Q&A / Tech

Ask Away with Jeff Smith: Understanding Vacuum Advance and Why You Need It

 

I’ve always thought that vacuum advance was something just for stock engines and something you didn’t use with a performance engine. In fact, I’ve heard guys say they’ve locked out their advance and just run a fixed timing amount. Is this a better way to go with a street engine? I have a small-block 350 Chevy in a ’67 Chevy pickup with a TH350 trans and 3.73:1 gears. It’s a nice little cruiser and not really a hot performance engine. Right now the engine has an Edelbrock Performer intake, a 600 cfm Holley four barrel carb, and an HEI distributor. What kind of timing should I be running and should I disconnect the vacuum advance?

J.K.

Jeff Smith: You’ve addressed several issues, but they are all related to ignition timing. First, on a street-driven engine there is no good reason to lock out your ignition timing. If we had to use a generic maximum advance that seems to work for most pump gas engines, somewhere around 34 to 36 degrees before top dead center (BTDC) is a good number. But with this much initial timing in the engine it will be difficult for the starter motor to crank the engine. You can get around this with an ignition cut-out so that the engine cranks and then you hit the ignition circuit, but this is unnecessarily complex for a street car.

The better plan is to start with a decent initial timing of roughly 10 to 14 degrees. This can be checked with the engine running at idle with a timing light. Make sure the vacuum advance connection is removed, and now rev the engine up to around 2,500 to 2,800 rpm. Ideally, the timing now should be somewhere around 34 to 36 degrees to total advance. With 34 degrees of total mechanical advance and 14 degrees initial timing, you have 20 degrees of mechanical advance—14 + 20 = 34.

Now connect the vacuum advance from the carburetor to the distributor and read the timing on the harmonic balancer while revving the engine to 2,800 rpm. This number will now be greater than 34 degrees with the addition of the vacuum advance. Let’s say it now reads 49 degrees, which would mean the vacuum advance is worth another 15 degrees. These are typical numbers.

First, let’s look at the mechanical advance portion of the timing—the 34 degrees total. This is determined by the weights and springs spinning around inside the distributor. This establishes the amount of timing the engine will see at wide open throttle (WOT). As you are probably aware, at part throttle the engine will create a certain amount of vacuum in the intake manifold. This is because the throttle is mostly closed and the engine is pulling against this restriction. Because the throttle is mostly closed, very little air is moving into the cylinders. So at light cruise, such as running down the highway, the engine is making much less power than it would at this same rpm at WOT.

With less air and fuel in each cylinder, the air-fuel mixture is not as densely packed compared to WOT. This less-dense mixture requires more ignition timing to complete the combustion because it takes longer to complete the combustion process. So we need a way to increase the amount of timing based on the load on the engine. This is how vacuum advance works. At part throttle, high manifold vacuum moves the diaphragm in the vacuum advance canister on the distributor to add more timing. But at WOT, the vacuum drops to near zero and vacuum advance is removed and the total timing then is established by the initial plus the mechanical advance.

So there are significant advantages to retaining the vacuum advance on your distributor. In the case of the HEI, you can actually purchase an adjustable vacuum advance canister that will allow you to custom-tune the amount of timing advance. For example, Pertronix offers an adjustable vacuum advance canister that bolts in place of the stock canister. Using a 7/32-inch Allen wrench placed in the vacuum nipple, you can change the amount of advance. Generally, the canister comes adjusted with roughly half of the total advance possible. The canister easily installs with two screws and then you can drive your truck with the unit adjusted as delivered. Each clockwise turn of the Allen wrench will add about 1.5 degrees of additional advance above 5 to 7 inches of manifold vacuum. The maximum is about 14 degrees of vacuum advance.

If too much advance is added, the engine will either start to knock or ping or perhaps it may surge slightly at very light throttle opening with high vacuum. If so, back the adjustment (counterclockwise) one to three turns and you will be very close to the ideal timing at part throttle. Once tuned properly, you may notice the engine does not require as much throttle to cruise at the same speed. This tuning should produce slightly better fuel mileage—assuming you can keep your foot out of the throttle now that it runs better!

