I have a C3 1977 corvette with 46,000 original miles. It still had all original parts. Carb, plugs, wires, distributor cap and rotor button. (all now replaced) Long story short I'm getting detonation at 2500-3000 rpm's. The carburetor has been rebuilt and the prior owner added headers and new dual exhaust with 40 series flowmasters. I realized the EGR valve was bad and replaced it. Still has the detonation. Timing set at 8 degrees BTDC per factory specs.

Any ideas on where to look next?

 

How's your fuel pressure? Have you changed your fuel filter?

 

With your vacuum advance disconnected and plugged, what is your total advance at 3000 rpm? Your dist should be "all in" at this point, is it?

 

Do you know for certain that the new EGR is actually functioning?

 

Headers and dual exhaust on my stock L48 '75 leaned it out so bad I had to completely re-tune the dist and carb. I was actually getting misfiring. Timing was the biggest factor.
1) Check dist timing: no vac - all in by 3000-3500 See what it is now. 36 degree total is suggested. Try less to see if it solves pinging.
2) Plug vac back in and see how many degrees it adds. Test car with and without vac can plugged at cruise. Limit vac advance if required.
3) Check EGR - push on diaphragm with fingers (idle with cold engine) should slow down or stall motor. If not passages could be clogged.
4) Check carb mixture - more difficult - suggest A/F ratio meter/wideband. Test it at idle, 2500 cruise, and full throttle (in car) Idle mixture adj is easy. Changing jets/rods on a Quadrajet requires some serious knowledge that is hard to come by these days. (You may also have APT adjustable part throttle on that year which raises the rods) Any of which could have gotten messed with or gummed up over 41 years.

One of those should find the issue.

 

It appears to be. Vacuum on throttle and no leak down.

 

Thanks so much

 

Will check ASAP

 

New filter, haven't checked pressure

 

Look at the MECHANICAL advance curve and components in the distributor. As well and the vacuum advance system and how it is plumbed. In the end result you can get adjustable vacuum advance 'pots/cans'.

Detonation with a low compression ratio is almost ALWAYS fuel and timing.

One thing we used to do in the old days....CAUTION! Requires finesse! Was at about 1500-2000 rpm with a warm engine....dribble H20 down the carb....just a little bit at a time.
Theory; Water hits the cylinders and pistons and flashes immediately to steam which dislodges coke and carbon from the piston tops. This Carbon is the nucleation hot spot for detonation....so it blows it out of there.

Good luck.

Detonation is also caused by too lean a mixture....so look at that too.

UNKAHAL

 

What octane gas are you using? If 87, try 91 or 93.

 

I could not possibly be that simple.....octane.

BTW most people do not understand octane....EG 87 octane fuel is MORE EXPLOSIVE at lower vapor pressures than 91.
Octane is 'the suppression of explosions'....it slows the flame front across the cylinder.

Unka

 

You can add 10 and 20 degree BTDC Mark's on the balancer. Need some ehite-out. Put timing mark on 0. Paint line at 10 degree. Or 14. Then put new mark at 0. Paint another at 10 or 12 or 14. So your second mark is at 20 or 24 or whatever BTDC. so that mark plus 6 or 14 or whatever that you can read on timing tab and you can set all-in timing. Then you can plug in vacuum and see how much the vac can adds at idle. You could also buy a digital timing light... and this ain' a big block. 34 degrees all-in. Any more gets no more power. Just detonation.

 

I agree with the first bolded part. I've seen no common evidence that the second part is necessarily true.

 

I didn't even post this because it may sound far fetched, but I find myself agreeing with L-46man, again (!).

Water injection to clean out carbon deposits is a well-known trick of budget tuners. I came across it with Corvair engines, and it worked great for both carbon deposits and cooling. I generated some impressive smoke when I first tried it. Some people even hooked up push-button water injection systems using spare windshield washer pumps (you need to pick the nozzle size very carefully).

What did the spark plugs look like when you pulled them out? Carbon in the combustion chamber will create hot areas for detonation and pre-ignition to start, away from the spark of the spark plug, and can also raise the compression ratio of the engine.

 

We used to do that in the 80s at a shop I worked at, one of the guys tipped his water bottle too far you know the rest lol
Used a spray bottle from then on with warm/hot water

 

Yes it can. If the engine was tuned for 91 octane and 87 octane is used, it could/will ping. All new cars are tuned for octane ratings but have anti-knock sensors to to protect the engine when idiots don't fill the tank with the required octane level. Btw, the fuel burns in the cylinder and does not EXPLODE.

 

Sir...this IS and explosion pure and simple....Burning Gasoline in free air MAY BE a 'burn' you enclose fuel in a cylinder, compress air at 14.5:1 air fuel ratio, ignite it...DEAR SIR THAT IS AN EXPLOSION.


PULEESE, I'm a Mechanical Engineer (45 years)...don't go there! I have built 109 cars including quite a number of Corvettes....... You are completely misunderstanding.

THE POSTER did not come off as a newbie, nor someone who would put the wrong gas rating in a Corvette...it was completely OBVIOUS to me this was either a STOICHIOMETRIC problem (A/F ratio) or a Timing problem.

If you are the kind of Corvette owner who would put 87 octane into a 9.0:1 comp ratio car...you probably should not be on this forum asking about detonation!

I have a C3 1977 corvette with 46,000 original miles. It still had all original parts. Carb, plugs, wires, distributor cap and rotor button. (all now replaced) Long story short I'm getting detonation at 2500-3000 rpm's. The carburetor has been rebuilt and the prior owner added headers and new dual exhaust with 40 series flowmasters. I realized the EGR valve was bad and replaced it. Still has the detonation. Timing set at 8 degrees BTDC per factory specs.

Any ideas on where to look next?

Andy did not seem to be an idiot, far from it....he's trying to cure a hidden problem. The stupid 'Pat' answer would have been 'put 104 octane AVGAS in it and see if it detonates.....this does not help the poster.

UnkaHal

ps edit. 'pinging' is a very short temporary form of DETONATION (pre-ignition) Andy quite correctly used the term DETONATION and gives a 500 RPM range of that Continuous detonation....that's not 'pinging' either.
Even cars that have knock sensors 'ping'.....that's how the piezo knock sensor WORKS...it detects pinging say 2-3-4 'pings' and ratchets back the timing. ALL CARS PING at one time or another, even with the correct fuel in them. Timing/Octane/Load etc....its an algorithm.

detonation is pre-ignition or the flame front of the EXPLOSION being out of sync with the timing and trying to drive the cylinder down, while it's still approaching TDC....hence pre-ignition.

 

Your the first engineer I've heard of that calls it an explosion. Its a controlled burn. I don't know who tuned his engine but he's not the original owner. If it was tuned old school by over advancing the timing and then backing it off to eleiminate the ping and it was done with 93 octane fuel, then 87 octane fuel will or may cause ping. Thatbis plausible whether you or others believe it or not. And I didn't call Andy an idiot. Fwiw, I too am an engineer.

 

Andy
Please report back when you checked current total advance with vacuum plugged.
And/or varied it to see if it stops the pinging.
Keep vac adv plugged during this.