another BB vacuum advance canister question

[Duke W.]

Re: another BB vacuum advance canister question

Yes I am that handle on the CF. I think I'm slowly learning all this. The number after 357 is "20". I know the weights and springs have been changed. A few years ago I asked on here where to buy the correct ones. But I was told they are no longer available and has been for many of years.

The OE 396/325 HP spark advance map is not ideal for your engine because you have a higher overlap cam. In fact it's not ideal for a production 396/325 because the OE spark advance maps were always very conservative to keep the engines out of detonation in worse case conditions.

So what you need to do now is characterize the centrifugal curve with a dial back timing light. In the meantime download my 2012 San Diego National Convention presentation. Links and the pdf file are here on the TDB, and it can be easily found on the Web with a Google search... duke williams corvette san diego.

BTW, those last two digits on the VAC bracket are the approximate total nominal advance in crank degrees. Joe got the 20.5 from a more accurate spec source. The tolerance ranges on these rather crude mechanical analog devices are about +/- one inch Hg and one or two crank degrees from nominal, but if you pick the proper one that complies with the Two-Inch Rule, they usually do a pretty good job.

[Keith B.]

Re: another BB vacuum advance canister question

Duke some updates for you to either correct me or help me some more. I installed a B26 VA can and increased the idle speed to 850 RPM with a manifold vacuum HG of 14. I double checked my timming and its 30* found using a dial timing light.

I have yet to drive the car due to condition of the roads. Am I on the right track?

[Duke W.]

Re: another BB vacuum advance canister question

14" @ 850 means that the B26 passes the Two-Inch Rule, but you have not provided full context for the 30 degrees you mentioned.

The are several "timing numbers" or what I usually call "spark advance" that characterize a spark advance map.

1. Initial spark advance
2. Total WOT spark advance
3. Total idle spark advance
4. Total cruise spark advance.

All these are fully explained in my 2012 San Diego national convention presentation that is on the Web, and you need to use your dial back light to measure them. But I said such a week ago in post #16. Did you do your homework?

Duke

[Keith B.]

Re: another BB vacuum advance canister question

Well Duke I will be honest and say I did not. But now that you reminded me I will go back and find your notes and read up.