Yamaha 6h5, 50hp, mods?

[janihei]

Hi, need some help to squeeze more power from my 50hp. The boat is 15ft v-hull, 400lb without engine. Engine is light, short rig without trim. Boat now has yamaha 30hp 3cyl engine, top speed 35knots, jumps on plane.

Plans
-Offset chamber head milling, how much?
-DIY exhaust tuner, shape? Milling the tuner at the gasket surface?
-Bigger carbs, 60hp or 70hp?
-Milling the intake surface of the block, how much? Or stuffers under the reed cages, or both?
-Port timing modifications?
-Ignition system, old style? Or newer style arrow box?
-Exhaust plate spacer?

[rgsauger]

If it jumps on plane, have you tried going up a pitch or two on prop or different prop for more top end?

[janihei]

Yes i have, 30hp has to go and is replaced with short shaft 50hp. Very suprised no one has any detailed info for 50hp performance modifications

[timberman123]

Yes i have, 30hp has to go and is replaced with short shaft 50hp. Very suprised no one has any detailed info for 50hp performance modifications

You could check hydrotech marine. They have kits with a modified exhaust, cilinder head for increased compression etc. https://shop.hydrotecmarine.com/prod...?productId=113

I am currently installing same type of on my 60 hp 2stroke with 90 hp carbs. Should increase hp to about 80 hp.

[janihei]

For me the hydrotec is too expensive, i live in europe and have to pay 24% tax to import parts, and shipping prices are in the moon right now. Thanks anyway!

[janihei]

Took some measurements today, quench seems to be around 0.094" (2.4mm), have to shave 0.06" out of the quench to make it 0.034". I think i will machine x ammount from the block and the rest from the head, dont know yet how much from each, have think! ��

[Chaz]

What size is the bore ...

At TDC , how far down the bore is the piston ...
(measure parallel to the wrist pin)

Does it use a head gasket or O-ring ...
(if gasket, how thick)

Stroke of engine ...

Rod length ...

Ex. port height ...

Cyl. head cc ...

[janihei]

Bore 67mm, stroke 66.mm, displacement 698cc. Pistons are at zero deck. Head gasket thicknes 1.4mm. Cylinder head volume unknown. Mocked the engine without headgasket and measured the quench thru the spark plug hole, 0.04" quench.