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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Correcting 9.5:1 back to stock CR.



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      09-10-2020, 10:21 PM   #1
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Correcting 9.5:1 back to stock CR.

So, If I picked up a set of JE pistons with the 9.5:1 compression, you can correct CR back to 10.2:1 when you have your head decked, right?

I just don't want to pay for the CP pistons at $1100+ just because they are set to 10.2:1.

I mean I'm going to get the head worked on anyways.

Pros, cons, remedies?
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      09-11-2020, 11:49 AM   #2
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You are going to have to take off a lot of material to go from 9.5 to 10.2 cr. I'm not sure, but you might also run into valve to piston clearance issues. Might weaken the deck surface..depending on how much actually needs to come off.

I'm sure there are some guys on here with much more experience on building N54's that will chime in.....but to me, it sounds like the right fix is to buy the correct pistons.

Or...why not just use the 9.5 pistons? For a boosted motor, most aftermarket guys run less compression when they start going crazy with the boost anyways....
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      09-11-2020, 01:41 PM   #3
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I just ordered a JE pistons kit with STD bore and 9.5:1 for $700(found after I posted this). Deal was too good to pass up. At $400 less than anything with 10.2:1 and wont require a cyl bore out, I'll just map my tune around the slight differences going to 9.5cr.
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      09-11-2020, 01:53 PM   #4
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Quote:
Originally Posted by KnighTT View Post
I just ordered a JE pistons kit with STD bore and 9.5:1 for $700(found after I posted this). Deal was too good to pass up. At $400 less than anything with 10.2:1 and wont require a cyl bore out, I'll just map my tune around the slight differences going to 9.5cr.
Lower CR in a boosted motor is a good thing.....
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      09-11-2020, 02:20 PM   #5
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I found it pretty interesting that JE only sells 9.5cr off the shelf. If you want stock compression, it's a special order. That would lead me to believe there was much less of a demand for stock CR when buying forged pistons.

Anyway, too hard to pass up. I am already sending back the piston rings I ordered a few weeks ago and those were $165. So these pistons really only set me back around $550 as they come with rings.

Molnar rods on the way to support them.
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      09-11-2020, 02:31 PM   #6
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Quote:
Originally Posted by iqraceworks View Post
Lower CR in a boosted motor is a good thing.....
It is... but with new fuels and technology running higher compression is ideal.
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      09-14-2020, 08:23 PM   #7
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Quote:
Originally Posted by FEDE/E92 View Post
It is... but with new fuels and technology running higher compression is ideal.

I wouldn't say it's ideal except for emissions. Even with DI, for knock resistance, the 9.5:1 will be much more tolerant of a bad tank of gas, will allow you to run more boost and timing, and will spool faster, contrary to popular belief.
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      09-15-2020, 09:44 AM   #8
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Ok, that is good to know.

Thanks.
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