? does 81 have vac/mech advance ?

[Jack]

There continues to be confusion regarding 81 vette HEI distributor and computer (engine management systems). I understand the 81 carb is computerized but my questions focus on ignition. I'm pretty good with first design HEI but I'm lacking when it comes to computerized C3 ... I think there's a few others here that would like to know also.

Would someone with direct experience with and knowledge of Original Equipment HEI distributor & computer for 1981 vette please chime in?

Did the 81 HEI distributor come OE with a vacuum advance?

Did the 81 HEI come OE with a mechanical (centrifugal) advance?

What role does the 81 vette OE Computer play in ignition? ... does it substitute/attenuate vac advance? ... does it substitute/attenuate mechanical advance? ... either or both or neither?

Step on up, 81 folks ... help a cretin out.
JACK

[mikejpss]

I was told by my mechanic to remove my OEM computer driven distr. and replace it (along with a new carb.) to a VACUUM ADVANCE dist.
At the risk of showing my ignorance here, I do not believe the 81's had a vacuum advance distributor. I also believe that the ignition- or firing- was done with an electronic spark inside the distr, but was retarded / advanced by the computer.
Once removed from the computer(old style dist installed) , firing is regulated by vacuum advance (or retarding) generated from the negative pressure caused by the carb as fuel was added to the engine,along with electronic ignition replacing the old points system.
Now I am completely brain dead.Hope you're happy cause I need another beer.

[Jack]

TTT additional input please?

[Yoda]

I wonder where Stingray6974 is hiding... I'll send him an invite, he can fill in all the spaces Mike does this kind of stuff for a living..

Bud

[Stingray6974]

Hi Jack,
The 81, and California 1980 305, distributor does not have a vacuum or mechanical advance. The timing and advance curves are run off a program in the computer. Once the engine hits 200 rpm and recieves a signal from the pick-up coil the computer takes over the ignition. The timing and fuel mixture are then controlled by various inputs from the MAP, TPS, coolant, O2, etc sensors. A failure of any of these sensors will cause the computer to go into base, or limp-in mode which uses a set timing and fuel mixture to keep the car running. The "check engine" lite will come on with a noticable loss of power and a code will be stored for that particular sensor. Any problem with the primary ignition such as the module, magnetic pick-up coil, or the Electronic Spark Timing circuit would set the dreaded code 43 which has no specific way of diagnosing. This problem was the most common reason for dumping the whole system and installing an earlier carb and distributor. These early systems were less than 30% accurate at self-diagnosis. There were less than 50 possible codes availible compared to over 1400 possible in current systems. Hope this helps.

Mike

[Jack]

Originally Posted by Stingray6974

Hi Jack,
The 81, and California 1980 305, distributor does not have a vacuum or mechanical advance. The timing and advance curves are run off a program in the computer. Once the engine hits 200 rpm and recieves a signal from the pick-up coil the computer takes over the ignition. The timing and fuel mixture are then controlled by various inputs from the MAP, TPS, coolant, O2, etc sensors. Hope this helps. Mike

I was hoping for the likes of the two Mikes to come along ... especially Stingray6974. Thanks to each & for Bud's invite.

Yet another question Stingray6974? ... what kind of module does the 81 350 & 80 CA 305 supposed to have? ... I'm guessing 7-pin ... but maybe 5 or 4-pin?
JACK

[Stingray6974]

Hi Jack,
It should be a 7 pin. It's been so long since I've looked I don't remember. The 4 pin is used on 75-79, 5 pin on 81-83 Chevy trucks with ESC, and the 7 pin on 80 Cali. vettes and 81-86 cars.

Mike