I'm trying to pick out a set of pistons for my L98 rebuild. My question is if a compression ratio around 10:1 or slightly higher would have any affect on the computer or TPI system? I'm going to be running it with a ZZ3 cam(.474/.510 lift, 208/221 duration@.050, 112 LSA),mildly ported stock heads, and a Hypertech Thermomaster chip. Any suggestions on what would be a good range for the compression ratio?

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89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."

 

The computer doesn't care about the compression (has no way of even being aware of what it is), as long as there's no spark knock.

10:1 is too high with that small of a cam. I would shoot for 9.5 min to 10 max. You will have to step up your valve springs to use that much lift, the stock L98 ones probably won't survive their first trip around the block.

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"So many Mustangs, so little time..."

 

I used a calculated 10.3:1 compression with my L98 aluminum heads and ZZ4 cam. I had to use 100 octane at the track, but 92 was fine for street use. The key thing is the aluminum heads. For iron heads, I would take RB's suggestions and run 9.5:1 to 9.75:1 compression, max.

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1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.25 @ 107.18 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
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-=ICON Motorsports=-

 

Thanks for the suggestions guys. I was kinda figuring 10 to 1 would be a bit too much for my setup. Now I'll just have to decide if I want to get the 9.3 pistons, or if it's worth it to spend double the price for 9.7 pistons. Btw, both sets I'm looking at are hypereutectics. Hey RB, I forgot to mention the valve springs, but I definitely plan on upgrading them. GMPP sells a set of H-11 springs for a decent price that are listed as being used with the ZZ3/ZZ4 cam. The only thing I have to figure out is if they require different retainers than the stock L98 springs.



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89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."

 

Compression ratio spreadsheet is in your email so you can run some what-if scenarios.

Pistons hat might work well with your combo include Keith Black 12cc dished hyupereutectic slugs that will keep your compression at about 9.5:1 with 64cc cast iron heads. With aluminum you might want to go about 1/2 point higher.