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6hp vs 8hp mercury

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olmo40

Regular Contributor
Is it as simple as changing th

Is it as simple as changing the carbie to turn my 6hp into an 8hp?? 06 Mercury
 
"If you grab a carb from a pre

"If you grab a carb from a pre-95 9.9 and another exhaust housing/tube, you can make it a 9.9. The 6/8's did not have a "tube" attached to the plate (and the exhaust opening in the plate is larger on the 9.9's).

The older 9.9 plates have been discontinued but the post 95 plates fit however, they also require replacement of the water-tube (hooks up different at the exhaust plate end) - but also requires a very long drill bit to widen the guide hole inside the exhaust housing - located in the middle of the leg (to accomodate the flare in the new water tubes).

Until 1995 the 6/8/9.9's all shared the 12.8 cu in head - after that they paired the 6's and 8's and the 9.9 started using the 16 cu in the same the 15 horse...

And if you put on a pre-95 15 horse carb/exhaust plate and tube, plus fiddle with the timing a bit and run a high flow prop hub you can squeeze about 12 horses out of the motor - but you will probably send it to an early grave - but hey, it can be done....

And although it's illegal in most of North America to increase the horsepower of a motor, it almost sounds like I have hopped up a few of these before
"
 
"Thankyou very much i will get

"Thankyou very much i will get on it .I was also told today if you buy a 15 Mercury make a kit to de tune it to a 6 or 8 but it is way fast because of the capacity ,however this defeats the purpose of a cheap hot up."
 
"After buying an 8hp Carb from

"After buying an 8hp Carb from the dealer im informed my Mercury is a Tohatsu???.However if i now get a 15 Mercury and put a 6hp kit in it ,the dealer says it will be way faster than an 8hp because of capacity ???.I am limited to 6hp due to licencing"
 
"David, you failed to mention

"David, you failed to mention that your motor was a 4 stroke. If that is the case, it is in fact a Tohatsu.

The 2 strokes were about 90% Yamaha. Hey, you could have bought a Johnson which was really a Suzuki...

Totally confused now
"
 
"That's a Yamaha powerhead

"That's a Yamaha powerhead/gears, everything else is Merc. Belonged to the 6/8/9.9/15 horse series. Shared it's powerhead with the 8 horse (both 12.8 cu in)....."
 
"Thanks Graham, so it off to b

"Thanks Graham, so it off to buy a yamaha 8hp carby .Any thoughts on detuning the 15hp just for interest??"
 
"Don't usually get asked a

"Don't usually get asked about a "detune", but simply using the smaller horsepower carb and swapping out the exhaust plate/tube on the 15 for the exhaust plate (only) from either a 6 or 8 would do it.

While I think it would provide a little better torque numbers if you hooked it to a machine to measure (a detuned 15 compared to a factory 6 or 8 because of the larger displacement) I can't really see it making any "noticable" difference.

One byproduct would be that it would probably last longer (wear less) and run cooler since the displacement to horsepower ratio would be on the order of 2:1 in the case of a 15/8 and and better than 2.5:1 in the case of a 15/6.

(rule of thumb any ratio greater than 1:1 is very hard on the engine parts and produces excess heat - diesel engines are usually engineered at 2:1 for longevity and heat dissapation)

But again, we are talking about a very small motor series to start with, that is well engineered and has years of outstanding history for durability...

It could be done, but why? The cost would offset any minute difference in performance. A complete tune-up once a year and the addition of a second prop to be swapped out as the weather conditions change would do far more for overall performance than trying to re-engineer one of these little motors... But it could be done...."
 
Thanks Graham good discussion!

Thanks Graham good discussion!!The only reason for the detune is to be legally 6hp and the fastest 6hp untill i get my licence .
 
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