Compression Ratio for boosted motors
Thread Starter
Teching In
Joined: Sep 2025
Posts: 1
Likes: 0
Compression Ratio for boosted motors Hey all,
currently I’m building an iron Gen IV 6.0L and am at the part where I get to finalize my build list prior to purchasing parts. Currently I’m debating going with a 10:1 static comp ratio or a 9.5:1.
I understand static comp plays a small role in the overall healthiness of the engine but just looking to get opinions.
The car runs E85 majority of the time (occasionally 93 is needed for a road trip but never raced or pushed on regular pump gas). For fuel it has triple 450walbro set up with injector dynamic 1050x’s (may need upgrade to 1300’s). And I will be putting in a full forged rotating assembly, boost cam, LS9 head gaskets, etc…
For power adders I’ll be running twin 66mm turbos. And am aiming to make a maximum of 1100-1200hp.
With good fueling and tuning higher comp boost motors are much more prevalent than 20 years ago but what would everyone personally choose? 9.5:1 or 10:1?
currently I’m building an iron Gen IV 6.0L and am at the part where I get to finalize my build list prior to purchasing parts. Currently I’m debating going with a 10:1 static comp ratio or a 9.5:1.
I understand static comp plays a small role in the overall healthiness of the engine but just looking to get opinions.
The car runs E85 majority of the time (occasionally 93 is needed for a road trip but never raced or pushed on regular pump gas). For fuel it has triple 450walbro set up with injector dynamic 1050x’s (may need upgrade to 1300’s). And I will be putting in a full forged rotating assembly, boost cam, LS9 head gaskets, etc…
For power adders I’ll be running twin 66mm turbos. And am aiming to make a maximum of 1100-1200hp.
With good fueling and tuning higher comp boost motors are much more prevalent than 20 years ago but what would everyone personally choose? 9.5:1 or 10:1?
On The Tree
Joined: Feb 2022
Posts: 196
Likes: 72
From: Grand Rapids, MI
10:1 to make the engine more fun off boost. Higher comp engines sound better too
lower compression will help you keep the heads down at 1200hp though
Half a point isn't going to be noticeable really
Youll need at least the 1300s
lower compression will help you keep the heads down at 1200hp though
Half a point isn't going to be noticeable really
Youll need at least the 1300s
TECH Fanatic
Joined: Jul 2022
Posts: 1,046
Likes: 396
From: Alaska
.
.
Serious question: WHY?
Once boosted, Manifold pressure is adjustable with the literal turn of a dial, or exchange of wastegate spring.
" Boost pressure is ALWAYS cheaper than machine work "
AIR Pressure is cheaper than pistons.
TECH Enthusiast
Joined: Sep 2019
Posts: 532
Likes: 630
If you're running on gasoline and air then a bit lower compression might be a good idea. Gas and air under high compression runs hot, and some pinging under boost can still break rings and cause other bad things. I still wouldnt go under 9:1 for a street motor, the days of the 7:1 compression small block with a roots blower are long gone.
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For a track application it’s plausible. You’re gonna rebuild often. Your tuner will earn his/her money on that spec…there is virtually zero room for error. The better the fuel you have access to, the better off you are with that static number. Camshaft choice dictates dynamic compression, which is all the engine sees anyway….it doesn’t know or care what the static number is, but static makes tuning window size.
Staging Lane
Joined: Dec 2022
Posts: 56
Likes: 16
For a track application it’s plausible. You’re gonna rebuild often. Your tuner will earn his/her money on that spec…there is virtually zero room for error. The better the fuel you have access to, the better off you are with that static number. Camshaft choice dictates dynamic compression, which is all the engine sees anyway….it doesn’t know or care what the static number is, but static makes tuning window size.