2.5i head swap on 2.7e or swap whole 2.5 in?

[brahbarian]

My current ride is a 85 325e. I have a whole m20b25 as a donor. Was planning on doing the 2.7i conversion and swap the "i" head on to the m20b27. But now I'm considering just swapping the whole m20b25 in. What do you guys think? 2.7i worth it?

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[EatsHondas]

The 2.7i will be less work and noticeably quicker than the stock m20b25. One project is a full engine swap while the 2.7i is swapping heads and pretty much all the 325i engine electronics over to the "e" block. Are you ever going to be interested in turbocharging it? The 2.7i combo usually lowers your compression ratio to the low 8's unless you take specific steps to alter it (pistons, machining, head gasket size, etc.)

An N/A 2.7i will still be more potent than the M20B25 in stock form, but it won't exactly take your breath away.

[brahbarian]

The 2.7i will be less work and noticeably quicker than the stock m20b25. One project is a full engine swap while the 2.7i is swapping heads and pretty much all the 325i engine electronics over to the "e" block. Are you ever going to be interested in turbocharging it? The 2.7i combo usually lowers your compression ratio to the low 8's unless you take specific steps to alter it (pistons, machining, head gasket size, etc.)

An N/A 2.7i will still be more potent than the M20B25 in stock form, but it won't exactly take your breath away.

Perfect. Now what are the cons on a N/A low compression engine as a daily driver? Eventually I will more the likely turbo it.
Also head gasket size, what's ideal size?

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[Regnar75]

i have a 885 head on an 86 ETA block with stock pistons, but full motronic 1.3 upgrade and a 2.7i chip. the low compression 2.7i is a blast to drive, even with the 2.93lsd it was fun and i could take a 325i (was also shifting out of 2nd at close to 70mph hehehe) now with the 3.73lsd its an even better car and a blast to drive.

Daily driving is perfect and you wont realize your driving a low compression motor. i read up on the conversion for a while before finally doing it, and most of the naysayers saying its "too low of compression" are always people who dont know first hand. you will recognize the people who have done it on the forums because we always try and defend this swap. i think the guys with 325i's are just mad because can ETAs beat them ;)

I say do it. i have no issues Daily driving, no issues passing cars on the highway, no issues autocrossing (and even being competitive), my car starts without any issues, idles perfectly.

hope this helps

[EatsHondas]

Perfect. Now what are the cons on a N/A low compression engine as a daily driver? Eventually I will more the likely turbo it.
Also head gasket size, what's ideal size?

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A little less power is the only difference. Increasing compression while leaving everything else alone will yield little results. Low compression is ideal for forced induction because the pressure inside the cylinder is lower while out of boost. The extra air that the turbo/super shoves in raises the cylinder pressure to the equivalent of a high compression N/A engine. Since N/A engines can only breathe what the piston will draw naturally, you want to run a high compression ratio to squeeze the air as much as possible.

A stock head gasket thickness is fine as long as the cylinder head has not been machined past its limit. There is a small dimple in the cylinder head that shows you the max you can safely machine away.

[SDbmwe30]

m20 2.5 Head on 2.7 eta block

I had a fully re machined 2.5 head put on a 2.7 eta block everything stock. Running the stock ECU and stock everything else. I've had a rod knock on now 2 eta blocks. First one on the stock motor and second on a donor motor i bough on craigslist. Am I just unlucky or is the 2.7 block not ready for the change in compression?

[2mAn]

I had a fully re machined 2.5 head put on a 2.7 eta block everything stock. Running the stock ECU and stock everything else. I've had a rod knock on now 2 eta blocks. First one on the stock motor and second on a donor motor i bough on craigslist. Am I just unlucky or is the 2.7 block not ready for the change in compression?

When you say "stock ECU" Are you running all the stock ETA components other than the head?