Determining Compression Ratio

[LingLing]

I know, I know. Subaru tech on a Mitsubishi site, but the math is all the same.

Since I didn't put the motor together and it's a hodge-podge of various oem parts I don't know the exact CR. So I did a bunch of looking and here's what I came up with.

-EJ22T block (Stock bore)
-Stock EJ22T pistons
-Stock STi rods
-Stock EJ22T crank
-EJ25 heads
-OEM Subaru headgasket

Using these numbers..

Cylinder Bore: 96.9mm
Stroke: 75mm
Head C.C. Volume: 47cc (I couldn't find a definitive answer, since I don't know what year heads are being used. I tried to choose something realistic)
Headgasket Bore: 97.5
Headgasket Thickness: .59mm (Assuming they used an oem EJ22T HG)
Piston Dish Volume: 28cc
Piston Deck Height: 0.016" (Again, I chose to use this because I couldn't find a definitive number)

I get a CR of 7.98:1

Now, what's tough is I don't know if they used a head gasket from an EJ22T or an EJ25. Difference in bore makes is approx. 2.5mm-3.5mm. Enough to skew the result.

[LingLing]

Shane,

In regards to our conversation yesterday. That's as close of a number as I can come up with. I've been trying to confirm the combustion chamber volume of the EJ22T and an N/A EJ25. This is the last number I am really unsure about.

[Shane@DBPerformance]

Do you know what the bore size of you current motor is version the bore size of the engine that those heads came off of? When people do the 2.0l WRX heads on a 2.5l turbo block, there are problems then just the raised compression ratio that happens if you keep the stock 2.5l pistons and run a stock thickness headgasket.

I don't know if there are any problems with your 2.2l combo though. Here is some info on the 2.0l heads on a 2.5l and service to fix it. http://forums.nasioc.com/forums/show....php?t=2011458

[LingLing]

I've seen two answers related to my setup.

1) Compression stays the same since the combustion chamber volume is the same.

2) Compression is lowered because DOHC EJ25 heads have a larger combustion chamber volume than the EJ22T.

Maybe I'll just go out and buy a few Haynes manuals today. IIRC they include all specs, such as this.