Marine engine vs car engine?
 

I have bought a couple of repo boats in the past,that have needed engines. I bought new GM crate engines and changed
to brass freeze plugs and installed them, Granted, they were
not performance boats, but these performed perfectly in both
cases. My question is this. After seeing the cost of high performance marine engines, why not buy say a Scott Shafiroff 540 ci 625 hp street engine that runs on 91 octane
fuel and costs $9950.00 ? I am wanting to get into a 24-28
hot rod boat with a single engine and don't see why this
wouldn't work. But since I don't know performance boats I
wanted experts thoughts and opinions. Thanks Steve.

Different clearances, and different cams are the main things needed in a marine engine. You need a flat torque curve. Being able to make 625hp last for 11 secondss at a time is not that hard. Being able to make it last for 20+ mins at WOT is entirely different. Marine engines are under under a much heavier constant load compared to automotive engines.

I understand the differnt cam requirement, but as far as
clearnces go I would assume a street engine would have
tighter clearances than a drag race motor. I'm also assuming
your saying marine engines built with tighter clearnces? I'm
assuming,because I don't know.Steve.

Like was said the setup is just different,
car engines are setup on the ragged edge
as far as rpm and hp because noone ever
puts their car at wide open throttle up a hill
for hours at a time .
Marine engines are set up looser .
Another factor is fuel, most boat engines are setup
to run the crappy gas found on or around where we boat
so they run lower compression than you would see in a car buildup.
Most car engine will never see wot for more
than a minute or two at a time if that.
marine engines are detuned and have better
internal parts than you would think they need
and have a lower rpm operating range so they can
live at WOT for a much longer period of time.
Taking that engine you posted as an example,
it would probably have to be detuned cam and compression
wise for a boat and you would end up with a 500-550hp
engine.
You CAN build your own for much less if you have a good machine shop to work with if you research the parts,clearences and marine builds other have done, the problem is finding that "good" machine shop..
These days it seems hitting the state lotto is easier.
I have maybe $6000 total in my engine and its a 509" making roughly 500-525hp. We built it last summer with help of local machine shop .
Its already run 4 poker runs on top of many boating hours
and roughly 4-5 hours total at WOT, idles at 700rpm and runs on 87 octane gas.

They should actually be LOOSE !!!!!! not tighter.
Thats what i always been told !!!!

HI i was told that a 496 is exactly the same as the 8.1 GM truck engine except for the cam ?????.

I cant imagine that a 540 making 625hp is running on the ragged edge, or has that radical of a cam in it.

A conservatively built performance "car" motor will work just fine in a boat, so long as it has top quality parts and is built correctly. There is no voodoo as everyone likes to think there is to building a marine engine. They are just built right, with conservative cam profiles, very good valve train components, ie iconnel exhaust valves, and fairly low compression ratios, around 9.5:1.
Drag race guys can make 800+ hp out of a 540 N/A, that is not realistic for a boat motor without a blower.
Do not expect to get any warranty using a car engine in a boat, and I would break it in fairly conservatively.
Also, if it is raw water cooled the rings are generally gapped a little different.

NO WAY, did u ever run your car at 5200 RPM for 20-40 min ???????

The clearence is bigger( wider ) on a boat engine because the heat would eat the walls in a couple of days....

BOAT ENGINES ARE LOOSE !!!!!! CAR ENGINES ARE TIGHT !!!!!!!!!!!!

+ u need brass frezze plugs not steel..and on and on ,,,lots of things are actually different when it comes down to it , RELIABLE is the key !!!!!