What is the difference?
 

What is the difference between marine motors and car motors? Basically what I'm asking is why would I spend 2-3 times as much money on a nice boat motor when GM Performance Parts sells brand new high performance engines for cheaper. This might be a stupid question but I'm new to the addiction and just trying to learn.

I believe one big difference is that they are built to different tolerances.

This is a fairly common and legit question. Do a search on this site and you will get lots of great info.....then fine tune your question.

Tolerances, CAM, need for torque, need for huge amount orf cooling due to 80-90% loads at all times, need all sever duty parts, need rust proof everything (gaskets/temp sensors/thermostats)...plus the coast guard stuff....can't have a spark in an engine room or you go BOOM

The GMPP motors are pretty good. They're made of solid parts that will last a long time. If you buy one, pull it apart to hone the bores to proper clearance, replace the valves with stainless or inconel & adjust stem clearance, put a more appropriate cam in it and replace the intake, you'd have a good, solid medium performance marine motor. Oh. yeah- brass freeze plugs too- no big one there.

If you're paying 3 times that for an equivalent engine, parts and power-wise, you're getting hosed. The GMPP 502 is about $6 grand street price. Figure $1500 to marinize it as I described. If someone is selling 450 horse 502's in the $15K to $20K range, they're not selling many (unless you're name is Mercruiser- different story)

$15K will buy you a very stout 540 built on a performance block with American-made 4340 crank, billet rods and aluminum heads, built by a reputable builder.

I ran a GM crate motor to 400 hours with no problems at all. Ran it hard regularly, and the mid range torque was great.

Some guys do get lucky. Many don't.

JE makes the pistons that GM uses in the GMPP engines. JE has two different specs for clearances on those pistons when used in marine applications- and both are larger than the automotive spec.

On valves, using a steel exhaust valve in a max-rpm marine engine is a recipe for disaster. The valve gets hotter than the steel can tolerate, the head tulips, then drops off and through your piston. You need stainless at a minimum and for hard use, you need inconel. Valve stem clearance is critical as well. Standard automotive clearances don't take into account the constant high rpm, max load that a marine engine sees. When the exhaust valve sees high heat, it expands, filling that tight clearance and binding in the bore. The piston then contacts it and it's all over.

If you have a cruiser/fun boat and you putt around at cruise rpm with the occasional momentary blast up top, you'll probably be OK. If you're running long and hard, they're not gonna' make it.

I agree with Chris...Low performance under 450HP and 4800RPM you might be OK, But after that no short cuts.

thanks again, Chris, that is the most direct, concise answer that I've ever heard---- now the wheels are turning - Jeff

Thanks for all of the good info. I have learned something new every day since I joined this sight.

You 'n me both.