So based on this, you can see that having a curve in the distributor along with vacuum advance is a good thing as it reduces low-speed timing where the engine might detonate with too much timing but it can also benefit from more timing at part throttle than what is required at wide open throttle.

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25 Comments

  1. Great article! Many people do not understand this and you explain it well!

  2. Fantastic! You just nailed it. Great article.

  3. i have a question I started up my 1970 ss chevelle it has a 350 create motor ,I have a loud whiney sound coming from my distributor ???? Bering maybe or Whatever ,Have you ever had this problem.BTW its a Mallory 85 series distributor. Ricky SS

  4. Ken Chorney says:

    I believe the old Crane adjustable vacuum advance cans for GM V8 point distributors used the allen wrench through the nipple to adjust the spring tension (i.e. the rate of advance vs. vacuum) and also included a limiter plate that installed under the end mounting screw that determined the total amount of advance the vacuum unit could add.

  5. Lots of good info, but one very important point left out of the article, was the source of the vacuum. In order to function properly, in the manner stated in the article, the vacuum canister should be connected to a *manifold* vacuum port (sourced from below the carb throttle plates), NOT “ported” vacuum (above the throttle plates). Manifold vacuum is highest at idle and part-throttle cruise – precisely when the engine can use the additional advance. Despite the misinformation rampant in aftermarket parts instruction manuals, ported vacuum is not the proper source for vacuum advance on a performance street engine. It’s purely to reduce emissions, is only in effect just off idle, and does nothing to aid engine performance or fuel efficiency.

  6. Robert Fortier says:

    great info.had many questions regarding my vac. advance ajust. Pertronic dist.,ford 302 crate
    in MK4 Cobra (manifold vac.&ported vac.) erratic at low rpm,in first gear,hope it solves my
    problem. Thanks Robert

  7. Good article. But you the way you are describing setting your timing confuses people. You need to set total @4000 rpms, with vacuum advance disconnected (to make sure mechanical advance is all in). Then your initial will be = total-mechanical. Only way to change the mechanical amount is to swap out for a shorter plate, weld your current plate, or buy aftermarket that has mechanical adjustability.
    Then hook up vacuum advance and adjust it like you mentioned.

    Thanks
    Ross

  8. Have to dis-agree. More dense fuel, earlier the timing. Less dense fuel does not need more advance.

    • Actually Steve, I was talking about the density of the air in the cylinders. At part throttle, the reduced density of the charge in the cylinder needs more time to combust. That’s why we add timing at part throttle – and why tuning with vacuum advance will improve part-throttle performance and mileage. Not all engines need a lot of timing. Some might need as much as 45 -48 degrees at part throttle. Others may need very little – it all depends on cam timing, mixture distribution, and about a dozen other variables. Hope this helps…

      • I disagree with you about a less dense charge of fuel mix needing more time to burn. this is backwards physics. The engine needs more advance to burn the enriched charge at the press of the accelerator. (more dense air/fuel) Less fuel/air, less timing.

        • You are a moron

        • Jeff Smith says:

          Steve,
          You are certainly allowed your opinion, but let’s look at this in a more application-specific sense. You contend a denser amount of air-and fuel in the chamber needs more timing (up to a point, that’s true this may be where the misinterpretation begins). But if that were true, then why do we need a load-based timing advance (vacuum advance)? The reason is that with limited throttle opening – the less-dense air in the chamber requires additional timing (greater advance) to produce peak cylinder pressure at the proper time to make the most torque possible at any given throttle opening. I think if you are willing to be open to the concept – you will see that this is why vacuum advance was created. The same is true with electronic control of timing. If you look at a typical timing map for EFI engines, you will see MORE total timing at partial throttle and LESS total timing at WOT at any given rpm.

