If I wanted to bolt a roots style blowers on top of my 600hp 509's. What are the pro's and con's of let's say an 8-71, 10-71, and a 14-71. Does it take more power to turn the bigger blower? I don't won't to run intercoolers, can I run more boost with the bigger blowers and keep charge temps down? The goal is 850hp. Bottom end's are ready for it, 8:1 comp ratio, 93 pump fuel.

the bigger blower makes less heat,really important if you are not useing a innercooler,but 8.71 at 7 lbs will really wake up a 509,been there,done that,run 34 total timing,with a locked out distributor.

So with out running an inner cooler, I should run the 14-71 cause it will allow me to run more boost with less heat, and ultimately make more power? Any negatives to running a 14-71, compared to an 8-71?

a 14.71 wont clear the distributor,8.71 clears easily,a 10,71 might clear,if you use a bds intake manifold.

If you want the blower to clear the distributor you would have to buy a intercooler and you're done... its only $$$$$$:

I believe if you were to run a 14-71, on a 509, it might be too much. Granted turning a large blower slow, can reduce heat, but if you turn it too slow, I believe they can be inefficient, especially at lower rpm, due to leakage past the rotors. Im no blower expert, but I think turning the blower around 1:1 ratio, give or take a little, would be best for marine rpm/boost levels. With a 14-71 on your 509, i would guess you would be underdriving it 25-30% or something like that. A 871 around 1:1 or maybe 10% over should get you the boost you want. Alot of that depends on cam selection and overall engine package. I have found the blower charts to suggest more boost than i see. Boost drops when you have the right cam, good heads and exhaust. Bolting a 871 on a 454/330HP engine might net 12psi at 1:1. But bolting it on a 454/500HP might net 7psi at 1:1. Im no blower expert by anymeans, so it would be best consulting with someone who is more knowledgeable with blowers.

I would think a 8-71 or 10-71 would be more than enough for you. 7-8psi 800+hp should not be a problem. A proper selected blower cam is important too.

Few guidlines to keeping a marine roots blower engine alive
Low Static Compression
Proper Timing
Big fuel System
Aluminum Heads are a bonus for heat dissapation
Cool water temps
I like running dumps off the back of the intake water jackets to cool rear cylinders
intercoolers
A/F ratio set safe, but don't flood with fuel to where your cylinder walls get a bath, and crankcase gets diluted with fuel after a a few runs.
Big Carbs are ok (within reason). Most will tell you its very hard to overcarb a roots blower setup. Its unrestricted airflow on top of the blowers what your seeking.

Two blower motors : 60 grand.
Two intercoolers : 4 grand
Simple math says 4 grand can save 60 grand.

Thanks for the help so far everyone! One of the reasons I do not want to run the intercooler is space. It's not so much a money issue as a space issue. Also will be running highly modified and ported steel heads by JimV, which was another concern with heat, and no intercooler. I thought by spinning a bigger blower I may be able run more boost even though there would not be an innercooler or aluminum heads. But yes motors where built with the plan to throw boost at them in a season or two. Pistons, cranks, cams, lifters, springs, have all been addressed.

If your at 8:1, with the iron heads, I dont think 5-6psi out of a 871 will be a problem without the chiller. The 600SC, was a 502 with a 420 B&M Blower. They were low compression, 7.5:1, with iron heads. I believe stock they ran 5psi, and were very reliable. No chillers on them.

At 5-6psi, I think you will be grinning from the power increase. Like Mike Tkach said, most 10-71's wont work without the chiller.

Is 5-6 psi going to get me to my 800-850hp goals though? Remember I am already making 600hp right now, but that's on 9.5:1. When we make the upgrade, heads will be coming back off for more extensive work and dropping comp ratio down into the 8:1 range. Would I be better off to run chillers, and then I can crank them on up to 8-9 pounds of boost, and Know I can achieve the hp goals?