Marine piston clearance

[TeamHawaiin]

I was wondering.............why does a marine engine require more piston clearance than a street motor? I have 330 hp GM marine max motor that I would like to pull out and put in a mud truck. Straight swap out, leave in the cam, distributor, carb. I would like to give this jetboat a 400 sbc, and remember 5 or 6 years ago while building another motor the engine builder asked if it was going in a boat. Never needed to ask why until now.

Also, I read the cam has to have a certain LSA on a jetboat. What my main goal to do also is to kind of use the boat as a dyno so to speak. When I'm finished playing with the 400, I want to be able to drop that in a drag car. Any info would be greatly appreciated

 

[jetboatperformance]

Several reasons not the least of which is the "Cold" block factor , marine engines (at least Jets and V-dives) tend to run cold no matter what Piston tops however run hot as the block does not swell or expand as a Auto application would "standard clearances" tend to produce piston and ring issues . Generally speaking a "seasoned" motor say from your truck with a few miles on it won't likely be an issue. Most manufacturers have clearance charts that show increased clearances for different apps ,eg N/A, forced induction , Marine etc . Check out the KB website Tom

btw ring end gaps change significantly as well !

 

[GT Jets]

That 330 BBC will work pretty well in a mud truck/bogger, just needs some RPM to get the job done..

To add to Tom's reply, cold shock comes into play, typically a marine engine is more prone to rapid temperature changes than that of an automotive application and if the piston to wall clearances were inadequete, the chances of hot siezing the cylinders are great.

Chevrolet used to use loose clearances on the Corvette small blocks in the mid to late 80's because of people driving them hard when cold, they actually used to have a slight knock when cold (skirt slap).

Also too, the bigger the slugs, the more the added clearance.

GT:monkey:

 

[TeamHawaiin]

Thanks for the info guys. I'm begining to wonder if Marine engines have a shorter life than automotive due to the larger piston and ring end gap clearance. Any input on that? I can imagine the emissions would be higher so you couldn't get away with this on a street vehicle.


What about the cam ? Do boat cams only have cam profiles to put there horsepower at a certain RPM ?

 

[cave]

The short answer is yes But depending on what your trying to achieve. You should call Tom of Jet Boat Performance Phone - 805-466-4719

He could get you in the right direction. Nothing sucks more than putting an engine together and not being able to get the most out of it. What Impellar do you run, brand and cut. bla bla bla. Then your tearing it down at seasons end to replace the bearings. If your lucky. Not saying this is everyone's experience. Just mine. I run just about every month and the boat gets used quite a bit. I picked a cam that worked with my Impellar, blower boost I wanted to run and the internals I have.

Jets run at higher RPMs than a car or truck for the most part. More wear and tear occurs do to this.

Tom does this stuff for a living. He can give you a pretty good idea on what cam would work good for what you want it to do.

 

[cheyenne580]

:champagne::champagne::drnkfr

 

[92 cole]

Alot of good info on the response and alot has to due with the piston metal composition you will use. A cast will expand different than a forged or hypereutectic piston. Lots of factors to piston to wall and ring end gap clearances. Engine and water temp creating the nightmare for the motor builder.

 

[TeamHawaiin]

Thanks again for the additional responses. It seems though now I'm not going to build a new BBC. I'm just going to do a vortec head swap on the sbc in it now and run it for what its worth. I will eventually get it built, and I will definately keep this in mind when I build it for marine application. Thanks again

 

[jetboatperformance]

Alot of good info on the response and alot has to due with the piston metal composition you will use. A cast will expand different than a forged or hypereutectic piston. Lots of factors to piston to wall and ring end gap clearances. Engine and water temp creating the nightmare for the motor builder.

This is true and good info ! Piston to wall clearances even vary between SRP and JE (different compositions) same manufacturer