Pro billet distributer. MSD 6 box. Ported vacuum (no vac to distributer at idle) 383 Chevy, 10.5 compression, AFR heads.

My engine builder said he could hear a high RPM miss so I started with the basics. Checked all the plugs and changed 2. Checked the timing and found 10 degrees initial but it moves past 50 as I get towards 4000RPM! I am running a fluiddamper with etched numbers so there is no way the damper slipped. Then I pulled the distributer and found everything was OK. There was a blue mechanical advance bushing installed and the mechanical advance was working smoothly. Vacuum advance was working smoothly. I reinstalled it, set the initial to 14 and checked it with the vacuum advance disconnected (vacuum gauge says about 18 inches total at high RPM). It hit 38 degrees max. Connected the vacuum and it maxed close to 50 again (the damper is only etched to 40).

I thought the vacuum was supposed to drop off as throttle approaches wide open, or is it only at wide open?
Is the vacuum advance really supposed to add 12+ degrees? Should I just disconnect the vacuum advance or is 50 degrees normal given the other settings?
And if the high advance is not the cause of the high RPM miss, how do I go about troubleshooting this?

Thanks
Chris
www.corvetteforum.net/c3/71roadster

 

The vacuum drops off at WOT. You need the additional advance the vacuum canister provides under cruise conditions. At part throttle (cruise conditions) the A/F mixture is less dense in the combustion chamber and requires more advance to finish burning at the right time. If the miss is at part throttle at higher rpms, maybe you want to limit the amount of vac adv. that 38* total, centrifugal may be a little high. It all depends on how your engine was built. The less advance it takes (WOT, no vac) to make the maximum power is a measure of your combustion efficiency.

RACE ON!!!

 

Chris - You need a Crane adjustable. I have mine set at 4 degees additional. I think that it can go up to 12 or so. Where you see problems is on a steady state cruise down the freeway if you have the 70 mph rattle on our 91 Calif super

 

Chris,

Lars put up a nice paper where he gives the specs on lots of different vacuum advance canisters - points and later HEI as well. Some of those cans can add as much as 20 degrees of vacuum advance. Mine adds 16 degrees and it really helps at cruise. 50+ degrees of total advance at partial throttle cruise is just fine by me and my 454. Could your high speed miss be due instead to fouled plugs, bad wires, or a jetting problem?

 

You can free rev the engine to the redline and you will not be at WOT because there is no load on the engine, other than internal friction, and it will show signficant manifold vacuum. WOT is near zero inlet manifold vacuum. That's why you simulate the WOT timing curve by disconnecting the vacuum can.

Your ignition map is okay. total cruise timing on my L-76 above 2350 revs is 54 degrees - 14 initial, 24 centrifugal which is all in at 2350, and 16 vacuum which is all in at any manifold vacuum above 8".

It gets 22 MPG on the highway

Higher idle and cruise timing is required beyond what is provided by initial and centrifugal due to lower flame propagation speed from the combinated effects of low inlet density and exhaust gas dilution, and the higher overlap cam you have the more timing the engine needs at idle and low to moderate load.

Duke

 

good info