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engine upgrade/swap

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I have a 93 Malibu echelon lx with the 350 ski mag in it. It sounds like I have head gaskets going out for the second time this year, first time the impeller went out and it got hot. Refinished the heads and repaired it. Now I am getting that same knock, but lately the engine has been "bogging" down after a minute or two when wide open for footing. I'm worried I might have bearings tying up. Anyway, I'm debating replacing the engine verses rebuilding it again (complete rebuild two years ago also). Figure vs putting all that money in a carborated engine, replace with a fuel injected engine, ls1,etc. Anyone done this? Any thoughts? Just trying to consider any options.

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If you want to upgrade your existing unit, not hard to do and end up with great results. The limiting factor you have compared to the newer (post 1996) boats are the Vortec cylinder heads. A pair of heads, ether cast iron or aluminum from a variety of sources (direct GM, aftermarket sources such as Edelbrock) will easily boost the power level to 310+ from your current 265. Assuming stock, you will also want to upgrade your spark arrestor and your prop. When you do the head swap, the exhaust manifolds will not match a newer style port so either grind to fit or upgrade options are the way to go. Additional power can be had upgrading the cam (go only slightly more aggresive, ZZ4 works great) or going to 1.6 rockers. The carb is fine for a boat application and can/will run just fine, it will deliver the power needed (750 cfm Weber flows well more than the 600 cfm needed at 5000 rpm), is easy to clean and tune but does come with the starting and warm up idiocincracies of a carb. A clean, properly adjusted one performes just as well as FI once past the warm up phase.

I have bumped my original Mercruiser 350 up from 265 hp to 400 hp with fast burn cylinder heads, ZZ4 cam, aluminum SM exhaust manifolds and a 4" exhaust system along with some carb/timing tuning and a free breathing spark arrestor (had to upgrade the prop, using a tuned Acme 3 blade). Boat has no problem actually pulling three footers so there is plenty of grunt in a SBC.

Your other option might be a crate engine, they are plentiful and there are a variety to choose from.

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The one good thing about staying with the carb engine is that it will repsond really well to any changes you make now and in the future. The newer engines have a program that's hard to crack to be able to use small upgrades in power. The ECM will only supply so much gas, even if the heads were considerably upgraded...

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Woodski, I am very interested in this setup you are talking about, but do any of you have any suggestions why the boat would be bogging down after a minute or two at wot? Would this have any connection to why I may have blown head gaskets again after only a month and a half? If I can figure out what is happening then I'd like to fix that and add these upgrades.

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I would venture a guess that the bogging is related to fuel delivery, not enough fuel getting to the carb when a high demand scenario is in play. Check the carb for float level, inlet screen debris, any other debris, fuel pump pressure and volume and if nothing there, check the tank and the vent valve (at the tank, it should be a check valve there, simple test would be to vent the tank from the fill hole to see if that cures it). Dumb question, is the spark arrestor clean?

I would think a bearing issue would simply knock and tighten up but not act like a bog and recover. If you think there is a bearing issue, have the oil tested at a lab (find on the web), that will tell you about the bearings. You could take off the oil filter and cut it open and check for bearing material on the filter element, easy cheap and fast to do. If you want to look deeper, I would first check blowby amounts, an indicator of ring condition. Next I would do a leak down and compression test, that should tell you a lot about the health and what you are looking at if you need to dig in to the bottom end.

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Before making any big decisions, figure out what happened or what is happening.

If it's a cooling issue, straighten that out of course, but get to the bottom first things first.

Good luck,

Steve B.

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I would bet on a fuel problem based off the issues you are listing. Follow advise of Woodski and you should be able to solve it.

Stick with the carburated 350 and update to Vortec heads and whatever profile cam is used on current Monsoon. That is a great package for low end torque and power. Attempting to get any more power will more than likely start diminishing low end grunt.

Going with an ls1 will give you very little return for a big spend.

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Woodski, could you send me a list of what you did and what heads you had and so forth so I know I'm getting the right stuff? Can I do those heads and cam without having to increase the exhaust size? Thanks

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I started with a bone stock Mercruiser 265 hp Tournament skier 350 CID small block. From there I added the GM 385 Fast Burn heads, a GM ZZ4 camshaft. That made a big difference. At the time, I port matched the existing exhaust manifolds. The boat needed a different prop and less restrictive spark arrestor (home made unit) at that point in time. Next steps included an Edelbrock Performer RPM intake manifold (actually identical to the GM unit but aluminum so that was done for weight reduction) and some carb modifications (calibration and response, no hp gains) and added cold air tubes to a home made plenum to get cold air to the carb. Added SBC Stainless Marine aluminum exhaust manifolds, 4" tailpipes and made my own exhaust system so it was 4" diameter front to back, which helped again but not as dramatic as the head/cam swap.

Yes, you can do the head/cam without the exhaust, but once you have spent the coin on the heads, I strongly recommend the tail pipes and bigger exhaust as a pretty cheap added bang/dollar. The cam probably adds the least as my boat has soooo much midrange which is not that needed (but fun). As an example, when footing, you actually "hurry" to stand on the feet rather than wait forever, the 2-5k pull happens really fast. I have been overshot on speed numerous times by a driver not paying attention. The throttle never needs to go to WOT for a footing run or even getting up on a deep water start. You can also accomplish a lot with 1.6 rockers and the stock cam, you can get those from GM, Crane, or other vendors.

In addition to gaining HP and response, my other goal was to reduce weight, hence all the aluminum components. If you don't care, there are cheaper cast iron versions of the parts available. You are really targeting the air flow rates of the post 1996 Vortec heads or slightly better. The fast burn heads come with larger valves so that is a help.

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Awesome! Thanks for the input. Looks like I'll have my evenings full for a while this winter. So you are actually still running your 600 Weber carb? I have a rebuild kit so I might as well use that as long as it's good enough.

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Yes, the carb is actually a 750 cfm unit so plenty of flow. It is the same as a Carter AFB, but uses Weber specific needles, jets and misc parts (the needles are three step v. 2 step for the AFB). There are some very good books that detail the carb and David Vizard wrote one on how to modify for best performance.

I keep thinking of doing an EFI install, but it just never really makes financial sense to do it. My boat is everybit as smooth as any of the other newer EFI boats on the lake, so the value just does not seem to be there.

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  • 5 years later...

Hey guys, resurrecting this old thread. 

I want to do a similar upgrade, but my 1995 Response has EFI - the TBI (throttle body). Has anyone done a Vortec head system with this TBI unit? Does it add significant HP? What intake and exhaust system would I need? Can I upgrade the injectors?

I'm trying to get my boat to go from 43 mph to 46 mph for barefooting - need more speed to simulate tournament wake crossings.


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@scotcher3:  Not a big problem, you can get parts to match the HP you are targeting.  Key change you need to make are the cylinder heads, the '96 and newer heads were the change that upper power output from 280 to 310 hp, all due to increased airflow.  You will also want to match the cylinder head to intake manifold bolt pattern, there are options available to match either bolt pattern.  You will want to determine the flow rate potential of your TBI unit to make sure it would not become the limiter in your upgrade.  As for the exhaust side, there are more efficient manifolds available, the existing ones will bolt up but check the port dimensions for proper clearance (there is enough meat to do minor grinding if needed), and you can increase the diameter of your exhaust system to improve flow although that will require specific additional parts (exhaust manifolds or at least risers, hoses, mufflers, tips).

As a side note, I would recommend a ZZ4 camshaft if you do the swap, it wakes up the mid and upper RPM range and is excellent for footing. 

Some good information:  https://www.dynamicefi.com/TBI_Fueling.php

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