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I hope that this is in the right section......

I want to start prepping for FI in the near future ( less than 6 months) Still using the M20 engine, I want to do the 2.7i conversion first. Does anyone know what the compression will be if I use an B27 short block with the B25 head and related parts. Reading from the strictlyeta website , all that was said was the compression would be low. How low??

Exactly how low is hard to say. I do know theres a guy on an e30 forum with a standard eta block and pistons with a standard 325i head thats running about 20 psi of boost. You would probably be around 8 or 7.5 to 1. Apparently, off boost driveability isn't a concern. I would do it if I had the parts sitting around ready to go.

Thank you Dan.

Are the stock 2.7L pistons beefy enough to mill them down to 8:1 CR?

Alternatively, is there an "off the shelf" thicker than stock headgasket to get close to this compression ratio?

Thanks!

11-12-1-722-735 is .3mm thicker, but I dont know if you'd need it. I wouldn't mill the pistons. Anything off the top weakens the upper ring land. Just bolting on a 325i head with a stock thickness headgasket would probably get you as low as you need to go.

I've done some searching around various BMW forums and utlizing a 885 head on a 87 and older 2.7L eta should yield a 7.9:1 CR. Perfect for what I want to do! Anybody have a different perspective on this?

Thanks,
Chris H.

I'd go with it. Thats a wee bit low, but I dont think you'll have any problems with it. Boost the hell out of it.

Can I ask how you came up with 7.9 to 1?

this doesn't answer your question, but may be helpful in coming to a conclusion.

this fella has done a fair amount of research on the topic of M20 compression.

http://www.bmwccn.no/rogaland/Mahle_m20.html

He's not quite right on super e pistons. I dont know about his numbers, but the diagram is way off. They are pretty much 2.5i pistons with a deeper dish and different dimensions for the 81mm stroke. They do slightly protrude above the deck at TDC, unlike standard e pistons. They do not have a symmetrical bowl as the diagram shows.

weird. i wonder how he came up with that stuff.

goes to show you that you can't believe everything you read on the web. i'd have figured that stuff was from some factory manual or something.

given the title of the page (Mahle Pistons or whatever), might all of these designs be aftermarket parts with slightly different characteristics?

It seems odd. I wonder if he just got the wrong info. Everything else looks correct, though I cant confirm any of the measurements he has for the various pistons. I dont recognize what he has pictured for the super e. Its nothing I've ever seen in an M20.

I'd go with it. Thats a wee bit low, but I dont think you'll have any problems with it. Boost the hell out of it.

Can I ask how you came up with 7.9 to 1?

Im actually happy with that CR being this is the first turbo motor I will be building. It will provide an extra margin of error against detonation with a slight loss of off boost power.

I didn't come up with the 7.9:1 CR. Several people from other different BMW forums seem to agree on this one. At first it was going to be between 7.5-8:1 was the answer. Then it was wittled down to the 7.9:1 from someone who measured the 885 head volume and eta piston volume, etc., etc.

Cool. I know theres one guy on e30tech that has/had a 2.5i head on a standard 2.7e block. He was running something obscene like 19 psi. Sounded like a good plan to me.