Found the limits of a stock M20B25 Bottom End

[SLEEPYDUB]

Well guys, I found the limit of my stock 240k miles M20B25 Bottom end this weekend. Here is a quick run down of my setup.

Stock M20B25 bottom
Bimmerheads 280/274 cam, HD valve springs, and HD rockers
Welded coolant channels on head
ARP headstuds w/ stock head gasket torqued to 70ft/lbs
T3 6266 .81AR
44mm wastegate
ID1000s and Walbro 450 on E85 ~19* timing
Ms2 using GM wasted spark
BR7E plugs

The engine was responding very well to boost. Up until about 28psi it was holding fine and pulling like a son of a bitch. It seemed just about as fast as my 2JZ E30 which made 550whp. It has zero signs of detonation and there were several minutes between pulls for cool down. Unfortunately I did not log the last run that let go as we were just having a fun 2nd gear roast up the on ramp when it let go. On this pull we were at ~30psi and in 2nd gear, pretty much standing on the Rev limiter.

Now im not saying the limit of the B25 is 30psi, but the limit for my B25 was 30psi while standing on the limiter (this car is a drift car and gets beat on). I'm curious to know what caused the failure. Was it my Rev limiter settings? Or was it just pure power that took it out? My Rev limiter settings are Fixed angle, -5* retard, Spark Cut @6800rpm with a 200rpm hysterisis.

I usually see failure at peak torque, not at Rev limiter. But me standing on it with such a hard cut definitely didn't help. Anyways, figured you guys would be interested in the carnage. Now I've got a 2.7L ETA bottom end that I will be swapping in place of the B25 in a few weeks and we will see how long it will last on a little less aggressive boost and Rev limiter settings.

Quick question since we're here. On the 2.7L bottom end, is it just about as strong as the B25? I know with lower compression ratio it should be "better" for boost. What should I expect out of this engine? It has about 20k miles on it since it was rebuilt.

[LJ851]

The 2.7 eta crank was designed for a 4700-ish rpm redline and is cast steel as opposed to nearly every other bmw crank (including M20B25) made from forged steel.

The only reason to use a 2.7 crank is because it's cheap.

[ak-]

Sorry to see, but thanks for the info and post dude. Always interesting. What was your peak TQ figure @ what RPM?
My rev limiter is at no retard, spark cut @ 6700rpm with a 100rpm hysterisis, but I never stand on it haha so seeing this is is relevant to my interests. I think my launch control is at -5* though with spark cut.
My bottom end is also OEM rebuilt. What rings did you use by the way on your 2.7 bottom end?

The 2.7 eta crank was designed for a 4700-ish rpm redline and is cast steel as opposed to nearly every other bmw crank (including M20B25) made from forged steel.

The only reason to use a 2.7 crank is because it's cheap.

Any opinion on the SETA bottom end, or should we assume the same being they redline to 5000(?)RPM?

[SLEEPYDUB]

The 2.7 eta crank was designed for a 4700-ish rpm redline and is cast steel as opposed to nearly every other bmw crank (including M20B25) made from forged steel.

The only reason to use a 2.7 crank is because it's cheap.

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.

Sorry to see, but thanks for the info and post dude. Always interesting. What was your peak TQ figure @ what RPM?
My rev limiter is at no retard, spark cut @ 6700rpm with a 100rpm hysterisis, but I never stand on it haha so seeing this is is relevant to my interests. I think my launch control is at -5* though with spark cut.
My bottom end is also OEM rebuilt. What rings did you use by the way on your 2.7 bottom end?

I dont know what kind of power i was making. This was all prelim street tuning before we stuck it on the dyno. Obviously never got to that point. The 2.7L bottom end that I picked up was rebuilt by an old BMW mechanic who stuck it in his 325e for a about 20k miles before the car was totaled and I obtained the engine, so no idea on the rings.

I am just using this 2.7 block for the time being, while I build a 2.8L forged stroker for future use.

