MicroSquirt.com read only forum (Jan.15,2018)

I've heard a few stories about people disconnecting the vacuum advance on a race engine, evidently out of fear that this can produce too much spark advance if you snap the throttle closed at redline. Are these fears justified? If so, should I do something to decrease the advance at low KPa and high RPM once I add spark control?

I suppose it depends on the vacume advance. They are limited in there amount of movement no matter how much vacume is supplied. If it is only capable of say 6° total then that is it.

I would think that most are disconnected on race cars just because it is just one more moving part to fail and it would rarely be used during a race as most times these are full throttle conditions. It may also have something to do with compression braking too. I think most road racers do not like much as it upsets the balance of the car so they depend more on the brakes for slowing.

Todd

Matt,

Most true race engines don't use vacuum advance. They don't need it since fuel economy isn't usually a consideration, and it's just one more thing to fail.

In fact, quite a few race emgines don't use mechanical advance either. The reason is that since most timing curves are at their maximum by about 2800 to 3000 rpm, and most race engine operate well above that, there's no real ned for changing advance. This does make for hard cranking and starting, and a rough idle, but also eliminates potential failures and makes it easier to be sure that the advance you set is the advance you get.

Lance.

Thanks for explaining that one. Looks like it's not something I need to worry about then - the Dart is not a pure race car, but I do plan on autocrossing it and figured I could very well snap the throttle closed at full power.