Replace Chevy 305 with Chevy 350 on Mercruiser 898.

[cecho]

My Mercruiser 898 has worn out valve guides. Since the engine is original from 1978 I'm thinking about replacing the Chevy 305 engine with a Chevy 350 and replacing the 2 bbl carb with a 4 bbl carb. I'm wondering what I need to look for when swapping engines. I know the engine is a two piece rear seal while most long blocks I see online seem to have one piece rear seals. What's involved in switching to a newer block? What about something with vortec heads?

 

[Scott Danforth]

Build the motor you want. Plenty of 2-piece RMS motors.

Vortec heads can be bolted to any SBC block

 

[cecho]

Ahh, I didn't know that about the heads. So I'm thinking 350 two-piece block, marine cam (maybe the cam straight from the current 305), vortec heads and intake with a marine 4 bbl carb then. Is the exhaust manifold bolt pattern different on the vortec heads?

 

[Scott Danforth]

There are better cams than what is in your motor.

 

[cecho]

I figured that would be that case, and I haven't figured out exactly what I want, but I've read I need to keep the HP under 300 so I don't blow up Alpha 1. Speaking of which, should I think about changing the gear ratio on the out drive? I have a torque shift prop now which I'll probably need to replace as well. I seem to remember the max HP for those to be useful was around 250.

 

[Scott Danforth]

Unless you go nuts building a stroker, dont worry about the drive

 

[cecho]

Unless you go nuts building a stroker, dont worry about the drive

Good to hear, and thanks for the advice. I'm not looking at doing a stroker, but I'm aiming for 300 HP which seems reasonable with vortec heads and should be a nice upgrade from the worn out 198 HP 305. If anyone has ideas on what parts to use I'd love to hear them.

 

[Scott Danforth]

The parts depend on the block. You have to determine if your building a 2-piece RMS motor or a 1-piece. From there, if you build the motor, get proper pistons to go with vortec heads.

Inpection of the crank and rods will tell you what you do next.

New forged scat rods are cheaper than redoing your current rods

A new crank is cheaper than grinding and polishing your existing crank.

Cam will be determined by the block and build (roller vs flat tappet, etc)

 

[cecho]

Thanks again for the help! I've done some more research regarding 1 piece and 2 piece differences.

The 2 piece RMS engines need a different flywheel and coupler than the 1 piece RMS engines, but the bell housing bolts are the same. I have an MC1 housing with an Alpha 1 upper and lower leg. What I need to know is will the newer flywheel style and coupler work with the MC1 housing? If so, replacing the coupler and flywheel will factor into cost of deciding to go with a 2 piece or 1 piece engine. My guess is the housing and coupler will work assuming the flywheel diameter is the same.

Should the coupler be replaced anyway? It is approaching 40 years old as I doubt it's been replaced by a previous owner.