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Old 06-26-2006, 11:41 PM   #1
kurion
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Default mahle pistons and CR question

Hi,

I going to build a 2.5 hybrid, stock STI block with Mahle 99.5 pistons and WRX heads. My question is what will the final CR be? 9:0 or 8.7?

I ask because threads I have across say that the mahle on a stock sti block + sti head combo raises CR to 8.5 :1 and thus using WRX heads would increase it to 9.0:1, since using WRX heads raises compression by roughly .5

However, other posts/threads have stated that those who have put in the Mahle pistons into a sti shortblock plus wrx heads have compression around 8.7:1.

Finally will a compression ratio of 9:0 be hard to tune/hard to run more than 22 pounds of boost off of 20g? thanks for any input.
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Old 06-27-2006, 12:02 AM   #2
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yes it will be hard to to tune if the cr is 9 u can purchase pistons from cp that will keep the stock cr for a little extra cash, and a few weeks wait. or u can just use a thicker head gasket

i talked to two tuner shops and they told me that with the fp green i would not be able to run 20 psi...don't know about the 20g.
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Old 06-27-2006, 09:20 AM   #3
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it will be more like 8.7:1 and you can run 22 psi if your tuner has any idea of on how to tune. i am running cp pistons with higher compression,and i am making more power with less boost and less timing which means more reliable motor. i am going in for another tune session to run 22-24 psi on my setup,and i am shooting for 440 whp and 400 wtq.my motor will still be reliable after this tune.
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Old 06-27-2006, 09:33 AM   #4
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ah thanks for the input/help, now all I gotta do is decide on cams haha
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Old 06-27-2006, 09:59 AM   #5
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i went with helix cams and springs. i love them ,and the got a nice idle with hardly any vacuum loss. the only thing i would do differently is not used double springs. they are a little stiff. so they make a little noise ,but not bad .i just like quite . great choice for the money,and they have a great torque curve
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Old 06-27-2006, 02:39 PM   #6
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The Mahle Power Pak pistons have a -22 cc dish the same as stock. Thus the same static CR. The listed CR by Mahle is wrong (unless you do a WRX-STI hybrid with a smaller combustion chamber, 50 cc vs 56 for the STI).
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Old 06-27-2006, 07:22 PM   #7
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Quote:
Originally Posted by bboy
The Mahle Power Pak pistons have a -22 cc dish the same as stock. Thus the same static CR. The listed CR by Mahle is wrong (unless you do a WRX-STI hybrid with a smaller combustion chamber, 50 cc vs 56 for the STI).
to the OPL
this is in the Piston FAQ: http://forums.nasioc.com/forums/showthread.php?t=999055

you're welcome.


and bboy...you replied in that thread and i recieve an email back from Mahle...you should check it out.
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Old 06-27-2006, 07:27 PM   #8
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Quote:
Originally Posted by bboy
The Mahle Power Pak pistons have a -22 cc dish the same as stock. Thus the same static CR. The listed CR by Mahle is wrong (unless you do a WRX-STI hybrid with a smaller combustion chamber, 50 cc vs 56 for the STI).
Templar said this:
Okay, I actually found my measurements on deck clearance when we put the Mahles in my car. The dish volume is the same (-22cc), however the deck clearance on the stock pistons is .380". The deck clearance with the Mahle PowerPak pistons was .305". This was with the 100mm pistons. I believe the stock bore 99.5mm Mahles have a little less deck clearance than that, but I haven't measured a set of those yet. This shorter deck clearance is where the slightly higher compression ratio comes from.

Tony Ford from Mahle confirmed the compression ratio via email. 8.5

stock CR is 8.2

piston dish on both stock and Mahle pistons are at -22cc
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Old 06-27-2006, 07:36 PM   #9
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Modaddict, thanks for the pointers, I had read your posts in the previous threads and hence I wanted some confirmation i.e 8.2 vs. 8.5. So, then am I correct in assuming that the final CR of the setup will be roughly 9:1? since 8.5:1 for a sti block and sti head combo, and an extra .5 if using WRX heads?

