Quick question about eta motors and CRs

Fri Sep 18, 2009 3:10 pm

Hi guys,
I'm currently in the process of building a 2.7L short block from an eta to go into my 323i. The thing is that according to the e30 Zone wiki about building 2.7's, the engine stamp says it should have a CR of 11:1 but the pistons are dished so i would've expected it to be 10.2:1 at most.

Can anyone shed some light on this confusion?

The stamp:

The pistons:

Fri Sep 18, 2009 5:43 pm

It's a shot in the dark, but the 11:1cr pistons are NLA, so maybe it's had new pistons and 10.2:1 were all that was available.
Also, IIRC, there were pinking issues with the 11.1 engine back in the day so maybe that's why the block has the 10.2 pistons in.

Fri Sep 18, 2009 7:50 pm

They look like the standard eta pistons to me,around 10.1 is about right,bearing in mind the current quality of fuel!
Presume you are using the 323 cam?
Opening the valve seats out to take 325 sized valves will give a bit more top end,but loose a little torque lower down...I am about to move my 2.7 into a different car and I am going to swap the current 325 cam for a 323 one that I have 'in stock',but I am not too sure about swapping the valves,as the bottom end grunt is the best feature of my engine!

Fri Sep 18, 2009 8:30 pm

daimlerman wrote:They look like the standard eta pistons to me,around 10.1 is about right,bearing in mind the current quality of fuel!
Presume you are using the 323 cam?
Opening the valve seats out to take 325 sized valves will give a bit more top end,but loose a little torque lower down...I am about to move my 2.7 into a different car and I am going to swap the current 325 cam for a 323 one that I have 'in stock',but I am not too sure about swapping the valves,as the bottom end grunt is the best feature of my engine!

Malcolm, bruv..... My 2.7 has got 325i valves (and inlet ports) in the 731 'head actuated by a 325i cam. It doesn't "kick-in" until about 2k rpm. IIRC you've said yours does it at 1.5k. Knowing what I know now, I wish I'd had Iain put my old 731 on when he built the engine, I think I'm losing a bit of mid-range. If I ever have to have the 'head off, I'll be using the small valve/port one.
I've read that the "323i cam is the best" theory is a fallacy. It's only 252deg duration, compared to the 325i cam's 260deg, and it's slightly less lift IIRC.

Fri Sep 18, 2009 9:08 pm

I am even more confused now!
I will have the engine on the garage floor tomorrow,do I just deal with the oil leak from the breather and shove it in the cabbie or swap the cam?
I'd like to try the 323 cam,get it re,mapped with a unichip and see what it will give...
See,I reckon I can swap the cam with the engine on the garage floor without removing the head,then stick it in the cabbie and let one of these 'mapping experts' loose on it and see what the result is.....but....but....help....help....

Sat Sep 19, 2009 7:02 am

IIRC there were 2 cams for the 325i's depending on if it had a cat or not.

The plan for this motor is rebuilt short block (rebuilding currently in process), then put the 323 head on the short block and into my 323i. Initially i intend to run the L-Jet fuel system for a bit, but then once it's run in and i have more money i'll move up to megasquirt.

Does anyone here have experience running the 2.7 on L-Jet?

Then down the line, depending on how it drives with the aformentioned set-up, i'll pull the head and do things like 325 TB port, headers, cam and all the other fun stuff.

Sat Sep 19, 2009 7:26 pm

Not too sure on how you can get the fueling right on L-jet,ignition timing will not be a problem,of course,the usual way with motronic is a custom chip from Ant!
Bearing in mind the cost of the work so far,it may be an idea to upgrade to motronic management whilst you have the thing in bits,dead easy to swap the front pulley etc whilst the engine is out.

Sun Sep 20, 2009 4:23 am

I've read that it runs ok on the L-Jet, a bit lean at the top end though, so i was going to use that for running in as you shouldn't really be trying to tune a new engine (if it runs rich the fuel can wash some of the oil off the bores) then once it's run in i'll move up to MS.

Sun Sep 20, 2009 12:09 pm

well after having a closer look at my pistons and this site http://www.bmwccn.no/rogaland/Mahle_m20.html i've determined that this motor actually has a CR of 9:1 which isn't entirely bad, it makes everything easier, such as: cooling, preventing detonation and boosting.