Here's something I discovered after quite a bit of research on Chevy crate motors over the last few days. I posted part of it on another thread.

It seems that the vacuum can Chevy supplies with the crate motors has too much advance (20 deg.) and the spring is way too soft. That brings the advance in too quickly. It's easy to get 52 -56 degrees in some situations. At the same time, the newer, more efficient heads (like the Fast Burn) don't like a lot of timing.

Many of the folks reporting good results with these motors either disconnect the vacuum advance, which has an effect on idle, engine heating and gas mileage, or they get a replacement vacuum can (either fixed or adjustable) with 10 degrees or less and a stiffer spring.

Of course, after that, they still have the debate about manifold or ported vacuum.

On my new crate motor, I'm taking this approach:

- I ordered a vacuum can recommended by Sallee Chevrolet's performance department - PN 1973681

- Install the new vacuum can, tune the motor, set the timing, then see what happens on the tach (idle), temp gauge, fuel gauge and the "seat-of-the-pants power gauge" with both ported and manifold vacuum.

- Disconnect the vacuum can, then take a look at the same four gauges.

Experimenting. Isn't that what hot rodding is all about?

Here's my guess on the results:

I didn't believe I'd ever say this, but . . . new vacuum can connected to manifold vacuum. (Don't have a heart attack on me, Denny)

Bear in mind that this is coming from a dinosaur (Vacuumus Nixis) who never used anything but a full mechanical advance distributor in ANY hot rod.