Mercruiser 5.7 Engine Improvements

mercruiser 5.7

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I have a 22ft Mariah fitted with a Mercruiser 5.7L v8 engine which breathes through a 2 barrel rochester carb. The top speed of the boat is around 40 mph on a good day. The boat is currently fitted with a 3 blade / 17 pitch prop. I have tried different prop pitches and this seems to be the best for the boat. Max revs are around 3800 to 4000rpm in water.
I am looking to increase the top speed by increasing the power of the engine.

1- Has anyone experience of fitting a K&N air filter or a perfomance carb spacer to this type of engine and what (if any) speed increse was achieved? ie is it worth speeding the approx £200 on these parts?:confused:

2 - Is it better to replace the intake manifold so that it accept a 4 barrel carb? I have seen a 2 bbl to 4 bbl conversion kit from the US which promises better performance - as per link below. Has anyone bough/fitted one and did it provide any extra power / top speed?:confused: How easy is the removal / relacement of the intake manifold / Is there anywher ein the uk which sells such an upgrade?

http://marine-performance-parts.com/2bbl4bblconversionkits.aspx

Any thought / experience gratefully received.
 

spannerman

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You are under revving as this engine should be 4200-4600 at WOT so your prop is too high a pitch as these are heavy boats.
Changing to a 4 barrel should help as it will breathe better.
I would not recommend an K&N filter, as the filter on a marine petrol engine isn't just a filter its a flame trap preventing flames in a backfire, a paper filter will just catch fire making the problem worse.
 

MapisM

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I am looking to increase the top speed by increasing the power of the engine.
1st of all, welcome.
I'm afraid I must give you a short but very strong suggestion on the above: DON'T.
For your boat, the 5.7 is plenty, and you only risk to spend a lot and gain a little, by going that route.

And anyway, Spannerman is spot on: your max RPM is way too low.
Do you have tabs fitted? And if so, do you raise them at WOT? And the hull is clean?
Because before thinking of any engine tuning, you must be sure that you're getting the best out of what you've already got, and in this case you clearly aren't.
What is weird is that 17" is already a rather short prop for that kind of engine/boat. If you can confirm the outdrive ratio, we can do some math and check the prop slip.
By mph, do you actually mean it, or knots?
 

dingdongs

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expect to lay out about £1000 for every 1 mph gain.
i'd personally put it towards fuel and if you want speed get yourself a lighter boat with the same engine.
as stated above the engine is not revving to the correct rpm.have you checked to make sure bilges dont have excess water in them and dont have 300 litres of onboard fuel and water to carry.
 

moonraker 36

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got to disagree..
i would change the manifold to take either the holly or rochester 4barrel.
the hardiest bit is you have to remove the dizzy to change the manifold.which mean you have to resett the timing..i use a rochester quadrajet 4bl and it's a very good carb.
the 5.7l is very easy very cheap to tune.
cost me about 3-3.5k to produce well over 320hp,so i would disagree with the quote 1k for 1mph gain.i got around 20mph (heavy 18ft boat).(not trying to say you will see 20mph just better performance in general)
the 4barrel should give you a better torque curve..its not all about hp.
 
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moonraker 36

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p.s
i found getting part's direct from the u.s
alot cheaper than the u.k
thats the reason why i tuned my engine in the first place.
needed a rebuild and it was cheaper to get performance parts from over their than standard parts from the u.k.
just a little tuning will give you a nicer, sometime's more fuel efficent engine.
 

Kipper

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got to disagree..
i would change the manifold to take either the holly or rochester 4barrel.
the hardiest bit is you have to remove the dizzy to change the manifold.which mean you have to resett the timing..i use a rochester quadrajet 4bl and it's a very good carb.
the 5.7l is very easy very cheap to tune.
cost me about 3-3.5k to produce well over 320hp,so i would disagree with the quote 1k for 1mph gain.i got around 20mph (heavy 18ft boat).(not trying to say you will see 20mph just better performance in general)
the 4barrel should give you a better torque curve..its not all about hp.

