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09-06-2004, 08:23 PM
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CC Member
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Join Date: Nov 2000
Location: Houston,
Tx
Cobra Make, Engine: Unique FIA
Posts: 2,064
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Not Ranked
Engine experts please help troubleshoot
Hi,
I am helping a friend of mine with an engine failure. The engine is a 351 windsor normal stroke with Edelbrock heads. approx. 3000 miles on a fresh rebuild. The pistons are Wiseco forged. As you can see, a chunk broke off of #1 piston. As far as we can tell the engine did not ingest an object causing this. In the circled areas you will note that the aluminum has been eroded away. The last photo is of another piston from this engine. It also has some aluminum eroded, though not so much as to cause a failure yet.
Question: What do you think is causing the erosion??
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All my ex's live in Texas
Last edited by Steve R; 09-06-2004 at 09:53 PM..
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09-06-2004, 08:28 PM
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Join Date: Jan 2003
Location: mpls, mn,
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Looks to me like you had a top ring butt together, is it a KB piston? What ring gap did you run? motor get hot?
Dave
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09-06-2004, 08:49 PM
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Join Date: Nov 2000
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Thanks Dave.
I just edited the post with the details. The engine was built by a local shop that I have not been impressed with because of another job I have seen them do. We had just removed the two pistons when time ran out for this evening. Good question as to the ring gap. That is my next step. I will probably have an answer for you tomorrow.
Steve
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All my ex's live in Texas
Last edited by Steve R; 09-06-2004 at 08:52 PM..
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09-07-2004, 01:57 AM
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CC Member
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Join Date: Dec 2001
Location: Castalia,
Oh
Cobra Make, Engine: EM cobra, 450 inch sbc running a best ET of 9.14..so far..ALL MOTOR...approx 800 horse.............ERA with 482 FE..All Aluminum Engine
Posts: 1,395
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Hard to say for sure, but top photo looks to have a few little pits on the top of the piston, these are caused by detonation....and detonation will destroy a piston this way..
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Jack
XSSIVE .....
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09-07-2004, 02:21 AM
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CC Member
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Join Date: Apr 1999
Location: cleveland,
OH
Cobra Make, Engine: CSX4000, 427
Posts: 1,999
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What compression ratio was the engine, and what type of fuel was he running, does he run?
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"After jumping into an early lead, Miles pitted for no reason. He let the entire field go by before re-entering the race. The crowd was jumping up and down as he stunned the Chevrolet drivers by easily passing the entire field to finish second behind MacDonald's other team Cobra. The Corvette people were completely demoralized."
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09-07-2004, 04:01 AM
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Member of the north
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Join Date: May 2003
Cobra Make, Engine: A Cobra
Posts: 11,207
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Looks like predetonation to me. Spark to high and crap gas. What kind of oil was in this? There is a good collection of deposits for only 3000 miles.
There are reasons lead was necessary for some engines, lubrication.
Usually I see it as a burned hole, not a chunk of aluminum.
I think Jack is right. What do I win?
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Last edited by trularin; 09-07-2004 at 04:04 AM..
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09-07-2004, 06:45 AM
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Join Date: Feb 2002
Location: Dadeville,
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Cobra Make, Engine: Sold my EM.
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Those pistons look similar to the #2 & #3 pistons on my 351 Windsor with Edelbrock Victor Jr. Heads when it failed with about 3,000 miles on it. I was running 10.5:1 compression and I couldn't rule out detonation because I couldn't hear anything over the sound of the side exhaust. The only other thing that might have contributed to my engine failure was a small vacuum leak in the intake manifold creating a lean condition for the two failed pistons.
When I rebuilt the engine, I went to forged pistons and a slightly lower compression ratio (9.8:1). I also installed a knock sensor with a visual display to warn me of detonation. I've got a couple thousand miles on the latest build including some very hard track time. So far, so good.
By the way, be sure and check the valve train above those failed pistons. Those missing pieces had to exit via a valve.
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Tommy
Cheetah tribute completed 2021 (TommysCars.Weebly.com)
Previously owned EM Cobra
"Never attribute to malice that which is adequately explained by stupidity." - Hanlon's Razor
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09-07-2004, 09:32 AM
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"Ping",,,, sure looks like the 'cause all right!
So Tommy, tell me about that knock sensor, I'd be interested in getting one for my engine.
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09-08-2004, 07:29 AM
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Join Date: Mar 2002
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too hot (not referring to water temp)
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In a fit of 16 year old genius, I looked down through the carb while cranking it to see if fuel was flowing, and it was. Flowing straight up in a vapor cloud, around my head, on fire.
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09-08-2004, 08:33 AM
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Join Date: Nov 2002
Location: Mt. Pleasant, MI,
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Abnormal combustion!!
auto10x
Bill
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09-08-2004, 09:04 AM
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Join Date: Nov 2000
Location: Houston,
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Cobra Make, Engine: Unique FIA
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Thanks for the feedback
Though detonation is a sound theory, a few questions come to mind.
