Ask Away! with Jeff Smith: Matching Up Vacuum & Mechanical Advance

I have a 1990 C1500 with a 1984 454 carb and an HEI distributor. I can’t seem to get it tuned right. I installed a Moroso timing curve kit with the lightest springs. These springs give me 23 degrees of mechanical advance.  With 12 degrees of initial timing, I’ve got 35 degrees in by 3,000 rpm.

Do I need to figure in my vacuum advance since my mechanical timing comes in so quick? I do have an adjustable vacuum advance. If I figure 35 total and 15 from the vacuum advance I’m sitting around 50 degrees total. Any help would be appreciated.  Stock heads with a mild cam and headers.  

M.S.

It sounds like you’ve got it covered. Your initial and mechanical advance numbers sound good with the vacuum advance disconnected. The quick answer to your question is yes, the vacuum advance will begin right after the throttle is opened assuming you’ve connected the vacuum advance canister on the HEI to a ported vacuum source. If the HEI canister is connected to straight manifold vacuum, you will have total vacuum advance along with the initial at idle, but it sounds like you have it connected to ported vacuum.

Vacuum advance is essentially a way to add timing to the engine for part throttle use that is directly related to engine load. If you’ve ever watched a vacuum gauge that is connected to the engine while driving down the road, the vacuum will drop anytime throttle is opened. A typical vacuum reading for a mild street engine on level ground at highway speed might be 16 to 18 inches of mercury (“Hg), yet as you climb a hill or accelerate and open the throttle, the vacuum will drop.

Vacuum advance adds ignition timing with the highest vacuum levels and then diminishes the advance numbers as the vacuum drops. With an adjustable vacuum advance, the actual amount of timing advance changes along with its rate. We’ve included a graph of these changes from the Accel instructions on its adjustable HEI vacuum canister.

With these adjustments, you are changing not only the amount of advance added by the canister but also how soon or how late the advance starts. For an Accel Adjustable Vacuum Canister, this adjustment is made using a 3/32 inch Allen wrench inserted into the nipple on the vacuum advance can that adjusts a spring. There are multiple ways to begin this adjustment. Usually there are 10 turns of adjustment. If you insert the Allen wrench and tighten the screw clockwise until it stops, this will produce the maximum amount of advance while fully counter-clockwise will produce the least amount of advance. The Accel unit offers a total of nine turns of adjustment.

One approach to this would be to turn the adjustment all the way in and then back it off counterclockwise five turns. This places the vacuum advance curve in the middle which would be adding 14 to 16 degrees of vacuum advance.

As you suggested, this would add around 15 degrees of advance but it will add that timing only at part throttle and only with maximum manifold vacuum. You mentioned that this would take your total to 50 degrees of total advance. That’s not always the case. This would only occur if the mechanical side is fully advanced and then only with the highest manifold vacuum.

Let’s look at some numbers. You mentioned that the mechanical advance is all-in by 3,000 rpm. So if you add 15 degrees of vacuum advance with the canister, the 50 degrees of advance would only occur with maximum manifold vacuum which means the engine is under very little load. This might not be too far off from what the engine wants.

This requires some explanation.

At part throttle, there is very little air and fuel entering the combustion chambers because the throttle is mostly closed. This means while the piston squeezes this air and fuel the same as it always does there is less air and fuel in the chamber. The air molecules are not as tightly packed as they would be if the engine was at wide-open throttle with more air and fuel packed into the chamber. These less densely packed air and fuel molecules will burn much more slowly than they would be at wide-open-throttle (WOT) when the cylinder is more tightly packed.

To achieve maximum power from less air and fuel, we will want to start the combustion process much sooner in order to complete the process to push the piston down into the crankshaft. The optimal piston position for leverage on the crankshaft is roughly about 15 to 20 degrees After Top Dead Center (ATDC). With a slow burning part-throttle mixture, it might take an ignition setting of 50 degrees Before Top Dead Center (BTDC) to complete the combustion process by 15 degrees ATDC.

At WOT, your engine may only want around 32 to 36 degrees of timing because the molecules are more tightly packed and they will burn faster. A more modern chamber might be able to complete that process in only 20 to 26 degrees of total timing at WOT due to a faster burning chamber design. That’s one reason why newer engines are so efficient.

With 50 degrees of timing (35 mechanical plus 15 degrees of vacuum advance) at part throttle. When the throttle achieves WOT, the engine vacuum will drop to near zero and vacuum advance will disappear because there’s no vacuum to pull on the canister. This will leave the engine with 35 degrees of initial and mechanical at any engine speed over the 3,000 rpm you specified.

Each engine is different and may want more part throttle timing but less WOT timing. Or it could be that it wants more timing in all cases.

With a big block like your 454 with a large 4.250 inch bore, this generally means it might want a little more timing especially at part throttle compared to a small bore engine like a 305ci small block with a 3.736 inch or a 283 with a 3.875 inch bore. But these are just generalities. There are entire books written about the variables that play into ignition timing and combustion.

The best tuning approach is to gradually add more vacuum advance while test driving your vehicle. You may want to spend time with each setting to really evaluate each change. Adding more timing will eventually create a surge or light bucking or the engine may ping or detonate at light throttle. This will tell you that you have reached the limit of that setting and you will need to back the vacuum advance down slightly. You may also discover that if you slow the mechanical advance curve down slightly you can add more vacuum advance. Testing all these variables can be very valuable in producing a very happy engine that will deliver its maximum power along with better fuel mileage.

To put all this into perspective, you are limited with the fixed settings of initial, mechanical, and vacuum advance because the last two are linear features. To illustrate this point, we did some tuning with a friend’s 469ci big block street driven El Camino. We added vacuum advance until we had to back off when the engine would detonate (ping) at one specific point during light acceleration. The engine wanted more timing in other areas but we couldn’t get there with vacuum advance because of the light detonation.

We eventually converted the engine to Sniper EFI, which allowed us to customize our computer-controlled ignition with a Sniper distributor. This created a digital spark map that was not constrained to linear operations like a vacuum or mechanical advance. This allowed us to reduce the timing where the engine had previously detonated at part throttle yet add timing in other areas in the cruise section where it wanted the timing. This created a situation where we were able to optimize timing during cruise speeds and generate a 16 to 17 mpg fuel mileage with a 468ci big block on the highway. We could never get there with a traditional HEI distributor.

As a final suggestion, don’t worry about the numbers. In other words if the engine wants 52 degrees of total timing at 3,000 rpm cruise at light throttle—that’s what it wants. Pay attention to what the engine is signaling and you will find that tuning is both fun and rewarding.