pinging under light load - Chevelle Tech
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post #1 of 33 (permalink) Old Sep 28th, 19, 12:56 PM Thread Starter
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pinging under light load

I am using MSD Streetfire ignition, I have 10 degrees base timing, 12* vacuum advance and 24*mechanical. Timing curve is all in at 3900 rpm which seems slow to me but at 2000rpm it is at 29* which seems to be quick, I would like to try and have the mechanical advance all in sooner but slower, anyone had this problem? I tried disconnecting the vacuum advance and the pinging was the same. It only seems to do it at low rpm under light load maybe it a bit lean transitioning into main jets, I will richen the idle circuit a little but I dont expect any change.
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post #2 of 33 (permalink) Old Sep 28th, 19, 8:27 PM
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Re: pinging under light load

Compression ratio? Piston to deck clearance, head gasket thickness? What octane fuel are you using?

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post #3 of 33 (permalink) Old Sep 28th, 19, 10:31 PM
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Re: pinging under light load

More info on engine, please. Sounds like the timing might be close, if not OK, and carb too lean. More info should help.

The questions above should establish the quench distance, which can have an adverse effect on things like pinging if it is radically wrong. If this is a new rebuild, the engine builder should know these things.
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post #4 of 33 (permalink) Old Oct 6th, 19, 12:26 PM Thread Starter
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Re: pinging under light load

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Originally Posted by Dave Ray View Post
More info on engine, please. Sounds like the timing might be close, if not OK, and carb too lean. More info should help.

The questions above should establish the quench distance, which can have an adverse effect on things like pinging if it is radically wrong. If this is a new rebuild, the engine builder should know these things.
I cannot remember the quench but I do know it wasnt huge, maybe 50 thou, I would have to find my build sheet. I will check float levels next chance I get, I think you might be right about the lean condition, if not I will have to do a compression test.
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post #5 of 33 (permalink) Old Oct 6th, 19, 12:30 PM
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Re: pinging under light load

And, vacuum advance, which source, please?
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post #6 of 33 (permalink) Old Oct 6th, 19, 12:31 PM Thread Starter
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Re: pinging under light load

I noticed a big rush of air when I took fuel cap off, I guess this was a vacuum inside the tank and not pressure, this might be starving the engine of fuel, it appears to have a new vented cap on it so dont understand why it is doing this.
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post #7 of 33 (permalink) Old Oct 6th, 19, 12:36 PM Thread Starter
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Re: pinging under light load

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Originally Posted by pnugene View Post
Compression ratio? Piston to deck clearance, head gasket thickness? What octane fuel are you using?
10:1 compression, aluminium AFR heads, I cannot remember the rest. it says 95 Octane on the pump.
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post #8 of 33 (permalink) Old Nov 17th, 19, 1:19 PM Thread Starter
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Re: pinging under light load

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Originally Posted by Dave Ray View Post
And, vacuum advance, which source, please?
Manifold vacuum
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post #9 of 33 (permalink) Old Nov 17th, 19, 1:33 PM Thread Starter
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Re: pinging under light load

I had a fuel hose with a kink so fixed that but still pinging, I then released the fuel cap to make sure it had no vacuum and it was still pinging.
Maybe I should try bigger jets in the primaries? it is a 383 SBC with holley HP 750 carb, roller cam and AFR Aluminium 195 heads. If it is a lean condition I assume that it would ping regardless of how the timing is set? I will take the tank off soon to check that end out then maybe increase jet size or colder plugs, it is driving me mad!
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post #10 of 33 (permalink) Old Nov 17th, 19, 1:55 PM
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Re: pinging under light load

Just for grins, try running without vacuum advance.

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post #11 of 33 (permalink) Old Nov 17th, 19, 2:19 PM Thread Starter
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Re: pinging under light load

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Originally Posted by oldcutlass View Post
Just for grins, try running without vacuum advance.
I did try that but it made no difference unfortunately
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post #12 of 33 (permalink) Old Nov 17th, 19, 3:43 PM
 
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Re: pinging under light load

If you are pinging at light load with only 29 or so degrees timing in something is way off the mark someplace.

Most well thought out N/A engines on pump gas will like closer to 40-50 degrees timing at light load (initial plus mechanical plus VA).

Of course this assumes ideal engine temps, tight squish, decent to excellent combustion chamber shapes....etc.

Do you have the cam specs of copy of the cam card?......

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post #13 of 33 (permalink) Old Nov 17th, 19, 5:48 PM
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Re: pinging under light load

You probably aren't on the mains at low RPM and light throttle either so jetting up isn't going to help. I'd pull the metering block off and blow air through all the passages just to rule the carb out but I don't think the problem is in the carb.

Unless you've made some modifications there is no way a Holley HP is lean on transition.

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post #14 of 33 (permalink) Old Nov 17th, 19, 6:18 PM
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Re: pinging under light load

Your curve info is good for the most part. the late total mechanical is workable with the weights you are running, but is touchy to attain, because it involves "tailing" the straight ramps of the weights.

Tailing is adding a curvature to the end of the straight ramp of the weights, and is touchy to do, best with a bent sander, slowly, a little bit at a time. having a NON-DIAL BACK timing light and degree'd balancer will help.

Depending on the amount of metal removed to make the actual tail, the curve speed is increased, even when the springs are not changed.

Please remember, these mods are only for changing (speeding up) a part of the upper degrees available from a set of weights and center.

Weights 1 is a three set cuts info picture, stock, small change, then more degrees faster as rpm's rise above a certain point. Weights 2 are all 3 weights stacked on each other, pay particular attention to the straight part of the weight that interacts with the center plate. The more curvature, the faster the degrees activate as rpm's rise. It is possible to do this so the rate speeds up for only a very small amount of the actual curve.
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post #15 of 33 (permalink) Old Nov 17th, 19, 7:03 PM
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Re: pinging under light load

Quote:
Originally Posted by ivorchevelle View Post
10:1 compression, aluminium AFR heads, I cannot remember the rest. it says 95 Octane on the pump.
Can you tell us your location ? Where can you buy 95 octane fuel ? is that Research Octane unit or Motor Octane unit,it makes a difference in the real octane number fuel you are using.With 95 octane and aluminum heads it shouldn't be pinging.

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