By
Jeff Smith
—
Updated
in , How To, Research, Tech 101
Where to hook up the vacuum advance on a distributor?
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If you want to initiate a heated internet discussion with a group of hot rodders, just ask their opinion on ported versus manifold vacuum for the distributor’s vacuum advance can. The discussion board will instantly light up with all kinds of opinions along with, unfortunately, very few facts. The often-used blunt dismissal is “you don’t need vacuum advance – it’s just for emissions or for fuel economy.” It is partially true that vacuum advance is a factor in fuel economy but to consider vacuum advance as unworthy of attention is misguided at best. This story will avoid opinion and instead use basic internal combustion physics and apply that knowledge to determine when either ported or manifold vacuum is the right choice for your engine. But let’s be clear: every street engine deserves some kind of vacuum advance.
To begin, we must first set the stage with some basic information. This story will deal with street engines that operate using a carburetor and a distributor fitted with both mechanical and vacuum advance. If the engine in question is not equipped with some type of vacuum advance, you are choosing to limit part-throttle performance and ignoring improvements to throttle response, drivability, and fuel mileage.
This discussion is aimed solely at street engines. Engines intended for competition, such as drag racing, don’t need a vacuum advance system because racers are not concerned with part throttle performance. On the other hand, a typical street-driven engine in urban areas operates at part throttle roughly 98 percent of the time. This indicates that part-throttle performance should be a major concern.
Before we get to initial timing, it’s worthwhile to cover a few assumptions. The first will be that your engine is in good tune and not suffering from fouled spark plugs, bad plug wires, or perhaps an engine vacuum leak. Almost as important is the assumption that the indicated zero timing mark – or top dead center (TDC) for timing checks is accurate. If you are in doubt or your engine has been the recipient of a mishmash of parts it would be a good idea to verify TDC since that is the reference point from which all subsequent timing numbers originate.
Let’s first break down the three components of ignition timing for a street engine. This includes initial timing, mechanical advance, and vacuum advance. When checking ignition timing, initial or base timing is set by physically moving the distributor body to set ignition at idle at a certain number of degrees Before Top Dead Center (BTDC).
Older production engines from the 1960s and 1970s often used single digit initial timing numbers. There are many exceptions to this, but most will feature initial timing around the 6 to 8 degrees figure. We’ve seen factory numbers for a 1967 350-cu.in. Chevrolet, for example, of a paltry 2 degrees BTDC. For even mild performance engines, especially ones with a longer duration camshaft, initial timing should be 10 to 14 degrees as a decent starting point.
This story will not get into all the variables of initial timing versus total mechanical advance, except to mention that initial has a direct 1:1 influence on total mechanical advance that you should keep in mind. This means it’s important to balance initial timing with mechanical advance to ensure that the correct total advance is achieved.
One way to tell if your engine responds positively to more initial advance is by hooking up a vacuum gauge to manifold vacuum before changing the initial timing. Let’s say your engine is currently set at 8 degrees initial at 850 rpm and idling with 14 inches of manifold vacuum (14” Hg). If advancing the timing to 14 degrees BTDC increases both the idle speed up to 925 rpm and idle vacuum jumps to 15.5 “ Hg, then it’s fair to say that the engine responded favorably to this change.
The second component of ignition timing is mechanical advance. This is the amount of timing added to the initial that is controlled by the distributor and based solely on engine speed. A typical performance mechanical advance curve will start at approximately 1,500 rpm and add all the timing to the engine by 2,500 to 3,000 rpm.
If our theoretical engine has 14 degrees initial and the mechanical advance adds 20 degrees, then the total of initial and mechanical advance would be 34 degrees of total timing above 3,000 rpm. The ultimate total timing will be determined by the engine configuration – some engines will want more timing to make best power at wide-open-throttle (WOT) while others may need less.
Now we can address how vacuum advance plays its role in all this. The reason vacuum advance is so important is because it alters timing based on engine load. The best way to understand vacuum advance is to think of it as load-based timing that is excluded from operation at WOT. At part throttle, the cylinders are not receiving a full load of air and fuel. This is because the inlet is restricted by the nearly closed throttle blades. Therefore the air and fuel inside the cylinder when squeezed will not be as densely packed in the combustion space as it would be if the engine was operating at WOT.
This less dense mixture will require more time to burn inside the combustion chamber to achieve best push on the crankshaft. This is just basic physics. If the air and fuel were more tightly packed in the chamber, it would burn more quickly. Think of the combustion process as more like a prairie fire across the top of the piston rather than an explosion.
