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Old 09-16-2004, 02:01 PM   #1
sponaugle
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Default Compression Ratios and Heads (STI, Spec C, etc)

Based on some of the results from my recent dyno tuning (http://forums.nasioc.com/forums/showthread.php?t=628790) , I thought I would extend the discussion about the compression ratios. Since I happen the have a selection of different heads sitting here in my garage, I took some measurements.

First, let's look at the specs of the three heads in question:

Head 1: Us STI Head. Visually, this is the 'small port' head with AVCS. Cams are the new STI cams, which are pressed on cam lobes. Lift on these cams looks very similiar to the SpecC, but duration is much less.

Combustion Chamber Size: 57ccs measured, Factory Spec: 58ccs
Intake Port Chamber Size: 130ccs
Exhaust Port Chamber Size: 100,115ccs (one is bigger then the other)

Head 2: US WRWX Head. Similiar casting to the above STI one. Small port, No AVCS.

Combustion Chamber Size: 51ccs measured, Factory Spec: 52ccs
Intake Port Chamber Size: 130ccs
Exhaust Port Chamber Size: 115,130ccs

Head 3: JDM V7 STI Spec C Head. Visually, large port casting, AVCS. Cams have higher lift and duration.

Combusion Chamber Size: 50ccs measured, Factory Spec: 48ccs
Intake Port Chamber Size: 151.5ccs
Exhaust Port Chamber Size: 113,130ccs.

Commentary

Looking at these three heads, the larger intake ports on the SpecC is obvious. The New US STI heads are based off the WRX casting, or a derivitive of that. They have the smaller ports, but a different combusiton chamber design. The chamber is larger and deeper, and the spark plug
thread depth is different. (longer) Obviously the SpecC heads seem the most desirable interms of stock flow possiblity. However, additional headwork on the STI heads may make them as good/better.

Second, let's look at how this data reflects on the compression ratio.
To determine the compression ratio, you need not only the Combustion chamber volume, but you also need piston dish, gasket thickness, and most importantly quench distance and volume (aka piston to deck height).

The head chamber data is derived from above, which we measured here. The piston volume of the stock STI pistons was also measured last night, and correlated exactly with other builders notes (22ccs).

The piston to deck height can be determined two ways. The first is to measure it. With the engine I have here, it measure out to about .015" or .38mm. Other people have measured it at .013" or .33mm, and a previous (less accutare) measure I made came to the same endpoint. The second method for determined this distance is to calculate it based on the compresssion height of the piston, the length of the rod, the stroke, and the total deck to crank height. Using that method, I came up with .39mm, which corresponds well to my other measurements. The gasket thickness can be measured with a crushed head gasket. I measured .6mm on both head gaskets when compressed.

Putting these numbers together results in the following table:



As you can see, the static compression of the stock motor using this method comes out to around 8.09:1, vs the factory claim of 8.2:1. This may be due to error in measurement, or more likly the translation from static to true dynamic or observed compression ratio. Either way, the relative compression values are still valid and educational.

Putting the WRX or SpecC heads directly on a stock block results in raising the compression ratio to approximatly 8.6-8.7:1. While this is by no means obnoxious, it is a significant change. In order to bring this CR back down, one could substitite the cometic gaskets which Axis Racing sells. These have a compressed size of about .051", or 1.3mm. With that change, CR would drop to 8.2:1.

Jeff Sponaugle
PDXTuning.com

*Thanks to Ron at Axis for a wealth of information on this topic!*
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Last edited by sponaugle; 09-16-2004 at 02:07 PM.
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Old 09-16-2004, 02:06 PM   #2
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Good Work! Hopefully this will put an end to all the "what will my CR be with an e257"
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Old 09-16-2004, 02:15 PM   #3
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Good work Jeff.. Thanks

Clark
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Old 09-16-2004, 02:47 PM   #4
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Informative as always.

Jorge (RiftsWRX)
www.ProjectWRX.com
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Old 09-16-2004, 02:57 PM   #5
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Thanks for the right up Jeff!

Richard
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Old 09-16-2004, 05:03 PM   #6
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yea, thanks for the info that is awesome, so does that mean your engine wasnt actually at 9.5:1 then?

EDIT:nevermind just checked the other thread

i love these informative posts.

thanks, keep up the pioneering!

