How to Take Advantage of Vacuum Advance for the Street

Words And Photos: Jeff Smith

“Race engines don’t use vacuum advance, so I don’t bother. It doesn’t really help anyway…”

Some version of that statement has been made hundreds if not thousands of times when bench racing sessions turns to engines and tuning. Unfortunately for those “experts” who espouse such information, they have just not put a lot of thought into that mindset. The truth is, all street engines can benefit from vacuum advance, and the more aggressive engines ironically need it the most. This story will dig into why this is and how to take full advantage of the benefits that simple vacuum canister offers.

Before we burrow deeply into the specifics, let’s first lay out a solid tech foundation for why vacuum advance is such a big deal. This discussion will focus on naturally aspirated engines running at part throttle. This is where a typical street engine spends as much as 99 percent of its time. Our goal is to adjust the timing and mixture to produce maximum power and efficiency at part throttle.

Light throttle acceleration creates high manifold vacuum because the pistons are pulling against a highly restricted inlet. This means the cylinders are only partially filled with air and fuel. In terms of volumetric efficiency (VE), we’re talking about cylinders filled to perhaps 10 to 15 percent of capacity. This means the air is not very densely packed compared to what it would be if the engine were running at peak torque at wide-open throttle (WOT) where the cylinders might be filled to 100 percent VE or more.

When a spark plug fires, the result is more of a process, not the instantaneous explosion you learned in high school auto shop. If combustion was instantaneous, there would be no need for advanced engine timing. To create maximum force on the crankshaft, the accepted rule is to produce maximum cylinder pressure when the piston reaches 15 to 20 degrees after top dead center (ATDC). In order to create that situation, we must start the combustion process much earlier — before top dead center (BTDC).

The combustion process has been described as like a prairie fire burning across a valley of dry grass. In an engine, the fire starts adjacent to the spark plug and burns across the top of the piston and combustion chamber. This takes time. If the grass (air and fuel) on the prairie (the combustion space) is spaced far apart, it takes longer for that fire to burn a given distance (the combustion space). In order for our slow-moving fire to complete its low-density burn, we must create more time for that process by beginning the ignition sequence sooner — which is why we advance the timing.

The problem with mechanical advance: it is strictly rpm-generated and not affected by load. Our attention is aimed at part throttle, low-load situations. A great indicator of load is engine vacuum. High vacuum indicates low load, while high load (WOT) produces nearly zero manifold vacuum.

In the early days, the idea was to connect vacuum advance to a manifold vacuum source, which added more timing to improve idle quality. Since this also increases hydrocarbon emissions, during the ’70s, the OE’s moved to ported vacuum advance that only added timing above idle.

Performance engines with big cams often need more timing at low speed to improve idle quality. Your particular engine will determine the actual amount of advance, so let’s look at a real world example. Our 383c.i. small-block Chevy used a flat tappet hydraulic cam with 230 degrees at 0.050 duration, a JET Q-jet, and an aftermarket HEI distributor. The tight torque converter in the TH-350 trans loaded the engine pretty heavily at idle. We know this because the engine idled at 13 inches of manifold vacuum in Park but dropped to barely 8 inches of vacuum in gear, and it tended to surge and misfire at idle in gear.

We initially set this combo with 16 degrees of initial timing with 18 degrees of mechanical advance that put the total mechanical at 34 degrees at 2,800 rpm. The engine ran great at WOT, but it never ran well at part throttle and generated barely 10 mpg on the freeway. That’s when we discovered the vacuum advance can had failed.

We replaced the original vacuum can with an adjustable unit from Summit Racing. This helped the engine with ported vacuum advance at part throttle, but it still struggled at idle. We checked the timing at 15 inches of manifold vacuum at 3,000 cruise rpm, and total timing was 50 degrees (16 initial + 18 mechanical + 16 vacuum = 50). To some, this might seem like excessive timing, but this is not unusual. Remember the discussion about needing more timing with a low density load in the cylinders. With the vacuum advance functioning, at part throttle, the engine immediately ran much crisper and fuel mileage improved, although we have not yet tested that.

However, the engine still didn’t idle well in gear. That’s when we decided to change from ported to manifold vacuum advance at idle by connecting to manifold vacuum. This added 16 degrees of timing at idle and immediately we had to reduce the idle speed because the added timing bumped the idle to speed from 950 to 1,200 rpm!

Adding the vacuum advance placed our total timing at idle at 32 degrees (16 initial plus 16 degrees vacuum advance). The only thing this changed was the idle characteristics. At part throttle, the engine has the same vacuum advance it had before, and at WOT, the vacuum plummets to zero and we’re back to our 34 degrees of total advance. In terms of idle quality, this simple move from ported to manifold vacuum increased the idle vacuum in Neutral 2 inches from 13 to 15 inches Hg, and the in-gear idle vacuum is much more stable at nearly 10 inches Hg. Idle speed now is 850 rpm in gear and jumps to 975 in Neutral because of the tight torque converter. What we really need is a different, slightly looser converter, but that’s a different story.

One caveat to using manifold vacuum advance is making sure the canister delivers its maximum advance at a vacuum level lower than the engine’s loaded idle vacuum. For example, our engine sometimes stumbled to 9 inches of vacuum in Drive. Our vacuum canister creates maximum advance of 16 degrees at 12 inches of vacuum. This creates a slightly erratic idle because when the engine vacuum drops below the max advance point, this slightly retards the timing.

This is not a huge problem with this engine, and the idle quality is still much better than it was since now it will idle and not stall at odd times, like when sitting at a street light. One way to improve our situation is to back out the adjustment on the vacuum canister slightly to lower the high vacuum value for maximum advance. Ideally, we would want the maximum vacuum advance value to be below the lowest idle vacuum. This can be accomplished by choosing from different fixed vacuum advance canisters, as most of the adjustable canisters add very little advance at the low settings. If you search the internet, there are a couple of forum discussions where all these canisters and their values are listed.

In our particular case, it’s possible the engine may respond to slightly less timing at idle. We will only know that after further tuning. If it does, we can accomplish that by retarding the initial timing, but that also retards the total, which is only 34 degrees when we really need 36 degrees. To remedy that, we will have to modify the mechanical advance curve to create 24 degrees mechanical advance, allowing us to lower the initial from 16 to 12 degrees.

This should give you a few ideas on how you can fine tune your combination to improve the idle stability and part throttle performance. There will always be a few die-hards who will shun the lowly vacuum advance canister, but for the rest of the enlightened world, there’s no reason not to take full advantage of this simple device. When a street engine spends a majority of its time at part throttle, you’ll find vacuum advance will make your car much more fun to drive. And that definitely makes it worth the effort!

HEI Adjustable Vacuum Advance Values

We created the following chart from our testing of a typical adjustable HEI canister. The adjustment figure is listed as the numbers of turns in from full counter-clockwise. All the vacuum levels are listed in inches of Mercury (Hg”) as shown on a typical vacuum gauge. The total vacuum advance is listed in crankshaft degrees. For example, at 4 turns in, the canister begins to advance timing at 6 inches of manifold vacuum, with a maximum of 14 crankshaft degrees with vacuum at 12 inches.

Parts List

Sources

Holley
Holley.com

Crane Cams
cranecams.com

Performance Distributors
performancedistributors.com

Pertronix
pertronix.com

Summit Racing
summitracing.com