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Old 11-18-2006, 09:33 AM   #1
gumby647
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Default Couple of questions about building RS turbo block

Hey all,

I've got an RS turbo and I'm trying to round up some parts for the bottom end and I have a couple questions and could use some feedback.

I like the feel of the motor with the high compression pistons when compared to an STI so I don't want to lower the compression too much. I just want to make it stronger and run a little more boost.

First off has anyone compared the STI pistons to RS pistons? I know the STI rods are shorter and the pistons taller, but I'd like to know where the extra piston height comes from. If it comes from more material above and between the rings they should be stronger (det wise), and it would be better for me to run the short rods and STI style pistons. If the difference is between the oil ring and wrist pin it wouldn't matter. Except I can get the shorter rods for half what a set of RS length rods cost.

I'm trying to avoid custom pistons for cost reasons and I'm also trying to avoid using thinner head gaskets and milling to bring the more common 8.5:1 pistons up over 9:1 because cam sprockets for these motors are non existant.

I've read on here that some of you guys are running high compression STI motors. How did you get the compression up?

Also how tight can you squeeze these motors? I know .040" is a safe number but what about .030"? I know the tighter you run the squish the less chance for det so I want to get it as tight as I can without having to do the job more than once.

Your thoughts are greatly appreciated,
DaveW
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Old 11-18-2006, 09:41 AM   #2
Starkiller
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If your talking about high compression hybrid motors, the 205 heads coupled with 257 block OEM pistons/OEM headgasket will raise the CR w/o any machining done.

As far as squish(do you mean quench?) is concerned, thats beyond my knowledge.
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Old 11-18-2006, 10:04 AM   #3
gumby647
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From what I gathered from other posts here I would be at 8.9:1 with the SOHC heads and aftermarket 8.5:1 STI pistons and STI gaskets which are .030" thinner than stock RS gaskets. My concern is that the extra machining to get to 9.5:1 coupled with the thinner gaskets will throw the cam timing way off.
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Old 11-18-2006, 10:11 AM   #4
Drink
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Machining the heads for a hybrid shouldn't have any noticeable effect on CR. I wouldn't feel comfortable using a gasket do change my compression, I'd sooner change out internals. Some builders swear by the .40 Cometic. I've heard mainly because you will get less detonation due to shotty pump gas . If it plans on seeing a big turbo with lots of boost, upgrade your headbolts. I know plenty of guys running BIG turbos on OEM HG, you just have to tune accordingly.

Last edited by Drink; 11-18-2006 at 12:21 PM.
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Old 11-18-2006, 10:15 AM   #5
gumby647
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Quote:
Originally Posted by DrinkAV8 View Post
Machining the heads for a hybrid shouldn't have any noticeable effect on CR.
You'll have to explain to me how making the combustion chamber smaller won't raise the compression ratio.
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Old 11-18-2006, 10:18 AM   #6
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Quote:
Originally Posted by gumby647 View Post
You'll have to explain to me how making the combustion chamber smaller won't raise the compression ratio.
If your machining that much material off, you are doing it wrong. Machine for the straight edge, not to dictate your CR. Do that with internals.
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Old 11-18-2006, 11:18 AM   #7
gumby647
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Quote:
Originally Posted by DrinkAV8 View Post
If your machining that much material off, you are doing it wrong. Machine for the straight edge, not to dictate your CR. Do that with internals.
I see what you're saying. If going for a flat surface I always lap the parts on a granite surface plate so the minimun amount of material is removed. But 9:1 STI pistons don't seem to grow on trees.
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Old 11-22-2006, 08:26 AM   #8
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So, custom pistons it is I guess.
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