High Reving M20?

[SickE30]

Is it doable to build a high reving M20 for turbo purposes? If so what would it take?

[Jordan]

Please define high revs.

Stock m20b25 internals will revolve to about 7200rpms. At which point something will fail, most likely a connecting rod or valve float.

[NickP]

Why do you need high revs if you're going turbo?

[E30godz]

the connecting rods are forged, i think they will last higher....to my understanding valve float occurs around 7200 and then rockers start breaking.

7 grand with turbo should be plenty man...any higher and the mods you will start doing to make power at those rpm's will interfere with a good turbo setup.

[bmwdude]

Re: High Reving M20?

Is it doable to build a high reving M20 for turbo purposes? If so what would it take?

There are a few good perspectives on this. It actually depends what you will be using the car for, and what charachteristics you like in your engine based on your style. In my case, I stroked the engine to 3.0L, so that would take care of the bottom end torque, and added a nice large juicy turbo for the top end of the spectrum. The turbo basically takes care of the range just where the stroker and stockish head start to fail. I would like to note, that at 4500-5000 rpm though, when the two forces blend together, the engine rocks!

One can write a whole article on this going on for pages and pages, so it will be difficult to explain this in a couple of lines. You can make your 2.5L rev nice and high, and then add a large turbo, but there will be more lag down low, and the car won't feel streetable. A setup like that would be great for the autobahn! Then again, for some mega bucks, you can buy the newer generation GT turbo's, that have an excellent low AND high rpm range capabilties, but you are looking at $1500 and perhaps even more for just the turbo unit itself. Turbo unit talk is an other topic on its own.

In a one liner, you have to decide what your budget is, what charachteristics you want your engine to have, build the engine to handle the durability aspect of what you ask of it, and last but not least, PROPERLY size your turbo so that the formula works in the end.

[SickE30]

By High Reving I meant like 8500-10,000 It seems that with a lower compression motor that high boost and higher reving capability would let the turbo go to work better. I mean obviously stock internals would not be an option. I am just asking is it doable and what would it take if it were.

[bmwdude]

By High Reving I meant like 8500-10,000 It seems that with a lower compression motor that high boost and higher reving capability would let the turbo go to work better. I mean obviously stock internals would not be an option. I am just asking is it doable and what would it take if it were.

In that case then, it would have to take, HD racing valve springs/titanium retainers; rocker arms can stay stock; ported head for those high rpms; extrude honed intake; larger throttle body; lightweight forged pistons (8.5:1 and lower), lightweight connecting rods; definately a forged crank; large turbo to handle those high rpms with efficient intercooler design; 3 inch exhaust; and if going over 9 psi, stand alone engine management. I'm sure I may have forgotten some minor details here.

Yes it is doable. Do bare in mind, whoever reads this also, that what I have listed, is for 10 000rpms. Otherwise, it would be overkill and not required for most other applications.

[schnitzer teil-jäger]

rocker arms can stay stock

rocker arms are a weak point on just spece30 engines(100%) stock hp
rocker arms are a paint on IT engines with a chip
rocker arms will stop you very soon over 8500
the camshaft will snap at 9000 if the belt is still on
w

8500-10k engines are 4 valve

[AndrewBird]

^^^ I agree.

While what you said bmwdude might be true, there is no way stock rockers will ahndle that. How many people on this board hav posted that they broke a rocker arm revving to high? More then I can count on two hands.

The timing belt is the other weak spot. You would pretty much have to go to a chain somehow, preferably a dual chain. Although I have no idea how you would do this. You could get away with a kevlar belt maybe, but I have no idea where you would get one or even if it would work being that it would not stretch at ALL.

IMO, 10,000 RPM is impractical. It would be easier and cheaper to go with an engine swap with an engine that has a lot more power. Or hey, go rotary! Then you can have Rpm's and power!

[bmwdude]

rocker arms can stay stock

rocker arms are a weak point on just spece30 engines(100%) stock hp
rocker arms are a paint on IT engines with a chip
rocker arms will stop you very soon over 8500
the camshaft will snap at 9000 if the belt is still on
w

8500-10k engines are 4 valve

hmmm..I didn't know that rocker arm life was directly related to engine speed. Interesting!

My car by the way (and almost a daily driver when the weather is nice), has about 350hp at the flywheel, and I can tell you right now, that rocker arms failure is unheard of, where power relatioship is concerned. However, I never rev the car past the usual 6900RPM mark, although when pulling smoke shows, you will occasionally hear me hit the rev limiter.

Korman I THINK built some high revving M20's on the early 90's. Perhaps they would know something about the M20 in an high revving environment.

[Mystikal]

An M20 will snap the stock rocker arms just north of 7000. Ask me how I know.

BTW, unless the engine is modified to the point of barely resembling an M20 anymore, revving it that high may be pretty useless. The design lends to a thick midrange, but it just can't flow well enough to make anything useful past 6000RPM. Everything is possible, just does it make sense when there are other, more logical solutions available.

[bmwdude]

An M20 will snap the stock rocker arms just north of 7000. Ask me how I know.

BTW, unless the engine is modified to the point of barely resembling an M20 anymore, revving it that high may be pretty useless. The design lends to a thick midrange, but it just can't flow well enough to make anything useful past 6000RPM. Everything is possible, just does it make sense when there are other, more logical solutions available.

I agree 100%. It is for this reason the 24V conversion is so recommended.

In my case, I used a different formula/philosophy, where since we know that the M20 (compared to todays mutivalvers anyways) is designed as you say for a thick midrange, with half decent top end, I just simply added a large turbo to take of that top end. Stroked to 3.0L for a nice healthy bottom end. Head is freshly rebuilt.....with minimal mods.

Question: When you snapped that rocker arm....how old was it???? Just curious....

Some people seem to be snapping stuff, while driving their 300 000km+ cars hard.....

[Mystikal]

You're right on the 300k part. Almost dead on, actually.

From what I've read though, even new motors don't like the scary side of 7000.

[bmwdude]

You're right on the 300k part. Almost dead on, actually.

From what I've read though, even new motors don't like the scary side of 7000.

For sure. There is a reason why our tachs are listed up to 7000.

To sickE30 who posted his initial question, you CAN do it, with plenty of research, proper engineering, and money, but why go through that trouble, when BMW did that work for you already? With the M engines.

Our M20's have their unique charachteristics, and make for a very balanced drivable car. There is nothing wrong with stroking it, camming it, turbocharging it, and don't listen to anyone who tries to talk you ot of it with non sense. But if you want something that has a specialized, unique charachter, in your case super high revving abilities, then the M20 engine is not the personality that will meet your needs.

You can make that E30 M3 4 banger rev 9000-10000rpm ;)

[BS87]

You can make that E30 M3 4 banger rev 9000-10000rpm ;)

You can do it with the M10 also, but it's just not feasable.