Ben
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Old 09-16-2004, 05:49 PM   #7
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Yep. Since Jeff and I have the same motors (or did until he got pistons) I guess we where both at 8.71 to 1.

C
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Old 09-16-2004, 05:54 PM   #8
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Wow, great info as always.

Oh, and noted at Imprezario as usual
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Old 09-16-2004, 10:45 PM   #9
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Thanks Jeff and Ron!

-st
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Old 09-16-2004, 10:50 PM   #10
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ok so now I am confused.

What do you end up for (compression wise) with Version 7 heads on the usdm 2.0 wrx block?
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Old 09-27-2004, 04:45 PM   #11
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I'd be happy to know if the cc's on the V8 heads are the same as the V7 RA.
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Old 10-21-2004, 04:10 PM   #12
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very nice work done!
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Old 03-07-2005, 02:18 PM   #13
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Nice Info!
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Old 03-08-2005, 01:30 AM   #14
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yahooo! just what i was looking for! great job even if im a little fewmonths late! haha, so now my question is this, what are the pros and cons of high and lower CR on a 2.5 with wrx heads? or another note derivatives such as sti cams, crower cams, jun cams, crawford cams...LOL
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Old 07-10-2005, 06:33 PM   #15
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Default EJ257 combustion chambers

Jeff, could you confirm that the EJ257 combustion chambers are 57-58cc volume? Quirt stated that mine were 50cc. That would mean a helluva lot of milling, and a mistake in my custoom piston spec. I hope I haven't Swain Tech coated a few ashtrays. That would really suck.

Thx,

S.
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Old 07-10-2005, 06:58 PM   #16
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Quote:
Originally Posted by n2xlr8n
Jeff, could you confirm that the EJ257 combustion chambers are 57-58cc volume? Quirt stated that mine were 50cc. That would mean a helluva lot of milling, and a mistake in my custoom piston spec. I hope I haven't Swain Tech coated a few ashtrays. That would really suck.

Thx,

S.
Steve,

The US STI has a combustion chamber volume of 56cc with a spark plug installed.

The US WRX has a chamber volume of 49 to 50cc with a spark plug installed.

I am not quite sure about the 50cc statement you got from me

Quirt
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Old 07-10-2005, 07:03 PM   #17
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Quote:
Originally Posted by Crawford Performance
Steve,

The US STI has a combustion chamber volume of 56cc with a spark plug installed.

The US WRX has a chamber volume of 49 to 50cc with a spark plug installed.

I am not quite sure about the 50cc statement you got from me

Quirt
I called before ordering my pistons...Chris asked you (without you coming to the phone) what the heads cc'd at, you stated 50ccs. I'm going to cc them myself when I get my burette back from work....stay tuned.
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Old 07-10-2005, 07:18 PM   #18
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Quote:
Originally Posted by n2xlr8n
I called before ordering my pistons...Chris asked you (without you coming to the phone) what the heads cc'd at, you stated 50ccs. I'm going to cc them myself when I get my burette back from work....stay tuned.
I am sorry
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Old 07-11-2005, 11:01 AM   #19
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Quote:
Originally Posted by Crawford Performance
I am sorry

...and I'm back to the drawing board. 56 +/- .2ccs

Measured each 3 times, head is level, etc.

I guess I'll be in touch for your ideas about where to go with this nightmare.
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Old 11-04-2005, 01:03 AM   #20
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Quote:
Originally Posted by ebeck
I'd be happy to know if the cc's on the V8 heads are the same as the V7 RA.
Jeff (or anyone),

Any idea about the v7 STi non-RA heads?
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Old 08-27-2007, 01:49 PM   #21
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Anyone have the excel spreadsheet this was calculated from?

I'm building an ej207 with the Tomei 2.2l stroker kit and WRX (ported) heads. I need to calculated CR with a few different head gaskets.
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Old 08-27-2007, 03:50 PM   #22
nhluhr
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added this thread to the forum sticky "Threads of note"
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Old 10-05-2007, 08:06 PM   #23
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Would like to know this also as I'm about to either buy new pistons and a block or just a block and I have JDM STI 7 (big port) cyl. heads.

cheers

poorman


Quote:
Originally Posted by gpatmac View Post
Jeff (or anyone),

Any idea about the v7 STi non-RA heads?
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Old 03-26-2011, 01:43 AM   #24
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what would be a piston choise for v8 jdm ej207 ?
wisseco,cosworth,CP,JE,etc?
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