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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?? http://www.clubcobra.com/photopost/d...g_9811-med.jpg http://www.clubcobra.com/photopost/d...g_9813-med.jpg http://www.clubcobra.com/photopost/d...n_burn-med.jpg |
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 |
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 |
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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What compression ratio was the engine, and what type of fuel was he running, does he run?
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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? :3DSMILE: |
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. |
"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. |
too hot (not referring to water temp)
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Abnormal combustion!!
auto10x Bill |
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 |
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 |
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 |
"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. |
FEguy Tom,
You beat me to the post:) What say you as to my questions to Fixit? Steve |
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 |
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:):):) |
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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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 :) |
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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