What are the differences between an automotive 350 chevy and a marine unit?

[roysbb]

I have found the 350 I want to put in the boat. I can buy a new GM engine from scoggin-dickey for 2069.00.
It is 4 bolt, vortec heads, 9.3 compression, roller cam, etc
I am thinking that with a simple cam and spring swap it should be about 350 HP.

The only issue is it says "Not intended for marine use".
What modifications should I make for reliable marine use?

Thanks, Roy

 

[porscheguy]

Brass freeze plugs
Marine gaskets, especially if open cooling.
I’m not sure if the gm marine engines are painted differently from those meant for cars/trucks.

 

[Scott Danforth]

long block or complete motor. you will only be using the long block

depends on the cam that is in the motor

if its a truck cam, you should be fine. if its a car cam, you need to replace it

head gaskets on marine motors are stainless
core plugs are brass or stainless.
circulating pump is brass impeller and stainless rear cover
intake manifold on marine motors are cast iron or dual alloy (bronze and aluminum)

all electrics, brackets, etc are marine only

 

[GA_Boater]

What cam are you going to use to pump up a 250HP L31 to 350HP?

What outdrive will be behind the motor?

 

[Scott Danforth]

the 350hp is most likely crank rated on the motor. which would equate to about 300 at the prop. however my guess is the motor is NOT a truck motor and is one of the performance motors which would be eaten alive in a boat.

 

[tpenfield]

Core plug, Head gaskets, Cam. As stated from a long block perspective those are the differences.

Regarding the particular engine that you are considering, it is hard to say what aspect(s) of the 3 the engine does not meet.

The cam is the more difficult/expensive thing to change out, the core plugs are the easiest. Cams in Marine 4 stroke engines have very little valve overlap so that the intake and exhaust valves are not open (or barely open) at the same time (i.e. top of the exhaust stroke). As Scott mentioned, a significant amount of overlap, would cause reversion of the exhaust/water mix back into the cylinders at idle speeds.

 

[Bondo]

I can buy a new GM engine from scoggin-dickey for 2069.00.
It is 4 bolt, vortec heads, 9.3 compression, roller cam, etc

Ayuh,..... Welcome Aboard,..... A new GM long block crate motor is a rock solid boat motor, Right outa the crate,....

It's got ssteel core plugs, 'n composite head gaskets,.....
Just bolt on the marine dressin's, 'n Go Boatin',.....
The truck cam in it is plenty,.... if ya go much bigger, reversion can be a motor bustin' Problem,....

It's not for a boat, because they don't want to guaranty a boat motor,....

 

[muc]

Do automotive crate engines come with a windage tray installed?

 

[Scott Danforth]

Do automotive crate engines come with a windage tray installed?

most do. some performance motors also used to come with crank scrapers years ago, however they fatigue and fall into the oil pan

 

[roysbb]

Thanks for all the info. Since I have an enclosed cooling system is overlap still an issue?

The cam I am looking to upgrade to is a GM roller (as is the one that comes in the engine)
and has less duration that the Mercruiser 350 260HP cam.

The GM cam 196/206 .431/.451 versus the mercruiser hyd flat tappet cam with 200/212 .410/,420

I am thinking I should be fine, right?

 

[TyeeMan]

Pretty sure I'm correct on this, overlap is an issue with open or closed system cooling because in either case you are dumping "cooling water" into the exhaust system and then out the boat. The only difference is, in a raw water cooled system you are dumping the very lake water that cooled the engine and exhaust risers etc.
On a closed cooling system you are still dumping lake water but this time its from the heat exchanger and risers. So you could still get some reversion with valve overlap.

 

[tpenfield]

Yes, regardless of open or closed cooling, the joining of lake/sea water and exhaust at the end of the elbow/riser is the same, so the reversion issue remains the same.

 

[aspeck]

The only issue is it says "Not intended for marine use".

If you do the swaps listed above you are fine as far as the motor is concerned. But GM doesn't want to guarantee a road vehicle in a boat application. No matter what modifications you do to the motor, the warranty will be voided as soon as it is installed in the boat.

 

[GA_Boater]

Even in a car, truck or a marine version, a cam change may void any warranty.

 

[jimmbo]

Just buy a long block from MerCruiser, or Volvo and all your concerns are laid to rest, plus you will have a warranty. As for truck cams, 20 years ago my uncle blew up his 4 cylinder 140 OMC(weird engine, half of a chevy SB V8. His 'expert friend' convinced him replace it with a SB chevy from a 1/2 ton truck, bellhousing bolted up and so did the coupler, and cobbled together an extra exhaust manifold and thru hull exhaust . Well even with the gearing for the 4 cylinder, this unit had very poor hole shot, had a decent top speed but took forever to get there. I know others will deny/argue this, but there is enough difference between a Marine Cam and a Truck cam to have an affect on a boats performance

 

[Scott Danforth]

Thanks for all the info. Since I have an enclosed cooling system is overlap still an issue?

The cam I am looking to upgrade to is a GM roller (as is the one that comes in the engine)
and has less duration that the Mercruiser 350 260HP cam.

The GM cam 196/206 .431/.451 versus the mercruiser hyd flat tappet cam with 200/212 .410/,420

I am thinking I should be fine, right?

yes, your type of cooling has absolutely nothing to do with reversion. it is the wet exhaust

For example on my BBC (with closed cooling), I was sucking waster in a full 24" up the exhaust pipe from the point where the water entered the exhaust back to the manifold. I ended up with custom exhaust with a cross-over and a custom curve in the dizzy to back off my base timing to below 8 to prevent reversion from cam overlap

unless you have dry pipes installed, you can still have an issue. (dry pipe exhaust for any boat, SBC or BBC will set you back about $2k)

what is the full cam specs, not just lift. need intake CL, exhaust CL lobe separation.

the largest cam you can run on a small block with wet exhaust an XM 270 HR http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=202&sb=2

in a jet boat you can run wild cams because the exhaust is not a wet exhaust.

 

[Bondo]

in a jet boat you can run wild cams because the exhaust is not a wet exhaust.

Ayuh,.... or selectively dry, or wet,....

If I had a mountain of money, I'd order up a GM HT 383, from my local GM dealer,.....

When it arrived, I'd haul it over to my local Machine Shop, 'n have 'em deck the block, 'n install D-dish pistons,.....
A tight quench, 'n static c/r 'bout just under 10:1 would make that motor, 'bout the Perfect boat SBC,......

 

[Whateft]

FWIW I put a 5.7 long block truck motor in my old 4 winns, lasted 2 years until I pulled it and junked the boat. I took all the marine stuff off old engine and put on new (actually made a OMC/Merc hybrid). Still have it i storage. I used the fresh water OMC cooler (I am in salt water) and that engine sang with no issues for the 2 years. Listen to the guys above and you should be good to go.

 

[thatone123]

Watermen on the Chesapeake bay used to use 440 motors right out of cars for their workboat engines. Up until the 80's some could also build workboats using rack o' eye, which meant no or few measuring tools. OF course these boats did not plane on step over the water.