Compression for a 408ci 91 octane build
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Teching In
Joined: Nov 2021
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Compression for a 408ci 91 octane build I'm getting ready to order a rotating assembly for my 408 build and I'm hung up on the compression ratio choice, 10.2 or 9.3 with 91 octane being the highest available locally. I MAY add 6-7psi boost with the "cheap" Vortech kit down the road. E85 is no longer available locally
LQ4 block .030 over
PRC ported 317 heads
Summit Ghost cam 222/233 .600/.575
Terminator X for engine control
70' Chevelle
10.2 would be a no brainier for NA, and I'm sure It could be de-tuned enough to live with the mild boost. At that point would a 9.3 compression engine actually be less of a dog though?
LQ4 block .030 over
PRC ported 317 heads
Summit Ghost cam 222/233 .600/.575
Terminator X for engine control
70' Chevelle
10.2 would be a no brainier for NA, and I'm sure It could be de-tuned enough to live with the mild boost. At that point would a 9.3 compression engine actually be less of a dog though?
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Joined: Jan 2008
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From: Ohio, Georgia, Nevada, Texas
We'd go with the rotating assembly that gets you to 10.2:1 since boost sounds like it's a maybe. Going with boost at 10.2:1 and 91 isn't all that bad. If needed there is a multitude of things you can do to keep the engine happy. Examples would be tuning/timing, octane booster, meth/water injection, etc.
Depends more on power level and torque curve when the boost is added than simply saying 6-7 psi. Anything north of 600whp is likely going to neuter timing on straight 91 octane at that CR. If you're willing to add a splash of water/meth injection (cheap insurance) then definitely go with the 10.2:1. Otherwise I'd probably go with the lower compression route with that octane.
Teching In
Joined: Aug 2021
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I have a 370CI LQ4 with summit forged pistons and rods. 317 heads.. CR comes out to about 10.2:1. I run a 7875 on 7 psi @ 12 degrees with pump 94.
If you monitor IAT's you can get away with it if you keep the timing out of it. I would recommend going with the higher compression but monitor IAT's closely, once you add boost.
If you monitor IAT's you can get away with it if you keep the timing out of it. I would recommend going with the higher compression but monitor IAT's closely, once you add boost.
