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Old 10-13-2014, 11:21 AM   #1
jt subie
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Default What pistons did you use?EJ255 on pump gas.

Wanted to see what pistons those of you with built EJ255's used running pump gas as well as your results. Did you do a rebuild with over sized pistons new short block with drop ins etc. What compression ratio did you go with. From what I've seen CP is the only one that offers forged pistons that retain stock compression for the EJ255 with stock heads. If you used EJ255/EJ257 8:2.1 pistons how much did the higher compression effect your tune?
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Old 10-13-2014, 04:12 PM   #2
theotherguy
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The only piston that I know of to keep stock ej255 CR would be the JE hybrid piston at -25.5 dish.
We run manley pistons at -22 dish on an '07 255 with D25 heads in our 2000 RS. I think it came out to around 8.7:1 comp.
I have some CP sc7422 and the sheet says -21 dish.
Some of the piston manufacturers, like Manley, show the compression ratio on 51cc combustion chambers and an 8.5:1 comp ratio, while CP uses 56cc combustion chambers and an 8.2:1 comp ratio. Have to do the math.
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Old 10-14-2014, 10:52 AM   #3
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Originally Posted by theotherguy View Post
The only piston that I know of to keep stock ej255 CR would be the JE hybrid piston at -25.5 dish.
We run manley pistons at -22 dish on an '07 255 with D25 heads in our 2000 RS. I think it came out to around 8.7:1 comp.
I have some CP sc7422 and the sheet says -21 dish.
Some of the piston manufacturers, like Manley, show the compression ratio on 51cc combustion chambers and an 8.5:1 comp ratio, while CP uses 56cc combustion chambers and an 8.2:1 comp ratio. Have to do the math.
Yup, this is the reason why it appears only CP has "stock compression" pistons. If you calculate the C/R manually with a 56cc combustion chamber using the dish volumes from Manley, you'll see that they are the same as CP.
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Old 10-14-2014, 11:17 AM   #4
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Yup, this is the reason why it appears only CP has "stock compression" pistons. If you calculate the C/R manually with a 56cc combustion chamber using the dish volumes from Manley, you'll see that they are the same as CP.
I have to admit I have no clue on how to do this math. My understanding is that with the pistons for EJ255/EJ257 the compression ratio advertised is what it would be for EJ257 heads and that with the EJ255 heads compression would be higher. For example 8:2.1 C/R on EJ257 heads would actually be around 8:7.1 with EJ255 heads. Basically I'm trying to see how those with 08-14 WRX's did with a slightly higher C/R how much boost they were able to run and how much power they were able to make on "pump gas"
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Old 10-14-2014, 12:38 PM   #5
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Originally Posted by jt subie View Post
For example 8:2.1 C/R on EJ257 heads would actually be around 8:7.1 with EJ255 heads.
what is the difference between the B25 heads that come with sti's vs D25 08+?
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Old 10-14-2014, 12:50 PM   #6
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what is the difference between the B25 heads that come with sti's vs D25 08+?
This is a area where I'm still trying to learn. However my understanding is the stock C/R for the EJ257 is 8:2.1 and for the EJ255 it's 8:5.1 but using pistons that retain the EJ257's stock C/R will still raise compression when using EJ255 heads.
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Old 10-14-2014, 07:48 PM   #7
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B25 have a 56cc combustion chamber, D25 have a 50cc combustion chamber.
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Old 10-15-2014, 02:55 AM   #8
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Manley pistons 99.5mm, stock compression, 35k miles daily driven at >500whp, zero oil consumption.
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Old 10-15-2014, 11:12 AM   #9
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Manley pistons 99.5mm, stock compression, 35k miles daily driven at >500whp, zero oil consumption.
Those are what I would like to use especially since they make "graded" pistons. But you have a EJ257. That's what I'm trying to figure out here. How much the higher compression on the EJ255 would effect the tune. Higher compression on pump gas is going to make the car more knock prone. Going from 8.5:1 to 8.7:1 doesn't seem like a big difference but I can't say for sure.
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Old 10-15-2014, 03:48 PM   #10
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stock compression, je fsr, dont order the size before you see what size you need to work with after the inspection of bores & hone, pick the ideal p/w clearance for the type of material the piston is made from, different material expands different size in heat, really do your research with that because if you just daily drive it, I wouldnt go too loose, it take forever to warm up everytime to good to go pulls, and if you go too tight of a gap for what your looking to do (example: you set it up for here and there pulls, medium to on the tighter side, and you go tracking full pulls in all gears for a 20-30 min road race session they will seize, if you need to go high like 100-100.5mm because of damaged cyl from stock bore size, id consider new case halves, its always nice to have as much wall as you can

btw, most of the time, dont go with drop in pistons unless you know that exact size of the bore of your new case halves or stock re-used block, 99.5 is stock yes but, alot of times the cyl comes a little ovaled, so the machinist will suggest to step up to 99.75 to give you that perfect p/w clearance your looking for


make sure bearing clearances are also just as perfect and consistent with each other for each style

