In the future, if you are working on a Firecore distributor and want advice, it would help if you mentioned right off the bat that it was a Firecore.

Rick, aka Sparky, AKA Sunroof GTX, IS Firecore, or at least a good part of Firecore. So if he has a comment it would behoove you pay attention.

I agree that the use of springs and vacuum pots is much akin to witchcraft. It is true, also, that electronically setting advance is another level of witchcraft. Electronic setting has one advantage over the spring/slot witchcraft and that is this: You don't have to understand how the black box works, you just have to understand that it does work.

R.

Is vacuum advance really needed? If so Why?

It provides additional advance at part throttle conditions when the engine can take the advance (the additional advance would cause pinging at wide open throttle).

The additional advance increases the engine efficiency at part throttle, does not affect WOT.

This is another reason to think about moving to EFI. Even the cheap EFI systems available these days can control the ignition and they have vacuum advance built in since the EFI system needs to know manifold vacuum. I helped install a low dollar MSD Atomic system the other day and it had vacuum advance built in. The nice thing about EFI is that you can program the vacuum advance for both rate and total amount.

I was curious on vacuum advance as the new FBO distributors have vacuum advance. Their mod years ago was adjusting the advance profile and eliminate vacuum advance on the distributor.

To limit the vacuum advance arm travel, I set my initial where I want it, use a timing light and a mighty vac hand pump to pull in an additional 10* and note the amount of vac required for the 10*, pop the cap off, pump the vac to noted amount, mark the vac advance arm with a sharpie right where it passes through the housing, pick a appropriately sized short roll pin and drill the correct sized hole through the arm where you marked it. Drive the pin in and you now have a mechanical stop on the vac advance.

^^^ Ding ding ding I do believe we have a winna.

Put a 5/32" ball in the vacuum hose. Vacuum advance worthless.

i like vacuum advance on anything that sees any street time. if the hose is just removed or plugged, the tune is not complete. take the time to add/subtract any advance you can use [with what ever method you choose], and you will be happy you took the time to do so.

I recently worked over the older MP electronic distributor in my friends GTX. With a Lunati Voodoo hydraulic FT,mildly ported 906 heads and a Holley SD with a 750 AFB, it was a bit sluggish on the bottom end. I put the FBO limiting plate and springs in it, and adjusted the vac advance for an additional 10* at idle. The difference was amazing, super sharp on the throttle response and my friend had to relearn how to take off from a light without spinning the tires. If you follow FBO's instructions, you can get your dist squared away and the advantages of having a vacuum advance that works properly will be obvious. My buddy said he picked up a little mpg to boot!

Easy way to do it on an old Mopar Distributor is to wedge a piece of ~.1-.125 header gasket material between the pin and the slot, takes out about 10-12 degrees of centrifugal and if done properly it can't go anywhere because the pins sling against it as the distributor speeds up. You could weld up the slot but back in the day this was easier. This allowed to advance the distributor and keep the total timing ~34-36 degrees.

It's real good and easy to try to see if it makes the difference you need, you could always weld the slot up later if you want.