Here are a couple thoughts:
First, how much power are you looking to make?
You can achieve a really scary ride with a legit 6-71, stripped and clearanced for gasoline. In a discussion with Littlefield Superchargers, when i picked up my GMC 6-71 a couple weeks ago, it should be producing right around 19 PSI at my current 10.9% overdriven. Years ago, in a conversation with Mert Littlefield, bigger blowers have longer rotors, have more internal leakage because of the longer sealing surfaces, and more rotor twist. Combine that with more internal friction and additional weight, in a location that is not jet boat friendly. The point is, how much do you really need?
Positive displacement blowers produce a lot of bottom end torque, something that jet boats don't utilize very well, until the impeller is fully loaded. You will really want to stick with an A cut stainless impeller. A smaller cut impeller might get you more top speed, but you will blow the pump dry on the hit. Yes, an inducer will help.
Build your engine for peak power in accordance with the impeller charts. However, you don't want the engine to make peak power at 7500 if the impeller is going to stall it at 6000.
The additional weight of a blower has already been mentioned here and in other posts. I have not had the opportunity to verify the weight of my 6, but when about to lift it into the truck, I was told, "It's only 60 pounds!" It didn't feel that heavy. Each size jump, 6 to 8, 8 to 10... adds an inch of length to the case and rotors. I don't see a significant difference there. What I can say is that a TBS 10-71, carb adapter, and twin Dominators feels HEAVY!
I'm not entirely sold on the cost/benefit ratio of the current intercoolers available for roots superchargers. Don't get me wrong, there are a lot of benefits to intercooling. Before I jump off that bridge, I am going to investigate Intake Air Temperatures (IAT) further. The harder you overdrive the blower, the more heat it will put heat into the IAT. Again, it comes back to how much do you really need?
I have been researching quite a bit about my own build. My findings are:
* Dart Big M standard deck block, 4.500-4.550 bore
* 4.0" stroke crankshaft. Longer strokes just add unwanted bottom end torque.
* Optimize the pistons for ring pack and pin height.
* Make the connecting rod length fit the block, pistons, and crankshaft.
* As with any engine, the best flowing heads are beneficial. I'm looking at the Brodix Dragon Slayer 320.
* Porting is not beneficial in a supercharged application until you really start pushing your limits. If you have to do a lot of porting, you are starting with the wrong heads.
Some things I feel are vital:
* A double keyway crank
* 4130 double keyway hub, so it doesn't split
* .187 or better yet .200 wall thickness wrist pins.
* I would not skimp on valves. I would strongly recommend Inconel exhaust valves.
Building an OE engine, I suggest:
* Bore the block the absolute minimum necessary to clean up. Overboring allows too much flex in the cylinders.
* Same with decking the block.
Cheers,
Joe
