93 Octane pinging under load

diy83sc · 12-23-2002, 07:16 PM

I've been using 93 octane from several pumps and find that periodically the engine pings under load. I've removed the cat, o2 sensor, replaced with euro pre-muffler. To compensate for the "possible" lack of backpressure, I tunned CIS by ear (ccw till idle drops off, then clockwise just till idle is smooth.) Have to say, that gradual accelleration is really sweet, but I don't like what I hear under heavy load.

Is this due to incorrect ignition timing?

Tony

Vipergrün · 12-23-2002, 07:48 PM

Yeah, check your timing advance. I'd re-time to factory spec, 5 degrees BTDC 950 RPMs, distributor advance hoses removed.

This said, I run my SC with about 30 or so degrees max advance at 6000 RPM's, which is about 9-10 degrees at idle, no problems with our 91 max octane crap here in Cal.

You could still be running too lean. From what I have heard, you want the mixture a bit on the richer side with the timing advanced.

Have you checked for exhaust leaks and the dreaded broken head studs issue? Evidently it is quite hard to actually head detonation on our cars due to their noisy nature.

Best of luck!

Gordo2 · Gordo2's Garage

I had the same problem when I first bought mine. Bumped up to 93 Octane and it still made no difference.

Threw the timing light on it, timed it to spec (5 deg) and the problem went away.

Gordo

Doug Zielke · 12-23-2002, 09:55 PM

Tony,
Are you sure that what you're hearing really is detonation?
A broken head stud can make a similar noise (my motor proved this for me.)
Before I found the broken stud on the #6 cylinder, I would have bet the farm the noise I was hearing was detonation.
Check all your head studs.

Superman · 12-23-2002, 11:14 PM

Yes, exhaust leaks can sound like that too. Set ignition timing to spec. Set mixture richer than spec (the best way is with a gas analyzer). Look for exhaust or compression leaks.

diy83sc · 12-24-2002, 05:22 AM

I am sure it is pinging / detonation. As I load the engine, I can vary the shaking of the bb's in the coffee can. The broken stud and the rest of the top end has been rebuilt this past summer. The last set lasted 19 years

I'll get a light on it after xmas. Have to work today and cook tonight.

Happy Holidays!

Tony

john walker's workshop · 12-24-2002, 07:09 AM

standard rule of hot rodding, retard the timing only enough so it doesn't ping. any more than that and you loose bottom end response. adjusting the mixture really only affects idle and low rpm. the rest is predetermined by the design of the fuel distributor.

RazorRacer · 12-24-2002, 11:13 AM

diy83,

Is there anyway you can record the sound. If you e-mail the sound file to me, I can post it via a link on this thread. I've been trying to heard exactly what detonation sounds like.

Many digital cameras have a sound record feature.

tops911 · 12-24-2002, 02:08 PM

on my '78 911SC I was getting pinging when I was acclerating. The timing was spot on at an idle. When checked at 6000 RPM I had something like 45 degrees advance. After I took the distributor out and took it apart I found the problem. The counter weight spring had come off on one side. Actualy the little ear that the spring hooked to broke off. I used some epoxy to hold the spring on, so far its held on fine. So you might want to check the timing for total advance as well. There is nothing like timing an engine at 6000 RPM's. Had to give the wife some very explicit instructions. Good luck