383 swap for newb.. help appreciated

[kawakazi]

Hi all. I'm new to this forum and looking for advice. I have a 89 Four Winns 245 Vista with an omc cobra 5.7. The engine was just recently seized and I'm looking at repowering with a 383 build. I know this is a novice question but will an automotive short block work in my marine application? I have just recently acquired a 383 short block that has zero hours and still in the bag. The man who built this short block did not skimp on any of the rotating assembly. Best of the best. So while this would save me tremendously on getting back in the water but give me all the extra ponies I need to put all this boat on top of the water. So if this works, what am I looking at? Im assuming that this sb is pre 87. Coupler issue? Can I just swap my heads, intake distributor waterpump and make it happen? Balancing issues; will my omc cobra flywheel work or will I need to have it all rebalanced? Any and all help will be greatly appreciated.
Thanks
Justin

 

[Bamaman1]

Re: 383 swap for newb.. help appreciated

Sorry. An auto engine won't work, as the motor mounts are different. GM rents out their factory to the boat manufacturers from time to time, and they spec out a different recipe for their metals for corrosion reasons. I was thinking a 5.7 marine engine from Mercury or Volvo Penta uses the same mounts as an OMC Cobra, however.

My boat mechanic was telling me he uses a certain Mr. Goodwrench industrial engine 5.7 that has the correct motor mounts for I/O's, and that it's a long block--available at any GM dealership. The engine worked fine, except that it couldn't be used in salt water. I'm not sure that the camshaft is the really correct for marine use--which is usually a low rpm/high torque camshaft. The GM industrial engine used to be very economical--especially if you had a friend that was a dealer that would sell it to you at cost.

You might can swap some GM high performance parts over to a marine engine, but the benefits may not be worth your efforts. As I said, marine engines turnover pretty slowly. A balanced engine would be nice, but they're engineered for higher rpm's. Put a 7000 rpm engine in I/O, and I'd be looking for your outdrive to disintegrate.

I guess most of us are stuck with marginal performing I/O's. I do wish some "mad scientist" would take time to engineer a nitrous system for an I/O--especially a 3.0 or 4.3. It'd be nice to hit the accelerator from a dead stop, then spray your engine for 4-5 seconds--until the boat planes out. I/O's just don't have the pulling capability of a straight inboard. There again a straight inboard doesn't have the speed of an I/O trimmed out fully. Just a thought.

 

[Bondo]

Re: 383 swap for newb.. help appreciated

Sorry. An auto engine won't work, as the motor mounts are different. GM rents out their factory to the boat manufacturers from time to time, and they spec out a different recipe for their metals for corrosion reasons.

Sorry,... That's Untrue....

The Castings are All the Same...

The man who built this short block did not skimp on any of the rotating assembly. Best of the best.

Without Specifics, that's an Unanswerable question....

Best of the Best for a drag racer, is Not the Best of the Best for a Marine motor...

 

[kawakazi]

Re: 383 swap for newb.. help appreciated

Sorry,... That's Untrue....

The Castings are All the Same...


Without Specifics, that's an Unanswerable question....

Best of the Best for a drag racer, is Not the Best of the Best for a Marine motor...

Thanks for the input. I don't necessarily mean this short block was a drag oriented build, I meant he used very reputable crank & pistons and rods and had it balanced. What specifics do you need? I'll be prompt with all info to get me back in the water. Thanks

 

[winterfunguy]

Re: 383 swap for newb.. help appreciated

Yes the motor mounts are the same between auto and marine. I would strongly recomend changing head and intake gaskets to marine spec ones because fresh wafer will corrode auto grade gaskets very quickly. Find out the compression ratio of the pistons used in the build you don't want high compression in a marine engine 10:1 is actually a very high marine compression ratio...remember that a marine engine is always under load. Camshaft selection is also critical, choppy cams with long durations are great in a auto application but will cause reversion in a marine application...reversion is when the engine sucks water from the exhaust into the cylinder during the valve overlap period because marine exhaust is wet. I have found the Lunati Voo-Doo 100, 101, 102 cams to be exceptional marine cams with good dock manners and a 5000-5500 RPM redline. I will caution against Comp Cams, they tend to loose lobes quickly. You will need to reprop as well to achieve desired RPM range once you are in the water. I would also recomend balancing the rotating assembley which includes crank, flywheel, balancer, pistons, rings and rod bearings...you MUST have the complete assembley balanced together you cannot put an unbalanced flywheel on a balanced "crank kit" and achieve a 0 balance engine. Hope this helps and feel free to ask questions.

