hei timing and proper advance

[72vega383]

I cant get my timing right on my car. Its driving me nuts. I run a 383 stroker on 89 octane pump gas. Stock Hei Distributor. Holley 600cfm vac secondaries.

I am going to order the crane advance kit. but I am not sure what to change out. On a light car like a vega when would it be ideal for the advance to kick in? My total advance would need to end up at 34 to 36 degrees but at what rpm should it be fully advanced. I want response and maximum torque as this is a street car. What should my initial timing be set to.

Anyone running a similar setup or any engine gurus im hoping you can help. thanks!

[cjbiagi]

Most HEI's have about 20 degrees advance built in so to end up with 34-36 degrees you need to run about 14 degrees initial advance. There are a lot of variables that come into play but a lighter car can handle a quicker advance curve so I would say it should be all in by 3000 rpm or so. Usually with those advance kits I would try using the stock weights (which determine the amount of advance) and use some lighter springs to bring the curve in quicker. You can use different combo's of springs, the 2 springs do not need to be the same as each other so you have lots of combos. There should be a instruction sheet with recommendations. You may also want to check the pins and make sure they are not worn and put a very small amount of grease under the weights so they move freely.

[dadhad1]

I agree with Clyde. I'd start at a 14 advance at idle under 1000 rpm and another 20 advance for a total of around 34. I'm not a big fan of using vac advance. I prefer a straight advance based on rpm for a Vega street Appl. Too much advance too early can harm your engine so be careful. I'd start with peaking at around 3500-4000 to start. If it pulls hard through the entire rpm range I'd leave it there. If you have bog issues or less than smooth acceleration and your timing is set right its likely going to be fuel/air delivery. Put on a light and make it right!

Mike

[cjbiagi]

We didn't talk much about about vacuum advance but I do believe it is best to run one on a street car. If it is properly calibrated and connected to a proper vacuum source there is really no reason not to run it. It will improve fuel economy as well as improve driveability and help with cooling issues. Here's a good source of info http://www.corvette-restoration.com/res ... ing101.pdf
Her's some great info regarding vacuum advance http://www.corvette-restoration.com/res ... v_Spec.pdf

[spencerforhire]

So far all the recommendations sound about right. I'm with Clyde on using vacuum advance on a street car; lots of advance at low to mid RPM helps the car feel "snappy". Under low load(high vacuum), you want as much advance as you can get away with without causing that nasty rattle of detonation. It's not uncommon to measure 50 deg total advance under light throttle where you have full vacuum and are getting the RPM up where the mechanical curve is coming in. As long as the TOTAL advance is kept to 34-38 during WOT(low vacuum) and high RPM you should be safe. The distributor gurus like to limit the total vacuum advance possible by shortening the travel of the vacuum advance rod by welding up the slot. Most performance ignition manufacturers offer adjustable vacuum advance mechanisms; you stick a small allen wrench in the hole where the hose goes to change the total possible advance.

[OldsStarfire]

"you stick a small allen wrench in the hole where the hose goes to change the total possible advance" This is a common misunderstanding. All that adjustment does is change the rate of advance, not the total advance. You have to find other means of limiting the total advance. The better kits have a little metal plate for this, or you can weld up the slot as mentioned. Those links Clyde posted are really great. I've read other write ups by that Lars guy and he has a really good understanding and does a good job of explaining. They really helped me better understand what is going on.

The vacuum advance units built into the distributers were designed for the correct amount of total advance from the factory. Of course when you hot rod a smog era motor you have to recalibrate the distributor. You can also find OEM factory vacuum advance units on ebay with different amounts of advance built in. Sometimes the advance is marked on the advance unit itself (x2). This is mentioned in one of those links.

[spencerforhire]

The following is directly from the description of the Accel 31035 adjustable unit-
"These ACCEL adjustable vacuum advance canisters for GM HEI are designed to give you instant horsepower and throttle response. ACCEL adjustable vacuum advance chambers for GM HEI permit infinite adjustment to both the AMOUNT AND RATE of advance. The advance chambers include an adjustment wrench and an instruction guide. To optimize your engine's timing curve, you need ACCEL adjustable vacuum advance chambers for GM HEI."

I really don't like being told I'm wrong, especially when I've done my homework on a particular subject. I installed this same part on my old 76 GT with the 400; it would rattle and ping like crazy until I got it adjusted.

[Smiley]

The Crane adjustable vacuum advance kits which come with a stepped cam limiter to reduce the total vacuum advance separately from the rate.
Each notch in the cam reduces the total 2 degrees.




Get a full advance curve kit with weights and bushings

[cjbiagi]

Spence is correct (as always). Where the misunderstanding comes from is the total advance is generally referring to the total built into the centrifugal advance. This is generally around 34 - 38 degrees. This is a combination of the initial advance setting, say 14 degrees and the amount that is added by the weights which would be another 20 to 24 in this example. The vacuum advance adds to this "total" by adding another 10 to 16 (or whatever the unit is intended to add) degrees for another "total" of around 50 degrees at light throttle. The adjustable units do indeed change the rate and amount added by the vacuum canister. So, it does adjust the total vacuum advance but it does not affect the total centrifugal advance, it's a matter of terminology.

[Smiley]

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.

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.

[spencerforhire]

Smiley- That's a great, in depth explanation of vacuum advance; should be required reading for anyone who's even thinking about tuning an older carbureted vehicle. I remember back in the 70's and 80's how much better almost everything ran as soon as you switched from ported to full vacuum. It was pretty well one of the first things done to try and wake up a "smog motor".
In my opinion, a properly adjusted vacuum advance setup is necessary on a street motor; I've seen too many just disconnected because someone couldn't figure out how to optimize them. I guess part of it might be the influence of racing; drag cars don't need vacuum advance, so some people think they don't need it on the street. The whole package(initial, vacuum and centrifugal) needs to work together to optimize power, driveability, economy, and longevity.

[gerbsinmd]

agreed, great post by smiley! I had done exactly what Spence said on my 77 starfire GT, switched to manifold vaccuum and it really woke up the V-6. That was lots of trial and error to find, but it made a huge diffference.