Hasbro SuperDork
7/26/14 5:39 p.m.

Had this in another BMW thread but felt is it didn't fit there.

For the last few months I've been toying with building up a 2002 or e21 and hadn't given the M42 (or M10) much thought, leaning more towards the M20B25. A little more weight up front but that can be fixed and much more power. As suggested in another thread why would the M20B27 be a good choice? Which would be the best to build, normally aspirated? I was thinking 170 to 190 whp/tourque, maybe more if rock solid.

What has to be done to turbo a modest bullet proof M10? What kind of whp/torque are we talking?

I've had a 2002 roundtail and a 320i Sport, loved them both but they needed a bit more testosterone.

irish44j PowerDork
7/26/14 6:45 p.m.

It's the tradeoff...a bit less power for better balance when you're talking the M42 vs M20.

As for me, my e30 was originally an M10 car and was fun but not quick. It's been M42 (with a chip) for 2 years now and while it's not going to win any drag races, when I drive it now I never think "I wish I had more power." Not that it really applies to street cars, but it routinely beats all the M20 e30s at rallycross - I mostly attribute that to it being better balanced and lighter.

My e21 320i (an actual 2.0) is sitting in the garage with a rebuilt M42 on a stand next to it, if that tells you how satisfied I am with the engine.

M10 can supposedly be turbo'd to well over 200whp on stock internals, but with a bit of engine work the M10 can hold way more than that. For that matter, the M42 can do about the same (there is an inexpensive low-boost (8psi) turbo kit out there for the M42). One issue you have is how much power the diffs can hold. In stock form the "word" is that limit is around 200whp.

I race an M20B27 e30 (Chumpcar) and though it has torque, I don't enjoy the low-low redline and the engine runs out of steam fast at the high end. And I've definitely been walked on straights by M42 e30s, I'll add.....THat said, it's a pretty bulletproof engine.

Overall though, I'd start with an M42 and see if that satisfies you. It's peppy enough in an e30, and would be even better in a 2002 or e21, in my opinion. Stock for stock the M20s are more powerful for sure, but they aren't THAT much faster (maybe fast enough to beat a late-model Camry, lol). But with these cars I find that making them handle well and balance well makes for a far better driving experience than having a bit more power. Just my opinion though!

Also will mention that you can find good M42/M44s with transmissions all day long for cheap. Each of my M42s+G240 combos, both in good running condition, cost less than $300.

Hasbro SuperDork
7/26/14 8:09 p.m.

Excellent response, thank you, Irish. Handling is top priority for me, too. I'll read up on M42s.

irish44j PowerDork
7/26/14 9:20 p.m.

FWIW, I have a big build thread for my e30 on here, with quite a bit of discussion about the M42 swap, etc. The M42 stuff starts about a year into the thread, IIRC. You may or may not get some tips, ideas, or insights from these threads, but have at it.


I also have another build thread on my (currently stagnant) e21 project and the more extensive rebuild of the other M42 that I'm going to put in that car.


mad_machine GRM+ Memberand MegaDork
7/27/14 1:14 a.m.

often maligned and usually ignored, BMW's M42 is supposed to flow air through it's head better than the vaunted S14 in the E30M3. there was a factory S42 engine and it was a giant killer.. it just got overlooked by the factory due to how good the S50 was

djsilver New Reader
7/27/14 5:49 a.m.

My wife had the odd last year '92 E30 vert with an M42. Very peppy engine in combo with the 4.10 or so diff in the vert. It turned about 4k rpm at 80mph and still got 30mpg. GRM did an M42 turbo project several years ago and you may find it in the archives. You can find them in the E36 318's and Z3's as well.

irish44j PowerDork
7/27/14 12:18 p.m.

for bonus points, the M42 can be safely used with the small-case (read: lighter) 3.91, which is pretty much the ideal ratio for it for everything, in my book. Plus most 3.91's out there are LSD.

mad_machine GRM+ Memberand MegaDork
7/27/14 2:20 p.m.

that is the beauty of the M42/44. As it is a lighter more compact engine, everything around it can be lighter and better balanced. With a few mods, you are not far from 325hp levels with less mass to move around.

On my old 318ti, owners of other 6 cylinder E36s were always amazed at the close to a foot distance between the electric cooking fan and the waterpump.. where as the I6 cars had to use a mechanical fan that would hit the radiator if the mounts were worn

Slippery GRM+ Memberand HalfDork
7/27/14 2:35 p.m.

What about an M20B27 block with a B25 (885) head? Thats a great combination that gives you good low end torque along with a higher redline and power up top.

I have those two with and a 330 crank that I always wanted to put together to build a stroker M20.

dcteague New Reader
7/27/14 7:50 p.m.

My first technical dive into BMW engines was building a M20B27 stroker - a great engine. I had purchased a 1990 325is and a friend had an old 1988? 528e that was in bad shape. I learned very quickly that the engine was the seta - much more rare than a typical 325e engine. The seta was also available in the 325 series. There are a few minor tweaks you can make to improve performance - but a simple swap will show gains. Its obviously a heavier engine than the m42/m44. I had mine mated to the M3 4.10lsd - which also added to the low end torque feel and it was a great track car. I could hang with a lot of cars that were surprised at my speed out of corners. I also recently spent some time looking at the M44/M42 for build options to put into a lotus replica - you can stroke these as well, but they wont' be the same as the m20b27 stroker. if you can find a good seta (they were known as Super ETA engines), i'd go that route.

Hasbro SuperDork
7/28/14 2:16 a.m.

I could settle for this.

 photo bmw323ifront.jpg

 photo bmw323irear.jpg

irish44j PowerDork
7/28/14 5:53 p.m.

^ that car is one I have pics of saved on my computer. It's one of my inspirations for "how I want my e21 to look when it's done" (aside from mine having an M42 in it).

Burrito Enthusiast
Burrito Enthusiast HalfDork
7/28/14 6:27 p.m.

I had an M20B25 swapped E21. It was an awesome little pony car. Great looks, intoxicating exhaust note, and better-than-some handling made it a very fun little car. If I had been the one that built the car, I probably would have kept it forever, but there was just too much PO dickery to make it a lifetime car. Well, that and the tin worm in the trunk. They are very much still on my radar, I'm just waiting for a specific survivor in my neighborhood to come up for sale (early slick-top in a light metallic green. Swoon).

Here's mine after I sold it. All the next owner did was lower it another half-inch and add a stupid Tackima roof rack:

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