New to e30s. Have an extra engine. Need help!

[Blaise135]

Hello, I recently purchased a 1986 325e that used to be used as a drift car. Had a giant turbo sticking out of the hood, has a welded rear diff, racing seat, stripped interior etc etc. however the previous owner blew up the turbo and returned the 325e motor to stock. It still has all of the oil lines for the turbo and what not and he gave me the stand alone ecu for it if I decide to turbo it again. Here is where my question begins. He also gave me an extra engine. Upon inspection I believe the extra engine to be a 2.5i engine (serial number on the head is 1705885, serial number on the block is 22426703, casting mark on intake side of head has a 2.5 over 2.7 mark.) I have it completely apart and was trying to decide the best route for modification of the e30. Do I put the 2.5i head on the 2.7e block? Or should I just completely build the 2.5i engine with forged internals and boost it? ignore the 2.5 and just throw another turbo on the 2.7?

[Andy.B]

I am by no means a turbo guru, but here is the quick breakdown until someone much more knowledgeable than me pipes up:

2.5i: good compression, big valves, higher redline, dual valve springs
2.7e: lower compression, smaller valves, low redline, single valve springs
2.7i (2.7 eta bottom end with 2.5i top end): lowest compression, the benefits of larger displacement and more low end torque AND all the benefits of the 2.5i's bigger valves and higher redline).

If you're trying to do a quick easy build, you can swap the 885 (2.5i) head and intake onto the 2.7 bottom end and have a nice, torquey, high revving, low compression engine that responds very well to forced induction.

[4x4_e30]

I am by no means a turbo guru, but here is the quick breakdown until someone much more knowledgeable than me pipes up:

2.5i: good compression, big valves, higher redline, dual valve springs
2.7e: lower compression, smaller valves, low redline, single valve springs
2.7i (2.7 eta bottom end with 2.5i top end): lowest compression, the benefits of larger displacement and more low end torque AND all the benefits of the 2.5i's bigger valves and higher redline).

If you're trying to do a quick easy build, you can swap the 885 (2.5i) head and intake onto the 2.7 bottom end and have a nice, torquey, high revving, low compression engine that responds very well to forced induction.

By low compression, do you mean 9:1?

Shaven 885 Head on ETA Bottom = 9:1 C/R

http://www.e30zone.net/e30zonewiki/i...Building_a_2.7

Use the standard Eta bottom end with a 325i head, the 885 unit. The 325i has strange pistons with a deep dish but a raised section at the front of the combustion chamber to accommodate it. Despite what experts say, the 325i head will NOT give a horribly low compression ratio. The difference in chamber volume is quite small. The 325i head has a chamber volume of @ 42.5cc and the 731 and 200 heads have a chamber volume of @38cc. It all depends on whether the head has been skimmed. But 4.5cc difference isn't much at all and much of the increased size of the 325i chamber is made up for by the bigger valves and surrounding raised areas around them. 325i valve sizes are 42mm inlet and 36mm exhaust.
A lot has been said about Eta pistons being incompatible with the 325i head. Most of it is nonsense as this has been done many times and it works okay. Of course it won't give results quite as good as using the matching 325i pistons but it still has a good compression ratio which is most of the battle won.

[Andy.B]

Hey, I never claimed to be an expert.

I should have taken the time to look up numbers.

However, the dished eta pistons vs the domed i pistons make a difference when factoring compression, contrary to what is said in the first link.

And the second link is a UK website, and AFAIK, the euro engines are different from the us engines.


I did learn something new though! I guess my budget 2.7i is a better combo than I thought...

That said, naturally aspirated, a budget stroker dyno sheets show it to be slightly less powerful than a stock 2.5i. If the compression difference is negligible, is the power loss due to the poorer squish provides by the dished pistons?

[Blaise135]

New to e30s. Have an extra engine. Need help!

So that last statement says that mix matching the 2.5i head and 2.7 block won't be as efficient as just using the 2.5i engine? So should I just build the 2.5 then or is there more potential with the mix matching of the two. I'm only looking to get around 300 hp and want to do it as easy and cheap as possible, whether that be forced induction or n/a. I'm also interested in itb's but they're so damn expensive

[Andy.B]

I think that without a huge amount of money, 300hp is going to be hard to attain with an m20. I know the eta bottom/I top with a turbo is a pretty popular and proven setup.

[4x4_e30]

However, the dished eta pistons vs the domed i pistons make a difference when factoring compression, contrary to what is said in the first link.

That said, naturally aspirated, a budget stroker dyno sheets show it to be slightly less powerful than a stock 2.5i. If the compression difference is negligible, is the power loss due to the poorer squish provides by the dished pistons?

I really don't want to explain this again. But what the hell.

When factoring the compression ratio, the stock ETA has "dished" pistons with a combustion chamber volume of 38cc and equals 9:1.

If I take that same exact setup with the dished pistons, same stroke, same height and put a head on it that has 37cc combustion chambers, how does the compression ratio magically have anything to do with the "i" pistons that were never included in any of the calculations, baseline or modified?

This is such a simple concept. And besides, side by side the "i" pistons are actually more dished out than the ETA.

Compression ratio: swept volume:combustion chamber volume + piston dish volume + gasket volume
Swept volume is the same, combustion chamber volume is less + piston dish volume is the same + gasket is the same

Also, how many dyno sheets of 2.7l strokers that had the head milled down .020"? I can't find any.

[Blaise135]

Looks like I'll be running the 2.5i engine as a whole itself. Already has forged rods in it also. Thanks for all of the info guys.

[Andy.B]

OK, sorry to clutter up your thread op.

I understand that two didn't heads with the same combustion chamber volume don't produce different compression ratios. I simply wanted to clarify that when factoring compression ratio, piston volume does matter.

And most people who build quick and easy 2.7i motors don't machine several thousandths off the head first. While I haven't measured it myself, I have a hard time believing that everyone has been wrong all this time, and a stock 885 head on a stock 2.7 bottom end had the same compression as a stock 2.7 eta. In fact, just looking at the two engines heads/pistons side by side tells you that is false.

I just want to avoid confusion for others. A budget stroker is a proven, easy to assemble, lower compression engine that responds well to boost. As soon as you start machining, mixing, and matching other parts, you need to measure and factor everything. Carefully.

[whitebulat22]

Here's the kind of build that gets an NA M20 to 300 HP (pump gas):

http://www.racehead.com.au/?p=657

I don't think I would wanna street drive that whole deal. If you're looking for 300 hp boost will be a lot easier and the stock bottom end will most likely be fine with that. The stock crank would not want the revs needed to get that kind of power NA.