Repower options....4.3

[jdtower23]

I currently have a 4.3 OMC Cobra....probably going to get a reman short block/long block....

Was thinking about changing to a 350....how difficult is this? What other stuff would I have to buy as opposed to using a 4.3 longblock?

Also, what's the most power you can put into a Cobra drive?

Thanks

 

[95yj]

Re: Repower options....4.3

Just an opinion here; don't buy a short block even though it looks like you'll save a couple dollars by reusing heads and cam or whatnot. Spend the extra money on the long block and by the time you're done assembling everything you'll be pretty close to what you would have spent on the short block plus gaskets, possible valve job, etc. without nearly as much headache.

 

[jdtower23]

Re: Repower options....4.3

Yeah, I'm pretty sure I'm going to get a longblock, even though I have heads that just had a complete valve job done on them....I could even sell my heads and make up for the difference...

 

[wca_tim]

Re: Repower options....4.3

I just considered the options you're looking at. Wound up going with another 4.3 long block and overall am pleased with the result. Why 4.3 instead of a small block? Fuel economy, and the fact that to switch to 350, need distributer, wiring harness (at least part), intake and carb, exhaust, tin (oil pan, valve covers, etc...) and some other odds and ends I'm sure I'm forgetting right now. All that stuff adds up, and with gas prices the way they are... in addition a solid 4.3 is a good running engine.. However, part of me wishes I went with the 350. would be nice to go faster... although, 49 with just me isn't too bad considering I can run all day on a tank of gas... Don't know about cobra drive, have a mercruiser. Good luck with the choices!

 

[martyh]

Re: Repower options....4.3

When I first got my boat I started thinking the same thing, because it?s also 4.3 powered. Throw a small block in it and go faster. Well, I talked with the machinist at my work about it and he said the same thing as everybody else. He asked me how much time and money I wanted to invest in chasin down parts or fabricating things to make it work. Not to mention it will be heavier.

He tells me the 4.3 is basically a small block less 2 cylinders. So, keep it simple and if I wanted to make more power just put some go fast goodies on the 4.3 and tune it to run great. So, following his advice, I am collecting parts as I go, and I am still using the boat while I am building the new engine.

This is what I am building.

I am using a 1988 4.3 engine block (mechanical fuel pump block)
Balanced rotating assembly, flat top pistons
1998 4.3 Vortec heads with 2.02 intakes and 1.60 exh valves
Ported heads to clean up the flow and springs to match the camshaft
Comp Cams XM256H or XM262H (have not decided yet) marine cam specs
Edelbrock 2114 intake manifold and a 4 Bbl carb of some flavor (yes, marine type)

Not really anything special about it, the parts are readily available, I got my Vortec heads off of craigslist, but you can find most of them on ebay. Best of all, for the most part, everything fits back in the same place. I think I will have to deal with carb linkage but it shouldn?t be that big of a deal compared to swapping in a complete V8 engine and fabrication many things.

Not sure what the hp figures will be, but it should be more than what it is now, and based on the heads, intake, and cam specs, it will flow to 5500 rpm, but I will keep it under 5000 rpm.

Hope this helps.

 

[jdtower23]

Re: Repower options....4.3

Anyone done the swap to a 350? Would like a couple more opinions before making a final decision.....

Also, seen a few Vortec 4.3 base motors, brand new for around $1995....comes complete with pan, valve covers and intake manifold.....would need to add an electric fuel pump....would this be a good option?

Thanks

 

[cooter2506]

Re: Repower options....4.3

Its all about how much you wanna spend on your boat. But if it were me I would go with the Vortec 4.3 gives you just about as much HP as a small block 5.0. Mine has a 5.0 and I wish I had a 4.3 just because of gas consumption.

 

[jdtower23]

Re: Repower options....4.3

Talked with Raul at Rapido....one thing i have a concern about with the new style Vortec is my shift linkage bolts to the intake manifold('88 OMC) He said a 93-96 Vortec may work since it has a different intake manifold than the newer ones.... so I'm going to confirm this and this may be the way to go....

 

[jdtower23]

Re: Repower options....4.3

Anyone else have anything? What do I have to worry about changing to a later Vortec 4.3....was considering the Vortec with 12 bolt intake.....

Thanks

 

[95yj]

Re: Repower options....4.3

If you go with the Vortec motor, do yourself a favor and buy the entire kit for the electric fuel pump. It's around a couple hundred dollars, but gets you all the wiring and hookups to make the fuel delivery USCG legal. Much easier than figuring out how to do everything yourself and IMHO it's worth the $$ for the trouble.

 

[jdtower23]

Re: Repower options....4.3

Thanks, I will do that....

Any problems I'd encounter with this swap? What about the flywheel?

Thanks

 

[wca_tim]

Re: Repower options....4.3

I went with a long block from rapido. Got them to do vortec heads on the old style block and went with Edelbrock intake and 1409 carb. went with whatever marine spec roller cam they put in, and aside from needing a little tuning work (the suggested starying point for the carb is still a little lean I think), it runs great. That and the fact that I hit a rock or piling or something near the ramp in the river (going slow) and snapped the lower drive shaft. Put that back together this afternoon and am up and running. am still breaking in, but have hit 49 without going over 4500 rpm. As I trim it up, the motor feels really strong at 4500 and wants to run right past there, and keps going faster... we'll see how far that goes after I get some more hours on it and an oil change so forth...

