Have a old B&M book here from the early 90's.

250 B&M overdriven 60%=140* intake air temp.

250 B&M overdriven 112%=230* intake air temp.

They said at 60% over, the temps were stable from 3000-6000 engine RPM. At 112 over, the temps shot up 40 degrees from 3000-6000. 190* at 3000RPM. 230 at 6000RPM.

They didn't show any specific temp numbers on the 420 blower, but it went up 60* by going from 8% under to 15% over.

So, with that being said. My guess is a low boost, underdriven large blower, just is not making much heat. If a 250 blower is making 140* intake temps at 60% over, I cant see the larger blower 5-10% under making much heat to truly warrant a intercooler in a low boost scenario. If the intercooler was 70% efficient, and the intake air temp was lets say 120*. Then the lake water going thru the intercooler was 80*. That's a 40* temp difference. 70% of 40 degrees is 28 degrees. So, in theory, before the chiller you'd have 120*, after the chiller 98*. But, you'd also have to drive the blower slightly harder to overcome the restriction of the core itself. Which may add more initial heat than not having the core there. Not saying the intercooler wouldn't help, but the return may not be as great as below.

NOW, take a 250 Blower, or even a 177, that might be making 200* air temp, and put the chiller under it with 80* lake water. You may be able to remove 125 degrees from the air charge, bringing it back down to the larger blower, non intercooled temps.

In a nutshell, what they recommended in this book, is that for High RPM use, or in a marine application, the larger blower turning slower works better. In a street vehicle, the smaller blowers work just fine, and may add a little torque.

Keep in mind this is strictly shop talk, and my intercooled theories and numbers could be way off. I have some other thoughts I will bring up later, and maybe the pro's can shed some light.

80* lake water? I'm in the wrong place! Most of my boating is in the Sound, 47*F.

sounds like you need a Procharger

That sound right!

After a long discussion with Ron (The Blower Shop) last week about my situation, turning 177's at high RPM my concern was also heat. I want larger but just dont have the extra cash. He stated that I should be able to tune these 177's on the dyno find were the engine make its best power & make the peak boost at 6# there. Should make 5-7 psi & run WOT in a marine application without heat being an issue without chillers.
That # was surprising to me, I was expecting him to say 3-4#. If I try & push the small charger over the 7psi it would create heat & become very inefficient quickly. That being said Im going to try to hit 6# & see this season what kind of heat they make after a hard run.

Glad you brought that up I was wondering if a chiller added any restriction to air flow. I just wonder the rate & how much harder you have to twist the charger to create the same manifold pressure with & without a chiller. That being said are they really worth the extra hassle & $$ if its kinda a break even #?? Im excited to run the small chargers this season & learn more about then. Even more excited to upgrade to 420's or 671's next winter!

Thanks for posting those #'s Im sure ill be revisiting them trying to figure it all out.

Following the same theroy, the larger blower driven slower would be a little easier on drives in the lower rpm ranges much like a centrifigal supercharger is. Just thinking out loud.

I went from a 250 B&M overdriven at 85%. Ran 7lbs of boost. Then went to a 420, running 6lbs of boost, underdriven at 7%.

Planing and low speed, it does feel like I may have lost a little bit of torque down low. But, I definitely picked up a ton of power up top. Boat gained a solid 5-6mph running less boost, and still not totally dialed in.

Which brings me to my next area of thoughts.

Lets compare two setups, I'll use mine for a example.

I ran 7lbs of boost with a 250, highly overdriven. No detonation, pump gas 93.

Now, most generically would say, 7lbs would be the practical limit for pump gas without a intercooler. But how do we really know that? 7psi is just a number, and there are lots of variables. 7lbs from a 177 driven at 100% over, isn't the same as a 10-71 at 10% under, from a power and detonation standpoint. So from what I see, boost pressures aren't always equal. Kind of why with a procharger you can get away with much more boost on the pump. Its simply a cooler charge.

My thought process is, by switching to a larger blower, I reduced a lot of heat in the intake charge. So, If I ran 7lbs with lots of heat, 7lbs with less heat shoudnt be a issue. Maybe even 8lbs.

Most guys wouldn't hesitate to run 8lbs or more because they have a superchiller. Without any actual temp numbers, we're all just guessing at whats acceptable.

In the book I have here, they did a test, with various GMC 8-71 blowers vs the 420 Megablower (2 lobe). While they said the tighter clearanced blowers made a tad more heat, there wasn't much difference between them all temp wise. They all got about 60* hotter going from 8% under to 15% over. Simply proving the faster you spin a roots the hotter things get. Of course compressing air in itself raises the temperatures, but seems that heat generated by the compressor itself is the big factor in the equation. As you "beat up" the air by spinning the rotors faster, the boost PSI on the gauge will read higher, as the air gets heated. But, the air has become less dense, therefore not making any more power. So, putting a smaller pulley on to raise the boost psi say 2psi on a 177 highly overdriven, wont net nearly as much power gain as doing the same with a larger underdriven setup.

I ran a 177 on my 454's. 115% over. Made 5 lbs or so on the gauge. Never a issue with it. In my boat, with those blowers at 5lbs, best I saw was 78mph. I then switched to 250's at 7lbs. Around 82mph. Then on to the 420's at 6lbs. 88mph. Engines never really changed internally.

As for the chiller question. Most who've installed chillers have noted about 1psi less on the gauge after the install. Whether it is due to the restriction of the core, or the fact the air became more dense, I don't know. I don't think ive ever saw anyone record a IAT to prove any theories. From what Ive seen here on oso in the past 8-9 years, bolting the chiller on in most cases did not increase mph by itself. Turning the boost up after the chiller install, did.

Theres a lot of power from boost, in the right setup. I could not believe on the procharger engine we just played with, what 1.5psi did for power. He did a different cam, with a looser LSA. He lost a little boost because of this. The owner was disappointed initially. But, Once we pulley'ed it to get the boost back to where it was with the old cam, plus an extra 1.5psi, the engine woke up BIG TIME. 75FT lbs/50HP at 3500 over previous pull. Picked up 101FT lbs at 3500, and 67HP over the old cam. His goal was more midrange. 101FT lbs x 2 engines in the midrange should be noticeable.

I completely agree, makes sense it spin slower maybe a lower boost # but more air by volume just because of the density. Everything spins slower works easier with less heat, sounds like a win win all the way round.

So I have still see a few of the well known engine builders with 700-800hp blower engines with no coolers. I think though in that respect its just a safer build to include the coolers from the start with the thought of reliabilty and larger margin of error for bad gas or bad tune up and the option to turn up the boost later on if desired.