Turbojack

I think since the late 70's I have had just about everthing. Now I have a procharger M4 with 8:1 compression on a 540bbc. I wanted to go 7.5 but was told with my setup the pistons would not work.

Is my low end sluggish? Yes, compared to other setups I have had, but I can still pull 4 llbs of boost off idle & goes to 11.5 @ 5800rpm. If I had gone quad rotor whipple I would have way more boost off idle but I also may be spending more time installing new drives.

As rat said it all depends on what you are trying to acomplish. This thread was stared with a question of what compression to run on regular 89 gas & 3-5lbs of boost. My answer is Max 8:1 You want more low end? Add more cubes. As I stated above you can make more HP before detonation sets in with a lower static compression ratio & more boost then with a higher static & less boost with the same octane gas. I think the 575 supercharged mercuriser engine only runs 7.5:1

IRONMAN

Thanks, I am still trying to decide weather to add a blower to a 509 or build a 572 - 598 NA motor. Either setup should yield 725++ Hp and be reliable.

ratman

i was originally going to go n/a but several engine builders told me that valve train problems plague hi output n/a motors because you need so much cam lift and duration. blower motors ran a mildrr hydrolic rollers for less valve train maint. if you only want to run 3 or so lbs of boost you may want to talk to scott @ intense maarine i think he told me the stacic comp on his 540's was 8.5 because was was only going to run a 871 blower. he would have run less compressions with a 1071. research yuor project well. goog luck

Infomaniac

89 octane is the real issue.

Regardless of what compression is best running on the engine or on the blower.

7.7:1 is doable with a 4" stroke and keep a decent quench clearance. A 4.250 stroke will up the compression a bit to around 8.4 using shelf pistons and 118 CC heads.

I would opt for the 7.7:1 if you are absolutely limited on octane. You can always boost up later.

cooltoys61

Don't forget, the cam you choose will have a large impact on your "real compression".

Pismo10

Oh no Ironman, you got the bug again eh...............

Infomaniac

This is correct.