Timing Question

gregguillaume · 12-01-2015, 05:44 PM

Engine in question: 1981 3.0L

Issue 1: It doesn’t pull particularly hard and I wondered if it was the timing. Sure enough, it was out of whack, but after adjusting as described below, it still doesn’t perform very well.

Ignition timing: I set the timing to 5 degrees before TDC with no vacuum attached (both vacuum lines to the distributor disconnected) per the Bentley manual. With vacuum(s) attached, the idle drops and the ignition is at about 4 degrees after (yes after) TDC. Is this normal? What should the timing be with the vacuum attached??

Also, the vacuum line to the side of the distributor diaphragm nearest the fan has some serious vacuum to it (a blue hose), and the other side has no vacuum at all, really (a red hose). Lastly if I put a hose on the side of the diaphragm away from the fan (the red side) and pull a vacuum, nothing happens to the idle or timing. Conversely, if I do the same on the blue side, the idle drops quite a bit. Is that normal?

I also have occasional backfiring when the engine is cold and I “blip” the gas, if that gives any clues.

Thanks for the help and advice.

Flat6pac · 12-02-2015, 02:05 AM

The vacuum line with idle vacuum connects to the inner side. That will vacuum idle ATDC
The line with no vacuum is the advance the vacuum comes in at high RPM.
Bruce

stlrj · 12-02-2015, 03:33 AM

Placing the high vacuum hose, the blue one, on the advance side where the red hose was should raise the idle speed up with ignition advance. Since it does nothing, you will need to replace the vacuum advance/retard module because the advance diaphragm is leaking.

Joe

T77911S · 12-02-2015, 03:43 AM

make sure the mechanical advance is working.
remove the vac lines and rev the engine to see if the timing advances.
if you have a hand vac pump, nice tool to have, pull a vac on the advance pot and see if it holds to verify a leak.
there is a good thread here on rebuilding the dist.

psalt · 03:47 AM

Your engine has both vacuum advance and vacuum retard. Vacuum advance is a good thing , it improves mpg under light load and light throttle. It uses ported vacuum (no vacuum at idle) because you don't need extra advance at idle. Vacuum retard is an emission device to reduce HC at idle, it uses manifold vacuum, and your engine will run cooler in traffic and cold start better with it disconnected and plugged.

Porsche chose to raise compression and retard timing on your engine to improve mpg specs after the oil embargo. Factory spec is 25 BTDC max because of the detonation risk on the specified 87 CLC octane fuel. A wide VIA hemi head engine with domed pistons needs much more timing to make best output, previous models had up to 38 BTDC, but were spec'd for 98 RON fuel.

The optimum tune for your engine will be limited by octane and heat. You can improve things a bit by disconnecting and plugging the vacuum retard, disconnecting the intake breather hose and using a remote catch tank, setting the WOT mixture to around 13:1, then advancing the high speed timing for higher octane fuel. Timing at idle is irrelevant, you want to measure the high speed timing and advance a few degrees at a time and listen and watch for signs of detonation. You won't get to 38 BTDC on an 81 SC, but you can improve the output if you know what you are doing.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by gregguillaume

Engine in question: 1981 3.0L

Issue 1: It doesn’t pull particularly hard and I wondered if it was the timing. Sure enough, it was out of whack, but after adjusting as described below, it still doesn’t perform very well.

Ignition timing: I set the timing to 5 degrees before TDC with no vacuum attached (both vacuum lines to the distributor disconnected) per the Bentley manual. With vacuum(s) attached, the idle drops and the ignition is at about 4 degrees after (yes after) TDC. Is this normal? What should the timing be with the vacuum attached??

Also, the vacuum line to the side of the distributor diaphragm nearest the fan has some serious vacuum to it (a blue hose), and the other side has no vacuum at all, really (a red hose). Lastly if I put a hose on the side of the diaphragm away from the fan (the red side) and pull a vacuum, nothing happens to the idle or timing. Conversely, if I do the same on the blue side, the idle drops quite a bit. Is that normal?

I also have occasional backfiring when the engine is cold and I “blip” the gas, if that gives any clues.

Thanks for the help and advice.

For a US '80 SC
idle rpm 900+/-50, 5° BTDC vacuum disconnected
6000 rpm, 23-28° BTDC vacuum disconnected, RoW 23-29°

check them both, as the mechanical advance can freeze up
it's normal for idle vac to retard the timing

stlrj · 12-02-2015, 06:37 AM

Quote:

Originally Posted by psalt

Vacuum retard is an emission device to reduce HC at idle, it uses manifold vacuum, and your engine will run cooler in traffic and cold start better with it disconnected and plugged.
.

Leaving the vacuum retard connected will enhance throttle response by advancing your timing 10 degrees instantaneously the moment you tip in the the throttle at any rpm. Also disconnecting it will tend to slow the return of rpms to idle when blipping the throttle.

psalt · 12-02-2015, 08:19 AM

Not my experience.
The timing is set to 5 BTDC without vacuum.
The only advance you "gain" with keeping the vacuum retard, is the the advance you took away. It is irrelevant to measured WOT performance.

I have never met a vacuum retard that was worth keeping.

psalt · 12-02-2015, 08:25 AM

Issue 1: It doesn’t pull particularly hard and I wondered if it was the timing. Sure enough, it was out of whack, but after adjusting as described below, it still doesn’t perform very well.

If you are having issues with WOT performance, one of the first things to check is the throttle linkage. Have an assistant hold the pedal down and see (and feel) if you are really getting WOT. The bushings wear and the rod ends may need adjustment.