325 Head on ETA

[cabrio327i]

Hi, could somebody say why exactly you cannot fit the 325 head onto the 2.7 eta block, i know theres been varius threads on which heads and do and dont work, but i would like to know the reason why they dont, eg too high a compression, or too low a compression, or the wrong combustion chamber shape ie the squish in the head, the reason for me wanting to find out is my ETA pistons are flat top with small cut outs for the valves, compared to the 2.5 pistons which have quite a large raised squish area on the piston crown,so the combustion chambers dont match, and i would say the compression ratio would finish up too low with the flat top pistons, Anyway folks suggestions welcome, but would like real facts if poss, cheers Dave

[E30Adam]

You can fit the 325i head on the eta block, just don't use eta pistons becuse it'll pink

[cabrio327i]

Hi Adam, Yes i realise you can use the 2.5 head on the ETA block with the ETA pistons, and people say has you do that it will pink, so this tells me that the compression ratio would be too high yes? But if you see what im getting at the ETA pistons are flat top, compared to the raised crowns of the 2.5 pistons so surely you would think that you would get a LOWER compression ratio, Cheers Dave

[Demlotcrew]

Yes but the 2.5 head could have a smaller chamber in the head. This is why the comp could be too high.

Andrew

[cabrio327i]

Hi Andrew, Yes thats possible, but ive compared both the 2.5 head and the ETA heads and they dont look that different, but this is what im thinking too, if you do get pinking with the 2.5 and ETA combo, SO what im trying is ive removed the squish area from the 2.5 head and virtual made it look like a Hemi shape chamber, in effect LOWERING the compression ratio and hoping it will work! wont know tho untill the better weather comes cause cant be bothered at the moment but got everthing ready to bolt on then. Dave

[XMon]

Check out http://bimmer.roadfly.org/bmw/forums/e30 and then do a search for STROKER motor. It's something that's commong practice in the US

[cabrio327i]

Hi Xmon, Thanks for the info, that site has some interesting stuff on there. Cheers Dave

[MasterBates]

The 325i head goes straight on without probs, but it's best to use the later (post mid '85) engine. The eta has flat pistons with a slight raised circular secion in the middle. The 325i piston does have a raised section at the front but the rest of the piston is actually dished.
Compression would be about 10.4:1 using the early engine (which is too high for regular unleaded, hence the pinking) but 9.4:1 using the later unit which is just right. The combustion chamber volumes are similar on all these old head but the 325i head has a slightly raised secion to clear the raised bit of the piston which would lower the compression slightly.
The Super Eta 325e was the very last of the Eta E30's which used a standard Eta bottom end with a standard 325i head, albeit with the wank Eta cam.

[cabrio327i]

Hi Masterbates, I was wondering where you were when your needed, at the moment im running the complete ETA but using the 325 ecu with manifolds and injectors,wating to put the 325 head and 284 cam later,like i say ive ported and altered the chambers in the head and will only know if it runs ok when i build it up, thanks for the help mate Dave.