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2871 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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Thanks, Dan'l. At this point, I'm leaning SBC.

You talk about a scratch build, but there are complete Mercruiser 350 MAG MPI engines on eBay for $5500-7500. Why wouldn't I just drop one of those in? They use isolated front and rear 4-point motor mountings. Can I use those and fab some brackets or rails on the stringers, or would that be too much motion for the u-joints in the driveline over time? I see the adapters you mention on

Some car guy already got the 455.....the boat had no engine when I bought it.

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Hey Dan'l,

The Mercruiser 4-point mounting does use two rear mounts on the bellhousing, but the bellhousing isn't as long so the mounts end up being too far forward. The mount itself is basically a vertical bolt with some rubber donuts around it. So I think the easiest thing to do would be to fab a tubular crossmember to rest on top of the stringers, with a dip to go under the driveshaft.

The Mercruiser engine packages are complete....ECM, fuel pumps (low and high pressure), fuel rail and injectors. In the 2000's they used cable throttle and shifter, analog gauges, separate audible alarm. You mainly need to buy the boat-side wiring harness, about $250. Would take off the power steering pump, plate the water pump or just pull the impeller.

I know what you mean about the weight, the Mercruiser website shows the package at almost 1000#, but I think that might include an outdrive vs. the inboard bobtail.

Thanks for that input. My neighbor built a 454 Chevy for an old Biscayne he restored, but ended up putting in a more period-proper 409. So he said he'd give me the 454 (I'd probably give him something for his effort). He said the build is nothing too special, but still crazy HP. I'd have to marinize it, and switch out the Olds pieces (logs/snails, water pump plate, bellhousing?), but maybe that is the way to go?
Thanks for that input Heatseeker. I'm looking for a pretty mild engine setup, so I'll go with displacement as king. I've got an 8.1L in my 25' bowrider, and coach everyone about getting the cubic inches whenever possible, need to take my own advice. Plus, the Mercruiser drop-in engines seem very heavy, like 300#. I guess they are looking for durability.

My last decision, then, is whether I take the easy path of the Chevy 454 vs. finding an Olds 455. How much does anyone think I devalue a '73 Glastron Carlson CV21 if I don't go with the original Olds?
You all are great, way above my pay grade for all the motor talk. Although I've been an automotive engineer for 40 years, it's mostly been body related.

Once we northerners come out of hibernation I'm going to ask my neighbor if he's serious about making a deal on his 454. His son seemed a lot more positive than him. In which case I'll need 2sangers bag-o-BBC parts.

Otherwise, I see a lot of LQ9 LS motors (6.0L, iron block, aluminum head, 345hp) online and I'm sure a lot around my home with still some GM plants in my area. That looks like a fine starting point, unless I should pay the money for an aluminum block (LS1)? How deep would I have to dig into the motor for a marine-build? I know about the bronze freeze plugs, but what about head gaskets and cam? Of course, starter, alternator, flame arrestor. And how to get my hands on manifolds or logs/snails for less than $1500?
Agree with you on the exhausts....I'm learning about redrilling some 351W Ford logs and snails to fit, weld in an O2 bung. So new cam probably not required, and a used engine should be loosened up nicely for running on the water. The lake is fairly small with a 45mph speed limit, so won't be staying in the throttle for long periods of time. This boat will be my Woodward Dream Cruise.

For the cooling system, CP Performance has a thermostat/crossover kit for the LS that looks pretty promising, with or without an additional pressure relief valve.

Really down now to iron block LQ9 vs. aluminum block LS1. Would be cooler to put the Corvette engine in it (especially with the Corvette motor covers) and lighter, or stay with the iron for longevity?
I live just outside Flint, MI.
This CV21 could be a collector's item when restored, so I'm OK spending a little more if it enhances it's value. I've looked up and down the LS engine spectrum and am thinking LS2. I'm seeing dropouts for $4500. I could save about $1K and still get the aluminum block with the L92, but that's a truck intake....nothing like the LS2 with its Corvette or GTO engine covers in the motor compartment!
Unless someone says the aluminum block isn't a good idea for a boat. It would be freshwater use only, and I'm even considering a closed loop cooling system to not have any corrosion, winterizing, or jet water pressure issues. Iron is stronger, but aluminum expands and contracts with heat about 2x as much as iron, so to have aluminum pistons inside an iron block doesn't sound like a good idea, either.
The block runs cold, but the pistons run hot, hotter than in a car since you're going to be burning more fuel at cruise speed in a boat. So extra-hot aluminum pistons in an iron block being chilled with raw, cold lake water will have a large temperature differential and need more clearance.
I'd prefer those same pistons in an aluminum block, better if warmed by a thermostat-controlled closed loop cooling system. I would think an oil cooler would be a good idea too since its more the oil than the water that will be cooling the pistons. I see there are even piston cooling jets for an LS engine to spray oil on the underside of the pistons.....anyone know if you have to tear down the engine for installation or just remove the oil pan on a stand? Maybe I'll look for a YouTube.
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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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.
True about the cam. Never going back to carbs, though.
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.
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.
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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