As Turbo said, you do have to weigh the effects of a blower.
Two very popular brands ar Pro Charger and Whipple. There have been many, many posts and arguments over that subject.

Anyway, to answer your question; yes. You can lower your static cr too much to where you're "lugging" the engine at cruise. Where that point is, depends on your particular motor/boat combination.

I have no idea what that symbol is you're using. But if its 89 octane gas that you're going to use.
I would rely on the equation that was given;

effective cr = [(boost/14.7) + 1]static cr

The problem is 89 isn't good for very much of a comp ratio. 9.5:1 would be really pushing it in my opinion.

DAVE

Steve

In a PC motor boost in the box is not boost in the motor. there is approx a 2# dp across the carb.

Still. 10# isn't anything to sneeze at.

DAVE

Thanks Dave, and yes, 10lbs. in a boat connected to an outdrive is defintiely dancing with the devil.

Just for yucks, I carry spare pulleys in my boat that would bring our boost to something over 10 lbs. I doub't I'll ever use them, it's more a "mental sobriety" check, kind of like, when I get real real stupid, I'll bolt these on

Take care, thanks again for the info on the drop from PC box to intake plenum.

Steve

at 8:1 with 6lbs.boost your total would be 11.3:1. I use the rule no more than 12.4:1 total comp. for 92 octane.anything beyond a total of 12.4 start buying Torco. My motor is 8.3:1 and 7lbs boost, I figure right under 12:1 total. Jay

I have never had a whipple but I thought they would build total boost at any rpm. Guess I am wrong. I thought that is what whipple advertises is instant boost. The motor I had in boat before this was twin turbo with wastegates. With that setup anything over 2000rpms If I got in throttles hard enought I could pull 10 lbs of boost. I know everyone says turbos are dogs & have lag but I saw more boost at lower rpm with my turbo setup then I do with this procharger setup.

As Dave F said the pressures I was talking about was in the box above the carbs. When I am crusing along at 35 mph -22000rpm's motor is at aprox. 5" vacuum & box has 3 lbs of boost. As soon as I nail the throttles motor raps to 4k 7 then starts pulling from there. I am not sure of boost at 4k.

As far as outdrive I have XY (I think, only difference between my drive & XR is top gears) & have no problem with drive as of yet. When I take off the most boost I can see is about 1-3 lbs. With the prop I have I almost always need to get in boost to get boat planed out.

8.5:1 and 6 lbs and 89 octane will be fine, just make sure yor fuel and timing is right and you run an intercooler. I run 8.75, 11-12 lbs, 91 pump gas and a huge intercooler with proper fuel and timing and it lives. Too low of CR will make your motor lazy on the bottom end. I would rather have an 8.5 and 5lbs than 7.0 and 9lbs.

I have also been of the opinion that Whipples would build boost at low revs. If the setup has a bypass, then this will definitely dump most, if not all, of the boost till it closes. Then, at whatever revs it closes, it should (in my thinking) provide near to its max boost. Granted, if the throttles aren't open very far when the bypass closes, then the indicated manifold pressure will be much lower than what the pump is actually producing.

I'm just guessing that if SteveDavid's motors make 7.5 psi at WOT, then at 3000 (or wherever the bypass closes) then it would make close to that same 7.5 psi IF THE THROTTLES WERE WIDE OPEN. Not recommending that he disintegrate his drives by trying this, but I'm thinking that the boost is there if he wants it...

Cobra- I feel if your blower was able to build say 5lbs boost at 2000rpms wide open throttle you would have more torque than with a higher compression & less boost. Am I wrong on this?

Yea but you're not at 2000rpm at WOT often. This rpm is usually part throttle. Boost is loaad and throttle position dependant.

This is a marine motor boost chart from B&M...Its on the conservative side but it shows a basic guide line.