305 to 300HP 350 conversion

[mudweiser]

Found a deal on a 1969 Chevy 4 bolt main, steel crank 300HO 350CID. I'm picking it up this afternoon. It's the new engine for my 1989 Bayliner 2050 Arriva. It had a tired 5.0LX and the Alpha 1 with 1:50 ratio. Is there any reason the outdrive will not handle the added power? I plan to rebuild the 69 engine. I know, it needs hardened seats, new coolant plugs, marine head gaskets and such. Is there any thing else i should add to my list when doing this conversion? I'm planning to use the distributor, starter, alternator from my mercruiser 5.0LX. Will they work? Any suggestions on a cam, intake and marine carb set-up for the new mill? Such as compression ratio, Hyperutictic pistons, cam, 383 stroker and vortec heads? Thining the old heads will make more power then the vortec.... Thoughts?

 

[RCSConstruction]

Re: 305 to 300HP 350 conversion

Personally, and that's if it's a real 69 4 bolt steely, 2.02 heads,HO cam, etcc.., I would fix it up and sell it or part it out to buy a already built marine motor. That motor would be a waste really when a regular ole 350 truck motor will be the best.
Just my .02

 

[John_S]

Re: 305 to 300HP 350 conversion

yea, start with a 350 of similiar vintage to your 305. Its been awhile, but that '69 engine doesn't have allot you would want in a boat engine. Along with valve seats, compression is too high at 10.25:1. Cam would need to change, too. Lowering the compression and changing cam will put it back to around 250-260hp. A stock marine vortec 350 w/4brl will get you 280hp at prop.

 

[Bondo]

Re: 305 to 300HP 350 conversion

Ayuh,...... I Agree.......

Also,.... The Dipstick is on the Wrong side,+ the Alpha Won't take the Power........

 

[riverjet502]

Re: 305 to 300HP 350 conversion

Well if you aquired the 350 block already I would use it. Run it through a quality machine shop to bring the block back square with the world so to speak. Forged crank is a good plus! I would run forged pistons as well, just not a big fan of the hyperutictic pistons. Make sure the machine shop understands that this will be a marine engine and not clearanced for auto duty... Contact crane cams and tell them what you want out of your motor and what type of heads you will be using and what your finial "cr" will be, with what type of exhaust you will use also. Be honest here! They will set you up with a proper cam. I would go with a aluminum edelbrock performer rpm heads, with stainless steel severe duty valves. ( Cast iron vortec's are good heads for the money but aluminum will save weight & disapate heat much much better plus these heads will take more cam lift then the vortec's. Gennerally the aluminum heads will let you run a full point hight on cr then iron before detination with same fuel, as a rule of thumb.) Then run a edelbrock air gap dual plane intake with a 650 marine holley on top. Your starter, alternator, & distributor will work fine. I probibly would swap the dist over to a MSD marine. You should end up with a lot more then 300 hp when done. Will your outdrive handle it? Good question? Depends on it's condition. If you don't go jumping wakes getting air borne or doing one hole shot after another and gently roll on the power it should take it. I know of some running well over 450 hp that have lastest. Just a few things to think about and ponder.. Good Luck !!

 

[wire2]

Re: 305 to 300HP 350 conversion

I took my 5.7 OMC up past 300 hp. Then had it in for new seals in the outdrive, tech showed me the new wear marks on the upper gears from the extra torque. Said it will be ok for a while, but may grenade one day.

If you're a hot-rodder like me and you want the performance, consider a bravo drive.

 

[John_S]

Re: 305 to 300HP 350 conversion

With a 1989 Bayliner 2050 Arriva, are you looking for dependable as stock motor, or a performance motor that will be limited by your boat?

While the steel crank and 4 bolt mains are a nice extra, keep in mind that all stock marine sbc are cast and 2 bolt. We are talking 5K rpm and less in a boat. While Bondo is the expert, I think you will have issues with flywheel/coupler/starter to work out. The newer vintage will get you away from the infamous 70's oil leaks. It was bad enough on the driveway, let along the bilge.

 

[wire2]

Re: 305 to 300HP 350 conversion

all stock marine sbc are cast and 2 bolt.

You sure about that, John? I was under the impression all marine and truck blocks were 4 bolt.

 

[bomar76]

Re: 305 to 300HP 350 conversion

You sure about that, John? I was under the impression all marine and truck blocks were 4 bolt.

You are wong in that assumption.

 

[180shabah]

Re: 305 to 300HP 350 conversion

yup, cast crank, cast pistons, and 2 bolt mains.

 

[John_S]

Re: 305 to 300HP 350 conversion

According to Dennis Moore in his SBC Marine Performance book, all production marine sbc were cast cranks and 2-bolt. I can confirm that the Merc service manual that generally covers all '90-'96, 305 and 350's, are cast cranks and 2 bolt mains.

 

[95yj]

Re: 305 to 300HP 350 conversion

Yup. I thought that the marine SBC were 4 bolts until I did a lot of reading. Then when I pulled my 97 260 HP anchor out of the boat, sure enough it was a 2 bolt. Same goes for the replacement long block.

Find a car to put that motor in and get a marine motor. You'll spend more time and money converting it than you will getting a good marine rebuilt. Besides, if it's still making 300 HP by the time you're done lowering the compression and changing to a marine cam, you'll be replacing Alpha 1's like crazy.

 

[mudweiser]

Re: 305 to 300HP 350 conversion

Thanks for the replys. The engine is here. It is what i thought it was. 1969 350 Block # 3932386, heads #3932454. Valves are 1.72 exh and int 1.5. I was mainly after the block, crank and heads.
Not to argue, i'm a why guy.. If you don't ask why, you will never get the real answer and understand it. So here it is, didn't the 5.7 Mag run like 265 HP, was the Alpha 1 built that close to HP specs to only handle that? I'm new to I/O's so i have to ask. Thanks.

 

[95yj]

Re: 305 to 300HP 350 conversion

The Alpha I has a max HP rating of 300 HP and even that can be pushing it if its a heavy boat or doing full throttle starts, etc.

 

[John_S]

Re: 305 to 300HP 350 conversion

"Valves are 1.72 exh and int 1.5."

I think 1.72" intake valves are 305 heads, not 350. The standard 350 valve should be 1.94, with a hp head having a 2.02. The 350 standard exhaust valve should be the 1.5".

PS: Being 305 heads, you don't want to run them on a 350. I'd recommend going to stock iron vortec heads, for the best value and hp.

 

[MikDee]

Re: 305 to 300HP 350 conversion

"Valves are 1.72 exh and int 1.5."

I think 1.72" intake valves are 305 heads, not 350. The standard 350 valve should be 1.94, with a hp head having a 2.02. The exhaust should be the 1.5".

That's my opinion also, actually I think the intake was 2.02, and exhaust valve was 1.62.

 

[mudweiser]

Re: 305 to 300HP 350 conversion

I found out they are actually 1968-1969 chevy 307 heads. These were used on several applications from 300 - 350HP. Still have to rip it down and see how everyhting is. Just wondering if anyone has a good chevy 350 set-up.

 

[stinger222]

Re: 305 to 300HP 350 conversion

I have 10 seasons on my Alpha, 6 seasons were with a 350hp 406, the other 4 seasons were with a 350hp 383...have not had to do anything to the outdrive except seal it and change the fluid every season. I only use synthetic drive oil. I also have a drive shower. I never "floor it" from a dead stop either.

 

[mudweiser]

Re: 305 to 300HP 350 conversion

What is a drive shower?

 

[180shabah]

Re: 305 to 300HP 350 conversion

A device that sprays water onto the drive to help cool it.