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I Wonder
 
mgro1959's Avatar
 
Join Date: Apr 2012
Location: Vancouver BC
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Machining 8:1 pistons to lower compression

Without having the pistons in front of me I do not know the thickness of crown material available to work with. My question to those with far more knowledge then myself is , Is it possible to machine the crown of 8:1 pistons Ruf? so as to lower to 7.5:1 Although I have been looking on line I cannot seem to find 7:1 or 7.5:1 pistons for the 3.4 cylinders unless they are in a PC set. It may require that I have to phone and these are considered speciality but they must exist without having to get made. I am looking at running with about 1 bar max. Would 8:1 be too much. Running with the CIS system.84 Euro 930 Kokeln intercooler, B&B heeaders and K2729 turbo. Thanks Rob
Old 01-30-2014, 09:47 PM
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Why would you do that? 8:1 is pretty much the lowest I would go on any turbocharged motor, even on 911 air-cooled one.
You are just hurting the off-boost performance and nothing more.

Just use the pistons as they are at 8:1 CR.
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Old 01-31-2014, 12:02 AM
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I agree to what Raceboy is pointing out.

In one of our cars we run more or less the same configuration and this is working to perfection.

One major point in configuration is the camshaft.

So static and dynamic compression must be in coherence to that lobe figures.

Most important is the lift in tdc.

To give you the best possible advice just let us know what cam you are using or you are about to use in this build.

Best reg.

Dirk
Old 01-31-2014, 01:17 AM
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Thanks Guys for your input. I was thinking of using the 964 cams. I also have a set of SC cams.
Old 01-31-2014, 04:36 AM
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Then without question you go for the 964 cam - better top end without any loss in low revs.

It is ideal for the 3,4 L engines and best with a static compression ratio of 8:1.

Our engine is an andial setup with Kokeln Intercooler pulling 420 HP ar 0,8 bar.


Best reg

Dirk

Last edited by Edelweiss; 02-01-2014 at 06:22 AM..
Old 01-31-2014, 07:10 AM
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I run 8:1 at 1 bar no problems. I would go with the 964 cams. These engines rev so fast you wont miss any thing down low. I ran SC cams, and it pulled like a tractor, switched to Elgin 330's and still suffered too much wheel spin in 2nd when boost hit. This year I have DR993SS cams, and can't wait for the driving season to see what they work like. Philip
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Old 01-31-2014, 05:51 PM
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If you have an RCT engine, it is already machined for that compression, so I won't advice to machine it further, compression wise it all depends on the amount of boost you are planning for it, for high hp engines lower is better, 7:1 is great., for lower hp engines you can crank the comp to 8:1., cheers, Stef.
Old 02-01-2014, 02:34 PM
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