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Detonation

 
Old 03-13-2019, 09:25 PM
  #21  
lars
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Detonation at light throttle cruise is typically caused by excessive vacuum advance. After verifying that your total mechanical timing (with vacuum advance unplugged) is 36 degrees, check and make sure that your vacuum advance is limited to about 10 to 12 degrees. Most stock units will pull in 18 to 22 degrees of additional advance from vacuum, and that's too much for modern pump gas. E-mail me for my papers on this if you need a complete description of the issues, and instruction on how to set it up.

Lars
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Last edited by lars; 03-13-2019 at 09:26 PM.
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Old 03-13-2019, 10:39 PM
  #22  
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Originally Posted by L-46man View Post
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.
Detonation and Pre-ignition are two different things. Pre-ignition obviously happens before the moment of spark/ignition (caused by hot spots in the chamber), and Detonation happens after the spark and usually occurs after TDC (when the cylinder pressure/heat causes autoignition of the remaining mixture).
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Old 03-14-2019, 12:26 PM
  #23  
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Originally Posted by lars View Post
Detonation at light throttle cruise is typically caused by excessive vacuum advance. After verifying that your total mechanical timing (with vacuum advance unplugged) is 36 degrees, check and make sure that your vacuum advance is limited to about 10 to 12 degrees. Most stock units will pull in 18 to 22 degrees of additional advance from vacuum, and that's too much for modern pump gas. E-mail me for my papers on this if you need a complete description of the issues, and instruction on how to set it up.

Lars
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LARS MAKES A GREAT POINT;

Non Corvette related, but an engine is an engine. I have a bunch of cars...One is a 1978 VW Scirocco I put a much larger engine in it and @ 10:1 comp ratio. Mechanical Fuel Injection CIS Bosch...can't change that at all. I live in AZ so in the summer months, with the A/C on and 91 Octane ,the best you can get around here, short of AVGAS...it would 'Ping'....not detonate. I found this annoying.

SBC's have a panoply of support parts and etc...Scirocco's 'not-so-much'....it was a TEMPORARY heat related problem anyway....summer only .
Variable adjustable vacuum advance cans and 40 different mechanical advance springs and weights are not available. Additionally,AZ emissions ( '78 no-cat legally) required that the timing be set Just-so!

Cheap, quick and dirty solution....Metered Vacuum Snubber on the vacuum advance line....tried a few, found one that worked....problem solved. Snubber retarded the vacuum advance momentarily (slowed the vac advance curve) and no pinging. Winter, I'd remove it and get the full
(hold onto your shorts) 118 hp! (orig was 74 hp!)

anyway....good luck Andy!

Unkahal

Last edited by L-46man; 03-14-2019 at 12:27 PM.
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Old 03-14-2019, 01:22 PM
  #24  
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Originally Posted by lars View Post
Detonation at light throttle cruise is typically caused by excessive vacuum advance. After verifying that your total mechanical timing (with vacuum advance unplugged) is 36 degrees, check and make sure that your vacuum advance is limited to about 10 to 12 degrees. Most stock units will pull in 18 to 22 degrees of additional advance from vacuum, and that's too much for modern pump gas. E-mail me for my papers on this if you need a complete description of the issues, and instruction on how to set it up.

Lars
[email protected]
I want to add to this that the 8 BTDC number on your cowl tag needs to be thrown out.....just as the 18-22 degree vacuum can need to be thrown out or modified. At 8 BTDC on your timing mark...you might have 30 degrees total (total being when the engine is revved up and the advance stops climbing)........get a Bosch ( I just bought one) digital timing light.....and dial in the timing to 36 degrees total.....now find the correct advance canister (I think the "B9" pulls 11-12 degrees) or modify yours with directions from Lars papers. Do not be concerned with your timing number at idle........it makes no difference. NOW...from here you can check your carburetor.
GM had it dead wrong on EVERY car they ever built back then as far as timing spec. There was not ONE car that did not run better with a curve and more total advance (Even the L-88!).......this was a combination or Big Gov and Lawyers.........
I think Lars' idea of the canister pulling too much is spot on.

Jebby
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Old 03-14-2019, 02:11 PM
  #25  
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Agree with Lars and Jebbysan
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Old 03-14-2019, 02:26 PM
  #26  
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I want to share this link because I just got this and although nothing special to some....I though it was about time I had one: https://www.jegs.com/i/Bosch-Actron/885/CP7529/10002/-1
I have been using straight strobes with timing tape for over 20 years because of what an insider at MSD told me once....about how a dial back can be off and cause a delay due to the MSD module....
But not all cars have modules and everything that came to me would need a timing tape because of my stubborn ***......so I bought a dial back digital....
The cool part is that it tells you RPM.......so you know exactly where your advance is coming in by reading the sweep at the balancer as you rev it....

Jebby
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Old 03-14-2019, 03:07 PM
  #27  
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Originally Posted by lars View Post
Detonation at light throttle cruise is typically caused by excessive vacuum advance. After verifying that your total mechanical timing (with vacuum advance unplugged) is 36 degrees, check and make sure that your vacuum advance is limited to about 10 to 12 degrees. Most stock units will pull in 18 to 22 degrees of additional advance from vacuum, and that's too much for modern pump gas. E-mail me for my papers on this if you need a complete description of the issues, and instruction on how to set it up.

