What does a 911 ping/knock sound like?
 

I just rebuilt my '79 SC engine and put higher compression pistons in it (9.3:1). The car is noticeably faster but I feel that it is pinging at low rpms. I retarded it to 1 degree static timing and I still hear the same sound, and the car feels slower. The problem is that I can't quite recognize if it is actually pinging, so, what does a 911 ping actually sound like? In my car, I get a rattle-type sound, can anyone give a good explanation of how to tell if the car is actually pinging?

Thanks

pinging at low rpms ?

under load or just driving?

we need more info to help you pining mostly happens under load.

the sound well ill let someone else answer that one.



"a screwdriver being drug along a chain link fence" just what came to mind

cheers ed

that's a pretty good analogy. I think the ping sounds exactly like a 1981 VW Rabbit Diesel, optimally timed. :rolleyes:

The pinging (or what I think is pinging) happens under load below 2000 rpm, not at high rpm. I am going to the kindergarten across the street with a screwdriver where they have a chain link fence in the morning.

at 2000 under load sounds like you may want to recheck timming and fuel mixture.

you dont want this sound around long
ed

The descriptions of the sound of pinging are good. For me it sounds distinctly metallic, like a chain rattling quickly in a can.

Something important to remember. You are actualy doing engine damage at a point BEFORE you can hear the pinging sound. On some engines, I can hear a change in pitch before the engine pings and I adjust them just under the change of pitch. The problem is that I can't hear that minute change in all of them, so I tend to be rather safe/conservative with my timing.

Low RPM with load is exactly where you will most likley hear pinging. Need to get it figured out real quick (fuel, timing, etc). Failing to correct it can be most costly.

angela

You might re-test running the motor on 100 octane race gas. See if the noise goes away.

If not, it may be something mechanical in the motor, other than compression and/or timing.

To confirm, pour some race gas into the tank and retest. If the pinging goes away, it was detonation (high octane fuel has a higher detonation threshold).

If it's a chain tensioner ready to die, the sound will be heard with or without a load on the engine. That is, in neutral. Louder at low rpm; noise goes away at higher rpm.

Sherwood

Okay, I gotta ask one more thing. How noticeable is it when your cars ping? does it stand out or do you have to listen for it? On my car, you definitely have to listen for it and the exhaust is stock, so I am still wondering if it is actually pinging. I haven't gotten a hold of race gas yet, but I tried retarding the timing up to 10 degrees and the sound is still there.

Many thanks for reading and helping.

Brotok,

If you retarded the timing and you still hear the sound, you may be hearing the rattle pins in the CIS injectors. The injectors have a loose pin in the nozzle to help atomization that you can clearly hear when accelerating at low rpm. Pinging is a higher pitched metallic sound, a 911 engine pings mostly at higher rpm, around the torque peak, at WOT, especially when the air and oil temperatures are high. The frequency is much higher than walking along a fence. You should also check and set your timing at high speed, not at idle. If the springs are worn in your distributor, you may get a false reading at idle or too much low speed advance. Set the high speed timing to 25 BTDC and if the idle timing is above 5 BTDC, your springs are worn out. Do you still have a distributor for the 8.5:1 engine, the later engine ignition timing curve and fuel curve are different.

"Static timing" was done on older cars with points (Kettering ignition). I still do it on my old Volvo:
Set points gap, move pulley to 10 deg BTDC, connect 12V bulb to coil primary and ground, ignition ON, rotate distributor slightly until bulb lights up, voila!

Dynamic timing replaced Static timing when CDI came in. SmileWavy

What distributor are you using? 001?
Was the curve checked/recurved for your set-up?
Octane?
Do you have a good Stroboscope?

Steve Weiner mentioned 3 things for a recurve: CR, octane and air temps.

I had mine checked/recurved for peace of mind. :)

I don't have any unusual noises above 2k RPM.

I have the 8.5:1 distributor, nothing has been done to it, except I did clean it a while back.

The injectors are brand new, do new injectors still make that sound?

I have a stroboscope but it has difficulty measuring timing at high RPMs. what happens is it will get the proper advance up to about 18 degrees at 2-3.5k RPM, and then it jumps down and starts reading 9 and up to 13 degrees from there on - do you think it is halving the actual advance at high RPM?

I run highest octane available in most of CA, 91.

Any more advice?

Thanks.

Brotok,

Yes, all CIS injectors make this sound. Sounds like your timing light is faulty, or your plug wires are arcing, try borrowing another light and check the high speed advance.

Can someone answer this part of the question? I am curious too.

I swear that I hear a rattle at lower rpms (fresh rebuild too but with 9.5 CR) but two very knowledgable guys rode with me and said it was not pinging. It only happens on throttle - even barely touching the gas. The sound is faint ... very faint.

Maybe we are hearing the same thing and it IS NOT pinging. I have new injectors too.

mca,

Read my earlier post. Pinging occurs at high load, it is a function of pressure and temperature. Pressure is low at small throttle openings because of low VE and cylinder filing. Pressure is highest at peak VE (% of cylinder volume actually inhaled), which occurs at the torque peak, 4200 rpm in your case. It is a staccato metallic rapping that increases in frequency with rpm, that disappears the instant you lift the throttle. If you hear a sound when the engine or air is cold, when you are "barely touching the gas", it cannot be pinging. My guess is you are hearing the chattering needle valves in the CIS injectors. I always hear them, but I know what to listen for after 30 years of working with CIS.

Ok ... thanks.

Don't stop diagnosing the ping, but a 915 will have gear rattle at low rpm under load.

You'll need to get reliable numbers because the timing is soooo important.
Compare your Stroboscope to another one.
Or test it on another engine.
Because the pick-up is close to other wires on the cap, try another location on #1.
When using #4 as a pick-up, does the readout still dance around?

In any case, you'll need a reliable Stroboscope and no, it doesn't halve the readings at higher RPM's.

As a base-line, and with a good scope, set the timing at 5deg BTDC.
Then have someone rev and hold RPM's.
Note the advance through various RPM's from 2k to 5k w/wo vacuum.

Or set the timing at max advance (~28 deg) at 5k and let the idle fall wherever.
Either way, note the numbers every 1k RPM with and without vacuum.

Post them here.

I found that vacuum still had some effect at various RPM's but others disagree; test it yourself.

Would love to see your numbers once you have reliable results with a good scope.