Mercruiser 5.7 Engine Improvements
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?
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? How easy is the removal / relacement of the intake manifold / Is there anywher ein the uk which sells such an upgrade?
Any thought / experience gratefully received.
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.
1st of all, welcome.
Originally Posted by mercruiser 5.7
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?
Yes aggree with all above dont spend too much money, tweak. Are you trrimming the leg up a few dgres at WOT as I would think you should be pushing mid 40's with a light load.
What leg do you have by the way, is it a Bravo 111 by any chance?
Sorry not a B111 with single prop so scrub that
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.
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.
Last edited by moonraker 36; 14-04-11 at 16:46.
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.
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.
Originally Posted by moonraker 36
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