1) None of the spark plugs showed any damage or irregular coloration. why?
2) The car is a Lonestar which has relitively quiet side pipes. I had been in the car shortly before the failure and heard no pinging.
3) other than that chunk missing, the tops of the pistons showed no signs of damage. Same with the combustion chambers.
4) the erosion I point out is away from direct combustion. You would think that if it was detonation, that the erosion would be on top near the edge.
Steve
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09-08-2004, 09:09 AM
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Detonation
With Keith black Hyp pistons, you have to use much wider ring end gaps due to the heat transfer through the top of the piston. If these are Wiseco pistons, then detonation, and not ring butting is the culprit. With the MTBE in the gas, the cylinder temps are way up compared too what they used to be 5 or 6 years ago. This causes ring gaps to tighten and piston to wall clearance to tighten, along with a easier tendency to detonate. Distributor curves are important here too. If someone is running a vacuum advance distributor in a Cobra,(light car under no load) then when crusing at freeway speeds all of the centrifical and vacuum advance is in. This could mean as much as 55 degrees of total advance-OUCH!!
Hope this helps. Tom
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Thanks Much. Tom
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09-08-2004, 09:12 AM
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Fixit,
I agree. The eroded areas look as if someone pointed a torch there. Yes, it seems to have been very hot there.
I have not been back to measure the ring gap yet. I know one of the jobs of rings is to transfer heat fom the piston to the cyl wall. What would stop them from doing that?
Have you ever seen erosion in that area before? I haven't.
Steve
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All my ex's live in Texas
Last edited by Steve R; 09-08-2004 at 09:22 AM..
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09-08-2004, 09:14 AM
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Join Date: Feb 2002
Location: Dadeville,
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Cobra Make, Engine: Sold my EM.
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"Slick" -
MSD Engine Knock Alert, # 8964 ( link to MSD
A small sensor screws into the block replacing a water plug. Wiring goes into the drivers compartment to a device about the size of a PDA. It displays a range of lights and gives an audible alert when the sensor detects vibrations associated with detonation. Both detection sensitivity and audible volume are adjustible. It took a little time and experimentation to adjust the sensor for my engine. I don't know that this thing is perfect, but it sure beats what I had before - nothing.
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Tommy
Cheetah tribute completed 2021 (TommysCars.Weebly.com)
Previously owned EM Cobra
"Never attribute to malice that which is adequately explained by stupidity." - Hanlon's Razor
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09-08-2004, 09:17 AM
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FEguy Tom,
You beat me to the post What say you as to my questions to Fixit?
Steve
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All my ex's live in Texas
Last edited by Steve R; 09-08-2004 at 09:19 AM..
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09-08-2004, 10:52 AM
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I have seen several pistons eroded exactly like that.
Last one I disassembled was a case of:
pump gas + 11:1 comp + too much advance + a dozen laps on a racetrack = a matching set of burned pistons
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In a fit of 16 year old genius, I looked down through the carb while cranking it to see if fuel was flowing, and it was. Flowing straight up in a vapor cloud, around my head, on fire.
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09-08-2004, 11:01 AM
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Cobra Make, Engine: Kirkham, 427
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Steve,
I had the unfortunate experience of seeing one of those pistons on my brother's 427. The driver forgot to turn on the electric fuel pump and went out for a few hot laps at Willow and the mechanical fuel pump couldn't keep up.
The engine leaned out and detonated.
Looks like you have the identical twin piston.
David
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09-08-2004, 11:14 AM
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Thanks Tommy, I'm right in the middle of determining what kind of "build" I'm going to do on my 427 FE. Knock sensor will be one item!
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09-08-2004, 12:21 PM
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Ok,
We are now going to shop for an air/ fuel ratio tester. One that can be used to tune a carb. Not an onboard unit, but a diagnostic one that is accurate and displays the ratio, not just LED's. Budget around $500.
Suggestions/ comments needed.
Steve
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All my ex's live in Texas
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09-08-2004, 12:55 PM
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CC Member
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Join Date: Apr 1999
Location: cleveland,
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Cobra Make, Engine: CSX4000, 427
Posts: 1,999
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Dave K.,
I did the same thing at R&G last year, forgetting to turn my electric pump on. My engine would stumble every time on the straight, and also every time half way down the drag strip. I still ran a 12.01 @ 118 mph. As far as I know, my engine is still OK. I do have coated pistons though, wiseco. Wiseco has a deal with Polydyn? or something like that, such that if you order the wiseco pistons with this coating from polydyn, they are about the same price as if you bought the pistons bare from Wiseco. Coated pistons are supposed to protect somewhat against short lean out conditions. In a boat, with continuous throttle, I don't think it would make any difference. Anyways, the coated pistons look nice. I'll try to put a picture of one in my gallery.
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"After jumping into an early lead, Miles pitted for no reason. He let the entire field go by before re-entering the race. The crowd was jumping up and down as he stunned the Chevrolet drivers by easily passing the entire field to finish second behind MacDonald's other team Cobra. The Corvette people were completely demoralized."
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