thanks.
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Old 06-27-2006, 07:41 PM   #10
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Quote:
Originally Posted by kurion
Modaddict, thanks for the pointers, I had read your posts in the previous threads and hence I wanted some confirmation i.e 8.2 vs. 8.5. So, then am I correct in assuming that the final CR of the setup will be roughly 9:1? since 8.5:1 for a sti block and sti head combo, and an extra .5 if using WRX heads?

thanks.
im not sure if it is Exactly a linear relationship like that. (-6cc smaller=.5CR) but you WILL be higher. even if you're 8.9...thats too high IMO for a turbo engine DD. but you could be 9.3 too. who knows. you would have to calculate everything to get an exact CR....even these listings in my other thread are just an 'educated' guess.. each engine varies. there is a lot to consider when you calculate 'TRUE' CR. You can probably contact them and see IF they make an sti replacement piston designed for the wrx heads. i havent heard of it but they might *fingerscrossed*
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Old 06-28-2006, 02:53 PM   #11
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Tony Ford does not know what he's talking about. He's reading off of a sheet. CR is pretty simple to measure/calculate and if the dish is 22 cc, the CR is stock 8.1-8.2 depending on what you use for a combustion chamber volume.

The STI combustion volume in the heads is 56 cc +/- 0.2 cc with a spark plug.
The WRX is 50 cc.

Spark plug takes up about 1 cc of volume.

The static CR that you can run on a turbocharged engine depends on the boost level you want to run, the size of your turbo (small turbo makes hotter air), and the dynamic CR (and dCR depends a great deal on your cams). There is a balance to everything. Subaru chose a balance that favored engine longevity over performance and came up with a CR of 8.2. If you have the right aftermarket components you can run higher CR and still have longevity, but not with the Subaru components.

You can run a CR of 10.5 with a turbo if you like, just don't plan on running much boost on pump gas.
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Old 06-28-2006, 03:01 PM   #12
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Jeff's got it right on the money here except for his combustion chamber volume seems to be without a spark plug. I've gotten from mulitple sources that the compressed gasket thickness is 0.6 mm. The bore size of the gasket is 101.2 for the STI gaskets.

The rest is just 2*pi*r^2 and the ratio of the total volume to the combustion chamber volume + volume contributed by the piston dish, deck height, gasket thickness.

http://forums.nasioc.com/forums/showthread.php?t=630797

I just went through this all with CP Pistons while ordering some custom pistons. They can tell you all the measurement if you like including the volume between the crown and the first ring.
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Old 06-28-2006, 03:54 PM   #13
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Quote:
Originally Posted by bboy
Tony Ford does not know what he's talking about. He's reading off of a sheet. CR is pretty simple to measure/calculate and if the dish is 22 cc, the CR is stock 8.1-8.2 depending on what you use for a combustion chamber volume.

No, he is just the engineer who DESIGNED THE MAHLE PISTON for the Subaru application,

You have again failed to include deck clearance into your CR calculations. It is sort of important. Do you think that a piston with a 22cc dish volume that sticks out the top of the block is still going to have the same CR?
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Old 06-28-2006, 07:45 PM   #14
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The dish volume is the same (-22cc), however the deck clearance on the stock pistons is .380". The deck clearance with the Mahle PowerPak pistons was .305".


I would consider this to be pretty definitive as to why the compression ratio of the Mahle's is higher...wouldn't you bboy?
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Old 06-29-2006, 01:35 PM   #15
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I'd have to do the calculation, but the deck clearance is spec'd 0.015". I guess you are working in millimeters. I personally find it hard to believe that Mahle would change the deck height without telling anyone. You would basically have no clearance to deck the block with a subaru gasket.

You think Tony designed the piston. Have you talked to him? I seriously doubt it.
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Old 06-29-2006, 01:47 PM   #16
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(99.5/2)^2*pi*0.38 = 2954 cubic millimeters or 2.954 cubic centimeters
(99.5/2)^2*pi*0.305 = 2731 cubic millimeters or 2.731 cubic centimeters

Difference of 0.223 cc or 0.223 mL.

On a combustion chamber of 56 cc that's a 0.4% difference in volume.

At most that's a change in the compression ratio of 0.04 units so you are looking at a new CR of less than 8.24. Not much different. A different spark plug could change the CR that much.
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