[performerrpm]

for the guys running e85, ive read you need around 10..1 cr to make use of it with a blower. what would you guys thinks better, i ask because this would be a deciding factor of which heads to use. the bottom end is a forged 4 inch crank/studded 400 block, bme rods and icon 10cc dish pistons. i plan to have .060 piston to head clearence, so with speedmaster 78cc head i would be at 9.75cr, or i could use the 96cc eheads which would put me around 8.5ish which is better higher cr and lower boost or more boost and lower cr? i was either gonna throw an xr288 cam in it because i own it already but i might get a lil wider lsa cam. i dont know that i should spin this thing much passed the 6500 mark, in which case i think a hr cam would be fine?

[darbikrash]

I’ll throw out some thoughts, not meant as gospel, just a perspective.

I think it depends on the type of forced induction. For a turbo car, I’d think the higher compression/lower boost approach would work better on E85 - the idea being the higher compression will net an engine that is more responsive when it is off boost, with less turbo lag as well.

With a blower - as opposed to a turbo, again I think it depends on what type of blower. A centrifugal builds boost slower and the boost comes on at a higher rpm than a twin screw, so for the centrifugal the higher compression approach might also make more sense.

For the twin screw, these blowers typically throw all the boost in right away, and the engine is under boost almost at any rpm off idle, so the engine will be responsive in virtually all circumstances. In this case I think the lower compression approach might be the best solution.

[charlie66]

Both will work.
The higher compression one will just have a smaller tuning window...