Like I tell my daughters when faced with a lot of political mumbo jumbo, go to the data!
For the original powerplant question, "old (big block) vs new (GM LS)", I plotted the horsepower "demand" curve of a Berkeley 12J jet pump against the horsepower "supply" curves of the following engines (by graph color):
ORANGE - 1969 Olds 455 @ 310 HP – my CV21’s missing engine
PURPLE - 1970 Chevy 454 @ 330 HP
BLUE - Chevy 5.7L LS1 (car intake, aluminum block) @ 350 HP
GREEN - Chevy 6.0L L96 (truck intake, iron block) @ 365 HP
RED - Chevy 6.0L LS2 (car intake, aluminum block) @ 400 HP
BLACK - Chevy 6.2L L92/L9H (truck intake, aluminum block) @ 400 HP
Some comments and observations.
1.) Pardon my squiggles, but the process is to overlay the engine HP graph over the pump graph in .ppt, hand trace the engine HP curve, then delete the engine graph.
2.) I know all these engines can be modified to make more HP, but I needed to reduce the variables and make an assessment of their starting points.
3.) There's a lot of talk out there about the low rpm torque of the Olds 455 engine. True, and very important when torque is what's needed to turn a car axle to make it go forward. But a jet boat moves forward when the weight of water (volume x velocity) being thrown backwards creates a forward thrust.....kinetic pumps need power to keep that water moving. So for a jet boat, the BBO has a lot of unusable torque at the lower RPM, and runs out of HP just when the pump starts needing it. Yes, it had the lowest HP to start with, but even shifted slightly higher the curves collide. For a propeller-driven boat, it may be back to torque rules.
4.) Of the two old-school big blocks, the Chevy creates horsepower later and longer, so probably a better inherent match for the needs of a jet pump.
5.) I'll assume my CV21 has an A-cut aluminum impeller, so for the "A" pump curve:
- The 5.7L LS1 makes its power too late, it will max out at a lower RPM than the Olds (the BLUE dot)
- The 6.2L L92 will max out at the highest RPM of all the engines plotted. (the BLACK dot)
4.) To really optimize the system, going to a cutdown "C", or even a more standard "B" cut, stainless steel impeller is indicated. Here the L92 (BLUE dot) and the 6.0L LS2 (RED dot) are about tied.
So my bottom line is that, once I get a good handle on the hull and interior restoration of my CV21, I will start shopping for either:
- A 6.2L L92/L9H engine. It still has the 100# lighter aluminum block and horsepower of the more expensive LS2 (about avg $2K more on eBay). For $500 you can swap on the LS3 intake and fuel rails to get the 5" lower intake height and cleaner top of engine appearance.
- A 6.0L LS2 engine, if I want to pay more to be able to showcase with the Corvette LS2 engine covers.
The impeller swap then becomes a stand-alone decision, since either engine would benefit.
I invite comments, or if others want to show their charts using the same process for other engines.
