Thread:

Difference between marine and auto engines

You've already touched on a few differences, but the one, maybe the biggest one, that you failed to mention is "skirt to wall" clearances.... An automotive engine runs hot compared to most boats. The block expands, as do the pistons, with temperature. In a "open cooling" application you're using the entire body of water as coolant, so the block never sees the sustained temperature needed to expand the block/bores. For this reason, you need to add a few thousandths to the piston skirt to wall clearances because the pistons do get hot and expand..... Many ventures with automotive engines, fresh out of the car/truck, are very quick failures. Additional piston drag plays hell on rod bearings.... Oh, and the oil systems are also quite different.....

Plainly said, most automotive engines die a quick death in a boat, especially a jetboat. Asking an engine to attempt to compress water is about the toughest thing you can ask an engine to do....AND, boats do not coast...
Ray

A chevy engine is the best way to go when you have a thin wallet. When you come up with your hull, just ask some of the guys @ MHB for some stuff..We all have "give away parts" and would be happy to get another boat in our area.

Originally Posted by Moneypit

You've already touched on a few differences, but the one, maybe the biggest one, that you failed to mention is "skirt to wall" clearances.... An automotive engine runs hot compared to most boats. The block expands, as do the pistons, with temperature. In a "open cooling" application you're using the entire body of water as coolant, so the block never sees the sustained temperature needed to expand the block/bores. For this reason, you need to add a few thousandths to the piston skirt to wall clearances because the pistons do get hot and expand..... Many ventures with automotive engines, fresh out of the car/truck, are very quick failures. Additional piston drag plays hell on rod bearings.... Oh, and the oil systems are also quite different.....

Plainly said, most automotive engines die a quick death in a boat, especially a jetboat. Asking an engine to attempt to compress water is about the toughest thing you can ask an engine to do....AND, boats do not coast...
Ray

ZZ4 crate motors, not marine,- tunnel jet boats- SEALED motors-40 to 60 minute WOT races, open cooling, How does this work?

my 3 jet boats
454 k30, replaced cam, heads and intake, over 100 hrs
350 Burb motor, intake and headers, 7 seasons
454 indmar marine #8 piston failure, destroyed block, less than 25 hrs, go figure

Daniel
Mash on it

Originally Posted by c-dub

Well I dont have a hull yet I am looking at a couple different ones. But what I am looking for is what people in my part of the world call a hot boat, big exposed engine, shoots big ass rooster tails out the back and goes fast to put it as simple as possible. Probably not the answer you was looking for but its not set in stone what the said motor would be going in just yet.

I also suggest BBC, but it's your $ and your (eventually) boat. Probably 75% of the guys here / and 75% of the boats you will be near on the water have BBC. Therefore, fun parts are abundant and inexpensive - and the guy beached next to you would likely be able to help with any problems that occur on the water. There's like THREE guys here with fast Fords, but that's about it.....

That being said, piston clearances, as mentioned is one concern - the next is valve guide clearance. Probably the two MAIN differences - but NOT deal killers. Several people here have taken a GM crate motor and shoved it in a boat without any issues. The only real difference to the oil system is usually a bigger pan and associated pickup/screen and windage trays always help since RPM is higher than street use. Pistons for cars are usually set around 3 to 5 thou, whereas boats and car drag motors are usually in the 7 neighborhood. 5 is fine in a boat AS LONG AS YOU GIVE IT SOME BREAK-IN. Same with the guides, usually a bit tighter on car motors. So, as long as a good oil (suggest synthetic of whichever brand you like) is used and you EASE INTO the RPM, you'll be fine. Tighter guides have more friction. Boats spend most of their life at 3000 or more, where are car on the street is usually 3000 or less for 95% of it's life.

So, for example, If the boat with car motor is driven real easy (below 4000) for 10 hours, with NO WOT RUNS.....then some occasional blips to 4000 and 5000, momentarily, for the next 10 hours, all of the metal/metal parts get to make their homes and mate together for long-term usage. If the tighter guides aren't allowed to do this, the higher RPM right out of the gate causes a bit higher heat, right? The heat expands both the guide(s) and the valve(s). So now those clearances become even tighter. If not allowed to make a home before high-rpm/WOT, valves can start to slow down and "stick". Hot enough, and they become "stuck". All sorts of nastiness starts to happen after that.

A few here have used, for example, GM ZZ502 crate motors - with zero changes other than oil pan. A similar break-in method was followed by just getting 25 or so hours on the motor before really asking alot of it.

Originally Posted by c-dub

Wow lots of good info here guys thank you. I understand both sides of the battle here, It makes sense why the wall thickness would be issue with how the cooling system is. On the other side of things I feel that a car engine with some modifications would be ok in a boat but with that being said I don't want to shoot myself in the foot by messing up from the start. Going fast would be nice but as long as this boat I am envisioning in my head can outrun the bastard jet-skis that plague Devils Lake I'll be very happy.

c-dub,

It all depends on the thickness of your wallet, and the patience of your wife

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