 

[kawakazi]

Re: 383 swap for newb.. help appreciated

Yes the motor mounts are the same between auto and marine. I would strongly recomend changing head and intake gaskets to marine spec ones because fresh wafer will corrode auto grade gaskets very quickly. Find out the compression ratio of the pistons used in the build you don't want high compression in a marine engine 10:1 is actually a very high marine compression ratio...remember that a marine engine is always under load. Camshaft selection is also critical, choppy cams with long durations are great in a auto application but will cause reversion in a marine application...reversion is when the engine sucks water from the exhaust into the cylinder during the valve overlap period because marine exhaust is wet. I have found the Lunati Voo-Doo 100, 101, 102 cams to be exceptional marine cams with good dock manners and a 5000-5500 RPM redline. I will caution against Comp Cams, they tend to loose lobes quickly. You will need to reprop as well to achieve desired RPM range once you are in the water. I would also recomend balancing the rotating assembley which includes crank, flywheel, balancer, pistons, rings and rod bearings...you MUST have the complete assembley balanced together you cannot put an unbalanced flywheel on a balanced "crank kit" and achieve a 0 balance engine. Hope this helps and feel free to ask questions.

Winterfunguy, thanks for your time and input. With all that being said lets get on the same page. My original omc 5.7 "seems" to be in great condition minus the seized crank. I plan on this being my donor motor to keep this all marine appropriate, assuming all this will be compatible with my 383 short bock; Heads, intake, distribitor, water pump, oil pan, cam, flywheel, starter, alternater, pulleys, timing case cover gear and chain and anything else I might be forgetting. Essentially I'm just swapping the blocks out of the omc. So these are my new questions. I do plan on reassembling this engine with marine gaskets and brass freeze plugs but will this cam, even though its a marine cam, be the correct cam for this new bore and stroke? or should I go ahead and purchase a new cam and lifters correct for this build. Will this omc distributor be fine? ..... I remember this guy saying something about wanting to build a naturally aspirated motor to run with the blown v8s when he assembled this block so I'm afraid it may have a bit of high compression ratio. prob about 10:1. This guy is now MIA so there is now way of just asking him. How could I check? and is there away I could compensate? i.e. thicker head gasket? I guess when I take this in to my local machine shop to have it rebalanced he may be able to confirm ratio. As far as re proping im kinda a dummy when it comes to prop pitch but that will be a later summer project. Right now I just want to get this thing breathing again on limited funds. The fact that I just happen to have a new 383 short block lying around was just a huge bonus.
Again, your help and assistance as well as all others will be GREATLY appreciated!

 

[winterfunguy]

Re: 383 swap for newb.. help appreciated

Kawakazi, 10:1 is definitely on the high end of the compression scale for a marine engine. You should be able to pull a piston and find the stamping number...generally on the pin bosses of the piston, you can then reference what the manufacturer specifies for C/R with specific heads. Do you know what Cam you have in this block? Rule of thumb on an engine build is to build the engine around the cam profile...not build an engine then throw a cam in, on that note I would probably install a new cam and lifters so you dont get reversion and destroy your new engine. Also remember that dock manners (idle quality and low speed operation..ie docking) become poor with long durations and tight center lines of the lobes. The Lunati Voo-Doo 60101 is a good choice if you are running stock exhaust manifolds and you can get away with the 60102 if you change the manifolds to a stainless marine or other performance manifold that has longer risers to help control reversion. GM has used MANY oil pans through out the years so try to get a similar year block or purchase an oil pan to fit the new block...remember most marine oil pans are 6 qts. If you are planning on running the engine over 5500 RPM consider going with a 7-8 quart pan because marine engines are run at full throttle for long periods of time and oil starvation can become an issue. you will not be able to use a 5.7 flywheel unless the 383 crank is an internally balanced version, 400 cranks are externally balanced and use a different flywheel. You can get an extra 30hp legitimately by using Vortec heads on your build they shine on marine engines, this will require a Vortec Intake as well...The Merc. Magnum intakes are a great design and can be picked up second hand for reasonable. The OMC distributor will be fine as long as you don't go crazy with RPMs and compression...you should be ok to about 5500 RPM. I wouldn't expect to run with blown engines without spending mad money, don't spin the outdrive past 5500 rpm or you will be putting in a new upper outdrive, drive showers help but spinning an OMC that fast creates A LOT of heat in the upper gear box. Another consideration for compression is this...8:1-8.5:1 regular unleaded 87 octane...8.5:1-9:1 midgrade 89-90 octane...9:1-9.8:1 premium 92+ octane. with fuel cost this is a serious consideration.