m10 b20 build questions.

[mason6556]

i have a 85 318i , i have been giving a lot of thought into building a N/A m10 for a while now, giving a lot of thought as to what i want it to be, and how i want it to run and feel. i know i want a high reving ( 7500+) high compresion motor, something that would be fun to drive on the street or a track. im not set on a HP number but more a drivers car. im sure people will say swap it turbo it supercharge it, but im not going to so if thats what your going to say, im asking you not to. now for the motor i know i want a 2.0 block a bigger cam maybe a 304 i know i have to get the head done to safly rev that high and get HD rockers, i found a complet 2.0 motor for 500 its got a little work done already. what im not sure about is if i should use the whole motor or put the 318i head on it i know that this messes with compression so if i did that what kind of pistons would i need to run or combo of crank rod and pistons. and if i didnt put the 318i head on what problems if any would i run into hooking up 2.0 motor to FI and having everything run right. also if anyone has any ideas that would be cool to.

[TobyB]

www.bmw2002faq.com for a start. Use the Google search option and start reading
about engine builds.

The pistons and head need to match if you want a compression ratio above about 8:1
without detonation and avoid metal collision issues. So if you want to use the 1.8 head with
a 2L crank, you'll need to find a piston that will work with the head, or change heads
to something that will work with 2L pistons. To destroke the M10, they moved the
wrist pins down in the piston, and left the rods the same length, so the 1.8 pistons
only work with the 1.8 crank.
I seem to remember that Ireland made some pistons to let you do this,
but I don't know a lot about them.
HD rockers are good, but the critical element is the valve spring- it needs to be about
20% stiffer than stock, but not much over that. I ran stock rockers to 7500 for years,
and the only times I broke them were when the springs were soft. And some springs DO
go soft...

Beyond that, it's just tuning. If you want real power, you'll have to ditch or heavily
modify the fuel injection. It has a rev limiter (in some iterations) and will run out of
airflow and meter way before 7500 for a 2 liter. So either aftermarket EFI or carb(s).
A 40/40 will certainly flow plenty to hit 7500...

Good luck! 7500 is a good target- besides the valve springs, the engine's reliable
to that speed, and is still very durable. And it will make a decent bump in power, too.

t

[mason6556]

Thank you for the reply and the information I new about the low compression when mixing the 1.8 head with the 2.0 block that's why I was wondering about different piston options I thought about useing the head on the motor it's a e21 head. I want to get the head ported and polished, I was also thinking about using larger diameter valves I thought it might help more with the higher revs . Any thoughts on that?

[TobyB]

E21 head is reputed to be very good- I've used E12's, but just because that's what I had.

Pistons are still available, and if you want to stay on street fuel, something in the 9- 9.5:1
range would be good. I ran a 'limited' race motor at 9.3:1 for years, and it made great power and
good torque.

You won't find much gain in head porting- there's some to be found there, but
not all that much. It's worth doing, just don't expect the world. And the valves
are probably not a limiting factor at 7500. But if you're having all the work done
anyway AND have to replace valves, that would be the time to do it. I don't think
it would make much difference below 6500, certainly.

hth
t

[petrolhead]

i have a 85 318i , i have been giving a lot of thought into building a N/A m10 for a while now, giving a lot of thought as to what i want it to be, and how i want it to run and feel. i know i want a high reving ( 7500+) high compresion motor, something that would be fun to drive on the street or a track. im not set on a HP number but more a drivers car. im sure people will say swap it turbo it supercharge it, but im not going to so if thats what your going to say, im asking you not to. now for the motor i know i want a 2.0 block a bigger cam maybe a 304 i know i have to get the head done to safly rev that high and get HD rockers, i found a complet 2.0 motor for 500 its got a little work done already. what im not sure about is if i should use the whole motor or put the 318i head on it i know that this messes with compression so if i did that what kind of pistons would i need to run or combo of crank rod and pistons. and if i didnt put the 318i head on what problems if any would i run into hooking up 2.0 motor to FI and having everything run right. also if anyone has any ideas that would be cool to.

Have you opened up the engine and checked what pistons you have there?
For any real high compression ratio (over ~10,5) you'll need custom made pistons. The more degrees you have on the cam, the more comp. ratio you need.

Thank you for the reply and the information I new about the low compression when mixing the 1.8 head with the 2.0 block that's why I was wondering about different piston options I thought about useing the head on the motor it's a e21 head. I want to get the head ported and polished, I was also thinking about using larger diameter valves I thought it might help more with the higher revs . Any thoughts on that?

What size valves do you have now? Biggest stock size on M10 head are 46mm inlet, 38 mm outlet, which is already about as big you can fit in there without problems.