2.7 ideas

[modernist]

Since my 2.7i has been burning oil, it's time for a bottom end refresh. Here's what I'm thinking of doing:

If I get M54B30 pistons and machine 1.5-2mm off the top (depending on the block deck height being 206.2 or 206.7mm), I can make them work with 140mm connecting rods from an M44B19 (the lightest of the 140mm rods that will fit an M20). Can also use M42, M43, or M50B25TU connecting rods. Certainly a better rod ratio than stock 130mm rods can offer. With the piston protruding ~0.6mm above the block, compression will be ~10.2:1; at ~1.1mm compression goes up to ~10.8:1. Seems like the rod ratio, and drastically lighter pistons and rods, would make it smoother than a stock M20B27. Since this would be a N/A engine I imagine the pistons would hold up fine.

Also, with an 84mm crank you could use the M54B30 pistons unmodified, with 135mm connecting rods, compression ~9.2:1. Deck the block 1mm and it goes up to ~10.2:1.

Just some fun ideas, nothing scientific. Tell me what y'all think.

[digger]

i'd do the m52b28 crank, eta rod, b25 piston job. the longer rod wont do anywhere near what you think and you want to match piston and combustion chamber. take some from the top of block to get clearances and optimize compression and i think its a better and proven option

[redlightpete]

You mention you have a 2.7i, so I understand that you have a m20b25 head and 81mm crank. One side of the m20b25 piston sticks up considerably past the deck height. Were you going to try to replicate this with your modified m54b30 pistons? Look at the crown shape IE uses to get 10:1, and that's with an 84mm stroke.

http://www.iemotorsport.com/bmw/E30-...iepistons.html

Also, was the m20b27 not smooth enough already?

[modernist]

One side of the m20b25 piston sticks up considerably past the deck height. Were you going to try to replicate this with your modified m54b30 pistons?

Nope, just thinking about things that might work. My budget isn't particularly big, so custom pistons are out of the question. I really like the idea of lighter pistons/rods. There are sets of brand new M54B30 pistons on eBay for around $200, and M42/43/44 rods are pretty cheap too.

Also, was the m20b27 not smooth enough already?

Mine has always felt a little ... agricultural. When I had the head off, the piston walls had very little visible cross hatching, and it burns oil, especially when I have it above 3000 rpm for extended periods of time, I can smell some burning oil when I'm driving hard. Drivability is very good because I've sorted out the fuel, electrical, and ignition systems, but despite this there's still some roughness at all RPMs. So I suspect the bottom end needs a refresh. The head is rebuilt with ~20,000 miles on it.

[redlightpete]

Mine has always felt a little ... agricultural. When I had the head off, the piston walls had very little visible cross hatching, and it burns oil, especially when I have it above 3000 rpm for extended periods of time, I can smell some burning oil when I'm driving hard. Drivability is very good because I've sorted out the fuel, electrical, and ignition systems, but despite this there's still some roughness at all RPMs. So I suspect the bottom end needs a refresh. The head is rebuilt with ~20,000 miles on it.

An m20 should be pretty smooth, especially compared to cars of that era. It is an inherently smooth motor configuration. What's the situation with your exhaust and motor/transmission mounts?

[modernist]

An m20 should be pretty smooth, especially compared to cars of that era. It is an inherently smooth motor configuration. What's the situation with your exhaust and motor/transmission mounts?

New/new. I'm just picky. It's not rough, just kinda buzzy below 2500 rpm.