But those old carbed 5.7's only put out 260 hp cant see a carb giving you an extra 60hp. 6 i would believe.
The speed of most sports boats is limited by its hull design not just the engine HP I know of a guy who has spent 100k on a supercharged big block in a 24ft boat but it will only do 69.2mph and he reckons that it no matter how much he spends.
I also know of a 23ft fomula that will do 60+ with a carbed 5.7 putting out 260hp thro an alpha 1. Its down to the hull design and depth of the V. How deep a v is that Mariha
 

MapisM

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The speed of most sports boats is limited by its hull design not just the engine HP I know of a guy who has spent 100k on a supercharged big block in a 24ft boat but it will only do 69.2mph and he reckons that it no matter how much he spends.
I also know of a 23ft fomula that will do 60+ with a carbed 5.7 putting out 260hp thro an alpha 1. Its down to the hull design and depth of the V. How deep a v is that Mariha

I'll second that 100%
Sorry moonraker 36, but I can understand your suggestion only if you are in the engines tuning business... :D
I can give you some numbers based on my 25' hull, which is actually built for speed and widely recognised as one of the most efficient in her size range:
450hp = 70mph (stock)
650hp = 85mph
850hp = 95+mph (Skeptic? Just look at this video).
And at anything above 75mph, she begins to be a handful to drive.
Not to mention the money required to get such kind of power increase, and to change outdrives every other week, if you're not double careful.

'Fiuaskme, I'd be scared by a Mariah 22' at anything above 50 mph or so.
And I bet that the OP can get close to that without touching the engine (unless it has a problem, of course).
 

moonraker 36

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i know its all to do with hull design.i wouldnt do any more tunning to my chevy because the hull wont take any more.
just simpley trying to say it as to be the cheapest way of getting a better engine(power torque or hp)for very little money..
the 5.7l with a 2barrel carb will prob be less than 200hp standard.4barrel produce 260hp standard.both engines are the same apart from manifold and carb.
the only way he will find out how the boat re-acts to the increased power is to have ago.even if you say 30-60hp increase for less than agrand got to be a bargin...

the leg..well like i say i havent had a single problem with the increased power(alpha 1).and its been like this for over 4yrs..thats not to say he wont of cause...you pay's your money you take your chances..

plus the boat will only go as fast as you want it to ..all depends on how many beans you give it.
 
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moonraker 36

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the guy is only talking about lightly tuning the engine.
i am not trying to tell him to give it the full works.but why have a 5.7l that is restricted in both hp and torque.you might aswell have the 4.3v6 engine fitted..

why not ask the mariah owners club..?as see what they think.?
 
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moonraker 36

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But those old carbed 5.7's only put out 260 hp cant see a carb giving you an extra 60hp. 6 i would believe.
The speed of most sports boats is limited by its hull design not just the engine HP I know of a guy who has spent 100k on a supercharged big block in a 24ft boat but it will only do 69.2mph and he reckons that it no matter how much he spends.
I also know of a 23ft fomula that will do 60+ with a carbed 5.7 putting out 260hp thro an alpha 1. Its down to the hull design and depth of the V. How deep a v is that Mariha

but the 2barrel put out less than 200hp.
 

moonraker 36

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You are under revving as this engine should be 4200-4600 at WOT so your prop is too high a pitch as these are heavy boats.
Changing to a 4 barrel should help as it will breathe better.
I would not recommend an K&N filter, as the filter on a marine petrol engine isn't just a filter its a flame trap preventing flames in a backfire, a paper filter will just catch fire making the problem worse.

and even spannerman say's put on the 4barrel.....and he's the man...!!!
 

sundancer9000

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speed

Used to have a MAXUM 1800SR3. 4.3 Mercruiser it did 56 mph great fuel economy and out accelerated yamaha waverunner jstskis!!!!! Guess this was a great production speed boat it was a 2003 model. NO ADDITIONS OR GO FASTER STRIPS NOT EVEN FURRY DICE LOL .
 

moonraker 36

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Used to have a MAXUM 1800SR3. 4.3 Mercruiser it did 56 mph great fuel economy and out accelerated yamaha waverunner jstskis!!!!! Guess this was a great production speed boat it was a 2003 model. NO ADDITIONS OR GO FASTER STRIPS NOT EVEN FURRY DICE LOL .

hehe...peddle to the metal...rock on...
 

lovezoo

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Friend of mine had a Mariah 21SC with a 350 Mag engine. That would do 50mph flat out. He also put a High-5 prop on it and squeezed a couple of extra mph out of it, so I would have thought faster is achievable for the OP.
 

maxumham

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I have a 2300SC Maxum, Alpha I, 17P prop, 5.7L 250 HP and had the same problem. Tried everything and could only get 3900 - 4200 rpm. While doing routine maintenance I removed the flame arrestor and found it to be somewhat fouled. I cleaned it using an aerosal carb cleaner and now it revs to 5000 rpm. In the pasted my PM on the arrestor was to impley whipe it down. However, over the years the buildup between the individual plates was enough to really restrict air intake. Try removing the arrestor and aerosal spray clean from inside to out with a good solvent carb cleaner.

Good Luck
 
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