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vacuum advance

AJ/FormSFormulaS clone 367/3.55s 3.09-1.92-1.40-1.00-.78GVFABO Gold Member
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Many times when setting this, the idle timing will creep up and up as various engine parameters interplay. So if it ends up that your particular combo requires a large amount of timing to get the t-port properly working, then the addition of vacuum advance, by way of full manifold vacuum may drive the engine into pinging at very small throttle openings where the vacuum advance will not yet drop out.
One solution to this might be to use ported vacuum, instead. In this case the vacuum might not be enough to even activate the can and so the pinging goes away.
A second solution might be to reduce the initial timing, and a third solution might be to get a smaller can, and a fourth solution might be to adjust the onset of the Vcan, and a fifth, is some combination of all of these.
I try to leave the initial timing alone once the T-port sync is established.
On the other hand, with a small cam and lots of compression. The T-port sync might generate a very small number for idle timing.Yet the engine might still accept 34 to 36 as power-timing.And most guys will want to bring it in really early.And without a Vcan or with a high-stall, there is almost nothing wrong with that. As your engine snaps up to stall it runs through this zone and is never stuck working there.But if you have a typical stock-type TC which fits well with the small cam engine, then you run into issues where the centrifugal timing overlaps with the vacuum timing at small throttle settings and there's that pesky ping again.
The 360 is a good example. With a 223 cam and 10.7Scr,and a stick and 3.23s.This combo packs a wicked street punch, once wound up a little. But the 2.66 x 3.23 offers just 8.59 as a starter gear. The T-port sync might call for 12* initial. The power timing will still be 34 to 36.So yo have to build a centrifugal curve to go from 12 to 36. That's 24* in the dizzy;no biggie. But that engine at 65mph, will be turning 2600 Rs and might like 50* or more timing. So I like to work with the 20*Vcan. Then 50 less 20 is 30 in the centrifugal.That curve is almost buildable. But back up to first gear. If you hook the can to manifold vacuum it will idle at 12 + 20 =32*It might put up with that at idle.But as soon as you roll on the throttle, you will force the engine to to be heavily loaded by the clutch and the 8.59 starter gear. And she is gonna rattle to no end.And it will only get worse with more throttle, until the manifold vacuum falls to a level that the timing drops out to a level the engine can accept.
On the otherhand, hooking the can to the sparkport, gives you a lot of throttle blade angle before the Vcan gets signal, and the signal is weak in the early part as the blade goes by and strengthens over time until the vacuum begins to fall.You can handily take advantage of this, by adding the Vcan timing which is varying from zero to 20* and back to zero, with manipulation of the gas-pedal.In this way you can add up to 20* of timing at any time in the low to medium loading parameter, and the beast will be more controllable than the typical on/off behavior that full manifold vacuum exhibits.
But notice that this applies to a stickshift car with hi-way gears, almost a worst case scenario.But this is what guys build cuz that's what we are stuck with in terms of having just a 4speed.
The thing to keep in mind is that I set the initial timing by the T-port sync. I let it tell me what the engine wants. I don't even pay attention to the timing lite or a vacuum gauge, at this stage.You may end up with idle timing anywhere between 10 and 20-25, depending on your particular combo. This target is primarily determined by cam size and compression, but there are other factors.