I think the rotor leakage can be a issue in say a street application, where low speed torque is really needed.

For example, a 250 blower might outperform a 420 until say 5000rpm from a torque standpoint. But in marine, the larger blower at sustained high engine rpm works better. You'll get more leakage at say 3500 rpm, but it's minimal compared to the gains you get say at wot from the cooler air charge from the larger blower turning slower.

As witnessed by many on the old 525sc with the 177's, upping the boost from 5 to 7 lbs, gained very little. It just adds more heat and shorter blower life.

Thats the buttons I use. You can take them apart at the snap ring and surface grind/turn the fat end to get the clearance/cam play right where you like it. The bearing wear is from debris of some sort, probably not the right stuff but a dirty cooler or a small amount of metal in the galleys. You might cut apart the oil filter to get a better look but all in all no worries too big there in my estimation.

With that theroy, not saying your wrong, in fact I agree to an extent, but you have ask youself how much am I going to run wot and for how long at wot as well I guess. If you do the majority of your boating at "cruise" rpms then the above issue could still be a possibility I would think?

I see what you're getting at. I think most guys when doing upgrades, look for one result out of their mods, top speed gains. The ability to turn a higher pitch prop at wot, just means better cruise speed. For example, lets say a large blower setup can turn a 30p prop to 5500. and The small blower set can turn a 28p prop to 5500. The larger blower setup will also cruise faster at say 3500 with the taller prop. We normally prop out boats for wot rpm range, and the midrange kinda is what it is

let's not forget the OP is not going with the 420s he is installig te 250s, and i'll bet the perfomance will be addictive! so the throttle will begin to be pushed to WOT more and more, for longer and longer, isn't that what we do? I think it is a great project, and just wanted to see OPs thought concerning the intercoolers. Either way he will love the power gain.

I agree. Plus we were getting off track a bit. I'd love to bring one of my 420 blowers up for the dyno session though. Since It would bolt on to his intake, and just require some pulley swapping, which I have extras. That would be pretty cool, just wish I lived a little closer to rookie. Kind of a supercharger shoot out , maybe even bolt chillers on and off. Really give his engine a workout!!!

Not getting off track at all. That's why I posted this in the tech section and not on my restoration/upgrade thread. This is what the forum is all about.

Exactly! We all want the big hp and top end numbers or at least think we do, but is it REALLY what we are looking for? Just my perspective. Is midrange where its at?

Where do you dyno and how much $$$ to dyno? I have a truck and time, just not a ton of $$$.

I hear that! Place I dynoed mine is in Joliet, IL. They have a DTS dyno. I want to say I paid 500 bucks.

It really wouldn't be much for me to pop a blower off, and I also have complete drive setups, basically its the setup that merc used on the 600sc. We would need to remove your tensioner and crank pulley, and bolt my crank pulley on. My tensioner mounts to the blower snout. In reality probably faster than changing jets. With my setup, its a 10rib belt deal, and pulleys made 6.5psi on my 468's. But, if we can get our hands on a couple 16 rib pulleys for the megablower, we can bolt them on the blower and still run the 10 rib setup, it works. Just to try different ratios.

When you gonna have it built and ready?