Lars
[email protected]
This ^^^^^
And get Lars papers. You won't be sorry!
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Old 03-14-2019, 10:35 PM
  #28  
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After you set the timing, go to manifold vac for the vac advance, and ensure the float level is correct with properly functioning carburetor things may be better.
and they may not.
my 77 in stock form detonated regardless of tune if it was hot enough and I got into the throttle hard enough and made it really work.

there is a lot of heat added to the intake charge through that cast iron intake manifold , heat riser, hot water temps, high underhood temps. All add to the possibility of detonation.
is your stock cold air duct still,intact?

the whole engine design from the lack of real quench on the pistons, to the stock timing, cast iron intake, and crappy cooling system , to the restrictive exhaust, make it detonation prone.

mine detonation was pretty much eliminated,when I changed to side pipes and added an aluminum radiator with electric fans, advanced timing , vac advance on manifold vacuum and rebuilt the carb.

i now have 10.6:1 CR with aluminum heads and can run 88 octane with no detonation. Many many changes to achieve this, but simple CR of the engine does not solely dictate octane requirements.
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Old 03-19-2019, 09:23 AM
  #29  
Andy Krotzz
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Default Thanks to everyone

Thanks to everyone for attempting to solve this problem. I am an experienced personal mechanic having had numerous cars and motorcycles over the years I have just never experienced the detonation I am experiencing. It's just going to take a little while to figure it out all the bugs out. The car has basically sat for about the past 15 or 20 years only being driven less than a hundred miles a year. So I have to assume I am dealing with rotten gas Vacuum leaks and any other numerous items they come from a car that's sat a not driven. I'm not sure how the prior owner even drove the car and I'm not sure it wasn't trailered everywhere it went.
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Old 03-19-2019, 11:58 AM
  #30  
MelWff
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Nobody has suggested that the outer hub of the balancer has slipped and you are not really setting the timing to 8 degrees but rather a much more advanced number
Can you any evidence that the balancer has slipped?
Do you know how to set the engine to #1 TDC by testing the #1 cylinder to see when the piston is really at TDC and then comparing that to the timing mark on the balancer?
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Old 03-19-2019, 12:03 PM
  #31  
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Originally Posted by MelWff View Post
Nobody has suggested that the outer hub of the balancer has slipped and you are not really setting the timing to 8 degrees but rather a much more advanced number
Can you any evidence that the balancer has slipped?
Do you know how to set the engine to #1 TDC by testing the #1 cylinder to see when the piston is really at TDC and then comparing that to the timing mark on the balancer?


If we are going down that rabbit hole...let's go into the sub-basement! Timing chain slop.

Unka
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Old 03-19-2019, 12:28 PM
  #32  
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why would you make a statement like that, "rabbit hole" ?
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Old 03-19-2019, 04:26 PM
  #33  
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Back in the day late 70's early 80's I worked at a Chev dealership as a line mechanic and that was the time when emissions really choked off horse power. Many times people would have pinging problems. Especially low mileage around town drivers. Most of the cars were still under warranty so this was our "cure" before looking into mechanical problems.
We had good success using "Top Engine Cleaner" and if I recall it was a GM product. Comes in a 1 pint can. Get the engine warmed up, rev it a little past idle and pour the first half of the can slowly down the carb as the engine runs. Then with the second half of the can basically drown out the engine. I do not know if GM still makes it.
Let it sit a couple of hours then start it up and let clean it's self out. Do it outside because it will really smoke for awhile.
9 times out of 10 it solved the problem which was carbon buildup in the combustion chamber.
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Old 03-19-2019, 05:02 PM
  #34  
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Originally Posted by 71 Vert LS1 View Post
Back in the day late 70's early 80's I worked at a Chev dealership as a line mechanic and that was the time when emissions really choked off horse power. Many times people would have pinging problems. Especially low mileage around town drivers. Most of the cars were still under warranty so this was our "cure" before looking into mechanical problems.
We had good success using "Top Engine Cleaner" and if I recall it was a GM product. Comes in a 1 pint can. Get the engine warmed up, rev it a little past idle and pour the first half of the can slowly down the carb as the engine runs. Then with the second half of the can basically drown out the engine. I do not know if GM still makes it.
Let it sit a couple of hours then start it up and let clean it's self out. Do it outside because it will really smoke for awhile.
9 times out of 10 it solved the problem which was carbon buildup in the combustion chamber.
Ha! See post 15. This does about the same thing as a trickle of water, only much faster. I had no idea this was a genuine GM-approved method, I always thought it was a bubba/budget tuner trick!
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Old 03-19-2019, 08:39 PM
  #35  
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Yeah it was our "first" fix because it only paid .5 hours of flat rate to the mechanic. We didn't exactly line up to do warranty work but I was low man the totem pole so I got many of the jobs. I don't how GM came up with those flat rate rates. I'd love to see any GM engineer bring the car in the shop, propose a solution to the service manager, fix the problem as of above and all in 30 minutes?
Drove me from car repair into a completely field which at the end of the day worked out quite well in the end. But I digress.
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