Found the limits of a stock M20B25 Bottom End

[varg]

240k miles M20B25 Bottom end
pretty much standing on the Rev limiter.
standing on the limiter (this car is a drift car and gets beat on).
Fixed angle, -5* retard, Spark Cut @6800rpm with a 200rpm hysterisis.
I usually see failure at peak torque, not at Rev limiter.

I'm guessing this failure has more to do with revving the piss out of an old and thus questionable but heavily modified engine than the amount of power it was making, you probably would've had the same result at 15psi. 6,800rpm isn't that high but it's high for a high mileage engine that was never intended to rev that high, and being a drift car with "bang bang" rev limiter settings it's plain to see the abuse involved. A full teardown would be needed to tell for sure but riding an old engine on its rev limiter and "beating on it" is a surefire way to blow it up, boost or no boost.

[SLEEPYDUB]

I'm guessing this failure has more to do with revving the piss out of an old and thus questionable but heavily modified engine than the amount of power it was making, you probably would've had the same result at 15psi. 6,800rpm isn't that high but it's high for a high mileage engine that was never intended to rev that high, and being a drift car with "bang bang" rev limiter settings it's plain to see the abuse involved. A full teardown would be needed to tell for sure but riding an old engine on its rev limiter and "beating on it" is a surefire way to blow it up, boost or no boost.

You're probably right. A safer rev limit setting could have potentially helped. Im about to put in a 2.7L with 20k miles on it, so we will see what happens to this one. I probably wont go as high as 30psi, but I will definitely be above 20psi, maybe even 25. We will see

[doug.gardiner@gmail.com]

Watching the follow up

Subbed :popcorn:

[Dagamus(NM)]

Nicely done. In for the future pics.

[SLEEPYDUB]

Got a chance to take the head off today. You can see pretty clearly that the piston has turned in the cylinder and it has just slightly hit the head. From what I can tell, when the piston released from the wrist pin, it allowed the piston to hit the exhaust valve while it was open, offsetting it just enough to indent the valve into the head. The cylinder wall also has some dents in it, I'm sure from the piston being jostled around.

Random thought on cause of failure: Could the piston ring fail due to the ring gap being too small, causing the ring to bend and wedge the piston?

I'm fairly certain I will need a new intake and exhaust valve. My only concern, is if the head is still usable with that knick in it? Do you guys think it can be repaired?

I'll be taking the engine out Sunday and we may tear it down to see what the inside looks like.

Cheers

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[Maad325]

Doesn't look like much detonation happened, Although the porcelain on the plug at the deepest point in the plug will show if any happened while you were at high rpm. I'm impressed the 7 heat range worked well at that high of boost.
4

[SLEEPYDUB]

Doesn't look like much detonation happened, Although the porcelain on the plug at the deepest point in the plug will show if any happened while you were at high rpm. I'm impressed the 7 heat range worked well at that high of boost.
4

Yea I didn't see any signs of detonation whatsoever on the plug or on the piston. AFR was spot on. I dont think our tune had anything to do with the failure. I just think I reached the limits of stock cast 30 year old parts

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[Maad325]

Yea I didn't see any signs of detonation whatsoever on the plug or on the piston. AFR was spot on. I dont think our tune had anything to do with the failure. I just think I reached the limits of stock cast 30 year old parts

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Yeah get forged rods and pistons and should hold a bit better

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[kev kev]

Found the limits of a stock M20B25 Bottom End

Dude the same thing happened to me at around 23psi it was # 1 rod just like yours 220k on the motor but mine occurred at 7000 rpm took out the intermediate shaft I am still trying to figure out
What went wrong


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[varg]

I am still trying to figure out What went wrong

2 options IMO, bad tune or just this:

23psi... 220k on the motor... 7000 rpm

You can't expect a ~30 year old high mileage engine to make ~3x stock horsepower and rev higher than it was designed to without eventually failing.

[mrsleeve]

Are you sure the m20b25 crank is forged? Do you have a link to something that shows it is a forged crank, because everything ive read says that the M20 cranks are all Cast with the exception of the 524td crank.

This is correct, at one point I had a TD crank sitting next to a M20 crank on my bench, nearly identical other than "ringing" sound that lasts much much longer when you tap on the TD crank. Its forged, and the only M20 variant that is. Yes they are hard to track down, especially in the states, but you can also look for 80's Lincoln (yes the ones made by ford) diesels as well, they ran the M20 TD as well for a while between 82 and 87 iirc. Though like the 524TD in the states is nearly as rare a hens teeth.

As far as the head being reusable, you will have to get it apart and see if you can port out that gouge, or at the very least smooth it out to keep the hotpots away, and if you have enough meat in there to work with to smooth it out. I would be more concerned about the cam retainer on the top side of the head. I have bought several e30s over the years for beans when the a kid lets the timing belt service go to long. One memorable one was a kid that thought the fart can was more important than the timing set, and it let go in 2ed gear at about 4-5k rpm. Broke the cam retaining caps on #1 and 2 and broke the rocker on the exhaust side of #1.

This was a timing belt failure with no damage to the combustion chamber other than bent valves. You let go under boost at much higher RPM. You had a rod break, I suspect from your very aggressive r3v limiter settings finding the weak point in your rotating assembly cast OEM rods dont like that kind of hammering especially 30year old ones that type of on/off limiter setting I could see mimicking detonation over pronged period of time not just a bounce off....... Look over the top side of that head very closely, I will be surprised if you dont have damage there

[SLEEPYDUB]

One thing that does concern me is #5 looks completely different from the rest. Keep in mind e85 was being used.

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[digger]

looks like heat marks on some valves have red/blue/purple colors? one thing aluminium cast pistons dont like is heat, the strength drops off dramatically with temperature. this is is why people use the 2618 alloy with forged pistons as it is stronger at higher temps.

one theory is the piston failed where typically NA engines dont under the same conditions could be the higher temperature

[SLEEPYDUB]

Here is the mayhem. This is what 30psi on E85 at 6800rpm bouncing the limiter on spark cut on a 240k miles motor looks like (I need to describe it this way so you understand the stress, I don't know if this would have happened in a different circumstance, such as just a normal pull not hitting the limiter).

I think the piston got its wrist pin rip out of it and the rod went to town inside the crankcase. I haven't seen any signs of malubrication, other than the oil pump drive being sheared in half, which was probably due to the rod doing the nasty with the rest of the block. After a few rotations, everything just got fucked. Windows were blown open in the block, windage tray was bent and beat, but those rod bearings were still in tact and looked reusable. The crankshaft journals are also in good shape.

With the stock 2.7 that's going in now, we will limit the boost to around 25psi and make the limiter settings a little safer just is case it was a major contributor to its failure. Any input appreciated. Hope this helps future boost jobs

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[Kershaw]

That's impressive.