Thread: Marine application

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I am in the process of deciding on whether to rebuild a 305 with some performance mods, or to replace it with a 350. Since this is a marine application all of the spark arrested parts from the 305 would have to be put on the 350 (carb, starter, etc, etc). Does anyone have experience in marine engines??

Does anyone know the internal differences between a marine & "land" engine - a marine engine runs higher rpm (3200 plus) for cruising.

To the best of my knowledge the heads are more along the line of high rpm car heads - true???

Can 305 heads be bolted directly onto a 350?

Sorry for all the questions but I can't find any forums that deal with marine applications - Any help or direction would be greatly appreciated.

Thanks

People say marine engines have heavy duty valvetrains and a different cam shaft to help the engine survive the high rpm cruising speeds. This may be true for some engines, but I do not believe it true for the SBC. The only difference in the structure of the engine is that all marine 350's are 4 bolt main blocks to help with reliability at those high rpm cruising speeds. You will need to change to brass freeze plugs as well as stainless steel head gaskets if you are going to use raw water cooling. The bolt ons like the intake manifold, carb, spark arrester, exhaust, distributor, etc... will all carry over from the 305 to the 350. I do not recommend you reuse the 305 heads as they have smaller combustin chambers and your compression ratio will be very high. You will need to get a new prop with more pitch to efficiently use the added torque of the bigger engine.

We did quite a few similar projects in the mid 80's . My first questions concern what is backing the motor up and how much of a gain are you expecting to see. The 305 marine motors were identical to their over the road bretheren, low compression cast pistons and crank. They will respond to the normal upgrades.

Q1 - What's behind the motor? If it's a small I/O then check to see the torque ratings. We tore the bearing casings out of a couple of Volvo-Penta outdrives after stepping on the 305's.

Q2 - Is it an enclosed engine compartment or open? The motors like better flowing exhausts and intakes, do you have room for both? With riser exhausts starting around $800, be sure you can fit in a free flowing exhaust system before you go too deep into the motor.

Q3 - How much of an improvement are you looking for - 10/20/30% or more? The switch to a 350 gives you 15% right from the start, the parts costs after the core charge are the same.

Regards, Mark

Thanks for the comeback people:

I'm not really looking for any major power upgrades - lets face it fuel consumption in a boat is high enough without the recent gas price increases & generally more hp = more fuel unless you get it by becoming more efficient

In my case i inherited a 25 ft boat that needs the transom rebuilt due to rot which means the engine & outdrive must be pulled - stringer OMC outdrive rated up to about 270hp I believe so I have some leaway re power (305 probably 220- 225hp stock) & the outdrive is being rebuilt.

The 305 4bbl hasn't been run for a couple of years but a local marine shop indicates the thrust is within spec but #4 cylinder only at 90 psi - others all 120 to 130, so I suspect sticky valves(?) & plan on pulling the heads doing a check plus a home brew dremel job on the ports to ensure good matching especially for exhaust

The engine is mounted under the rear deck & if I wanted to run the exhaust through the transom it would be possible (as opposed to through the leg & out the prop) but it would really have to be cost effective - what gain vs what cost.

The 350 swap would only be an option if I could find a unit cheap, in good shape with total cost less than a minor tweak on the 305.

Bottom line - would like to find 250 - 260hp, clean up the heads & recam??)at reasonable cost, while still being fuel efficient, & end up with a unit that will be reliable.

Thanks again for any thoughts

You can buy a brand new 260 hp 350 from GM for $1200. Change out the freeze plugs for $15 and head gaskets for $65 and you'll be set. I have one in my boat.

Get the marine back exit headders, u cut 2 holes in the back, put some lining in the holes, put the headders through. Some are water cooled ( water is passed over the tubes cooling them off ). They make small block boats rumble.

Originally posted by 76GMC1500
You can buy a brand new 260 hp 350 from GM for $1200. Change out the freeze plugs for $15 and head gaskets for $65 and you'll be set. I have one in my boat.

As long as the boat has a heat exchanger instead of raw water cooling, none of that is needed, the plugs or gaskets that is. As the heat exchanger runs regular antifreeze through the engine, the exchanger box has the water pumped over it, cooling the antifreeze. the water pumped over it is the raw water, in effect, it is a water cooled radiator. Our boat has a 488 Mercruiser I/O ( 3.7 L 4cyl ) with this system, it's an '84 Imperial VC230, don't know how common this set up is tho, and he did not say if his boat had it.