Hot rod a mercruiser???

Not GT40 related so please delete if off-topic or posted in the wrong place......

I have an old 1989 Malibu Skier ski boat with a Mercruiser 5.7 (carb'd). I believe this is basically a chev 350 with a bit of dress up for a marine environment including water jackets for the exhaust, add'l water pump, etc. It has iron heads and an iron inlet manifold and a carter carb. Supposedly it puts out 260hp, and it certainly feels it.

I'd like to get some more hp/tq out of it. I'm thinking better alu heads, a different cam, and a good alu inlet manifold should get this thing above 350hp. As is, it scoots my boat down the lake at a solid 47mph and will rip any water skiers arms off who wants to try to hang onto a WOT start. I'd like to be able to get above 55mph, and I think 350hp should do it.

What do I do to this engine to get it cranking out some more hp/tq? Anybody here ever hot-rodded a mercruiser engine?

Thanks.
 

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Keith

Moderator
Alloy heads may not be a good idea, as there is bound to be some electrolytic activity despite being used in fresh water, and you could see your expensive heads 'melt' away. I wouldn't use anything aluminium on the engine at all (apart from the carb). If you can convert it to keel cooling, (dry exhaust) you can run what you want.

I used a Mercruiser/Chevy 350 in a Chaparral ski boat on the great Tennessee River and played around with the exhaust/cam/carb/pistons to get a decent bit of grunt. Getting the grunt is one thing, maximising the rpm is another, so playing around with prop dimension/pitch is paramount. Getting the motor to rev to say, 5,500 with power trim/tab optimised should see 55 mph plus on smooth water.

Is it a shaft or a leg? If a shaft, a pair of Bennett Trim Tabs will optimise the boats 'angle of attack' and if a leg, then correct leg trim is vital.

However,I see it's a ski boat, so it will likely be a shaft, with a hull optimised for producing a 'trick wake' for skiers at around 25-30 mph, in which case it may not be able to get out of it's own way for ultimate high speeds unless you can fiddle with the spray rails.

Lots of fun though... :)
 
The Malibu Skier is a direct-drive boat. Engine's in the middle with the shaft straight out the back. Down here in California people mod these motors a lot. Heads, intake, carbs, and exhaust are frequently changed for more speed. After you mod the motor you will have to play with the prop size a bit for optimum performance.

Personally, I wouldn't worry about tabs. They would help get the bow down at lower speeds, but at higher speeds you want them pulled all the way up as you want as much boat out of the water as possible. IMO 47 mph on the water feels pretty fast. Not sure I would want to go any faster.
 
What is current RPM ? If you have 4" exhaust elbows, exhaust should be good for 400 HP. Your program sounds good, alum. heads are fine for fresh water use. Bias the setup towards torque, and about 5500 rpm max. Mercury thunderbolt ign. is fine and reliable and I don't think 1989 had a rev limiter.

best regards, Doug
 
Thank you gents, good information, and much appreciated.

Hardy, yup, I hear ya - good to be aware of any electrolysis issues... The boat's being used in fresh water only, so I'm hoping/assuming electrolysis is not as big of an issue as in a salt water environment. In contrast, on my sailboat (Fisher 37 in salt water) I have zincs up the yazoo and all kinds of electrolysis defenses. As far as tabs and hull configuration and weight, this boat was built in the days when wakes were frowned upon (how novel!) and skiers wanted as flat a wake as possible....so she's very flat bottomed and gets up on a smooth plane very easily.

Rich, that makes sense - I was figuring that prop size/pitch would have to factor in somewhere to take full advantage of add'l hp/torque. I'm planning on calling Acme to get their expertise (should be helpful). You're right, 46-47 is pretty darn fast in a basic ski boat, it's just that I get passed by certain 50+mph boats when we (my kids and I) occasionally engage in a bit of high speed drag racing on the lake (when completely safe/flat water/no other boats around/etc.) and my kids hate to lose....

Doug, current rpm seems to be about 4,200. She should turn higher than that obviously...might be some limitation in the throw of the throttle cable or something - need to check that out. There's no rev limiter, but she should turn upwards of 5,000 at least as you suggest. I think with cam, heads and manifold I should be able to get close to 400tq.

Zollis, thanks! Yes, she's a lovely old ski boat that was lived an easy life.....until now perhaps!!
 
