building a marine engine Questions

ok this is my first boat 24ft bayliner. full throttle it will only tack out to about 3500 RPM ive been told that it should run up to 4200 -4500 once it planes out. i tryed a smaller less pitch prop and lost a lot of speed but never picked up the RPM's im also using oil i havent got to a compression check but i think it will show a warn and tried engine. its a 5.0 volvo penta (AKA GM 305 right?) what i would like to do is pull a 350 out of a car in a junk yard and use the block. i intend to replace the soft plugs with brass and use my 305s water pump (marine) or replace it if needed. i also have found that the 5.7 and 5.0 use the same cam so i would use the 305's cam if its not worn.
heres were i have questions beyond these things. are the heads differant for marine if so would using my 305 head kill my top end badly in a boat?
anything else i should address or watch for. this is my attempt to save some money and move to a bigger power plant in one pass. whats your opinions?

Re: building a marine engine Questions

i havent got to a compression check but i think it will show a warn and tried engine.

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Most marine engines don't wear out. They blow up or die from corrosion, but seldom wear out.
Before you start looking for another engine, I would suggest you do some fuel system tests as that is the most common cause of Low WOT rpm but there are other things as well. Here is a list of possible causes.


Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation (Full tuneup done recently)
• Remote control cables and linkage for proper travel to open throttle plates fully.

Re: building a marine engine Questions

I would also strongly suggest running a basic compression test before changing the motor. You might just have a few burnt valve seats or incorrect valve clearances..

Gary

Re: building a marine engine Questions

i intend to check out and not have to pull the Engine, My question was based more on problems i would run into building an engine as i stated above.

Re: building a marine engine Questions

Basically, I've heard marine motors use stainless head gaskets, besides the items you mentioned. I would use the cam the 350 came with, don't go changing it. If anything the heads may be better on the 350 with bigger valves.

Re: building a marine engine Questions

Re: building a marine engine Questions

so its safe to say a marine head is the same as a car/truck. i heard a tale of bigger water passages and ports for the exhaust cooling. this sounded like a story to me but i didnt know any differant. on the cam ive read the cams for marine engines have a grind for marine applications and the stock cam for a volvo 5.0 and 5.7 are the same i figure that would keep the engine performing as a marine engine should.

Re: building a marine engine Questions

its a GM its stamped on the centerhead.

Re: building a marine engine Questions

Re: building a marine engine Questions

So yes, the heads from the 350 will work fine. They do not need to be marine specific.

Re: building a marine engine Questions

305 heads will have smaller combustion chambers and likely smaller valves. THis will raise your compression ratio and I would not do this in your application. I have put some 305 heads on a 350 (FYI, the 305/350 share the 3.48" stroke, but the bore is larger on the 350) in my S10 to raise the compression ratio but that's my street machine.

Re: building a marine engine Questions

chevy 350 block froma corvette, new rebuilt heads fro cylinder head exchange from a chev 1500 pickup, a set of brass freeze plugs, all other parts moved over except new gaskets of course, and engine rebuild kit. did it, usin' it, works great, now officially a MERC 260HP. good luck, paul

Re: building a marine engine Questions

Haut Medoc said:

I would not put 305 heads on a 350, that will not help with performance at all.....
The cam should be OK, though......​

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Actually it would raise your compression and WILL give you performance. I dont think it would work well on a boat though. that type of mod is usually for street cars.

the only difference between a marine motor and a car motor is stainless steel head gaskets and the core plugs. other than that maybe a RV/truck cam for more torque but otherwise the heads aand block are the same.

make sure your outdrive can handle the extra HP and torque the 350 will put out
Rob

Re: building a marine engine Questions

Just my 2 cents but if you were to go to all the trouble to build a new motor correctly. That is get a block cleaned, checked, and machined the last thing I would save money on was the cam. There are far too many available from reputable companies for reasonable prices, and believe me, cams can fool you. Too many times I have seen guys rebuild a motor reuse the old cam and for some unknown reason the cam wipes out 6 months later. Second mistake I heard was a junk block from a car. Use only a truck or Corvette block. Cars only have 2 bolt mains, trucks and vettes use a four bolt. Also the casts themselves are different they hve thicker water jacket and cam galley walls, which leads them to be less prone to cracking between freeze plugs and thru the cam galley. As to using the 305 heads. If you get them cleaned and magnafluxed and they are not cracked, there are pistons available with the correct dish in them for using the 54cc chambers of the 305 heads on a 350 motor. They will give you approx. 9.5 compression. However yes these heads will hurt the top end of the rpm range because they have the smallest intake and exhaust runner of any that Chevy made. Look into a set of Edelbrock Performers, they are aluminum, they will be brand new, and you can get them for about $900 a set. I realize this is hard on a budget but it costs about $450 to have a set of heads cleaned, checked , new seals, and a good 3 angle valve job. That's my 2 cents, I may be new to boating but I have 15 years of street racing and drag racing under my belt, have worked in a machine shop, and have scattered more motors than I care to think about. We used to have two sayings at my buddies machine shop 1) Horsepower is easy to make it is just sometimes hard to pay for and 2) With the hood closed "she" really can't see how much money you spent.