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Old 09-16-2005, 04:26 PM   #1
G.Subramaniam
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Default Upping CR in a RS with EJ25 heads

Per recent research by Matt Monson, the thinnest gasket
for a SOHC EJ25 is 0.020 inch, which is the stock gasket

The stock 0.020 inch gaskets give a 10:1 CR
with EJ25 SOHC heads

Retaining the stock heads, the only way to up CR cheaply is to mill the heads
How much can the heads be milled without interference problems and / or affecting cam timing

The following post in NASIOC says that bumping CR by milling heads or adjusting gasket thickness risks valve interference and valve timing

http://forums.nasioc.com/forums/show...9&page=2&pp=25

06-25-2000, 04:03 PM #41
RalliSpec
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'Changing compression ratios by shaving heads or using thinner gaskets are not recommended. You will not get any significant change in CR. Thinner gaskets are less reliable. Shaved heads still risk valve interference and also compromise valve timing as Joel mentioned"
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Old 09-16-2005, 05:30 PM   #2
Matt Monson
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That is the same thing that Quirt and Bill told me. In light of this, I would say that what you really want to do is change the pistons. While it will be more expensive than just milling the heads, it will be more reliable as well. Depending on the mileage of the engine, you could potentially change the pistons without cracking the case. But really to do it right, you should take the block apart and hone the cylinders and then put the pistons in with fresh rings.

I am going to maybe take my chances in the near future. I have 25k on the block that is in my car. Cylinder wear can't be too great on that, and I may just throw some pistons in and see what happens. Just be aware that I have spare engines and don't really care if I break something. If your car is your daily driver, you would be better served being more conservative than I...
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Old 09-16-2005, 05:39 PM   #3
G.Subramaniam
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Default

Does this mean that you no longer recommend the Frankenscoobie approach ? of EJ25 shortblock with EJ22 heads ?


----
But really to do it right, you should take the block apart and hone the cylinders and then put the pistons in with fresh rings.
----
This in a shop will run about $2000 in labor and $500 for pistons
which runs very close to getting a new STI shortblock with 11:1 pistons
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Old 09-16-2005, 06:50 PM   #4
Burnout
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I've had no probles so far with my heads milled .010". Not that many miles on it yet about 500, but i will be taking it on a little trip this weekend to see how it does before i drive back to NC 2200mile.
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Old 09-16-2005, 08:44 PM   #5
Boom05rs25
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Quote:
Originally Posted by G.Subramaniam
Does this mean that you no longer recommend the Frankenscoobie approach ? of EJ25 shortblock with EJ22 heads ?


----
But really to do it right, you should take the block apart and hone the cylinders and then put the pistons in with fresh rings.
----
This in a shop will run about $2000 in labor and $500 for pistons
which runs very close to getting a new STI shortblock with 11:1 pistons
2000 dollars 11:1 in sti block If they truelly have 11:1 piston it is no wonder they have killer pinging problems...You sure there 11:1??????
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Old 09-16-2005, 11:12 PM   #6
G.Subramaniam
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The stock turbo STI has 8.2:1 pistons
I was talking about a NA STI with custom 11:1 pistons
NA cars can run 11:1 with 91 octane
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Old 09-22-2005, 03:41 PM   #7
CirrusWRX
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Quote:
Originally Posted by G.Subramaniam
Does this mean that you no longer recommend the Frankenscoobie approach ? of EJ25 shortblock with EJ22 heads ?
I'm not going to answer for him, but I don't think that's what he's saying because that's not what you were asking.
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Old 09-22-2005, 03:47 PM   #8
012.5RS
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could always throw a turbo on it... that will up the cr.
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Old 09-22-2005, 04:35 PM   #9
Matt Monson
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Quote:
Originally Posted by CirrusWRX
I'm not going to answer for him, but I don't think that's what he's saying because that's not what you were asking.
You can speak for me on this one, since you understood what I said.

To reiterate, it has nothing to do with the idea of changing the CR. What it has do to with is changing the overall dimensions of the deck height, HG and heads themselves because it alters the timing. Ergo, one should change CR using internal variables like piston dish and cylinder head volume. The Ej25 bottom end with Ej22 heads does this by
using a cylinder head with 7cc less volume than the Ej25 one...
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