 

[whywhyzed]

Re: Repower options....4.3

I went with a long block from rapido. Got them to do vortec heads on the old style block and went with Edelbrock intake and 1409 carb. went with whatever marine spec roller cam they put in, and aside from needing a little tuning work (the suggested starying point for the carb is still a little lean I think), it runs great. That and the fact that I hit a rock or piling or something near the ramp in the river (going slow) and snapped the lower drive shaft. Put that back together this afternoon and am up and running. am still breaking in, but have hit 49 without going over 4500 rpm. As I trim it up, the motor feels really strong at 4500 and wants to run right past there, and keps going faster... we'll see how far that goes after I get some more hours on it and an oil change so forth...

I'm a day late and a dollar short here, but with adding that much power, I wonder if you should have ordered a bit different lower unit ratio...

 

[wca_tim]

Re: Repower options....4.3

Not sure about different gear ratio - isn't that determined by the upper gearbox on the drive unit? Am playing with prop options now. Currently have a 23" laser II. We were on the river this evening topping 1 - 2 ft waves this evening going around 45 (gps) into the wind at 4200-4400 rpms. not sure how to get most out of the set-up. Would consider a different gear ratio (have 1.84 now), but I just don't have a lot of experience with prop set-ups and this is my first i/o. THinking about the gear ratio versus prop pitch and design changes is confusing for me. I assume that it's all about having the engine in the right rpm range to make the most power and translate that power into forward motion most efficiently which would then be a balancing act between gear ratio and prop choice? I don't have a clue how to think about where the sweet spot there is or what the best range of prop pitch and % slip for a given speed is and if this would then dictate the optimum gear ratio for a given set-up. I do know that it runs a LOT faster with a significant positive trim with the current set-up. Once I get the carb jets / metering rods fattened up enough in the mid range and see how far it really pulls hard, I'll think about another prop or possibly different gear ratio. Who knows... All I know for sure is that my 12 year old daughter and I were grinning ear to ear this evening tucked in behind the windshild zipping down the river...

 

[MikDee]

Re: Repower options....4.3

Sounds Good! What you need to do is find out how many RPM 's your motor will do at WOT, then prop up figuring -200rpm per each increase of pitch (normally 2" difference) till you reach 4600rpm WOT lightly loaded. This is where the gear ratio rears it's ugly head, because it's hard to find most props over 23" pitch, especially because they get smaller in diameter by design, and eventually will loose grip, and slip too much, or blow out in a turn, unless you have them on a real light boat. With a 1.5 ratio reduction gearing, instead of a 1.84, your prop would be turning faster, per engine rpm, putting more load on the engine bringing your engine rpm into it's proper range.

Then again, I thought the 4.3 had the same 1.5 X1 ratio drive as the V8's?

 

[wca_tim]

Re: Repower options....4.3

seems like the more i trim it out, the faster it goes and the further the rpms go up. easily would go past 4600 at WOT; running close to 50 (49.2 gps) with the first / older 23" laser 2 on it. Seemed like I could still trim it up a little further. That was also before putting the new lower unit on (without the whale tail that came on the original), and newer prop (another 23" laser two, but with plastic hub that will hopefully give before the drive shaft and in a lot better shape overall) it was a little slower getting up on plane, but once it planes out, seems to have a good bit more get up and go. I have a eeling some of the lower rpm power will get a lot better with jet changes - it's currently got a significant flat spot in the bottom end of the rpm range. it was too rough and windy this evening to get a good gauge of how much real difference the wing and prop changes made.

How does blade surface area play into all of this? ie. does a little more slip with higher pitch blade get you anything?

I'm guessing the boat weighs around 2200 - 2300 as it sits with a full tank of gas, etc...

ps. sorry to hijack your thread there JD...

 

[riverjet502]

Re: Repower options....4.3

Why not reduce your compresson ratio down to 8-1/2 : 1, freshen up your heads, Back your timing up to @ 28-30 degrees total, then install a blower say with 4-6lbs of boost... Then you could go hunting for small blocks???? All on pump gas...

 

[wca_tim]

Re: Repower options....4.3

riverjet, Thanks for the thought...

believe me there is a copy of an article where they got 500 horsepower out of a blown 4.3 pinned to my bulletin board right next to the brochure and information from procharger. Based on the homework, could very well run low boost assuming wind up with hp in mid 300's and very likely net something like 65-70 mph without just completely destroying the outdrive or fuel economy.

I put a long block in it and have the block and rotating assembly from the engine I got with it on the back porch. if I get a wild hair (and have enouhg play money this winter), I may run it over to the machine shop and see what kind of shape the blocks in. have the block and crank reworked, and then rebuild the powerplant using current (nearly new) heads with new lower end and procharger with intercooler. That would clearly require a different upper outdrive with say 1.5:1 gear ratio as well... probably another 5k+ by the time done with it...

Of course, might also keep this one the way it is for family ski-boat with good fuel economy and look for a good find that's even better suited for higher speeds and open water... something a little longer, say 21-25 foot with a fast, shallow hull with a pad and bigblock / bravo drive or maybe outboards... just have to wait and see... until then, see ya on the water...