High Revving M20B25?

[HennaE30]

So I just picked up an 89 325is that has no drivetrain. I have a couple extra M20B25 short blocks sitting around my garage so I'm going to drop one of them into it. I'm kinda looking for something off the wall.

What is a realistic number for how high a forged B25 can rev? some people say 8500, others say 10k. What about on original internals with HD rockers?

Also what kind of HP could you see from revving this high? I know at that point a lot of it is about fuel flow.

Just something I've always wondering. Thanks.


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[digger]

with serious engine work and a big $$$ valvetrain you should be able to rev to 9k above this you are in likely uncharted territory and it will be how many hrs does it survive. my gut feel is the crank and harmonics will become an issue before the valvetrain once the valvetrain has been upgraded. the tech available in valvetrains is exceptional these days and you can get anything custom made with enough $$$ but the cranks are long and flexible. you can get better vibration damper suited for the application and fancy billet cranks but steel is steel as far as flexibility goes so these measures only prolong the inevitable

biggest issue will be finding the need to rev that high, chances are the power at those rpm will be well and truly nosediving.

i would limit stock internals to 7k and thats with low mileage rockers and valve clearances in order with non worn cam lobes

[HennaE30]

with serious engine work and a big $$$ valvetrain you should be able to rev to 9k above this you are in likely uncharted territory and it will be how many hrs does it survive. my gut feel is the crank and harmonics will become an issue before the valvetrain once the valvetrain has been upgraded. the tech available in valvetrains is exceptional these days and you can get anything custom made with enough $$$ but the cranks are long and flexible. you can get better vibration damper suited for the application and fancy billet cranks but steel is steel as far as flexibility goes so these measures only prolong the inevitable

biggest issue will be finding the need to rev that high, chances are the power at those rpm will be well and truly nosediving.

i would limit stock internals to 7k and thats with low mileage rockers and valve clearances in order with non worn cam lobes

That's what I thought. Even with larger injectors and MAF it would still nose diving? I was thinking of it more as a "because I could" thing. I wouldn't get up that high around town as a DD and I would shift right before the power drops off on the track.

P.S. Dude you are an M20 whizz.


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[digger]

That's what I thought. Even with larger injectors and MAF it would still nose diving? I was thinking of it more as a "because I could" thing. I wouldn't get up that high around town as a DD and I would shift right before the power drops off on the track.

P.S. Dude you are an M20 whizz.


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those would be the least of you worries.

[HennaE30]

those would be the least of you worries.

Lol. I may hit you up during my build with questions and stuff, if that's cool with you


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[Coupelx]

You need the airflow to warrant spinning one that high which means lots of cash in port work.