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 Post subject: Need help with figuring compression ratio
PostPosted: Thu Jan 25, 2018 8:25 pm 
First Gear
Joined: Fri Mar 25, 2016 2:48 pm
Posts: 191
Location: Beautiful Downtown Goode, VA
Hi guys, so it is beginning to look like WRXish might be getting its 3rd engine in less than 18 months...I have a couple of options for how to approach it. I have a nice set of EJ25D heads that I definitely want to reuse on whatever engine gets built. So now the only thing left to decide is what to use for the short block. I have the original ej22e that came out of the car that is really tired and needs to be gone thru...thinking about having it taken to one size oversize on pistons and putting new pistons in it and calling it a day, with a new oil pump and new bearings. Probably be a fairly cheap way to get a decent bottom end. The other option is to buy a 2.5 liter STI short block from subaru and put my heads on it and drive on down the road. The question that occurs to me with these options is what kind of compression ratio will I get? I would prefer to stay with something that will run on regular fuel particularly since fuel prices are rising again and all the other cars I own take premium :( . Advice or suggestions gladly taken from you guys who know way more about this stuff than me...please speak up. Remember, I am going to remain NA going forward...no turbo for me.


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 Post subject: Re: Need help with figuring compression ratio
PostPosted: Fri Jan 26, 2018 7:56 pm 
First Gear
Joined: Fri Mar 25, 2016 2:48 pm
Posts: 191
Location: Beautiful Downtown Goode, VA
can someone please weigh in on this...approximate comp ratio would help...especially using the sti short block and ej 25d heads from a 98 engine...I have no idea what that combo would give me compression wise, but certainly it should be a pretty bulletproof engine in NA form. Can someone please help me out?


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 Post subject: Re: Need help with figuring compression ratio
PostPosted: Fri Jan 26, 2018 9:07 pm 
Vendor
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Joined: Thu Dec 11, 2003 11:34 am
Posts: 2479
Location: Missoula
Using this calculator

http://www.csgnetwork.com/compcalc.html

and the following numbers

bore 99.5
stroke 79
HG dia 101
HG thickness .56
head 47cc
piston dish 22cc
deck clearance .3

I get 9.1:1

I'm not sure on deck clearance and have heard 46, 47, and 50cc for the heads. Headgaskets are the thin 11044AA642 for 04-06 sti.

Instead of spending money on a new sti block, you should be able to use any n/a 2.5 with the 610 gaskets and wind up with a 9.7 (phase 1) or 10:1 (00+) CR.

_________________
peter@functionauto.com


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 Post subject: Re: Need help with figuring compression ratio
PostPosted: Fri Jan 26, 2018 11:59 pm 
First Gear
Joined: Fri Mar 25, 2016 2:48 pm
Posts: 191
Location: Beautiful Downtown Goode, VA
Many thanks for that Peter...I am assuming that since 10:1 was stock for a regular gas engine that should work just fine. I was afraid I would end up with a much lower than 9:1 compression with the sti block. That actually surprises me that it is that high. Anyway, those are good numbers to work with. Thank you for your efforts. I appreciate it very much. :)


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