CR question for the Gurus

[mike-tracy]

I have quite a mix of parts, I don't have any idea how to estimate the CR. Oh and the motor isn't assembled yet.

Closed deck early EJ20G block, bored out
1996 EJ20G WRX heads
Wiseco 98+ WRX EJ20 pistons (labeled 8.35 CR) .020 over
EJ20K OEM metal head gasket
OEM STI rods & ej20g crank, factory clearances

Any advice is welcome

Thanks,
Mike

[soundman353]

Well you need to do some more homework. To calculate compression ratio's you need to know cylinder volume and chamber volume. To calculate cylinder volume you need to know the bore and the stoke. The formula for cylinder volume is pi divided by 4 times bore squared times stroke. You need to measure the combustion chamber (all so know as chamber volume) on the heads. Add the gasket thickness to the chamber volume. Then take the cylinder volume plus the chamber volume divided that by the chamber volume and you'll have the compression ratio. That is if your pistons are flat. So get some numbers and do some math.

[mike-tracy]

Thanks for the reply. OK I cheated a little, I used:

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

The piston information I gleaned from where I bought them at:

http://www.nolimitmotorsport.com/wiseco ... ubaru.html
(I have K588M925 x4 for pistons)

Hopefully I did something wrong, because my estimated CR is:
8.76.... !

Here's the numbers I put into that site:

Enter Bore/Stroke Designation Type 1 = Inches 2 = Millimeters: 2
Enter Cylinder Bore Size: 92.5
Enter Piston Stroke Length: 75
Enter Head Gasket Bore Diameter: 93 (not sure about this one?)
Enter Compressed Head Gasket Thickness: 1.37
Enter Combustion Chamber Volume In CCs: 49
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-'): -9
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-'): -0.35

So questions about the above information: what is the stock bore size for an EJ20(g/k) head gasket, 92,92.5,93mm? Does a piston deck clearance of 0.35- (as listed on the site I bought the pistons from) mean they protrude from the deck 0.35mm or are under by that same amount at the top of the stroke?


Another wrench into the puzzle is both my block and heads were decked! One thing at a time, I'm trying to get an good CR number pre-decked.


using this calculator, my CR is a nicer 8.49... but of course you input less information
http://www.race-cars.net/calculators/co ... lator.html

[mike-tracy]

Found some Cometics here: Ebay that are 2.03mm thick (stock is ~ 1.3). This lowers my CR (discounting the decked heads and block) to 8.27, much better IMO.

Thanks for letting me think out loud.

Note to self, do all this BEFORE you order parts next time

[mike-tracy]

I was mistaken in my 1st calculation for the pistons deck clearance. The pistons I went with don't go above the deck, so using the 1.37mm ej20k (or ej20g for the matter) head gaskets, my CR is 8.57 (my mistake was using a negative sign there)

Good thing I didn't order the 2.03mm gaskets, I would be down to 8.09 CR (which is fine and dandy, but I'd like a reasonable off boost performance).

I also change the combustion chamber CC to 46 for my 96 wrx heads, and bore of 92.5mm for the HG's.

[soundman353]

My math teacher always said "double check" your numbers.... he was right. I glad you didn't get it together before you found out. I don't know much about turbo motors. What is a "good" CR for them? I am guessing that depends on which turbo you choose, right? I do hope your build goes well.

[MikeyMeyagi]

if it was me, id go w the 8.7:1

[mike-tracy]

if it was me, id go w the 8.7:1

When punching the numbers with my "corrected" input, if I were to put cometic 1.016mm gaskets in (thickness as suggested by the ej20g/k guru Marnix), I'd get that 8.7x CR number. For the time being I'd like the lower figure, since my "EM" will be a robtune chip.

[mike-tracy]

My math teacher always said "double check" your numbers.... he was right. I glad you didn't get it together before you found out. I don't know much about turbo motors. What is a "good" CR for them? I am guessing that depends on which turbo you choose, right? I do hope your build goes well.

You're right, I'm glad I went over these one last time. As far as turbo motors go, I'm new to them as well. I would think that Subaru in it's infinite wisdom sells turbo cars here with CR's between 8.0 and 8.5 for a reason (reliability and stability?).

[Florin1]

I hope you blow your motor...

[mike-tracy]

I hope a drunk hits both your cars while parked, lol

[Florin1]

he wouldn't make it past the 90* corner. Unless that drunk was me... But i don't drink and drive. neener