Pinging that is not pinging

[Jay Laifman]

On my 73.5, when I really stand on it, I can hear what sounds like pinging. It usually only comes on at high rpm at full throttle. It does not come on at low rpm or trying to get load on the engine in lower rpms.

I have had two mechanics listen to it. One was Schneider Autohaus, who rebuilt the engine a few years ago. The other was Pete Zimmerman of Redline - years ago. This sound has in fact been there since I bought the car 26 years ago, when it had very few miles on it. When we rebuilt the engine, the pistons and heads showed no sign of any pre-detonation. The timing is set well, and retarding the spark doesn't change it. And neither Schneider nor Zimmerman thought it was pinging. Schneider tried to find if a shield or something was vibrating, but couldn't find anything.

So, I'm wondering if anyone has any ideas of what to look for. Could it be something internal to the muffler?

Thanks for any thoughts.

[mrar]

Jay,
I have a similar problem with one of my cars, have listened to it with a electronic stethoscope and its not detonation. A few weeks ago after passing a car hard under acceleration the sound became louder,so have stopped driving this car until I have more time to investigate.Interestingly enough when not under load I can run up to the redline with no noise.
Sean.

[73-T]

mine is doing the same at present. It may be that the engine is running a little lean. I remember a few years ago that my mechanic adjusted toward a richer mixture and there was no more pinging and the car ran cooler as well. I've had the air box replaced recently and now its pinging again. Been meaning to get it looked at again so until then I also "try" to stay off full throttle=no ping.

jeff

[Jay Laifman]

Well they both insist it is not pinging, and like I said, no evidence of long term pre detonation on the pistons or heads when the engine was rebuilt. I don't know if being lean causes pinging. But, here is an interesting observation. I had to replace the WUR on my CIS. But the WUR from 73.5 are no longer available. Instead, I have an 009 on there, which is from the 74-75 cars. I've compared the charts for the pressures between the 73.5 and the 009, and the 009 runs a tad richer at low temp but a tad leaner at operating temp. What WUR are you running?

[73-T]

I don't know the type WUR I have. Never replaced it in the last 22 years of my care. Could it be something with the dizzy retard/advance plumbing or adjustment?

[Jay Laifman]

Some new additional information. This noise comes on at 3,500 rpm, in any gear. But it has to be full throttle, or close to full throttle. Cruising along at above 3,500 rpm, or less than full throttle acceleration won't cause it. Going up hill or going down hill, it happens at the same full throttle rpm - i.e., different load doesn't seem to change where it comes on.

The car is running very well and very smoothly. When the sound comes on, the acceleration rate doesn't change.

I was remembering that I got a new OEM muffler about 20 years ago, but the sound was there before the that muffler change.

[Jay Laifman]

Ok one more level of information. The pinging noise ends at 5,500 rpm. I confirmed this in 2nd and 3rd.

[edmayo]

The timing is certainly a suspect here, even though you say it is properly set what about the speed or rate at which the curve increases in that 3500 to 550 RPM range. The total may end up being okay, but the curve may be too quick in the mid-range. Any chance of getting a load of race gas to try? or borrowing another distributor?

[Peter901]

Jay a ??? does it do it when the the engine is cold or only when hot/warm

[Jay Laifman]

Ed, Thanks for the suggestion. I will do what I can to see how the advance curve moves using my timing light.

Peter, You know, I don't know. I've never pushed the car hard like this when it is cold. But, I'll give it a shot for information.

FWIW, it has done this since I got the car 26 years ago, with 40k miles on it at the time. The top end was rebuilt a few years ago and I would have expected any obvious weaknesses in the valve train or shields would have been noticed.