Some very valuable tuning tools are greatly misunderstood. Take the lowly vacuum advance canister. Many enthusiasts dismiss this device as superfluous—or worse—only for emission-controlled engines. Frankly, they couldn’t be more wrong. There is no excuse for not making use of vacuum advance for a street-driven performance engine. Sure, there are those who will claim their engine runs fine without that cheesy little can hanging off the side of their distributor. But the reality is that they are sacrificing throttle response, part-throttle power, and fuel mileage. So let’s dive into what makes this little mechanism so important.
Our discussion will be aimed at street engines that spend roughly 90 percent of their time at part throttle. Cruising down the freeway, most people don’t realize a performance carburetor or throttle body is only cracked open 10 to perhaps 20 percent of full open. With the throttle nearly closed, the engine is ingesting only a fraction of the air it could inhale at wide-open-throttle (WOT).
That small volume of air at part throttle means the density of the mixture in the combustion chamber will also be very low compared to WOT. Logic dictates that the air and fuel particles will not be tightly packed in the cylinder. Instead, they will be spaced farther apart. After the spark plug has fired, the combustion process will be much slower than it would be if the mixture was more tightly packed. As an analogy, think of the piston top like a wide plain of dry prairie grass. If the grass is closely packed it will burn very rapidly. But if there are large gaps between the clumps of grass the burn will take longer to move across a given area. So lower density air and fuel in the cylinder will also burn more slowly.
To optimize torque at any engine speed, internal combustion engines must maximize cylinder pressure so that it occurs roughly 15-17 degrees after top dead center (ATDC). With a slower burning mixture density, we must start the combustion process earlier to maximize torque at a given rpm. This means advancing the ignition timing. As an example, a generic tune might be 14 degrees BTDC initial with another 6 degrees of mechanical advance for a total of 20 BTDC at 2,000 rpm. This timing varies only with engine speed. But for light throttle situations, we need some way to adjust timing based on engine load.
A partial throttle opening creates manifold vacuum—let’s use a number of 15 inches of mercury (Hg). As we’ve seen, this creates a light mixture density in the chamber that requires more ignition timing. So to optimize part-throttle power, this engine might need another 14-degrees of timing for a total of 34-degrees BTDC.
There are several ways to get there. We could just add more initial timing, such as 20 degrees and then bring 14 degrees of mechanical advance all in by 2,000 rpm and this would supply the timing for part throttle. But that might make the engine hard to start when it’s hot. Or, we could just bring the mechanical advance in very quickly so it’s all in by 1,600 rpm. But when we stab the throttle at 1,800 rpm with all this advance in our 10:1-compression big-block with iron heads and a big 850-cfm carburetor, what happens? If you think the engine might rattle its brains out from detonation you’d be correct. But if we use timing based on engine vacuum, we can have our part-throttle timing cake and still let the engine eat and not rattle when we stab the throttle even at very low engine speeds.
This is because vacuum advance is a load-based timing device. When the load is low, high manifold vacuum moves an arm in the vacuum canister adding a select amount of timing. Under WOT, manifold vacuum drops to near zero and vacuum advance just as quickly disappears, leaving just the combination of initial and mechanical advance. So it’s just a matter of experimenting with the combination of mechanical and vacuum advance to come up with a package that works best for your car. We’ve just given you an excuse to experiment with your car and have some fun!
As a general rule, the lower the static compression, the more timing the engine will want to generate maximum cylinder pressure at any given rpm. Conversely, higher compression engines sometimes require less timing. This is a gross oversimplification, but it serves as a basic starting point. Engines with long-duration camshafts tend to bleed off cylinder pressure below peak torque, so these engines generally will want even more ignition timing at part throttle where the cam timing numbers are less efficient at filling the cylinder.
Carbureted engines with long-duration cams will benefit the most from vacuum advance. If your engine idles at 10 inches Hg or less, you can start the tuning process by setting the initial timing to at least 15 degrees BTDC. If your car is equipped with an automatic transmission, placing it in gear may pull the manifold vacuum down even further. In this case, you should consider hooking the vacuum advance line to straight manifold vacuum. This is counter to what you may have been taught in high school auto shop, but it’s actually a really good idea.
All engines with long-duration cams exhibit low manifold vacuum because of the large amount of overlap where the intake and exhaust valves are open at the same time. This produces that desirable choppy idle. Unfortunately, this increased overlap also introduces large amounts of exhaust gas into the intake manifold, causing poor idle quality. The best way to improve the idle is to add more timing.
But there’s a trick to making this work. All vacuum advance cans are not created equal. There are literally dozens of different vacuum cans that start at different levels, delivering various amounts of advance. The trick involves finding a vacuum can that offers the best (not necessarily the most) amount of advance at a low enough vacuum value to be of benefit.
