318i M10 block bore limit

Post Fri Mar 27, 2009 9:57 pm

I have a lot of e30's on my plate right now, but sometime in the future I'd like to make a light cheap reliable and fast E30 for the odd track event and fun country road trashing.

I keep thinking of a 318i m10 engine and would it be possible to rebore the block to fit 535i pistons and and M3 crank making it 2,3 liters.

Anybody know if the standard block can take such a increase, is the 318i and M3 block essentially the same casting?

Could it even take a reboring to the size of the 2,5 liter M3?

Post Sat Mar 28, 2009 12:40 am

Hi

92mm is possible, with care. There are 2.5 m10 but they use the complete s14 bottom end. To use the s4 crank you need to machine the timing cover for a bigger oil seal.

The oil ways vary between the m10 & s14 blocks for the drain from the head.

Jason

Post Sat Mar 28, 2009 3:48 pm

Jason speaks the truth - 92mm is as far as you can go. There's all sorts of stuff you can do with the stroke - s14 crank will fit as Jason says, can get you to 2.3L, but for what you want it's unnecessary.

Fot trackday use, lose weight off the car. My e21 when finished will have fibreglass bonnet boot and wings, cage and seats with no interior. Bias bar pedal box so no brake boosters etc, and I have a nice 2L M10. The engine has S14 throttle bodies via an adaptor plate (used to be Jasons ITB's!), and made 128bhp with a 284 cam, original 8.1:1 compression pistons, and a serious problem with something head gasket shaped leading to oil in water and vice versa. I am awaiting higher CR pistons from the USA, and the engine will be rebuilt. Should produce plenty more than 128bhp.

It's not just power that makes a car fast, you need brakes suspension etc too. My current E21 with 109 dyno bhp's is pretty good fun on track, and embarrases lots of faster cars, sometimes they even spin trying to get away from me

It's possible to spend thousands on an M10 and it's possible to get 100 bhp/litre out of them, but not with reliabilty. Lose a bit of weight off the car and it's the same as 10 more bhp.

I'm going to go on a diet myself once I finish the car - could lose 20 or so kilos myself!

Jim

Post Mon Mar 30, 2009 5:09 pm

The cylinder bore is the same from 1.5-2.5 litres the capacity is changed by crank stroke with correct rods and pistons and combustion chamber (head) to make it all work (stick with same block and head). The problem is finding correct stroke crank to make CC you want which is near on impossible for popular sizes in good condition.

Post Mon Mar 30, 2009 8:09 pm

drjim has a spare 2L crank, but it needs a 6 bolt flywheel as used on 2002's rather than the 8 bolt e21/e30 version.

Post Mon Mar 30, 2009 10:52 pm

Thanks for the information.

I have just realised that the 318iS M42 has a cam chain, I had mistanken it for having a belt as the M40. So I'm a bit in a mind shift towards this engine due to its superior valvetrain, but I fear it will be an expensive shift as cams etc. are quite expensive for this engine.

But its nice to know were the limits are for the M10, I already have a 318i e30 and it's engine will become in excess once my big project takes off, hopefully some time in the fall.

What about rpm limits on the m10? The M30 does I think 6200 rpm max standard, were does the M10 stop? There is a lot of headroom in the bottom end when compared to S14 and M88/S38 rpm limits. I guess the valves and rockers are the limiting factors.

Post Tue Mar 31, 2009 12:03 am

If you go above 7k the rockers break after a bit above 8k they break quickly! You can get expensive steel ones to stop that happening. I think e21jason had a source for them. Lighten the car and live below 7k - I will have a rev limit of 6.5k on mine - no point going higher for trackdays.

If you are building a race M10, lighten the valvetrain, fit steel rockers and head for 9k.