67 Corvette - another vacuum advance/timing thread with a few questions

[Donald H.]

With the recent threads regarding vacuum advance cans and running temperatures and general engine performance I decided to finally check my car. I've had to read and re-read the information from Duke and Lars for it to hopefully sink in to my limited brain capacity.

This is my 67 L79 with as far as I know stock engine internals and distributor (1111196). The only things I've done to the ignition/engine electrics since I've owned the car is install a single wire points elimination kit, new spark plugs and new spark plug wires, all done within the last year.

Following is what I did and found on my L79:

Idle – 750rpm

Initial timing 10^o BTC

Vacuum measured at idle – 14.5-15 in. hg (idle mixture set to max vacuum measurement)

Vacuum can on the car in the beginning was a B33, readings were Start @ 4 in. hg and full in at 10.5-11 in. hg

Vacuum can was replaced with a B26 can (per Duke's recommendation) which is rated as Start @ 5-7 in. hg and full in at 11-13 in. hg

Timing measurements were as follows:

Total advance at idle (with vacuum can connected) – 26.8^o (this conforms to correct parameters 10^o initial and 16^o full in vacuum advance per service manual specification)

Total advance (with vacuum can disconnected and plugged) at various rpm levels:

• 900 rpm – 15.2^o (this is a little high per specs of 10^o initial plus 0^o centrifugal at 900 rpm)
• 1200 rpm – 19.5^o (this conforms to specs of 10^o initial plus 9^o centrifugal at 1200 rpm)
• 1500 rpm – 22.4^o (this is a little low per specs of 10^o initial plus 15^o centrifugal at 1500 rpm)
• 3000 rpm – 30.2^o (no service manual specs for 3000 rpm)

I can't explain the 900 rpm extra 5^os. I sort of suspect the 1500 rpm reading was a combination of holding the rpm steady and getting the dial back timing light dialed in.

Should I be satisfied with the tuning as set currently? Any reason to mess with changing the advance springs?

Also, per Duke's paper, he says WOT timing should be in the 34-40 degree range for small blocks. How do I measure the full advance? I don't particularly want to jam the accelerator to the floor and hold it there while trying to dial in timing light to get a reading for timing. Is there some lesser rpm where the centrifugal advance will be all in to measure at?

Thanks,

Don

[Patrick H.]

Re: 67 Corvette - another vacuum advance/timing thread with a few questions

Your springs might be a bit light on the bottom end, giving you that extra 5 degrees. Try putting a rubber band over the weights to stop them from moving, and I’d bet the extra 5 degrees disappears.

The easiest way to find the timing at WOT (or the top end) is with an advance timing light. However, if you raise the throttle by hand and don’t see any additional advance beyond 3000rpm with your current lightyou’re probably not going to see any suddenly show up at 5000 rpm. If 3500 and 4000 are stil around 30 degrees you have your answer.

To get the 34-40 total you will have to advance your initial timing to 14 or 16 degrees rather than the 10 its at now.
You might also consider swapping out whichever of your springs is the lighter one to the next heavier one to eliminate that early advance, but do the rubber band test first to be sure.

I hope that makes sense.

[Duke W.]

Re: 67 Corvette - another vacuum advance/timing thread with a few questions

WOT is simulated, as far as the spark advance map is concerned, by disconnecting and plugging the VAC signal line. Do that then free rev the engine a few hundred RPM above the point where no more advance is added from centrifugal, and use a dial back light to set it in the 36-40 range, the higher the better, but it may be limited by detonation.

IIRC the max '67 L-79 OE centrifugal is 30 @5100, so you would have to rev it to over 5000 to use the above method to set total WOT advance if the OE springs are still in place, however, it should be all in at 3000-3500 with the lighter springs from the Mr. Gasket 928G spring set.

Engine performance will definitely benefit, especially at the low end, from a more aggressive centrifugal curve with lighter springs.

Your data indicates that centrifugal starts at less than 900, and that's okay as long as it doesn't detonate, and, assuming it still has the OE springs you didn't rev it high enough to achieve max centrifugal. For any given spring setup you must determine the start and max points first, them map it out in 500 to 1000 RPM increments.

Just spend the 10 bucks on the Mr. Gasket 928G spring kit, to bring full centrifugal in earlier, test all three sets and use the one that brings it all in the fastest, then use the total WOT advance method described above and if it doesn't detonate, you're done. If it does detonate you have to either slow the centrifugal or reduce total WOT advance lower in the 36-40 range. Finding the optimum requires experimentation.

Duke

[Donald H.]