Total Spark Advance

[Joe C.]

As I understand it, total spark advance is the sum of initial, vacuum, and centrifugal advance. In the case of a 327/365 engine, this is 10 (minimum) + 16.5 + 24 = 50.5 crankshaft degrees.

My engine is timed at 12 initial, with a 236 vacuum can, and reproduction autocam, weights, springs. This translates into 52.5 degrees total, IF everything else is AOK. The advance limit bushing IS in place. Well, I just now installed a harmonic balancer degree tape to check the advance map because I suspected "pinging". It's very difficult to hear "pinging" if you are hard of hearing (too many years spent driving with the screaming siren of the fire engine in my ears), and even more so if your Corvette is equipped with sidepipes!

I hope that I'm wrong, and I don't understand how the pistons didn't melt down with 68 degrees of total spark advance!!!!!! How and why, I dunno yet, but the autocam/bushing is my choice as the culprit, as of now. More to come.

Joe

[Duke W.]

Re: Total Spark Advance

As I have said a zillion times before there are THREE "total timing" "specs.

Total WOT timing = initial + full centrifugal (There is no vacuum advaance at WOT.)

Total idle timing = initial + full vacuum + maybe a few deg. cent. if the curve starts below idle speed.

Total cruise timing, initial, plus centrifugal at cruise revs, plus full vacuum advance.

Since the 327/365 achieves full centrifugal at only 2350 total cruise timing above this speed with be the sum of initial + full centrigfugal + full vacuum advance.

If you suspect a problem, check centrifugal and vacuum with a vacuum pump and dial back timing light. The 236 VAC max advance is 16@8", and your max centrifugal should be 24@2350.

Duke

[Jack H.]

Re: Total Spark Advance

What you're missing is the relationship between centrifical and vacuum advance. When you stand on it (WOT or close to it), manifold vac drops like a rock, it's then (delta RPM) that the centrifical weights throw out and that advance mechanism kicks in.

When cruising along (steady RPM), the centrifical weights collapse via return spring force, and you're left with the advance contribution of the vac system. At relatively constant speed, the intake's vac pressure peaks...

That's why there are two advance systems...one covers the 'faults' of the other.

[Duke W.]

Re: Total Spark Advance

There's no specific relationship between vacuum and centrifugal advance. Each is a totally independent system that is tuned to the specific characteristics of each engine configuration. For a given configuration, there is an "ideal" advance for each speed and load condition.

On vintage engines this is achieved by vacuum and centrifugal advance. (On modern engines it's a digital look up table based on the above and other parameters like engine coolant temperature, inlet air temperature, and can be modified in literal real time by detonation detection.)

It's an equation with two independent variables - engine speed and load as represented by inlet manifold vacuum.

If you plot it on an X-Y-Z graph you get a surface.

Duke

[Joe C.]

Re: Total Spark Advance

Duke,

I thought that you knew me better than that. The reading was taken @2500 RPM, steady state (ie full vac advance + full cent advance). Something very wrong with the autocam!
Will get back with further details > more anal.

Joe

[Michael H.]

Re: Total Spark Advance

Joe,

Not trying to weisel in on your conversation with the Duke but it would be a lot easier to understand exactly what you have there if you run your tests and post the results with the vacuum adv temporarily disconnected and plugged. That part can be looked at later. You should have somewhere in the range of 36-38* total at high RPM with the vac line disc and plugged.

When you built the engine, did you make sure that the timing mark and plate were indexed reasonably close?