M20 2,5 Modifications

[naser]

Hi
have recently moved into the world of e30's and m20 engines from 2002's and m10 engines by buying a 81 325 touring. Was wondering how strong the standard bottom end on a m20 is. How high can the bottom end be revved if proper mods are done to the head? Do the rockers need to be modified for aproximatly 7000 rpm? I am looking to modify for about 7000 rpm and 210-220 hp. Am considering running on 3 twin weber 45's or 40's. Is there any easy way to get higher comp from a 325, for example a cheap piston swap of some sort. What are the main limiting factors on the m20 for increased RPM? Anybody know how much a "new" engine from BMW costs? Are the slippy diffs in e30 the clutch type like in e21s and 2002s. Thanks for the help and its great to see so many e30 fans and e30 activity online,
Naser

[kam-325i]

The most common route for that kind of power from a 2.5, is to do the 2.7 conversion. Its along the lines of a engine from a 325e or 525e with a 320i head. look at the 2.7 conversion, as this is the route i am going to go with a goal Bhp of approx 190-200.

[gareth]

220 bhp is a lot for a M20B25. i suspect you're in with a good shot with the carbs if also followed up with suitable head and cam etc. however, a M20B35 (535i or 735i) converions will be a lot less wonga and have the same power and will (probably!) be a lot more economical to run and a huge amount cheaper to do. this will give you the required (desired!) power from a standard lump.

have a search on here for M30 conversions and have a look on www.e30.de

[Widge]

You mean an M30B35 - it's a different block and a physically bigger engine to the M20. And I thin you will find economy with and M30 is not good, not that you can be worried about economy if you are talking about 220bhp @7000rpm with carbs. I would do a 2.7 with 2.5 pistons and head, on carbs with a hot cam etc head work, cos if you are going to the trouble of rebuilding you might as well get the extra 200cc's

[Turbo-Brown]

Why oh why would you bother with webers?!

Use throttle bodies and enjoy some semblance of fuel economy and an engine that pulls cleanly everywhere in the rev range

[Widge]

Well I would do throttle bodies and MS, but he obviously likes or knows about webbers, they do make a good noise! And they give good power if emptying your tank as quickly as possible is also a priority.

[gareth]

that was a typo! meant to type M30B35...!

regarding fuel economy, i used to have a 170bhp pinto in a mk2 escort rs2000. that would do 15mpg on a run and could drop to 4mpg if REALLY giving it welly. that was a 2 litre, bored to 2.1 with a big valve head, full rally cam and twin 45's.

i expect triple 45's in a heavier car would be horrendous.... but VERY fun!

nothing sounds like 45's or 48's! TBs are close but not the same...

[naser]

cheers for the info,
a 2.7 conversion seems like a good idea. The problem i have is i dont know how much a m20 bottom end can take, is it posible to reliably rev a m20 to 7000 rpm with standard rod, pistons and crank without worrying about iminent engine failure? I thought about an m30 conversion, my uncle had a e30 alpina b6 3.5 and that was a real monster, but i wonder if it still isnt to heavy for the e30. How much porting and head work can be done on a m20 head. From what i see it isnt so different from an m10 head. Although it does already have many of the improvements you would do to an m10 to go over 7000rpm. Webers do give the car a great sound and feel, and properly set up they shouldnt be to bad in fuel consumption, then again even throttle bodies wont give me good mpg the way this car will be driven...........
Thanks for the answers
Naser

[Ant]

could always turbo the M20, use a stock bottom end, boost to 0.5Bar and enjoy