This is confusing me. a given engine, say a 454 will flow(through put) a certain amount of air. My thinking (and I may be completely wrong) is that forced induction stuffs more air into the motor than it can pass in a naturally aspirated state. this extra volume of air causes an increase in pressure in the intake manifold (psi) which we call boost.

Given the intake volume is a fixed volume at a given rpm and say a 177 blower fills that area with 177 cubic inches of air on every revolution and a 420 blower fills that same area with 420 cubic inches of air with each revolution the 420 puts 2.4 times as much air into the same space as a 177.

With that thinking on the identical motor, if you gear the 177 to make 1 psi of boost at 3000 rpm the 420 would make 2.4 psi of boost at 3000 rpm with the same gearing.

This would be why the 177 gets refereed to as a heat pump, it has to spin faster compared to engine rpm to make the same boost as a 420, spinning it faster causes more heat..

So if my thinking is correct (which it may not be) if the 420 makes more boost than a 177 at a certain rpm it WOULD make more boost sooner (proportionally)

Generally, the larger blowers, turning slower, can make less torque , and less boost , in the lower RPM's. Couple reasons. First, is rotor leakage. When the rotors are turning very slow, the blower can be somewhat inefficient. This is why, you wouldn't want a 14-71 blower, on a 454 that it going to be towing a trailer. At 2000rpm engine speed, it would be horribily inefficient. For something like that, the smaller 177 blower would work better. Rotor speed is up at that lower rpm, and the blower is efficient there, also, its not taking a bunch of power to turn the blower there either. Nor is the rotor speed fast enough, to create a bunch of heat.

Now, a 454 , that is going to see sustained periods of high rpm, like in a boat, the larger blower shines. Where the 177 starts making lots of heat, the larger blowers just are not. As you compress air, it naturally heats up. Thats minimal, compared to the heat though, that can be generated by the high rotor speeds beating the crap out of the air. As the temperature goes up, so can the boost reading. Just like how your tires increase psi when they get hot. While you may see a higher boost reading on your gauge , if the air is hot air, it's not good air. Hence why, the power isn't there with a small blower running hot. This is why intercooling works. A nice cold charge showing 7psi on the gauge, will make much more power, than a hot charge, showing 7psi on the gauge.

Generally speaking, my thoughts on sizing a roots blower for an offshore engine, would go something like this

454ci =6-71
502ci= 8-71
540ci=10-71
572ci=12-71
598ci=14-71

Keeping the blower in a 15% under, to 15% over range, is generally a good ballpark. Of course there's all kinds of variations and modifications to the blowers that can have an effect on things, as well as the overall build. A 502ci with stock heads, stock cam, stock exhaust, might make 10lbs of boost with a 871 at 1:1 ratio. Put that same blower on a 454, with fully ported heads, a big camshaft, free flowing exhaust, it might only make 8lbs of boost. The boost charts don't take any of that into consideration when looking at them. The boost chart shows for my overdrive shows I should be seeing 8-9psi on a 540ci, and would make like 15psi on a 454. Personally, i think the blower sizing should be more looked at for HP level, than just cubic inch. The blower has no idea what size engine its on, it just knows how much air it has to move. A 800hp 454, is certainly moving alot more air, than a 600HP 454. To do that, you need to speed up the blower. The engine is an air pump, and so is the blower.

Without putting much thought into it I figured that the larger blower would of been better at lower rpms just because the smaller one hasn't spooled up yet, but that makes sense. Thanks.

intercooled , chiller help situation , hell even methanol injection

Anything you can do to cool the charge down, is a win.

Icdedppl has 10-71 blowers on his 540s. Hes underdriving them, making 6 or 7lbs of boost. We were out 2 weeks ago, running the boat. I watched his intake air temps while he drove. The intake temps, were 135ish at full throttle, and most of the rpm range. They dont even begin creeping up when you hold the throttles down for a while. No intercooler on his setup.

135* isnt all that bad. Most on the internet hear the word "roots" blower, and immediately start talking about 200+ intake air temps, usually supported by zero actual data, jusy what they heard. The style and size of the blower has alot to do with it.

Overall, im not super impressed with my superchiller intercoolers. My intake temps are in the 110-120 range, which for a roots is not too bad. Realistically, they prob knocked off 20-30 degrees from my iat's. I got a good deal on them from a friend, but not sure id invest 3 to 4g on them for my setup if i was buying new. The superchiller also puts a big restriction in the intake tract, which requires you to speed the blower up to overcome that.

Id love to see what the intake temps are , in say, a non intercooled 177 blower setup.

MT - somewhere in the back of my brain remembers it said that vaporizing fuel removes somewhere around 60*F from the air going in the motor.

not a boat , Detroit the crazy guys intercooled w 200 degree coolant , how dumb is that / BTW sbc marine motor carb makes frost on a hot day

Nice

I'll let you know intake temp of a 177 in the spring.

i may be wrong but i don,t think detroit uses the same coolant to cool the intercooler.when you say detroit,who are you talking about?