Found the limits of a stock M20B25 Bottom End

[SLEEPYDUB]

Got an update here for you guys.

So we swapped in a stock 2.7 that had supposedly been rebuilt with 20k miles. The engine looked good and healthy, and it worked well until yesterday. Engine combo was Stock 2.7, 885 head with 280/274 cam, HD Rockers, SS Valves, HD Dual Valve Springs, stock headgasket, ARPs, 6266, E85.

So we have been running about 19-22psi on the engine, with occasional spikes to 25-28psi at times depending on the weather. The spike was really hard to control. Engine held together great and made crazy power, no smoke, no issues, up until noon yesterday.

I went sliding with some friends, only on 10psi which is more then enough. On the first turn at our spot I slide in 2nd gear, just like the last time I had a failure. I tag the limiter a few times, nothing crazy though, just up and down tapping it. Halfway through the first corner while tapping the limiter the engine lets go and sends #6 rod and its parts through both sides of the engine and the oil pan. The night before I was beating the shit out of it at 22psi, which is probably over 450whp on this setup, so maybe i weakend the rods and bolts and the limiter on 10lbs finally did it in?

I have yet to take anything apart other then removing the spark plug, which showed no signs of damage or detonation. The rod bolts you can clearly see are snapped, which ill see if they stretched when i get it all apart.

I have a bare 2.5 block and 2.9L forged parts waiting to be dropped into the new block, so we will be working on that in the weeks to come. Ill post the pics that I have of the new carnage once I have time.

In the meantime, what the hell is going on? Or am I just making too much power for the bottom ends to handle? I dont think it was detonation or my timing, which was about 23* @ 5000 and increased to 28* @ 7000 which the engine never revved past 6500. Although the timing is aggressive, if it were to be the issue I would think it would pop the headgasket before snapping a rod... My rev limiter settings are set to 6500 and 200rpm hysteresis, spark and fuel cut, using progressive limiter.

[varg]

Who knows? Needs a teardown tell more. Rebuilt engine? Could be something as simple as reused rod bolts failing, AFAIK B25 and B27 rod bolts are the same and they aren't to be reused. Measure the remaining bolts for stretch. I want to say it could've been preignition too since drifting is the kind of thing that can heat up spark plugs and valves enough to cause it, but though I can't read the strap and the insulator in that picture, the spark plug looks undamaged and doesn't scream "too lean" or "too hot". Your timing is aggressive at 5,000rpm, but not extremely so - I'm running 20° at the same boost and RPM, advancing by 5° between 5 and 7,000 seems unnecessary though, I have an increase of only 1.5° between 5,000rpm and 6,500. Still doesn't look like detonation or preignition killed it.

[SLEEPYDUB]

Who knows? Needs a teardown tell more. Rebuilt engine? Could be something as simple as reused rod bolts failing, AFAIK B25 and B27 rod bolts are the same and they aren't to be reused. Measure the remaining bolts for stretch. I want to say it could've been preignition too since drifting is the kind of thing that can heat up spark plugs and valves enough to cause it, but though I can't read the strap and the insulator in that picture, the spark plug looks undamaged and doesn't scream "too lean" or "too hot". Your timing is aggressive at 5,000rpm, but not extremely so - I'm running 20° at the same boost and RPM, advancing by 5° between 5 and 7,000 seems unnecessary though, I have an increase of only 1.5° between 5,000rpm and 6,500. Still doesn't look like detonation or preignition killed it.

Thanks for the response. This helped ease my mind.

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[varg]

I'd like to see a better pic of the spark plug showing the ground strap more clearly and more of the porcelain. You can tell a lot by looking at a plug if you can see the right parts of it clearly. Reading plugs is still routine in drag racing, and it's how we used to tune our race bikes, datalogging and dynoing a dirt bike or even a track bike was unheard of for smalltime guys and even the professionals would look at the plugs thoroughly after testing.

[nando]

Being an unknown rebuild i honestly wouldn't be shocked if they reused the rod bolts.

[SLEEPYDUB]

Here is a pic of the strap. I'm fairly certain timing was way too aggressive. E85 probably saved the top end, but going that far past MBT probably took its toll on the rods.



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[varg]

Plug looks fine, no evidence of overheating despite being a range colder than I would go for such a high stress build. However since E85 burns slower, cooler, the plug was OK in this case. I wouldn't say your timing advance "way too aggressive" since a few extra degrees vs a 93 octane pump gas tune are the norm with E85, it's just the addition of 5 extra degrees after 5,000rpm that I think is overly aggressive, but - if your offset/first tooth angle is different than mine you may have more advance than you think. My initial theory of reused rod bolts looks more likely by the day.

[DesertBMW]

cylinder #6 runs the hottest and will first deal with detonation, head gasket didn't give out but something else has to

[SLEEPYDUB]

Plug looks fine, no evidence of overheating despite being a range colder than I would go for such a high stress build. However since E85 burns slower, cooler, the plug was OK in this case. I wouldn't say your timing advance "way too aggressive" since a few extra degrees vs a 93 octane pump gas tune are the norm with E85, it's just the addition of 5 extra degrees after 5,000rpm that I think is overly aggressive, but - if your offset/first tooth angle is different than mine you may have more advance than you think. My initial theory of reused rod bolts looks more likely by the day.

Well the plug is a BPR7ES, the same plug everyone runs as far as im concerned, so i'm not sure why that would be too cold of a plug when stock is a 6 heat range. We were thinking that the top end didnt detonate per say, mainly because of the E85, but the load was just too much for the stock rods to handle. The night before I could have weakened it because I was running a consistent 22psi and that was definitely over 450tq. Id like to believe that it is due to rod bolt failure, especially since both engines let go in the same gear, while on the rev limiter. I guess more of a tear down will show additional theories.

The reason why I want to find what caused the failure is because Im building a 2.8 forged stroker to put in it, and I dont want to run into the same issue on an expensive engine.

Thanks for the help guys

[varg]

Meant to say a range hotter.

[SLEEPYDUB]

Carnage from the 2.7.
Cant tell what caused it.

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[SLEEPYDUB]

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[grinchxvx]

Oiling issue maybe? I starved my old m20 of oil and it looked just like that when it went through the block


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[SLEEPYDUB]

I dont suspect lubrication being the culprit, but the piston does have some discoloration where the wrist pin meets the skirt, looks rainbow. That may just be from so much heat from timing, but you can see there wasnt any detonation.

[LowR3V'in]

read up on putting oil squirters in an m20.