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Old 09-04-2008, 07:49 PM
BogiesAnnex1 BogiesAnnex1 is offline
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Originally Posted by Regal Beagle
Thanks for the reply Bogie. What do you consider a highly cammed engine?
And do you hook your advance to ported or manifold vacuum?
When a cam is getting around 220 - 230 degrees of duration, you're getting to the practical limit of having vacuum advance. However, a cam with less LSA is more likely not to make effective use of vacuum advance than a similar duration cam with a 112-116 degree LSA. This, also, depends a lot on vehicle weight and overall gearing as well as type transmission and if automatic converter stall.

Stiffer gearing has less need for vacuum advance as the engine is revving higher which makes it easier to have the centrifugal do the work. Higher gears and a heavier vehicle can use vacuum advance if street driven as the engine isn't spinning so fast. But you will need to customize the vacuum advance.

You can push vacuum advance further into a long duration cam, but you need to have an adjustable "can" so you can make it sensitive to the realistic manifold vacuum numbers. There's no sense in having a vacuum advance that functions with 18 inches of vacuum on idle when you have an engine that only makes 10.

A loose converter lets the engine rev up which once again allows the use of the centrifugal system at low speeds that also allow high engine RPMs.

However, regardless of cam, if you spend a lot of time cruising low and slow and don't want to constantly run in the lower gears to keep the revs up, a vacuum advance can be effective to squeeze the best performance possible under these conditions. But again, as the cam gets bigger, the vacuum will be less, which means the systems response needs to be customized to reality.

Whether or not to use manifold direct or ported vacuum is really dependent on set up. A lot of factors become involved these include the effectiveness of squish/quench, better needs less advance; location of the spark plug, centered in the chamber needs less; mixture turbulence, more needs less, mixture ratio, to a point more needs less, the weakest mixture in a cylinder needs more, this is usually the lower plane of a two plane manifold, single plane manifolds require specific solutions; headers will want more advance as they tend to over-scavenge the cylinder, however, with anti-reversion cones probably less is best. All these kind of things come into play, which makes exact specifics a learning situation.

Full up race engines tend to dispense with advance mechanisms starting with vacuum not because they have some trick unknown to Detroit' engineers, rather it's because as manifold vacuum goes down a vacuum system becomes less and less functional and it brings durability problems and failure modes that out weight it's usefulness. In far out motors, the mechanical advance can be eliminated again for durability and reliability issues that out weigh the usefulness of an engine that runs so fast so much of the time it just needs full advance. These type engines usually separate ignition and fuel flow from cranking, such that at start-up the starter is just spinning the motor without fuel or ignition. This eliminates the chance of backfire when just starting to crank it. Then the fuel is turned on to prime the motor then spark is added to fire it. With modern electronics the choice is there to have a starting retard or running full advance, or to even have electronic advance which has proven to be much less failure prone than mechanical mechanisms on the race track.

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