[lukeADE335i]

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.

That's correct - it's the turbo diesel crank that people hunt down as it's forged although I'd imagine they're pretty hard to get now.

The whole M20 family was thought of as weak when it came out due to being BMW's first engine without a forged crank. That said, the fears were a bit unfounded as the cast crank in the M20 has shown itself to be pretty tough.

30 psi is a fair bit of boost!

[digger]

M20b25 has a cast crank. Cranks aren't usually the weak point. The rods/Pistons etcdont like detonation nor simply too high cylinder pressure that a knock resistant fuel with high boost generates not that in this case it's necessarily the root cause

[nomansland92]

Ya you might want to get your self a 524 crank, thas what ive been running at 25psi, so far so good knock on wood. The stock cranks even the cast ones can holds a lot power, but they dont high rpm. Ive heard of people using the 524 crank up to 9k though.
I personally dont like using spark cut with the cast rockers they can be prone to snap

Sent from my SAMSUNG-SM-G891A using Tapatalk

[LJ851]

M20b25 has a cast crank.

I stand corrected, my mistake.

[SLEEPYDUB]

M20b25 has a cast crank. Cranks aren't usually the weak point. The rods/Pistons etcdont like detonation nor simply too high cylinder pressure that a knock resistant fuel with high boost generates not that in this case it's necessarily the root cause

Undoubtedly the cylinder pressures were through the roof. I was standing on the limiter in 2nd when it let go. Im sure thats the cause of it.

I personally dont like using spark cut with the cast rockers they can be prone to snap

I have the Bimmerheads HD rockers so they should be good for it. I think the spark cut may have been part of the cause of my failure at the limiter though

[digger]

Undoubtedly the cylinder pressures were through the roof. I was standing on the limiter in 2nd when it let go. Im sure thats the cause of it.


I have the Bimmerheads HD rockers so they should be good for it. I think the spark cut may have been part of the cause of my failure at the limiter though

often the cause of rod coming out the block is over rev or similar and or resulting oiling issue (often tensile type failure) which might be nothing to do with the boost (more likely compression failure), it pays to do a good diagnosis (including intact cylinders) otherwise you might suffer the same fate especially with the same quality of parts

[SLEEPYDUB]

often the cause of rod coming out the block is over rev or similar and or resulting oiling issue (often tensile type failure) which might be nothing to do with the boost (more likely compression failure), it pays to do a good diagnosis (including intact cylinders) otherwise you might suffer the same fate especially with the same quality of parts

Well RPMs never exceeded 6800, so I cant imagine that it would be due to an over rev. Oil level was full, but doesnt mean the oil pump couldnt have failed. We will know more once we disassemble the block.

[nomansland92]

Sorry to see, but thanks for the info and post dude. Always interesting. What was your peak TQ figure @ what RPM?
My rev limiter is at no retard, spark cut @ 6700rpm with a 100rpm hysterisis, but I never stand on it haha so seeing this is is relevant to my interests. I think my launch control is at -5* though with spark cut.
My bottom end is also OEM rebuilt. What rings did you use by the way on your 2.7 bottom end?



Any opinion on the SETA bottom end, or should we assume the same being they redline to 5000(?)RPM?

Id like to know more about your launch control settings. Im a little scared to use spark cut because ive heard it can snap the rockers. I use fuel cut but i cant build enough boost to launch like i want so od like to use ignition if possible to do it safely. Do you have a tips for setting it up and how much boost are you getting out of it?

Sent from my SAMSUNG-SM-G891A using Tapatalk

[potatomash]

Im of no assistance to the matter, just simply impressed and entertained :)

[SLEEPYDUB]

Im of no assistance to the matter, just simply impressed and entertained :)

im glad my experiment could provide some entertainment :)

[sampayne]

Im of no assistance to the matter, just simply impressed and entertained :)

Same here. Sub'd to see more info after teardown.