1972 454/270 centrifical advance - "all in"?

[Robert B.]

I have a low mile 1972 454/270 hp - automatic roadster. The timing is current set at 11 degrees BTDC at 900 rpms. The centrifical is stock and a 22 degrees. I have 21 to 21.5 inches of vacuum at idle (900 rms), and 18.75 inches of idle in drive at about 650 rpms. My vac advance is a B22 with shim that only lets it advance to about 15 degrees. I have no detonation with these specs.

I am playing around with springs and I currently have two med to light springs in mechanical advance.

At what rpm would "all in" with the full mechanical advance? I ma pretty sure I am coming in around 3000 - 3200 rpms full about 33 degrees total.

Thanks

[Duke W.]

Re: 1972 454/270 centrifical advance - "all in"?

Get the advance in as quickly as possible - as low as 2200 without detonation, but the centrifugal curve is short for emissions. It would be best to grind the slot out to achieve about 28-30 total all in by 2200-3000 depending on what you can achieve with available springs.

The B22 VAC provides 16 deg. at 15" Hg, so I don't understand what you have "limited". You should also convert it from ported to full time advance and remove the "limiter" assuming the VAC is reasonably within spec.

With 28 centrifugal and 8-12 initial, total WOT advance will be in the range of 36-40. Run it as high as you can in this range - detonation being the limiting factor.

With full time vacuum advance and initial in the above range total idle timing will be in the range of 24-28, which is in the correct ballpark.

Your engine was designed to operate on current 87 PON gasoline. There may be a limit on how aggressive you go on the centrifugal with 87, and if there is detonation with the most aggressive centrifugal curve you can achieve, use mid-grad or premium - whatever it takes to quell any detonation.

If you have not seen this, google my name with Corvette and San Diego and download my national convention presentation. It has all the background on how to set up the spark advance map, starting recommendations for various OE engine families, and parts references for springs and shim kits.

I've been looking for reliable manifold vacuum data on Corvette hydraulic lifter big block engines with automatics idling in Drive. Your recorded 18.75" at 650 in Drive seems high considering that the cam has nearly as much overlap as the L-79 cam. Is it possible that a lower overlap cam was installed at some point in the engine's life?

Duke