Cliff,

Nice boat!
Your Merc is probably same as the 350MAG in my 1987 Formula F-206. Different hull configuration but I hit 60 MPH with 270 HP.
Adding another 100 HP will take more than an intake and carb swap. Cam will need to be changed and maybe performance exhaust manifolds too. Then to take advantage of all that new power a change in prop pitch and/or diameter will be needed.
I have a book at my office that covers performance upgrades for Small Block Chevy marine engines. Can't recall the title or author. I'll dig it out and get back to you.

If you really wanted to you could take the easy way out and purchase a complete 383 CI, 425 HP engine from Chevy that has the perfect cam and lots of torque for marine use. I have talked to a guy who installed two of these in his classic 28 foot Cigarette. He said it really woke it up.

And if I was you I'd look into installing a full fresh water cooling system. This way only coolant flows through the block, intake, and exhaust manifolds. Not raw lake water. Also make winterizing a snap.

I was thinking....your hull probable has a "hook" built into it at the stern to keep the bow down and wake flat. You may find the boat plowing at high speed which will just slow it down. Take a yard stick and hold it against the bottom parallel to the keel at the stern and see what you find. It'll kind of look like an upside down spoiler.
 
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The book is titled "Small-Block Chevy Marine Performance" by Dennis Moore from HP Books.
I believe the book is out of print but maybe you can find one. Well worth it if you can.
ISBN 1-55788-317-3

Last I knew Dennis had a store on E-Bay for marine performance parts.
 
You are not even making your 260 HP until you hit 4600-4800 rpm. Check the top of the carb at wide open throttle to see if throttle plate is indeed wide open. If yes, you could try a prop with less pitch to raise the RPM and give you more power and probably even a stronger hole shot, but probably not much more speed. In any case, your current prop will probably be OK if you mod the engine to get more power and thus more RPM.

---Doug
 
Been around boats all my life, and I don't think you will have a problem with an aluminum intake. These boats are normally stored "dry" all their life, and just "wet" when being used.

I had an X45 with 350 hp. Redline was at 5200 but would only turn 4800 probably due to all the crap (I mean stuff....) that I had onboard. I bought a new acme prop with a little less pitch (don't remember whether the diameter changed, and I really liked the performance. Out of the hole faster, same top speed (a blazing 42 mph) as it would now hit the redline.

Maybe your boat is overpropped and can't pull the redline? It just might be that coming down a half inch in pitch may actually increase your speed....
 
Specs on my 350MAG state a max operating RPM of 4800.
I have the original factory 21" pitch prop and it's a dog out of the hole but gives me max top end speed. Swapped it to a 19" pitch and made a world of difference but lost a little on speed.
 

marc

Lifetime Supporter
Cliff you forgot the most important question. How much do you want to spend?
 
Cliff, I would like to suggest that the time to do the mods is about now. Then when the nice summer sunshine arrives it is all ready to go.

Some option to look at is to remove the existing engine and replace it with a 502 c.i. fuel injected one. I have seen one of them and, man it goes well.











Z.C.
 
Ha! Yes, you're absolutely right, now is the time to get it done. I'll be anxious to get the ski boat out on the water by some point in May so the window is closing.

For the time being I have the boat nicely cleaned up and fully maintenance'd. I've done a ton of work to this boat, everything is in beautiful shape now. The remaining piece to work on is the hotrodding of the engine. There's a ski boat site I check out sometimes and the consensus there seems to be to replace the cast iron heads with the newer Vortec aluminum heads, go with a better aluminum intake, port match everything, add roller rockers and a different cam matched to the valves and Vortec heads. The word is that should take the stock hp from 260 up to closer to 330-340hp.

I did put on an electronic ignition system and a few other modest upgrades, and this definitely helped. Now the engine will rev to about 4,800 now when previously it was max'd at about 4, 200. Top speed has gone from roughtly 44 to about 49 though I did see 50 on the gps for a short stretch (tail wind??). I think with the upgrades above I should be able to touch 55....hopefully at least.

Yes, replacing with a 502ci FI engine would have an even bigger impact! Both on my wallet, and the engine mount and hull stringers in the boat. As much as I would like to have that much engine, I think the hull and stringers wouldn't be up to it!

Thanks.
 

Jack Houpe

GT40s Supporter
Cliff I did an efi conversion on a buddy of mines BBC, we got a fast throttle body carb replacement off eBay for $870 and I sold him an old electromotive tec2, with new fuel pump and all the goodies he's into to it for $1800 and it made one heck of a difference in power and drivability. Can't see why you could do the same and maybe put a blower on while your at it.
 

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Jack Houpe

GT40s Supporter
For some reason now I can upload pictures from iPad but only one at a time.
 

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