MustangGT221,
I just got done DesktopDyno'ing a 5.7 liter engine to beat out the specs I found on a Mercury Tow Sports MAG MPI 350. I used as much information as I could find on the engine and was able to succesfully go well above the hp figures they claim.

I used my numbers for a set of Ported TBI heads which will flow about as good as a decent set of aftermarket heads, a fair assumption since watercraft engines can be designed much more racier and thus can cost more. I also modeled the MPI system and for an exhuast I chose a small-tube header with an open exhaust. With the same compression of 9.4:1 I designed a cam that gave me a peak torque of over 400 lb-ft at 3500 RPM.

One thing I can interpolate is that just because an engine is rated for certain HP at certain RPM doesn't mean that it peaks there. For the life of me I could not get the engine to make peak HP that low with that good of numbers.

But if we think like a performance-minded engine builder, it seems only natural to make power beyond our target rpm, but still govern the engine not to go past X RPM. Thus we have increased the engine's performance and still met the goal of Y power at X RPM.

I hope that answers your question, Justin.

Aloha,
Greg

 

again like most others stated..there are no emissions restrictions...as well usually you will find better camshafts ans better bearings and the heads on these motors tend to be old stlyel with big breathing...it is easy to make 300hp with a 302 roller block using just a cam and heads....and I would assume hp of these motors are based on flywheel HP...not at the prop...especially for my boat..I have no prop...lol

 

Some weird comments here???(not your question)

Anyway, please link the 260 HP 302.........most 350GM and Ford 351's are in that range. The 302's are usually around the 200HP range.

As far as overrated, I don't get it??????

Junk 302's with basic E-7 heads are 225 hp with full smog equipment. GT-40 crate motors are rated at 320HP........so even at 200-260(?) I don't see what's overrated.

Same with the GM/Ford 5.7/5.8...........the 260 HP ratings are very close to the basic street motors. (the last LT-1's were 275 HP and the last 351 smogged Ford was 235ish) So to have an uncorked Marine motor at those levels seems well within reason.

The peaks are usually around 4500RPM..............that's not neccessarily redline. But like any motor, once past max power you're just making noise.


Check out the HP levels............http://www.psengines.com/PowerSource_marine_page.htm

Nothing seems out of line to me.

 

there has been a lot of talk about the diffences in marine motors and why.
first off most marine in board engines are built for a power band of 4.500 to 5,500 rpms. horse power is always fly wheel rated gross and propshaft as net power with a minor lose at the prop. it self.

if a motor can not turn in that 4.5 to 5.5k band. if it over revs the prop is too small in pitch and to low the prop is to high a pitch. diferences also come from hull design. a trawler/cruiser hull being made for ride and comfort. it will no matter what you do still only run a max speed designed for. a case would be a 28 foot lobster boat i had with a 6-71 dd diesel 238 hp would ran all day a max of 12 knots. when repowered with a small light perkins of near the same power it would do 13. so hull design has a lot to how they run at top sped. trawler types are always slow as they don't plane just push though the water. while a speed are fast off plane and run at higher speeds some of 45 to 70 mph. "stock" they run on top of the water and don't push though like a trawler

a base marine engine is a street car/truck motor with several things different , in most cases the block in a low output gm based will be a 2 bolt main while a upper power will be a 4 bolt. crank shafts are different some being forged steel or cast hardened better to encrease it's life time of use. rods are different and stronger again mostly forged or powder forged, the pistons are still cast. the heads have bigger valves and big chambers excp. on higher performace engines as used in racing. core plugs will always be brass and the gasket sets are special marine
induction is still carb based. for low power motors. but for many years the companys have been slowly changeing over to e.f.i systems due to smog laws for the last 20 years. yes guys marine motors are smog motors. weather a 2 stroke or a 4 stroke beleive it or not it's cold hard fact.
camshafts used to be much higher in performance then now as if one listens to a stock motor you no longer hear the big cam roll they used to have. lift and duration used is still about the same as a old 3/4 race just it's not heard in the idle.
parts such as the spark arrester "airfilter" starter motors, alternators and distributors are special marine type that reduce and remove any chance of a stray spark ignighting gas fumes in the bilge areas and starting a fire or worse. use of non marine parts is illegal and can cause your boat to be put out of service by the law either local/ state or coast guard.
the exhaust manifolds have a water jacket around them to help reduce heat in the bilge area and another cause of fires.
so as one can see a marine engine is. but still is not a automotive engine just some thing based off a automotive/truck block.
another thing about marine smog motors while right now we have to deal with e.f.i. in a very short time we will also deal with cat. converators. and yes they are in the test phase right now in calif. along with the u.s.coast gaurd. this will be law on both 2 and 4 stroke motors. hense the seeing of more and more 4 stroke outboards in the market.
another thing that is in the law writeing stage right now! is out lawing though hull "load" exhaust. don't blame them there. ever overnight in a marina and listen to the noise as they all start up to leave at 5 and 6 am?

as to why you see more gm based over ford based it's the old dollar cost problem. ford based motors cost more then gm."check out some of the sites attached here read mexico built" mercruiser did use fords for a while "the last they still used was the inline 4 of 3.7 liters that was 1/2 of a ford 460." but the gm based unit was less costly to get. a few companys still use fords but if one notes and looks you find them in ski boats with prop shafts not with i/o units. those few are useing for transmissions either a v drive unit that lets the motor sit back wards in the hull so there is more room inside for people or have a mid engine useing a standard inboard transmission and a long prop shaft. also the inboard makes a better wake for sking and wake boarding
heres a list of companys that can give more information and anwsers.
http://www.gm.com/automotive/gmpowertrain/engines/specialized/listing.htm
http://www.gm.com/automotive/gmpowertrain/engines/specialized/marine/index.htm
http://www.rbbi.com/index.htm
http://www.fordpowerproducts.com/FordPowerProducts/marine.htm
http://www.mercurymarine.com/
http://www.volvo.com/volvopenta/global/en-gb
http://www.cmdmarine.com/
http://www.yanmarmarine.com/
http://www.detroitdiesel.com/
http://www.cat.com/cda/layout?m=51373&x=7

marine motors most also run under much different conditions. from a low smooth idle like for in no wake zones. to mid and upper end cruise conditions where the power most be used in a very effecent manor. that all comes from valve sizes, camshaft design. carburation/ efi. hull design. prop sizeing and pitch

another thing a raw water cooled cast iron block will only last 7 to 10 years in salt water thats why you see a lot of companys now useing closed cooling/antifreeze cooled motors now. forget alloy motors unless they are fresh water cooled . marine motors and out drives suffer big time from corrossion esp from stray/improper electric caused by bad grounding inside the hull .

for those of you who don't under stand my captchas. it's a real captains license like a 100 ton master near coastal for operating passenger boats to 200 miles offshore by the coast guard ., which lets me operate up to a 100 ton gross weight boat thats about 125 feet in length.

 

thanks for a great post!....