Compression ratio for 2.5 shortblock swap, 11.5 or 10.3

ciper · Post by **ciper** » Wed Jun 18, 2008 5:16 am

I want advice. I'm thinking I should go for the 10.3 CR unless a thicker gasket is available. Advice?

I used the specs from http://wac.addr.com/auto/obs/turbo/ejcalcs.html with the calculator http://www.bgsoflex.com/cr.html -
* Cylinder head Volume (CC) = 40.00
* Piston Top Volume (CC) = 14.50
* Cylinder Bore (Inches) = 3.9
* Cylinder Stroke (Inches) = 3.11
* Deck Height (Inches) = 0.000
* Head Gasket Thickness (Inches) = 0.060
# Computed Compression Ratio is 10.2 to 1

If I use the SOHC gasket thickness of 0.023 # Computed Compression Ratio is 11.3 to 1

For fun I entered the stock EJ22 specs -
* Cylinder head Volume (CC) = 40.00
* Piston Top Volume (CC) = 14.50
* Cylinder Bore (Inches) = 3.8
* Cylinder Stroke (Inches) = 2.95
* Deck Height (Inches) = 0.000
* Head Gasket Thickness (Inches) = 0.060
Computation Results:
# Computed Compression Ratio is 9.4 to 1

Background - This is in my beater Legacy. I've driven it like I stole it every day for the last 130k miles. It runs perfectly, good compression, passes smog great but cylinder #3 is consuming oil at the rate of 1qt per 350 miles. I got the EJ25 for free since the spark plug broke and put a few dents in the piston top (easily removed with the dremel).

Same post can be found here http://forums.nasioc.com/forums/showthr ... p=22679362

ciper · Post by **ciper** » Wed Jun 18, 2008 7:19 am

I found a Subaru compression ratio calculator!
http://www.jonesboy.com/Media/Matts_CR_calc_v1_01.xls

Using it I get 10.33 for DOHC gasket and 11:5 for SOHC gasket

douglas vincent · Post by **douglas vincent** » Wed Jun 18, 2008 8:55 am

I would be tempted to do some minor porting of the heads and go for the high compression with the PP6.

Arctic Assassian · Post by **Arctic Assassian** » Wed Jun 18, 2008 4:40 pm

what kind of heads are you using?

ciper · Post by **ciper** » Wed Jun 18, 2008 6:28 pm

This is meant to be an inexpensive daily driver. I'm using the stock heads.

Both of those ratios seem too high for regular gas. Are those of you who did this swap using premium?

BTW using a piston volume of 21cc gets a 9.4:1 comp ratio. That not only happens to be the stock ratio but the EJ257 pistons are 21cc IIRC.

ciper · Post by **ciper** » Wed Jun 18, 2008 10:10 pm

A well respected VW builder/tuner made a suggestion today. If I use one DOHC gasket and one SOHC gasket together I get 9.6:1 compression... (assuming the resulting thickness is .083). He said they have built many turbo motors this way and would help me fix it if it leaks. The stock CR of the 90-96 2.2 is 9.4 where the 97+ 2.2 is 9.7! This is only .023 more than the DOHC gasket so would my manifolds fit properly?

Three SOHC gaskets gives me 10:1

I know the newest design of EJ25D gasket from Subaru is MLS, Do you know if the SOHC gasket is of similar material and could mate properly in this configuration?

ciper · Post by **ciper** » Thu Jun 19, 2008 2:56 am

Who has a 2.5 swap? What fuel are you running and what gasket did you use?

4subaru · Post by **4subaru** » Sun Jun 22, 2008 6:14 am

i got the 2.5 bottom end with my 90 2.2 top, i got some thicker gaskets from a company in Ohio, but they were made for the swap as well, they line the water jacket holes up better. I just ran 93 in it and it was fine. Only problem i had was the oil pump in the 2.5 wasn't up to pair and after 6 nights of racing on it, i blew a hole through the block

Arctic Assassian · Post by **Arctic Assassian** » Sun Jun 22, 2008 10:28 pm

You know cipher, I have a 257 block just sitting here, and an extra set of heads, and gaskets of varying thickness. I think I'm gonna start thick, see how it runs, and then put a thinner gasket in there.
10.4:1... This should be fun!

ciper · Post by **ciper** » Mon Jun 23, 2008 9:12 pm

Arctic Assassian: The 257/255 block has deep dish pistons (22CC) which will lower the compression to more normal levels. Which heads will you use and where did you get the varying thickness gaskets?

4subaru: What was the name of the place that made the gaskets and how much did you pay?

