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Old 08-12-2002, 10:13 PM
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After tuning it for the change....yes. The ported vacuum source on any carbuerator can be anywhere above the throttle plate and depends on the design itself but basically if it you look at it this way you will understand;

Vacuum advance is for one thing...mileage and response at part throttle.

Race engines use only mechanical advance for this reason, if you introduce any vacuum advance into the equation you can have the vacuum advancing the timing when you least need it....like coming into a corner and releasing the throttle (high manifold vacuum). Also at top end the engine will start to generate some manifold vacuum (due to restrictions) which could cause some advance. This is very dangerous as detonation at this rpm will destroy the engine quickly (6000+). Your ported source will be at atmospheric under the same conditions.

Ideally you want the best of both worlds, vacuum advance when your just cruising around when the engine can use it and pure mechanical advance when you are hard on the throttle....ported vacuum is as close to a linear response to engine load as you can get and is only active when the throttle is open...at idle you get nothing.

Therefore you can run more initial advance in the distributor without the vacuum advance functioning and messing up your highly tuned mechanical advance setup. Usually what would happen is you would have to reduce initial timing to compensate...this kills off the line torque because as soon as you whack open the throttle the vacuum disappears and your left with a compromised timing for the engine.

I may not be explaining it all that well but here is a rule of thumb that applies to all high performance engines.

-Tune your engine using only mechanical advance on the dyno to get the maximum torque.

-once the engine is installed in the car you hook up the vacuum advance (to ported vacuum) and keep adding advance until it pings...then back off with a safety margin for changing weather conditions etc. Of course you need an adjustable vacuum advance canister. The major problem is load is what determines how much timing the engine can stand unless you are using really low compression )like 6-7:1 and big heavy cars can't handle as much vacuum advance as lighter cars under part thottle load.

Soooo....manifold vacuum will add advance in a non-linear fashion and the engine has to be tuned to compensate (reduced timing). You can see this if you hook up two vacuum gauges, one to ported and the other to manifold and then drive around. You'll see that anytime the load is removed vacuum skyrockets....but rpm stays the same and your mechanical advance mechanism is controlling that. this could cause detonation when you least expect it and least need it (high rpm, low load).

Am I making sense?

Really when you have it setup just right you gain a lot of "snap" from the engine since the timing is where it needs to be anywhere in the rpm range and load and the vacuum advance only kicks in when needed (cruise) and when it does, it does so more slowly and predictably. The mechanical advance mechanism is more in control of the timing that the engine sees.

Usually what happens is you end up dialing a lot more mechanical advance in and reducing vacuum advance. The term "put all your timing into the distributor" is very appropriate for power. Non-ported vacuum advance came about because manufacturers were already compromising the tune of the engine anyway by retarding the timing and the engine could handle that. But if you start trying to give the engine the timing it needs and retain the non-ported advance it will throw a wrench into the whole works. This scheme reduces emissions.

Try it and you will see, it helps to have access to a distributor machine or you can hook up a drill and digital tach to the distributor and tune it on the bench.

So what do you think?
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