With tinder grass that is farther apart (part throttle), the flame will burn more slowly across the prairie than the same grass that is closer together and therefore more dense. When operating at part-throttle, we want to pull the most cylinder pressure out of that less dense mixture. To do that, we must light that air-fuel mixture sooner in order to produce maximum cylinder pressure. By advancing the timing with vacuum advance, this achieves this goal.
Manifold vacuum is an excellent indicator of engine load. So with the throttle barely cracked open, manifold vacuum will be high – 14” to 16 “Hg – compared to WOT, when vacuum drops to roughly 0.5 “Hg. So this range of manifold vacuum can be used as a great modifier when hooked to a simple canister with a moving diaphragm that works against spring pressure in the canister to move a link rod connected to the base plate in the distributor to advance the timing.
Factory vacuum advance canisters offer a fixed number of degrees of advance based on the give vacuum signal. As an example, a stock unit may offer 12 degrees of additional timing when exposed to manifold vacuum levels of 12 “Hg or more. There are some aftermarket adjustable vacuum advance canisters that use a tiny Allen wrench to adjust the rate and amount of timing the canister can offer.
Now, let’s say we have an engine with 14 degrees initial timing and 22 degrees of mechanical advance that is all in by 2,800 rpm. The distributor also has a vacuum advance canister that can offer an additional 12 degrees of advance at a vacuum signal of 12 “Hg or higher. If the engine is cruising on the highway at 3,000 rpm then this engine will have a total of (14 + 22 + 12) 48 degrees of ignition advance at that point.
This may seem like way too much ignition timing, except that we need to remember that the throttle at 14 “Hg is nearly closed, so the mixture density in the chambers is very low. So we need that much additional timing to ensure that the crankshaft receives the maximum push from each cylinder even with that lower cylinder pressure. After some experimentation we may discover that the engine might want more or less timing to maximize fuel mileage. That’s part of the testing and tuning procedure.
Under light throttle conditions, we’ve seen engines with older, less efficient combustion chambers demand as much as 50-52 degrees of timing while others that are more efficient only require timing in the low 40-degree range. If too much timing is applied at light loads, the engine may surge or even detonate. The only way to know what your engine wants is to test it under various combinations until you find the right set of tuning values.
Now we come to the real contentious stuff. The debate centers around whether straight manifold vacuum is better (or worse) to connect to the vacuum advance canister than ported vacuum.
Here’s where we should define ported versus manifold vacuum pickup points on a typical carburetor. The straight manifold vacuum outlet on any carburetor simply directs intake manifold vacuum to this connection. Ported vacuum differs only in that manifold vacuum appears at the outlet only after the throttle is slightly opened. Otherwise there is no difference.
It appears that most arguments about ported versus manifold vacuum are focused on this as an either-or situation. Our position is that the application will dictate this selection. The only difference between the two is whether the engine would need additional timing at idle. So for an engine with a large camshaft with lots of overlap with very low manifold vacuum (below 9 “Hg, for example) might benefit from additional timing at idle.
For the rest of the performance world the use of ported vacuum would be of a greater benefit. An engine with an idle vacuum of 14 “Hg for example would not benefit from connecting its vacuum advance canister to manifold vacuum that would and add perhaps 12 to 16 degrees of timing.
We’ve actually tested this with more timing at idle and then monitored the engine’s idle emissions. Too much initial timing can cause idle misfire where the hydrocarbon count (HC) rises as the engine struggles to idle. One way to evaluate engine performance at idle is by looking at the CO2 percentage. This is a great evaluation of combustion efficiency. Too much ignition timing will result in lower CO2 percentages. This is counter-productive since higher CO2 percentages are the goal for proper idle efficiency. Lower CO2 percentage indicates poor efficiency.
For a typical street performance engine, there’s generally no need to hook the vacuum advance canister to manifold vacuum since the engine doesn’t need the additional timing. In these cases, ported vacuum advance is the wiser choice. There could be some cases where low initial timing numbers are needed for the engine combination where straight manifold vacuum to the vacuum advance might help drivability but those cases would likely be rare.
Total engine timing for power and efficiency becomes a very application specific. Today, most mild street engines make best power while only needing 34 to 36 degrees of total timing at WOT. It wasn’t all that long ago that weak combustion chamber designs and big domed pistons demanded 40 to 44 degrees of timing. Conversely, a late model production engine like an LS, Ford Mod engine, or Chrysler Gen III Hemi may only demand 24 to 26 degrees of total timing at WOT to make best power. These engines still need more timing at part throttle but electronic timing control handles that part of the equation.
Hopefully this overview of ported versus manifold vacuum has pointed this concern in the right direction. You can prove to yourself which application your engine will want, but for the majority of mild street engines, ported manifold vacuum is the best option.
