mechanical advance distributors

[chevy72blu]

I started this thread after a discussion I got into with Billla about non-vacuum advance dizzy's. I bought a brand spankin new MSD pro-billet mechanical advance distributor a while back for my BBC bracket/street motor (it still hasnt been fired). I was told by an "old school" racer/friend of the family that vacuum advance isnt desirable on a bracket motor because it will not always come in at the same time and thus be less consistant. I've since been told that this will cost me power, mileage, and eventually limit the life of the engine. Fuel mileage really isnt a concern, but the first and last of course are. Being that it is "mechanical advance" rather than vacuum advance, I would assume that the motor will still receive the right amount of advance at higher RPM via the springs and weights (I still need to learn more about this). Is this not the case? I just want some more feedback on the issue.

Thanks guys!

[VetteVet]

Here's some feedback for you.
http://www.lbfun.com/Corvette/Tech/v...Adv%20Spec.pdf

[cableguy0]

in wide open throttle conditions vaccum advance has no part in the timing. vaccum advance is there for fuel economy at cruise speed. when you go wide open and vaccum drops the vaccum can doesnt advance.you have the right distributor for your app.

[WorkinLonghorn]

If you don't mind driving around town or at cruise speed with the timing seriously retarded,yes,that's the right distrib.

[cableguy0]

theres no reason his timing needs to be retarded just because it doesnt have vaccum advance.set the total mechanical to be all in at around 3000rpm depending on the cam.stock distributors with a vac advance usually have springs so heavy the mechanical advance wont even move until 4 or 5000rpm.get your distributor curved to your motor.

[Billla]

To be clear in my comments - I noted that non-vacuum advance wasn't appropriate for a street engine. If the usage is all race - or you're willing to drive something that doesn't run well on the street, then it's fine. Any racing engine that gets driven on the street is a compromise - the jetting you're using also impacts longevity, etc. I think maybe I gave a snap answer to a quick question and didn't take the time for full context.

Mechanical advance even on a stock engine starts early - not at 4-5000 rpm - but I agree (as I've noted many times) that a custom-curve is required.

[68GMCCustom]

if it will only see track time then yes a mechanical dizzy is correct. if it will also see street time too....in addition to the other issues, your engine will also run hotter w/o vacuum advance.

[Green Machine]

Vacumn advance is not "required" on street driven engine, but it surely would be a good idea if you drive some miles on the street. Another 10-15 deg advance at idle and light cruise will let it run cooler as mentioned, better mpg if it matters and lilely keep the plugs cleaner.
At the track just unhook the advance line and plug the carb port if you want to eliminate possible vac advance variations affecting the ET

[chevy72blu]

Thanks for the replies guys! As much as I'd like to keep my truck as a "race only vehicle", I love it too much to not drive it around and show it off. That makes sense about the motor idling and cruising cooler. Hmm, I still need to do some more thinking on the issue.

I was under the impression that there was a disadvantage to plugging the vacuum advance on the dizzy and carb. If there isnt, there is no reason to not swap mine, and just recurve it when I go to the track.

I've been thinking more and more that I should just dyno the thing and answer a lot of these questions for myself.

[Billla]

Originally Posted by chevy72blu

That makes sense about the motor idling and cruising cooler.

I was under the impression that there was a disadvantage to plugging the vacuum advance on the dizzy.

The cooling impact is minimal at best; and there are disadvantages as noted - you just need to decide if the disadvantages are significant enough to warrant a change.