Ignition spark advance question

[Tom Sarno]

I just had my motor rebuilt and used a cam with more lift and duration. I was wondering if anyone had a chart for determining the advance curve, or is there someplace that I can send the distributor to to get it recurved. Thanks

Tom

[Patrick H.]

Re: Ignition spark advance question

Tom,

I've had great service with Bill Clupper, available here on the board.

Patrick

[Duke W.]

Re: Ignition spark advance question

The vacuum advance characteristics are primarily determined by idle vacuum, so if you post your idle vacuum/idle speed I can give you some guidance.

Is it a small block? If so, the initial plus total centrifugal should be 36-38 degrees and you can try different springs to get all the centrifugal in as quicly as possible without detonation.

Duke

[Tom Sarno]

Re: Ignition spark advance question

Duke,

From reading your responses to questions posted over the years, I was hoping that you would offer me some advise.

I have not yet installed the motor so I cannot give an exact vacuum number, but I used the same specs as the motor that comp cams dynoed their 262H cam on. The only difference is they used Dart II heads and I have the GM Vortec with the springs changed to match the cam. They give the figure of 17.5hg @800 and 18.5hg @1000. It is going into a '74 with a B-W T10 and a 331 rear.

Also, if I have 36-38 total is that initial + mechanical + vacuum or is it just initial + mechanical. How much Vacuum advance should there be and is it ported or manifold vacuum to the distributor.

Thanks for sharing your expertise
Tom

[Duke W.]

Re: Ignition spark advance question

The 36-38 degrees is the maximum advance you want at WOT. Of course at WOT there is very little vacuum, so this figure does not include the vacuum advance. It is initial plus total centrifugal.

You want a vacuum can that provides about 16 crankshaft degrees at least 2" BELOW what the engine idles at, so the vacuum advance will be "locked" full at idle speed. If it idles at 16" you want a vacuum can that provides full advance at 14" or less, and the vacuum signal line should be routed to a full manifold vacuum source - one that shows manifold vacuum under all operating conditions, including idle. Your OEM vacuum can and signal line probably does not meet this spec.

Your total idle timing, which is the sum of initial, vacuum, and any centrifugal (if the centrifugal curve begins below idle speed) should be in the range of 25-30 degrees.

The vacuum readings claimed for the 262H cam seem a bit high, so test when the engine is installed.

Send me an e-mail and I'll send you a list of vacuum cans for the HEI so you can pick one that is matched to your engine's vacuum characteristics.

Duke

[William C.]

Re: Ignition spark advance question

Duke, '74 was not HEI unless he has swapped distributors.

[Duke W.]

Re: Ignition spark advance question

Yeah, thanks for the correction, Bill. I had not yet had my full ration of morning coffee, so my brain wasn't up to operating temperature. I know that the first year for the HEI was 1975.

Duke

[Duke W.]

PS

Send me the e-mail, Tom. The list includes vacuum cans for both single point and HEI distributors. I can't add an attachment to e-mails from the Board.

Duke

[Tom Sarno]

Re: Ignition spark advance question

Duke,

As Bill mentioned I am useing a point set-up but it is converted to electronic.

If I have 30 centrifical and 16 vacuum, then initial will be 6-8 but show 22-24 at idle when the vacuum is connected and cruising would 52-54. Am I undersanding this correctly?

Thanks for your time
Tom

[Duke W.]

You got it! *NM*

[Tom Sarno]

The E-mail

Duke,

The info you sent was exactly what I needed. Thank you

Tom