I agree with Crewcheif on that weight placement piece of it being a big issue. Marc has it also with the RPM requirements of the jet drives, although I think 5000-6000 RPM is typically the target @ max HP you want to aim for and then "impeller" the pump up to challenge the motor. I know Marc likes his engines to run like at 7000-7500 RPM, so that's a different animal to tackle, and when you start messing with this kinda edgy stuff you need to add pressure taps into the suction housing and bowl areas for guages to monitor potentially disaSSterous pressure levels.
Not quite as simple as "spin it faster, push more water though, go faster".
On V-Drives, the advantage is built in with the engine backwards to begin with. Most of them equipped with the TH400 or the Powerglide are set up with manual valve bodies. The boats, props, gear ratios in the V-Drive case, etc are setup for the boat to fly according to the engine setup in High gear. Low gear is typically only used in the No Wake Zones, etc. TH400's add that third gear that can get crazy, insane fast and require nuts the size of bowling balls to do it. I had a RaysonCraft with a TH400 that I never even hit third gear in before selling it.
Also, on V-Drives, you're not dealing with an enclosed pressure pump. Just an open propeller that, the harder and faster you spin the most agressive cut you can, the better and faster. Faster spinning on THOSE, though, also wears out the V-Drive cases, bearings, bushings, seals and shafts also, so there ARE limits there.
So there you have it. For the most part, the Jet Drive is simply designed not to need all that. It's impeller and inherent design is intended to work at the max efficiency it can whether idling in a NWZ or flat-footing it at max speed. Think of how the centrifugal clutches work on Snowmobiles, Old Rupp Mini bikes, older Kawasaki Mules, etc. that use the goddam huge belt.
That's kinda how a Jet Drive operates.
Also, understand that if a transmission were used on a jet drive, when it shifts from say .65:1 to 1:1 or whatever, that is also 35% more HP needed to spin it at the same RPM........
Ray
