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Dropping compression. New pistons or heads

Old 01-25-2019, 07:03 PM
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Dropping compression. New pistons or heads

Currently running 10.2:1 on a 358 with a set of trimmed down fast burns. 60cc chamber. Id like to get down to 9.5:1 or under for some forced air. What cc chamber would put me there or should i just swap slugs? Currendly running ross forged pistons. A head swap needs to be a raised port for a 1206 intake gasket to match my intake

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Old 01-25-2019, 08:12 PM
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Re: Dropping compression. New pistons or heads

An 8cc chamber increase will drop your CR by .81.
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Old 01-25-2019, 08:43 PM
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Re: Dropping compression. New pistons or heads

Originally Posted by NoEmissions84TA View Post
An 8cc chamber increase will drop your CR by .81.
So going to a 65cc chamber with I think a .010 thicker gasket should get me close to where I think I should be? I'm unsure of the adverse effects of running a thicker gasket, or if my math is correct??

Thanks for the input
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Old 01-26-2019, 05:57 PM
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Re: Dropping compression. New pistons or heads

You're welcome. I don't know how thick your head gasket is now, but you should try to keep the quench close to .040".
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Old 01-27-2019, 11:11 AM
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Re: Dropping compression. New pistons or heads

As an example:
4.04 x 3.48
60 cc head
5 cc piston
.040 x 4.100 gasket
.025 piston below deck
Result is a CR of 10.26:1
A 64 cc head nets 9.8
A 60 cc head with a .060 gasket nets 9.78:1
The input values are a best guess but typical SBC. Verify your piston valve relief/dome/dish volume. Going from 5 cc (2 valve reliefs) to 9 cc (which I haven't come across) drops the CR to 9.8

Last edited by skinny z; 01-27-2019 at 11:17 AM.
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Old 01-27-2019, 11:21 AM
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Re: Dropping compression. New pistons or heads

How much boost and what kind of poweradder?

low boost i’d send it as is at 10:1. I would try thicker gasket first. Ease up on tune, keep engine cool and good intercooling

High boost throw c16 in it or e85
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Old 01-27-2019, 11:29 AM
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Re: Dropping compression. New pistons or heads

Gootta go back to my build sheet from a few years ago. Im pretty sure it was a.028 head gasket. I was thinkin of throwin some new afr's on whick would gain me 5cc oin the chamber. Ithimk at least 10psi intercooled. Id prefer not to swap pistons. But i will need be. Tryin to stay at 93 octane. E85 not too common around here.
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Old 01-27-2019, 11:30 AM
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Re: Dropping compression. New pistons or heads

Originally Posted by Orr89RocZ View Post
How much boost and what kind of poweradder?

low boost id send it as is at 10:1. I would try thicker gasket first. Ease up on tune, keep engine cool and good intercooling

High boost throw c16 in it or e85
Cog drive vortech. Intercooled
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Old 01-27-2019, 11:40 AM
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Re: Dropping compression. New pistons or heads

Originally Posted by 2slow5.0 View Post
Gotta go back to my build sheet from a few years ago. I'm pretty sure it was a.028 head gasket. .
It'd be tough to hit only 10.2:1 with a .028" gasket / 60 cc chamber all else being equal and standard SBC. Before you get too far you'll want to verify the values. With the above data listed a .028 gasket gets you over 10.6:1. Change the valve relief volume to 7 cc and a .028 gasket nets 10.3.
But like I said, there are too many variables to just guess. And I'm even guessing on the bore and stroke.
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Old 01-27-2019, 11:58 AM
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Re: Dropping compression. New pistons or heads

Originally Posted by skinny z View Post
It'd be tough to hit only 10.2:1 with a .028" gasket / 60 cc chamber all else being equal and standard SBC. Before you get too far you'll want to verify the values. With the above data listed a .028 gasket gets you over 10.6:1. Change the valve relief volume to 7 cc and a .028 gasket nets 10.3.
But like I said, there are too many variables to just guess. And I'm even guessing on the bore and stroke.
Yea. Why im tryin to find the build sheet. Its an oddball shortblock. 3.50 stroke, 5.85 rod and a 4.040 bore. I had to have the pistons made back when originally pieced together. But unfortunately Ross doesnt have the record. I do remember cc-ing the heads and calculating the 10.2:1 with Ross. I know the build sheet is here somewhere.
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Old 01-27-2019, 12:00 PM
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Re: Dropping compression. New pistons or heads

On a ebl flash I never had any detonation issues on 93. Even goofin up and runnin too much timing at first
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Old 01-27-2019, 01:14 PM
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Re: Dropping compression. New pistons or heads

If its 10.2 on a .028 gasket then go with a .040-.051 and let it eat. Not even concerned with 10 psi intercooled
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Old 01-27-2019, 01:31 PM
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Re: Dropping compression. New pistons or heads

Originally Posted by Orr89RocZ View Post
If its 10.2 on a .028 gasket then go with a .040-.051 and let it eat. Not even concerned with 10 psi intercooled
I've deffinetly gotta dig in and see exactly what I got. I figured it could be a good excuse to upgrade the heads/ cam
Id prefer to not tear the bottom end apart if possible
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