side note for anyone interested: David Vizard has a very recent video that touches on the ignition subject and expands on a couple points, (manifold vacuum for the win). Again, thank you for helping us less experienced guys.

[SteveZumpf-bdhb]

You are 100% correct on vacuum advance. It was changed to ported for emissions purposes only. It is interesting that a more efficient engine doesn't always produce the "cleanest" emissions. The only difference between manifold and ported is at idle and deceleration. At cruise they are about the same. It is alarming how many guys don't understand how vacuum advance works or why we use it.

[novakenss]

Great timing curve, mechanical and vacuum advance tutorial. I was a tune up mechanic in my youth and quickly learned to move vacuum advance from ported to manifold. The smog laws in 1966 in California moved them back to ported to lower emissions. Engines ran hotter and got poor mileage and had sluggish street performance. I've converted several friends' engines to manifold vacuum advance with great results. Some are harder to start when warm since there's more advance while cranking. Opening the throttle stops that. Street driving with this makes for cooler running and better MPG. Last note: I lived in Denver for a decade, and we could run nearly 40 degrees total there with no detonation. Higher altitude makes the difference. I'm in Texas now at 700 ft and 35 degrees is my maximum total now. Thans for the fine video.

[allhailinternalcombustion]

I agree about using the knowledge from the past. If you're building a regular production engine, naturally aspirated, someone in the past already designed one, all you have to do is follow their blueprint. I remember the old "Chevy Bible" and the Chevrolet interchange manuals, they were worth their weight in pun intended.

[sunset]

Thanks for the explanation. I now understand how it all works. I was unsure about the vacuum advance.

[kevinclemence]

Liked the video. I'm somewhat passionate about ignition timing and have studied it extensively over the years.
FTR, I've a manifold vacuum preference too. The ported vacuum as you know was a crutch used in the early smog days and was a way to try and clean up emissions. I noticed the engines you referenced with your "good book" were all pre-smog examples. Keep up the good work.

[WayneB-rvxs]

I am using both ported and manifold vacuum at the same time, each side has a check valve going into a tee, the tee is attached to the distributer vacuum can where an aquarium air vent is set in that line to vent off the vacuum for when the engine is shut off, mine takes 53 seconds after shut down for the diaphragm to click shut.

[Xshadowolf]

amazing finally after countless videos you finally made it make sense vac advance on ported because it keeps timing at idle and when there is no vac the centrifugal advance takes over the work .... never seen it that way so much easier to understand thankyou i keep carbon fouling plugs on my 383 stroker after about 30 miles so i am trying to dial it in and you helped me on this mission thankyou !

[erikalarson]

Not to go too far off topic but this got me thinking about the other parts of the ignition system; plug wires, distributor cap, rotor, spark plugs. Recurving the distributor would be good too. Heat range of spark plugs and adjustable vacuum diaphragm. Maybe with an engine on the test stand or dyno.

[richb]

You’re a very good instructor and do a great job explaining the details.

[yurimodin]

great video, ported vs manifold each have their place depending on what your timing curve needs.

[jchavins]

The distributor on my 67 gto had a single advance stop on the underside of the weight plate and the weights had a different shape. For some reason the gto had a rubber bushing on the stop. Aftermarket timing kits supplied a steel bushing. Chevys that I saw had the same type.

Your chart shows the total timing graphed at 35 degrees not 30. In the late 60's and early 70's the recommended total timing on both small and big block Chevys was 38-42 as
I

[noahdunaway]

I’m a 73 year old drag racer and have not been able to find a vacuum advance diaphragm that will last very long on my HEI distributor. I’m about to ready to order a HEI distributor from Progression Ignition that features the ability to program everything with your phone app.

[UncleVom]

The vacuum advance really is important in a street vehicle, they spend most of their time at something less than half throttle and at a steady state like highway use may be considerably less.
People need to realize this and be truthful with themselves and the actual application of their engine.
A part throttle dyno run may show the benefits of layering more vacuum advance on top of the total mechanical in such cases, there may be free torque and fuel economy to be had and better driveability. You did hint at this, but there are numbers to be had.
In modern engines there are ECUs, multiple sensors, and advance tables that play the game, but really it's not that much more functionally advanced than the good old distributor.
Anyway thanks for another interesting video and congratulations on breaking the magic 10k subscribers, it must feel good for you and your team.

[Wheelgauge-btox]

30+ years in heavy industrial transportation and you are 100% correct on split washers👍

[robertwest]

On another note, I never knew the factory Chevy timing curves were ridiculously slow and had too much mechanical advance and not enough initial advance. I didn't know that they were this bad until you showed the factory specs. Modifying it like you have makes a big difference in getting off the line 💪

[tomstrum]

Great helpful for Detailed explanation on the "Ported" vacuum advance issue...