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?

 

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

 

I've been running 10.6 to 1 with E85 and 20 psi. I know others have run 11 to 1 compression with boost on E85. I definitely wouldn't go below 10 to 1 compression.

 

Running 10.5:1 with 20+ PSI on E85. Below 10:1 you're throwing power away.

 

I would not build a boosted motor these days with less than 11:1 (no less than 10.5:1).

 

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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.

 

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.

 

For a street car higher compression helps out of boost throttle response.

 

What about running 11.5-12.1 compression? On E85 of course, I don't think 93 could handle that?

 

Boost or NA?

 

Boost? I know NA is fine.

 

Sure you could run more compression but my tuner buddies generally suggest keeping it in the 10.x:1 range.

Mine is 9.5:1 drives fine out of boost.

 

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.

 

That's my exact thoughts as well, when I do go boost im gonna have to change my heads lol. My compression is 11.9 with my setup.