Last edited by Irv Weissmanhowerton; 10-15-2014 at 03:54 PM.
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Old 10-15-2014, 08:03 PM   #11
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Originally Posted by Irv Weissmanhowerton View Post
stock compression, je fsr, dont order the size before you see what size you need to work with after the inspection of bores & hone, pick the ideal p/w clearance for the type of material the piston is made from, different material expands different size in heat, really do your research with that because if you just daily drive it, I wouldnt go too loose, it take forever to warm up everytime to good to go pulls, and if you go too tight of a gap for what your looking to do (example: you set it up for here and there pulls, medium to on the tighter side, and you go tracking full pulls in all gears for a 20-30 min road race session they will seize, if you need to go high like 100-100.5mm because of damaged cyl from stock bore size, id consider new case halves, its always nice to have as much wall as you can

btw, most of the time, dont go with drop in pistons unless you know that exact size of the bore of your new case halves or stock re-used block, 99.5 is stock yes but, alot of times the cyl comes a little ovaled, so the machinist will suggest to step up to 99.75 to give you that perfect p/w clearance your looking for


make sure bearing clearances are also just as perfect and consistent with each other for each style
Yep daily driver. It would likely be a new block and as you mentioned 99.75mm pistons. Doing this research early
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Old 10-16-2014, 08:05 AM   #12
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Quote:
Originally Posted by Irv Weissmanhowerton View Post
stock compression, je fsr, dont order the size before you see what size you need to work with after the inspection of bores & hone, pick the ideal p/w clearance for the type of material the piston is made from, different material expands different size in heat, really do your research with that because if you just daily drive it, I wouldnt go too loose, it take forever to warm up everytime to good to go pulls, and if you go too tight of a gap for what your looking to do (example: you set it up for here and there pulls, medium to on the tighter side, and you go tracking full pulls in all gears for a 20-30 min road race session they will seize, if you need to go high like 100-100.5mm because of damaged cyl from stock bore size, id consider new case halves, its always nice to have as much wall as you can

btw, most of the time, dont go with drop in pistons unless you know that exact size of the bore of your new case halves or stock re-used block, 99.5 is stock yes but, alot of times the cyl comes a little ovaled, so the machinist will suggest to step up to 99.75 to give you that perfect p/w clearance your looking for


make sure bearing clearances are also just as perfect and consistent with each other for each style
This is sound advice. The machine shop may tell you that all you need is a new cross hatch, and you can keep 99.5mm pistons (more cylinder wall, yay). I'm running. 003" ptwc on mine. No smoke ever on cold start, but I don't intend on riding it super hard for 30 minutes consecutively.

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Old 03-21-2017, 12:57 AM   #13
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reviving... I have a question .. Looking at the CP Pistions 99.75. They have 3 different kind of compression ratios.. 8:2:1..... 9:0:1.... and 10:0:1... Can someone explain what the differences mean and will do or not do
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Old 03-21-2017, 09:59 AM   #14
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reviving... I have a question .. Looking at the CP Pistions 99.75. They have 3 different kind of compression ratios.. 8:2:1..... 9:0:1.... and 10:0:1... Can someone explain what the differences mean and will do or not do
On 93 octane
8.2:1 - won't detonate
9.0:1 - probably won't detonate
10.0:1 - will detonate

moral - stay with stock compression ratio unless you are running E85 100% of the time
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Old 03-21-2017, 10:55 PM   #15
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On 93 octane
8.2:1 - won't detonate
9.0:1 - probably won't detonate
10.0:1 - will detonate

moral - stay with stock compression ratio unless you are running E85 100% of the time
wont detonate, like wont work fire correctly or will explode in there. Ill be running 91 oct and my stock block was honed n bored to 99.75..
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Old 03-22-2017, 10:18 AM   #16
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wont detonate, like wont work fire correctly or will explode in there. Ill be running 91 oct and my stock block was honed n bored to 99.75..

You want the low compression Pistons with 91pump.

(Detonation is when the air/fuel mixture in your cylinder burns to fast & kinda explodes in there-makes a huge pressure spike & hammers on the bearings & can crack Pistons, etc. bad things happen... The higher the compression, the faster it's all going to burn, with pump91, stick to lower compression, these engines don't seem to like higher compression like some other motors, such as a older Honda does)
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