bmwe21.net forums

quick one gents

what if any is the difference between an eta block and an ordinary m20b25 block? It is just the block dimensions that im concerned with and not the crank c-rods etc.

Willy

Block is completely the same, head is different but not visible from the outside (4 bearings instead of 7 I believe). Bore is the same, stroke is different (and so are crank, conrods and pistons).

the eta (latin symbol for efficiency) was designed as a fuel efficient engine stroked to provide torque at low rpms and limited high rpm, 5000 tops. this was ensured by a low compression ratio and a restrictive cyl. head design with small ports/valves/inlet manifold.
you can whack a 325i head and manifold on it to release some power but not much if any more than a standard 325i engine. you need to fit 325i pistons and 130mm rods from a 320i and deck 2-3mm off the top of the block using an adjustable cam pulley to get the most from it.... but then you have excessive side loading on the pistons reducing your powers/causing wear. its all swings and roundabouts.

the eta block looks the same as a 325 from the outside. bore is the same, but it is stroked for the extra 200cc which is ultimatly limiting revs wise for a perfomance application reguardless of what head you put on it. you can deck the block and fit 325 pistons in it, adjustable cam pulley as i said above....... but then you bounce it off the rev limiter once too many times..... bang.
the ball ache, cost and limits of a 2.7 conversion means its a pointless way of getting 20bhp over a standard 325i unit (id rather a safe standard cheap m20 325i at 170bhp you can abuse with confidence)
but then an m50 192bhp conversion wont cost you much if any more than a 2.7 m20. and be reliable.

temporarily built the eta crank into the 2.5 block with the eta rods and the sport pistons played about with a straight edge and a dial guage and have come to the conclusion that yes the eta block and the 2.5 block are the same and i need to skim 2.0mm from the surface of the block to bring the piston height back up to the same level as a standard sport.

is this not similiar to what alpina did with the c1 2.7??

thanks murran i was intending on building a good engine for my e21 and have pretty much all the bits gathered up anyway so i have my heart set on following it through.

plus i already have an e30 325i sport and would love the old girl to tramp on past it!!

Willy

Yep, that's what Alpina did as well. How much exactly you need to skim off the block depends on parts used but that should be somewhere between 1.3 and 3.0 mms

left the block off with the machine shop Comiskey engineering he is going to skim 2.1mm from the surface hone the cylinders and check my eta crank for wear. real good lads.
any advice on parts that i should change apart from the usual when building the engine?

Always worried about taking metal off the block or cylinder head because you dont know what has been done previously. Have you checked the thickness of your cylinder head? if its been machined before you might find its at its limit, so when you go and take metal off the block too you might run into issues with the timing belt.