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2946 Views 33 Replies 7 Participants Last post by  rbay
Hello Everyone,

New to this forum. Posed this question to the Minnesota Glastron Carlson forum, looking for any and all input from all of you:

I recently purchased a 1973 Glastron Carlson CV21 to restore....I think I can keep the interior and exterior pretty close to original. It has a newer Berkeley jet pump, but no engine. Two paths engine replacement could take and stay the same 320 hp range at about the same engine "ready to drop in" cost are:
1.) Build and install an early 70's vintage Olds 455 big block like original. I'm pretty certain I would use a Holly Sniper EFI fuel system and an HEI distributor, but otherwise pretty old school. Would integrate into the boat's electrical system, gauges, etc. like the original. It came with Olds 455 logs/snails, water pump plate, and bellhousing. Servicing it would be a project for the owner/mechanic or hot rod shop.
2.) Install a used early 2000's Mercruiser small block, like the 350 MAG. Would have modern ignition and EFI. It would take about $400 worth of wiring, switches, and alarms to integrate it into the original dash and controls. I don't know how easy it will be to connect the bellhousing engine mounts to the pump. Any Mercruiser service center could service it for me or any new owners.

So when thinking about reliability, driveability, sensory, fun, resale, servicing, what would be your vote? I know fuel economy will favor the newer engine, but Michigan summers are pretty short and I don't get to put that many hours on my boats, so that takes a backseat to the other factors. Or am I missing some things to consider? I know there are higher powered options, but 50-55mph is fast enough for me, wife, and puppy to cruise around the local lake. Key to me is confidence we will get back to the ramp without a tow.

Thanks for you thoughts.....option 1 or 2? I know there are a lot of car guys trying to make this same old vs new motor decision on their classic rides, too.
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I've come to the same conclusion, 2sangers about wanting an LS. I've looked at a couple of the videos.
I've started watching the LS1 and LS2 engines on eBay, but I need to get the tarp off the boat and do a deep dive on how much work needs to be done before I can think about adding the power, particularly if I need to rebuild the stringers.
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

Slope Rectangle Rainbow Plot Triangle

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.
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The Berk chart is a bit antiquated.
My SBC (mostly a 96 suburban 5.7) would run 4650/4700 rpm. That's up with your green dot 6.2 truck motor.

My 454, with an American Turbine AA (slightly Bigger cut than a Berk 2A) pulls another 200+ rpm than the chart.
JBP s impeller Dyno is more accurate, in my opinion.

There's a bit of a difference using corrected Dyno charts that the manufacturers use, they Dyno with all installed accessories, and catalytic converters/ mufflers and 10' long exhaust.

For instance - '96 suburban motor is rated at 255 HP. In the boat, it's at 325 HP, and by the Berk chart, should only turn 4100 rpm. I turn 4650/4700 rpm.
Summit and Jegs rate the same crate motor at 330 HP.

What ever power plant you plan on running, I would have a larger impeller and cut it down to reach YOUR desired rpm.

Remember, it's all horsepower hours. And rpm means ruins peoples motors.

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I'd check out Tom's impeller charts -

I'd check out Tom's impeller charts -

Hi Dan'l,
Appreciate the input.
Before jumping to the other pump curves, if I use your observation that the HP curves published by the OEM's are low for various reasons, and adjust all the HP curves up by 75 HP (shift the pump graph down 75 HP), I come close to your observed RPMs realized in the attached. Since I am really only using this information to make A vs. B comparisons to decide on an engine and maybe an impeller, and not absolute RPM predictions, I think I'll keep the analysis at this level for now. It does further suggest, though, that:
  • The BBC and all the LS engines outdistance the BBO by greater amounts.
  • The 6.2L L92/L94 is a nice fit and at max HP with a "B" cut SS impeller.
Rectangle Slope Plot Font Triangle

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Sorry, meant to say L92/L9H engine in the previous reply. The L94 has Displacement on Demand.....don't think I want to get involved with that one. The L9H is a L92 with Flex Fuel (ethanol) capabilities, which I would not run but suggests the fuel system more resistant to the ethanol now in most gasoline grades.
You don’t need to worry about any of that nonsense in my opinion. You’re going to change cam before it goes in the boat, and you won’t use the OEM EFI system.
True about the cam. Never going back to carbs, though.
1973 Glastron Carlson CV21-
That big boat is going to want a big impeller.
An A at least. Maybe even a AA or A2.
While a B impeller will work, an A impeller will give you better out of the hole for a big cruiser, at the expense of a lower top speed.
Jet boats operate on thrust. Nothing else. Smaller impellers have less thrust at the same rpm. On a big boat, more thrust is your friend.

Just my $0.02
OK, Dan'l.
I bought the boat then immediately set it on a storage lot. Hope to have the winter tarp off the end of April. I'm thinking it has an aluminum A impeller on it right now, but can't check until you know if I can tell without disassembling the pump? (This is my first jet) So that will be the first impeller the engine would see. If I'm only getting low 5k RPM, I would think I would want to go to a smaller cut to let the engine rev into its power band a bit more.
This boat is extremely lightly made vs. my 25' 8.1L bowrider.
True about the cam. Never going back to carbs, though.
Correct, but aren’t you going to run a Holley or other aftermarket EFI stystem, not the factory system?
I would have used an aftermarket EFI if I went BBO or BBC. For an LS, I would stay OEM GM. Tons of hardware and software to use. Biggest issues to work out:
  • the O2 sensor(s). Don't use (open loop EFI) vs. use (limits exhaust manifold options)
  • the fuel return. I think there are also non-returning options.....extra lines with fuel under pressure generally frowned upon on a boat. Affects the entire fuel delivery system, including need for cooling if returned fuel doesn't go back to the tank.
The tag on the pump would tell you what it was shipped with some 50 years ago. Likely still an A, but all bets are off, lol.
On your particular application, if it has an A impeller, cutting it down would yield negligible performance gains.
If it's been sitting a length of time, it would be a good idea to go through it and freshen it up. Then you know exactly what impeller is in it, or change it accordingly. Wear ring time too.
Amazing what a fresh pump is capable of.
Without hard data, I would think a 6.0 and an A impeller would push it to the 55 mph neighborhood.
At least that's where I would start.

OK. The seller and the owner before him said it had a newer pump, but the history of the boat is sketchy at best (can't get a good answer where the engine is, or why it was removed), so it will be Christmas in May when I get all the covers off it and dig in....
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