Here’s an example. Our pal Eric Rosendahl has a 468ci big-block that idled just barely at 8 inches Hg in gear. The initial timing was set at 12 degrees with 24 degrees mechanical advance with 36 degrees total at 2,800 rpm. This sounds like a good combination, but the engine struggled to idle properly in gear because the converter was a little tight for this engine. We tried adding more initial while limiting the mechanical advance, but adding 4 degrees initial did not seem to help. Instead, he chose to do two things. The first move was to switch from ported to manifold vacuum.
Next, he checked the amount of vacuum advance supplied by the can at idle. All we did was reset the initial timing at 12 degrees, and then recorded the amount of timing at idle with the vacuum can connected. This measured 22 degrees, which meant the can added 10 degrees. As soon as we changed to manifold vacuum, this added 10 degrees of timing at idle. That simple change improved the idle vacuum a solid 2 inches to make 10 inches Hg and the engine no longer felt like it struggled against the converter.
You can take this tuning even further by checking the vacuum can’s total advance. This can did not quite add all of its timing at idle because when we checked it with our Mity-Vac pump, we realized it maxed out at 12 inches of vacuum while our engine was only idling at 10 inches. Another move we could make would be to add an adjustable vacuum can to the HEI distributor. This would allow us to modify not only when the advance begins and ends but the amount as well. Several companies make adjustable vacuum advance cans for both HEI and standard Chevy distributors.
This work is neither difficult nor expensive. We’ve seen many examples where just adding vacuum advance to the engine and experimenting with vacuum cans delivered excellent driveability improvements. So we have a very effective tuning tool that costs very little to use. It doesn’t get much better than that.
01. Our big-block test car is a ’66 El Camino with a 468ci big-block, a mild Comp hydraulic roller cam, an Edelbrock Performer RPM dual-plane, and a ProForm main body Holley 750-cfm carburetor. The engine made 580 lb-ft of torque at 3,800 and 527 hp on the dyno. Now our goal was to improve the part-throttle performance.
02. Our first change was to merely swap the source of vacuum for the advance. We switched from the ported manifold vacuum (note plugged nipple on the metering block) and moved the vacuum source to straight manifold vacuum at the base plate.
03. The best way to check total timing if you don’t have a dial-back timing light is to use these simple timing tapes from MSD. Each tape is spec’d for a specific balancer diameter to maintain accuracy. Place this tape so the zero lines up with TDC mark on the balancer and then use the TDC position on the pointer to read the amount of timing.
04. This vacuum canister produces 13-14 degrees of advance but requires 14 inches of vacuum to deliver its maximum timing. At 2,800 rpm, the combination of initial and mechanical deliver 36 degrees. With light throttle and more than 14-inches of vacuum, the total timing can move to 50 degrees. This is not an excessive number. If the engine begins to surge at light throttle, then merely reduce the vacuum advance total.
05. This is a tunable vacuum advance can from Accel for an HEI distributor. By placing a 3/32-inch Allen wrench in the nipple, you can adjust the total amount of added advance.
06. Here is a vacuum canister installed on an HEI distributor. The rod from the can is connected to the trigger pickup. As the rod moves, it swings the pickup in the opposite direction and advances the timing. The distance the rod moves in the slot (arrow) determines the amount of timing added. On non-adjustable vacuum cans, you can limit timing by welding or brazing the slot. Increasing the length of the slot will add timing.
07. The best way to know how well your car idles is to use the combination of a tach and vacuum gauge. Here, we have idle vacuum of almost 10 inches with an idle speed of roughly 900 rpm. On this small-block with a somewhat lumpy cam, we improved idle vacuum in gear by nearly 3 inches by adding vacuum advance timing at idle.
08. Adding timing to improve driveability isn’t limited to just carbureted engines with distributors. This JET EFI screen displays a stock timing map for a stock 5.3L GM EFI truck engine. The numbers across the top horizontally are rpm while the vertical scale is a representation of load. So horizontally across the top of the scale roughly four rows down would be light throttle advance. Note that this curve delivers 46 to 57 degrees of timing at very light load at 2,200 rpm—three rows down from the top at that rpm. Remember, this is a stock LS engine that doesn’t need tons of timing to be efficient!
09. With these mild timing changes along with leaning out the idle circuit on the carburetor on our pal Eric Rosenthal’s big-block El Camino, we’ve managed to greatly improve throttle response while also improving fuel mileage. A carb’d 468 doesn’t pull down outstanding mileage anyway so even a small improvement is a win.
|Crane HEI adj. vac. can and springs kit||99600-1||Summit Racing|
|ACCEL HEI adjustable vacuum canister||31035||Summit Racing|
|PerTronix HEI adjustable vacuum canister||D9006||Summit Racing|
|Summit HEI adjustable vacuum canister||850314||Summit Racing|
|ACCEL GM points dist. Adj. vacuum canister||31034||Summit Racing|
|Crane GM points dist. Vacuum adv. can and kit||99601-1||Summit Racing|
|MSD timing tape||8985||Summit Racing|