1978 Johnson V6 235hp motor upgrades??

[gmacp58]

My first and only boat is a 1978 17' Starcraft bowrider fiberglass trihull that I have owned for 31 years. My engine was just rebuilt, other than that it is all stock except for the starter that I replaced. Are there any upgrades that I can do to get the optimum performance from my engine? I do have a hydrofoil installed, and that definitely increased the performance. I also picked up a 4 bladed 23" prop (same pitch) that I have not had the chance to check out yet. I am having the transom rebuilt as well as having the hull repaired and painted. What is the best paint for my hull performance and durability.
Thanks for your time! Gerry

 

[jimmbo]

That extra Blade will certainly kill your RPM and Top Speed. What RPM were you getting at WOT with the 3 blade? That engine will run best when propped for around 5400 - 5500 rpm

I will assume your engine is a 78 also, 149 inch block. In 1980, OMC upped the displacement to about 160 inches and they were sitting on the fence as to making it a 260HP. But when the 1980 motors were introduced it was a 235, There is more power to be had from the engine, however hopping it up will have an affect on how it runs at lower speeds, its acceleration, and its Longevity. Scream and Fly would have many threads about Hopping it up. It isn't too hard to make an engine scream at the top, but it may be a real PIA to operate otherwise, especially on a Non-racing hull
Regarding the hydro-foil, I assume you are referring to a piece of Plastic that you attached to the Anti-ventilation Plate? Myself, I would remove it, take it to the deepest spot on the lake/river and toss it overboard. Your engine should have absolutely no problem planing the boat very quickly. If the holeshot is poor, you might have too much pitch, an engine that isn't firing on all cylinders, or a bunch of saturated foam. Once all those are found to be in good order, if looking for more top end, you could raise the engine on the transom(about 1 - 1 1/2 mph for every inch higher at speeds above 50mph, but you will need a higher performance prop that can run partially out of the water. BRPs RAKER, or Mercury Enertia are great props for lots of Recreational use, Not Cheap , but great props.

Paint? some of the 2 part Epoxy Paints go on very thin and are suprisingly durable. If you are in salt water than you need Anti-fouling paint.

 

[Faztbullet]

That is the 149ci flatback motor and was powerhead rated at 235hp but is only about 205 at prop. When you say it was it rebuilt what was done?? Was all the pistons replaced?? This model had the high ring pistons from factory which are NLA. Motor makes it best HP at 5500rpm and since that boat is not a performance hull, only thing I would due is install fiber reeds(not Boyesen) and use a engine conditioner such as carbon guard or ring free.

 

[Chris1956]

You have 235HP on a 17 foot trihull? Trihulls by their flat-bottomed nature plane with low HP motors.

With the power you have available, I would expect speed in high 50's, which would be plenty scary in that hull. If you are not seeing that, I would think the hull or trim is holding you back.

It could be that your hull simply will not trim enough to carry the bow. Or you could have a hook in the hull. Is that hull even rated for that much power? Maybe you get deformation of the hull, when power is applied?