        • the richer mixture takes less time to complete the burn because the oxygen is completely consumed before the fuel is all used. less time is required and is then referred to as ‘faster’. rate of flame front travel during the combustion event is another story and confuses the fast/slow conversation. air/fuel charge density is mostly a function of manifold pressure (WOT creates higher manifold ABSOLUTE pressure in the induction manifold) the term ‘vacuum ‘ might better be referred to as ‘pressure differential’. if not, the effect of ambient pressure on the calculation is missing. think about air/fuel charge density at WOT in Denver!

  9. I have a problem that’s driving me crazy. I have a mint 77 Monte Carlo with a 3.73 rear & a 700R4 tranny built to withstand 475 HP. The cars weight is 3,900 lbs. I was running a stock HEI distributor wired into an older style MSD timing computer, 6AL, & blaster 2 coil . I had engine problems so I just ordered a new crate engine which is my garage on a stand. This engine was dynoed before it shipped and is rated at 454 HP at 5,800 & 477 torque at 4,200 RPMs. The cam’s lift is 225/231 at 050. The carb is rated at 750 cfm. They set the base timing at 12 degrees BTDC & total timing is 34 degrees BTDC. I’m ditching the MSD timing computer & will leave the 6AL box & separate coil. The distributor they gave me is the MSD Streetfire. I spoke to the engine builders & they said to run without vacuum advance. Well after reading all your expert articles, I know that’s a mistake. Here’s my dilemma & question. I wanted to purchase the MSD pro billet 8360 distributor. I feel I can easily wire this into my previous MSD equipment. The problem is this distributor has a vacuum advance locked in at 16 degrees with no way of adjusting it, even with after market parts. is this setting to high? If no, then what should I set my base & advanced timing to? If you think having a non adjustable vacuum advance is not a good thing then what distributor should I get to go with my set up? Thank you, Jamie

    • That 16 degrees is a good place to start. As an example, at a cruise rpm of around 2500 rpm if the mechanical advance is nearly all the way in, you might have 30 degrees of total timing – adding that vacuum advance of 16 would produce 48 degrees of timing which is a good place to start. You might find that it’s near perfect and should improve your highway fuel mileage. The vacuum advance should also improve part-throttle response. I would just duplicate the advance curve from the previous distributor and go from there. Likely it will be very close to what you need.

  10. Correction on my question above. The distributor I wanted to purchase with the non adjustable vacuum advance is the MSD 8360.

  11. Wow that’s twice. I meant the 8361.

  12. Ross York says:

    Contact MSD and they can get you answers on any timing issue. Their website also has great videos with David Frieburger on timing.

  13. Great write-up. I understand vacuum advance for example cruising and light throttle. What actually happens on the deceleration for example down a hill and no throttle but the revs remain high. Is Vacuum advance still present? I understand with late model EFI cars is that advance retard occurs on Deceleration. Thanks

  14. Hi Jeff, I have a new rebuilt 383 and I am having trouble getting it to run crisp and with good throttle response I am trying to determine how much machanal I need so I use the right machanal advance stop bushing could shed some light on this for me thanks

  15. This is all good and well, but as a modern parts guy, I know for a fact that many units are faulty out of the box, or the fuel is not up to the octane numbers that they used to be. I understand that many guys on these forums go for the $400+ units, but the daily customers that I work with don’t have that kind of bread, and they are left with little knowledge and a shabby running engine.

    In my daily driven 383 I do not use timing advance. It has 202 valves, Holley 700dp, warm plugs, & HEI. Driven on the street and cross country I have no issues with cruise idle, throttle response, fouling, pinging/detonation, or other drivability problems.

  16. If you’re running 10.5:1 compression you will detonate with vacuum advance hooked up. On pump gas and 10.5:1 compression you can’t have a max total timing more then 28 degrees. I have a cracked cylinder in my garage to show you what happens when you allow the advance to reach 48 degrees or more on a high compression engine. If you have a high performance engine you don’t want to ever see more then 38 degrees. Look up the timing on a 2002 z06 vette. Total max timing on it is 28 degrees and it has 10.5:1 compression.

  17. Pingback: Ask Away! with Jeff Smith: Vacuum Advance and How it Relates to Part-Throttle Timing - OnAllCylinders

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