Fuel economy is the reason... (archive)

My own digging on the subject supports what you state here. The vacuum advance/retard mechanisms are designed to give maximum advance under cruising/light load conditions. This gives better fuel economy and emissions.

A mechanical-advance-only dizzy sets the advance only as a function of engine speed, and does not take load into account. Take a glance at the advance function graphs for modern electronic fuel injection and you'll see that optimum performance requires a very complicated advance control system. The reason why mechanical-only dizzys are generally used on old performance cars is that in general the drivers did not care about fuel economy or emissions, and really only wanted the best full-bore performance.

You can make a mechanical dizzy work with any compression ratio or carb; it's just a matter of setting up the curve mechanism. You can home-brew this yourself using factory dizzys, or you can get an aftermarket unit like Mallory that's made to be tweaked more easily.

Mike

The deal is on the downdraft carbs, that port at the bottom of the carb has vacuum on it only when the throttle plate is only slightly opened. I hooked up a vacuum gauge to the thing any drove around for a while to reach this conclusion. And I actually took my carb apart this weekend and saw it's little hole which sits right next to the progression holes on the primary. When the throttle is closed. The hole is above the plate and there's no vac. As the plate opens and exposes the hole you get vac and as the plate opens even more you loose vac, just like you don't get any fuel through the progression circuit at wide throttle opening.

I suppose you could hook vac adv or retard modules to it to suit your needs, but it isn't going to do a hole lot to your perfomance as it's only doing anything at cruise. Maybe you'll get better gas mileage or less emissions.

Manifold vac is large with throttle closed and small with throttle opened wide.

I don't have any experience with DCOEs, but dividing that vacuum signal by 4 isn't going to do squat to any of these vac modules, so you would have to string in some kind of equilizing circuit to get little benefit and drill the port if it isn't on the carb already. I lot of work for little or no gain.

Follow Ups:

[ Follow Ups ] [ 2002 Message Board ] [ Msg. Board FAQ ]

Questions, comments, or problems, please visit the Roadfly help desk.