Vacuum Advance question

[suburble]

I'm running 18* BTDC base timing. More is better, until you get knocking, then back off 2 degrees.

What's your setup?

Also, what's your total timing at idle with the advance hooked up?

Thanks,
-Josh

 

[rich weyand]

350 flat-tappet crate, 12-300-4 Comp cam, Edelbrock 1406 on Air-Gap manifold, Hooker headers through dual Flowmaster 40s, Accel cap/rotor/coil. Base timing at idle is 17 BTDC with vacuum disconnected, at 3000 rpm it's 37 BTDC with vacuum disconnected. As I recall, the stock HEI distributors on these trucks pulled in about 7.5 degrees above 11" or 12" of vacuum, so at idle with manifold vacuum connected I'm running about 25 BTDC. At cruise at 2500 rpm, with the centrifugal advance in and the vacuum advance also in, I'm running about 44-45 BTDC.

SBC or BBC makes no difference. All GM V8s like about 36-38 of total advance (vacuum advance disconnected), plus vacuum advance up to 52 BTDC, so some stock distributors from GM ran up to 12 degrees of vacuum advance. 1969 Chevy 350 quads f'rinstance.

 

[suburble]

350 flat-tappet crate, 12-300-4 Comp cam, Edelbrock 1406 on Air-Gap manifold, Hooker headers through dual Flowmaster 40s, Accel cap/rotor/coil. Base timing at idle is 17 BTDC with vacuum disconnected, at 3000 rpm it's 37 BTDC with vacuum disconnected. As I recall, the stock HEI distributors on these trucks pulled in about 7.5 degrees above 11" or 12" of vacuum, so at idle with manifold vacuum connected I'm running about 25 BTDC. At cruise at 2500 rpm, with the centrifugal advance in and the vacuum advance also in, I'm running about 44-45 BTDC.

SBC or BBC makes no difference. All GM V8s like about 36-38 of total advance (vacuum advance disconnected), plus vacuum advance up to 52 BTDC, so some stock distributors from GM ran up to 12 degrees of vacuum advance. 1969 Chevy 350 quads f'rinstance.

Thanks. Good information. Maybe I'll just try bumping the timing for more advance before I mess around with other diagnostics.

-Josh

 

[rich weyand]

More advance gives you more horsepower and better mileage, all the way to the limit.