I currently have a mechanical distributor in my car that I got for free. I've heard that a vacuum advance will get better MPG's. I get about 15 city driving right now so how much of a difference would vacuum advance really make?

 

Tough to give an exact number but 3 MPG might be a safe number to throw out there. Could possibly go as high as 5 MPG. The reason it makes a difference is the extra advance allows you to cruise with less throttle therefore using less fuel.

 

^^^

Don't have exact figures either, but I've run with vac off while dialling in the mech advance, difference in mpg was about 30%. Forgive my ignorance, but there are mech advance distributors that don't have the vac can?

 

Mechanical advance works strictly off distributor rpm. The weights and springs control how quickly the advance comes on and how far it advances. For best performance, timing should be at full advance by 3000 rpm.

Vacuum advance is for part throttle operation. As the engine load changes, the vacuum advance advances or retards the timing strictly based on the intake manifold vacuum signal from the engine load. At WOT the vacuum signal is very low. At an idle or just off idle, the signal is high.

For a street car, you want a distributor with a vacuum advance because you're driving it at part throttle most of the time. For the dragstrip, vacuum advance is an unknown variable. Mechanical advance only or a locked out distributor works better.

 

Awesome thanks guys. Looks like that's in the next paycheck. Would this help with highway mileage too? Or is highway not really part throttle

 

It should help with any kind of driving except for WOT. It will be not different than mechanical only at that point as there is little to no vacuum.

 

Hey gsoleski, is this on your 84 z28? That should be a computer controlled distributor, all advance is handled by the ecm. I think the dist. helps the ecm run the feed back carb.

 

sorry forgot to mention that. The computer is long gone