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Old 06-01-2010, 01:50 PM   #1
audi5000
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Need a straight answer!

Okay so ive been looking into the stroker 325i engine all day.
i know this has been probable covered a million times but i just need a straight answer on this one question i have.
Its about the compression ratio with the 2.7 E block with I head.

here is says.
Quote:
get an Eta or S-Eta block and use the I head/ECU. This is a super easy swap. The compression on this setup is around 9:1 or so on the S-Eta block while the Eta block gets about 9.5:1. If you use I pistons on Eta rods the compression is really too low.
while here it says.
Quote:
so with the Super ETA bottom end what kind of CR would it have?
"8.5:1"

Which is true? i dont want to swap and have a low CR.
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Old 06-01-2010, 02:08 PM   #2
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Super eta bottom end > eta bottom end.
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Old 06-01-2010, 02:09 PM   #3
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I would say that you'd be looking at nearly 9:1.

With an 'e' bottom the compression will be higher than a stock 'i' and slightly lower if not equal to a stock 'e'.

Axe is right, the only difference will be the super e pistons.
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Old 06-01-2010, 02:14 PM   #4
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okay, so then i wont have ridiculously low compression?
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Old 06-01-2010, 02:35 PM   #5
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I'm not sure how correct this information is, but i found this:

Quote:
i pistons eta rods, eta crank, i head, decked block around 1.16mm-1.25mm good for at least 8.8:1 to 9.2:1

But you will need either an adjustable cam gear or a offset gear made by MM
http://www.e30tech.com/forum/showthr...t=48765&page=3

More Guesstimates on CR, i dont think anyone has actually measured it precisely, they are just guessing.
Quote:
"i" head on a regular eta bottom end should be good for 8.2 or 8.3:1, cant remember for sure which one.

"i" head on super eta bottom end is good for 8.5:1.

Standard "i" compression ratio is 8.8:1.

Standard eta compression ratio is 9.0:1.

Standard super eta compression ratio is 8.5:1.

Numbers get a bit mixed up in my head, but this should be accurate. I had this conversation the other day with a guy who wants to lend a hand building my engine, and we discussed the different compression ratios of stock and frankenstein motors a bit.
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Old 06-01-2010, 02:38 PM   #6
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see, everywhere ive read said you would run into issues with the I pistons.


Quote:
An 'i' head on a eta bottom end does not make a super eta, it makes a low compression (like 7.5:1 i think) 2.7i

then i read stuff like this

Quote:
The problem with putting a 325i head on a 325e block is the 325i (885) head has larger combustion chambers (but more efficent). Swapping it straight on will drop the CR a decent amount, so theres **** all power gain in reality, they cancel each other out. Using oem piston/rod combinations will leave you a few mm too short or too high, so you NEED custom pistons or rods to get a decent CR without destroying your valvetrain when you start it up...

there are some compression numbers in that link you left

"i" head on a regular eta bottom end should be good for 8.2 or 8.3:1, cant remember for sure which one.

"i" head on super eta bottom end is good for 8.5:1.

Standard "i" compression ratio is 8.8:1.

Standard eta compression ratio is 9.0:1.

Standard super eta compression ratio is 8.5:1.


those all sounds too low?
if i deck the head to what it said, would that help my CR?

Last edited by audi5000; 06-01-2010 at 02:43 PM.
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Old 06-01-2010, 02:45 PM   #7
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ill do some more reading maybe i can find a happy medium with around the stock eta compression.
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Old 06-01-2010, 02:52 PM   #8
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What are you planning to use?

Stock ETA 2.7L bottom end will give you low compression with an 'i' head. (~ 8.2 : 1)

You have 3 options to up compression:
1) Custom Pistons $$$ (Best Solution) - you choose what CR you want.
2) SETA Pistons
3) i Pistons, e rods. But you need to shave the block n get an adjustable cam gear.

Crz_Azn has done #3 i believe, but his dad is an engine builder and made his own cam gear, and did some other mods to the pistons i think.
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Old 06-01-2010, 02:55 PM   #9
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i was looking to get the Standard eta compression ratio is 9.0:1.

-im not interested in getting custom pistons.
-However, if i used the SETA pistons would would be my CR?
if its close to what i want ill use them.
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Old 06-01-2010, 03:21 PM   #10
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The confusion here is the head you are using.

The I head on the e bottom. You need to use the i pistons on the e rods and the i pistons need to be shaved. Or you need to use custom pistons $$$. or you need to find super eta pistons (rare). Those are the 3 normal options. An I head on an E bottom with nothing else done, will lower the compression which is fine if the car is going to be boosted.

When I built my 327i I used the e bottom and a modified e head, Modified to accept the 2.5 cam, double springs, this included drilling more oil journals.

When this modified e head is used... the stock 9:1 compression of the 'e' is retained, except the engine is running like an 'i' because the computer harness, intake and all associated parts are from the 'i'. This head would likely conflict with the I bottom.

It gets complicated but just look at everything and write it out on paper, makes sense eventually.

The absolute best NA is to stroke an I with the e crank, have the bottom bored out, special pistons made. Then tune it. These are 2.9 strokers.
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Old 06-01-2010, 04:49 PM   #11
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Quote:
Originally Posted by InfiniteDice View Post
The absolute best NA is to stroke an I with the e crank, have the bottom bored out, special pistons made. Then tune it. These are 2.9 strokers.
Technically the best NA is to use a s50(i think) crank, custom pistons, not sure what rod.

Similiar to the 3.1L stroker motors MM builds.
But we're talking $$$$$ when it gets to that.
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Old 06-01-2010, 04:58 PM   #12
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Or swap an s50 in and call it a day
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Old 06-01-2010, 04:59 PM   #13
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okay so i i put the i pistons and the e rods, how much would i have to shave the pistons? and where can that get done for a good price?

but lets say i did just throw the I head with a shrick 284 cam, and 19lb injectors on the Stock E bottom end.
would i gain or loose performance compared to keeping the I bottom end?
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Old 06-01-2010, 05:30 PM   #14
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If you are trying to do this on the cheap, then your answer is already here:

Quote:
i pistons eta rods, eta crank, i head, decked block around 1.16mm-1.25mm good for at least 8.8:1 to 9.2:1

But you will need either an adjustable cam gear or a offset gear made by MM
However, if you are a hands on type, then do what InfiniteDice did with modding an E head. 9.0 compression, no decking of the head or any funny stuff like that.

Quote:
Originally Posted by audi5000 View Post
okay so i i put the i pistons and the e rods, how much would i have to shave the pistons? and where can that get done for a good price?

but lets say i did just throw the I head with a shrick 284 cam, and 19lb injectors on the Stock E bottom end.
would i gain or loose performance compared to keeping the I bottom end?
Why are you shaving pistons? You can't shave it too much, or it gets weak. In any case with E rods, I don't see why you need to shave the pistons, regardless of crankshaft choice.

Sure you can do the Schrick cam, but its gonna be pretty low compression(as already mentioned a million times). Wilder cams are only advantageous with higher compression, otherwise it is a pretty inefficient part to throw in.
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Old 06-02-2010, 12:47 PM   #15
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Just to throw this out there.

a Super ETA (1988 325e) has 8.5:1 CR

It uses different pistons in the block but comes with an "I" head (885 casting). The only difference in the head are the valve springs (single instead of double) and the cams.
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