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Building my EVO IX Engine, what compression should I use?

Old Dec 6, 2018, 08:22 AM
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Building my EVO IX Engine, what compression should I use?

Hey guys, just pulled my motor and am having it built. I am wondering if I should run 9.0:1 or bump it up to 9.5:1?

It's a street car & 93 octane only. I have a FP BB Red, full bolt-ons, AMS VSR, AMS FMIC, Ported TB, and GSC S2 Cams.

Would there be any benefit to go to 9.5:1 or should I keep it simply at 9.0:1

Thanks in advance.
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Old Dec 6, 2018, 08:55 AM
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My old Curt Brown engine was 2.0L 10:1 running 93 octane and ~33-34psi daily (~65K miles). New Curt Brown engine is 9.5:1 2.15L and will run the same boost. I would recommend you run the 9.5:1, but that is just me based on my experience. As long as you run a conservative timing curve and have the knock tables set aggressively to pull timing at the first sign of knock you should be ok.
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Old Dec 6, 2018, 10:23 AM
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Leave it at 9:1. That 1/2 point of compression isn't worth anything compared to running more boost or timing.
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Old Dec 6, 2018, 12:13 PM
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If you're just going to run gas and no ethanol you'll always get more power out of lower compression with more boost than higher compression and less boost. Keep it 9:1.
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Old Dec 6, 2018, 12:16 PM
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Originally Posted by ayoustin View Post
If you're just going to run gas and no ethanol you'll always get more power out of lower compression with more boost than higher compression and less boost. Keep it 9:1.
I've heard the opposite until detonation manifests. I believe Buschur Blocks are all 10:1.1 unless specified otherwise. I have a choice now of 9.0:1 or 9.5:1 and I'm currently torn.
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Old Dec 6, 2018, 12:44 PM
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Originally Posted by High_PSI View Post
I've heard the opposite until detonation manifests. I believe Buschur Blocks are all 10:1.1 unless specified otherwise. I have a choice now of 9.0:1 or 9.5:1 and I'm currently torn.
You've heard incorrectly. Buschur gets away with 10:1 by running ridiculously low timing which isn't necessarily good. Pump gas = 9:1 comp

A FULL point of compression I nets about 3% more power on a kill mode tune that has the fuel/octane to deal with it. So, for arguments sake, 1/2 a point would be 1.5%. Well, pump gas never gets a kill mode tune, it knocks first. And the higher compression will only make it knock sooner.

Originally Posted by ayoustin View Post
If you're just going to run gas and no ethanol you'll always get more power out of lower compression with more boost than higher compression and less boost. Keep it 9:1.
This

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Old Dec 6, 2018, 12:47 PM
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You'll make more power (boost for boost), have better engine response and turbo spool with a higher compression setup. The 1/2 a point in compression isn't going to allow you to run more timing and boost of any significance. Heat is the killer so make sure you have a good intercooler core to support higher boost levels.
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Old Dec 6, 2018, 01:21 PM
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Stock compression is 8.8:1. Unless you think a stock engine is laggy and drives like crap off boost, stay with 9:1. Nothing is more of a buzzkill than being stuck with lower power because you bumped compression too high and now you're knock limited.

Higher compression = more likely to knock. Lower compression is less efficient. A half point is not going to be noticed by even the most attentive driver.
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Old Dec 6, 2018, 06:17 PM
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See "John Bradley"'s response...

9 to 1

https://www.evolutionm.net/forums/ev...77-10-1-a.html
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Old Dec 6, 2018, 06:58 PM
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Thank you all for posting. Ive decided not to rock the boat and go with 9.0:1.


Thanks again!
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Old Dec 10, 2018, 09:13 AM
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Originally Posted by hutch959 View Post
See "John Bradley"'s response...

9 to 1

https://www.evolutionm.net/forums/ev...77-10-1-a.html
Awesome find.
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Old Dec 10, 2018, 01:45 PM
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The discussion was anecdotal showing a dyno graph of a 9:1 setup and then imposing boost limitations of ~7-8psi lower assuming a 10:1 to make theoretical comparisons all while using crappy 91-92 octane gas. Like I previously stated, I ran 33-34psi daily for ~65K miles on 93pump with 10:1 compression using both a GTX3576R and GTX3582R. The difference going from stock 8.8:1 to 10:1 was significant enough to cause the turbo to surge in 3rd gear due to the increased spool because of the higher compression. I posted about this many years back and you can see max boost on this dyno run was 32.8psi, but plenty of street logs showing 33.9psi even in the middle of summer.

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Old Dec 10, 2018, 02:21 PM
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Originally Posted by 240Z TwinTurbo View Post
The discussion was anecdotal showing a dyno graph of a 9:1 setup and then imposing boost limitations of ~7-8psi lower assuming a 10:1 to make theoretical comparisons all while using crappy 91-92 octane gas. Like I previously stated, I ran 33-34psi daily for ~65K miles on 93pump with 10:1 compression using both a GTX3576R and GTX3582R. The difference going from stock 8.8:1 to 10:1 was significant enough to cause the turbo to surge in 3rd gear due to the increased spool because of the higher compression. I posted about this many years back and you can see max boost on this dyno run was 32.8psi, but plenty of street logs showing 33.9psi even in the middle of summer.

Yeah. Aaron at ER really just has no clue then I guess...?
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Old Dec 10, 2018, 07:30 PM
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Originally Posted by letsgetthisdone View Post
Yeah. Aaron at ER really just has no clue then I guess...?
Did I incorrectly characterize the data presented in the post you referenced? No need to choose data collected from a 9:1 motor to theorize the performance of a 10:1 motor when data from an actual 10:1 setup is presented.
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Old Dec 11, 2018, 10:55 AM
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Originally Posted by 240Z TwinTurbo View Post


Did I incorrectly characterize the data presented in the post you referenced? No need to choose data collected from a 9:1 motor to theorize the performance of a 10:1 motor when data from an actual 10:1 setup is presented.
I must be missing something, but I don't see where you made 648whp (What Aaron made on 92). All I see is 573, on 110 octane.
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