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Old 01-07-2009, 07:54 PM   #1
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Default increasing compression

I have decided to increase my static comp. from the usual 8.8:1 weiscos to roughly 9.5:1..yes the weiscos that FM sell are actually lower then advertised"no big deal"
I have done some research on this matter,not inc already haveing been around alot of 10:1 -11:1 turbo motors but these have all been on race fuel "c16"
As some of you may know I have been using 93oct with 50/50 water/meth, and have had very strong and consistant results for the last 3 years.
I normally dont share to much R&D during my build but thought this may be interesting to some of you..
I am doing this not so much for max power, but rather trying to get more off boost power and better spool response
I have witnessed some very strong results with comp. increase on hondas and the evo 8's which btw is very similar cumbustion chambers size, piston size etc
I also have seen good #s from ls3 and ls7 with very high comp. and they have run over 10psi with no problems, to name a few more.
Your opinions and facts would be appreciated on this matter as I will post final dyno#s and other data logs on this site..
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Old 01-07-2009, 08:05 PM   #2
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We had a huge discussion on this a while back. Huge debate of what's really happening when changing compresion ratios. What leads you to believe that increasing compression will increase spool? If you're still gonna run a bunch of boost like before I wouldn't get in too big of a hurry to get the compression up.

Nitrous helps spool and off boost power too.
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Old 01-07-2009, 10:44 PM   #3
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Recently an Evo specific tune/build shop underwent some testing with compression ranging from low 8s to low 10s and they found major spool increase..A couple of other well known tuner/builders chimed in and said they had the same results..
Typically they saw a 300rpm earlier spool from the factory 8.8 to an extra 1 point in compression with no other changes and an added benefit of 20whp across the board.
I spoke to someone that worked on the earlier eclipse's with Shepard racing and the more comp. they ran the faster the car went in every aspect of the track.
So I did enough homework to convince myself to undergo this on our own miata..
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Old 01-07-2009, 10:56 PM   #4
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Hmm, well, I've seen cars drop compression and gain spool too. Anyways, sounds like you've already made up your mind. How much boost are you planning to run at this new higher compression? If you have a tuner that can dial in a high comp boosted engine it should be fine. But as you know it will be less forgiving.

I think it goes without saying that for the average, turbo DIY'er, a low com, high boost motor is preferred as it's more forgiving to a less than perfect tune.
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Old 01-07-2009, 11:12 PM   #5
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I agree.Lets say I'm cautiously optimistic
My low boost is set at 21psi 1st/2nd gear and high is 32-34psi 3rd/4th gear,,
I will not touch anything on the tune other then remove 1-2 dgrees of timing above 14psi
Worse case scenerio..remove pistons after dyno pulls..lol
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Old 01-07-2009, 11:48 PM   #6
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You can always JBWeld some quarters to the tops of your pistons.
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Old 01-08-2009, 09:44 AM   #7
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You can always JBWeld some quarters to the tops of your pistons.
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Old 01-08-2009, 09:55 AM   #8
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Quote:
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Recently an Evo specific tune/build shop underwent some testing
Wait, the shop underwent testing?

They conducted testing, the motors underwent the testing.
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Old 01-08-2009, 01:55 PM   #9
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Wait, the shop underwent testing?

They conducted testing, the motors underwent the testing.
shop underwent some test on "THERE EVOS"...hahaha
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Old 01-08-2009, 02:12 PM   #10
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Ok, I'm still a newb here, but as far as I can tell, the compromises you make to a decent turbo set up in order to run higher compression outweigh the slight benefit of quicker spool. The "20hp across the board" is secondary, as on a FI car there are plenty of quicker ways to pick up that 20hp, like getting a larger trim snail and sacrificing 300rpm of spool for example.

To me, if spool is so important, it's much easier to build a regular setup on a low compression motor with the initial turbocharger matching your needs. But then again, I've turboed 0 EVOs.
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Old 01-08-2009, 02:18 PM   #11
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So go call wiesco and ask them to get you some 9.5:1 pistons....
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Old 01-08-2009, 02:29 PM   #12
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Originally Posted by Braineack View Post
So go call wiesco and ask them to get you some 9.5:1 pistons....
Why bother going to that expense?

MX-5 Miata Forum - Building high compression pistons



bwahahahahhaaahahahaaaaaaa
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Old 01-08-2009, 03:56 PM   #13
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If you where running 15psi I would do it. But 34psi. Hmmmmm. You are braver than me. Hope it works for YA!

Good Luck!
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Old 01-08-2009, 05:28 PM   #14
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9.5 shouldn't bee too drastic of a bump from where you are currently running. At least you know what your doing enough to find out safely what the limits are. Most people that come in here talking about running higher compression haven't done their research on the subject.
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Old 01-08-2009, 06:13 PM   #15
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FWIW, I had JE make some pistons for me. 9.5:1 040 oversize if memory serves. No extra charge.
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Old 01-08-2009, 08:05 PM   #16
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i run 8.6:1 pistons...and after the head deck and 1mm increase the car is insanely strong off boost. I was really worried about this with a big turbo, but its no biggie at all.
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Old 01-08-2009, 09:09 PM   #17
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Quote:
Originally Posted by gospeed81 View Post
Ok, I'm still a newb here, but as far as I can tell, the compromises you make to a decent turbo set up in order to run higher compression outweigh the slight benefit of quicker spool. The "20hp across the board" is secondary, as on a FI car there are plenty of quicker ways to pick up that 20hp, like getting a larger trim snail and sacrificing 300rpm of spool for example.

To me, if spool is so important, it's much easier to build a regular setup on a low compression motor with the initial turbocharger matching your needs. But then again, I've turboed 0 EVOs.
If it were that easy...Every time you change something on the turbo or the turbo itself its a trade off..I have tried many turbos and or combos and the current turbo has the best of everything. So I decided to see if there was more left in this particular combination and the next best place to try and find power everywere in the powerband was an increase in comp.
The next logical turbo up would have been an hta3582r but the loss in spool and transient response was not worth the increase in peak power.
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Old 01-08-2009, 09:10 PM   #18
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So go call wiesco and ask them to get you some 9.5:1 pistons....
they are doing a set for me now...
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Old 01-08-2009, 09:25 PM   #19
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Would you care to share how the 9.0:1 pistons are actually 8.8:1? Just curious how you came to this conclusion, I know once I bolt the head back on this engine I won't even think about them unless I have a problem.
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Old 01-08-2009, 10:45 PM   #20
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wiescos have a -4cc dome volume and the stock head is 52cc,head gasget is 0.8
my pistons are .005 in the hole w/84mm bore=8.8
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