Arctic Assassian · Post by **Arctic Assassian** » Tue Jun 24, 2008 1:43 am

I've got SOHC 2.5 gaskets from an 05 and up motor. I've got DOHC Sti gaskets, and I've got aftermarket gaskets that are huge! I'm not sure what theyre from. Good to know about the pistons, I thought that's what kind of motor you had.

4subaru · Post by **4subaru** » Tue Jun 24, 2008 8:00 pm

it was cometic gasket inc, they were about 105 for the pair, and they can be order in whatever thickness you want. like i said before, they are design for using a 2.5 block with 2.2 heads

ciper · Post by **ciper** » Tue Jun 24, 2008 9:42 pm

I sent a message to Cometic. I asked them for a .08 inch gasket

4subaru: Do you remember the size they sent you?

ciper · Post by **ciper** » Wed Jun 25, 2008 8:20 pm

Ok guys help me out here. I want to make damn sure I order the right size. I don't trust the numbers I posted originally.

Secondly give me your advice. Is a 9.7:1 ratio good if I plan to use 87 octane gas?

Response from cometic -
We can gladly make the gasket thickness what ever you like as long as it is a possibility for us. However, I cannot confirm nor deny if your calculations are correct or not. When it comes to volumes I can tell you it is never a safe bet to assume anything or trust information you find on the internet. In theory your numbers make sense but like I said I cannot do the math to verify.
Kevyn Kistner
Technical Sales
Cometic Gasket

siouxbe · Post by **siouxbe** » Wed Jun 25, 2008 9:14 pm

I am already driving my 2.2l SOHC heads with the STi short block set-up and I love it!
Use the 4 layer steel OEM Subaru 2.5l headgaskets and you'll be able to run 87 oct. pump gas no problem!
My dyno test showed 125 ft./lbs. of torque at the wheels!

siouxbe · Post by **siouxbe** » Thu Jun 26, 2008 5:19 pm

IIRC the C.R. is closer to 10.3
BTW the SOA part # for the H.G. I used is 11044AA610 4 layer multi-steel 2.5
I carefully drilled holes to match-up to the 2.2L heads.
End result is it works GREAT!

ciper · Post by **ciper** » Mon Jun 30, 2008 7:40 pm

I would like to order the gasket but I need your help to decide on a final target CR and then the appropriate thickness to achieve it.

Just to make things clear it will be a stock 91 EJ22 NA with a phase two 2.5 liter shortblock.

siouxbe · Post by **siouxbe** » Tue Jul 01, 2008 12:15 am

Since you appear to be at Sea-Level, I would say go with the 4-layer MLS Subaru head-gaskets...but I think you'll still have to run premium. The phase II 2.5 short block in combination with your 2.2L heads will have a higher C.R. than with a STi short-block and SOHC 2.2L N/A heads that I am using and I am at altitude of 5,280 feet

ciper · Post by **ciper** » Tue Jul 01, 2008 12:36 am

I don't want to run premium thats the issue! The only MLS gaskets available from Subaru are .152 and .058

douglas vincent · Post by **douglas vincent** » Tue Jul 01, 2008 2:25 am

Isnt high comp and 89 octane a contradiction?

Arctic Assassian · Post by **Arctic Assassian** » Tue Jul 01, 2008 8:41 am

Thats what I was kinda thinking...

ciper · Post by **ciper** » Tue Jul 01, 2008 10:50 pm

So any advice on a target CR? I'm going to shoot for 9.7:1 unless someone chimes in.

Secondly how should I have the gaskets made? Should they be of EJ22 design (to match the heads) but with the larger bore? Or should I try to match the block?

douglas vincent · Post by **douglas vincent** » Tue Jul 01, 2008 11:25 pm

If you match the block bore, compression will lower... Match the 2.2 head, compression will be higher....

I have done both, both worked.

ciper · Post by **ciper** » Tue Jul 01, 2008 11:43 pm

Thats not what I meant to ask. I plan to use the 9.96 bore of the EJ25. My question was regarding cooling/oil passages.

Is an increase from .152cm to .215cm going to cause an issue? Perhaps too much change in timing since the heads are father apart or maybe exhaust/intake manifold fitment.

douglas vincent · Post by **douglas vincent** » Wed Jul 02, 2008 12:25 am

If I remember correctly, the only time I had issues with the cooling/oil passages was when using ej22t gaskets on ej22na heads.

I have used 2.0 DOHC heads and 2.5 DOHC heads on 2.2, no problems with the gaskets, and 2.0 heads on the 2.5, no problems. But as for the 2.2 na heads, not so sure.

I wouldnt sweat the extra thickness, as its spread out over the entire belt.

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Compression ratio for 2.5 shortblock swap, 11.5 or 10.3

Compression ratio for 2.5 shortblock swap, 11.5 or 10.3