Setnev Posted June 16, 2019 Share Posted June 16, 2019 I have a set of B25 STI heads and I'm planning on putting them on my 07 XT. I know the 07-09 EJ255 has a deeper dish piston than the 05-06 but have a smaller combustion chamber. I think the calculated compression ratio is about 8.11:1 with that combination. A couple of questions... 1.) Can anyone share their experiences with this combo? 2.) Can I run the base stock tune with this combo until I get it tuned for the lower compression? If so, what can I expect in terms of performance, or lack there of? Link to comment Share on other sites More sharing options...
Outblack Posted June 17, 2019 Share Posted June 17, 2019 You’ll end up with 8.7-8.8:1 cr It needs to be tuned, you’ll have a more responsive car off boost but on pump gas you’ll most likely be tuned for less boost Link to comment Share on other sites More sharing options...
Setnev Posted June 18, 2019 Author Share Posted June 18, 2019 (edited) You’ll end up with 8.7-8.8:1 cr It needs to be tuned, you’ll have a more responsive car off boost but on pump gas you’ll most likely be tuned for less boost I found this: https://imgur.com/a/yHDgTk0 The 07 has the deeper dish pistons and has D25 heads like the 08+ WRX. According to this, I'll be dropping compression. Edited June 18, 2019 by Setnev Link to comment Share on other sites More sharing options...
Outblack Posted June 18, 2019 Share Posted June 18, 2019 Ahh ya I read that wrong Link to comment Share on other sites More sharing options...
Gabo Posted June 18, 2019 Share Posted June 18, 2019 You’ll end up with 8.7-8.8:1 cr It needs to be tuned, you’ll have a more responsive car off boost but on pump gas you’ll most likely be tuned for less boost My current motor is a 9.0:1 compression motor, flex fuel tuned. You'll be limited in how aggressive your tune can be (ignition wise) on Cali piss 91 octane but that's about it. I'm running almost identical boost levels in 91 and E85 and by similar I mean maybe a 2-3 psi difference if that. My high boost setting fyi is 25 psi. A higher compression motor will most definitely be more responsive off boost. Sometimes I make it all the way to work without hitting any boost Link to comment Share on other sites More sharing options...
Setnev Posted June 22, 2019 Author Share Posted June 22, 2019 I hate that I only have access to 91 here in Northern AZ. E85 is available in AZ, but I have to drive 2 hours to get it, IE not worth it. Maybe I'll just save my heads for when I move to Washington. They've got E85 everywhere up there. Link to comment Share on other sites More sharing options...
Gabo Posted June 25, 2019 Share Posted June 25, 2019 I hate that I only have access to 91 here in Northern AZ. E85 is available in AZ, but I have to drive 2 hours to get it, IE not worth it. Maybe I'll just save my heads for when I move to Washington. They've got E85 everywhere up there. I guess in this regard CA has an advantage. E85 is becoming more common every year. I have at least three stations on my way home and that's because I haven't bothered to look for more. Link to comment Share on other sites More sharing options...
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