RPM at WOT on a Inline 6 Mercury 90hp

[kmk89]

I have an 1978 inline 6 Mercury 90hp on a 16' bass boat. Just installed a tachometer, water pressure gauge, and water pump and took it out the other day. Water pressure at WOT is about 8-10 psi and about 2 psi at idle, which seems good. Engine has good even compression, but at WOT, the tach only says 4500 rpm and I barely hit 40mph by myself with the motor trimmed up a good bit. With other two people on board, I'm lucky to get 36 mph. My question is, what rpm is this engine rated for and what prop should I get to increase the speed? I currently have a 19" pitch prop and it's not stainless. Poking around on the forums, most people with a 90hp and a bass boat my size seem to get 45-50mph. Is something wrong with my setup or are people just over exaggerating?

 

[Barnacle_Bill]

WOT is suppose to be 4500-5000 RPMs. Dropping down to a 17" pitch would get you up closer to 5000 and might help a little with your speed. Make sure the motor is mounted correctly and the tach set for 6 pulses

 

[mr 88]

Some of these post that you read may be from guys with newer engines [1985 or so is when they changed the rating system] that are rated at the prop. Your engine in todays terms is about 75-80 hp and was rated at the crank. So that may be why your seeing them with higher speeds and maybe running a SS prop which adds a couple mph in most cases.As stated above try a 17" prop to get your rpms up,the 6's do not like to be lugged and thats what your doing when you have more than one in the boat. The 90 was never known as a speed demon motor,very reliable when tuned right, I have one on a 18' aluminum almost flat bottomed stripped to nothing boat and run about the same speed as your hitting,never GPS it,so I am not sure but thats what it feels like.I threw a 17'' SS that I had lying around and it made a big difference vs a non cupped one. You could feel it picking up the back end vs the aluminum non cupped and I do have PPT on it but the props were like day and night.I have a 15' cat with a 1500 Tower on it and use that for my speed runs,thats why I never checked the hunting boat

 

[Texasmark]

I recall back in the '80's in the sales brochures where they advertised the 90 as the workhorse and the 115 was their light hull get up and go engine. I do remember the rpms were deliberately limited to 5000 recommended max and I guess they did this with carb jet changes or something more dramatic like reeds and induction chamber changes. Also as I recall they upped the ratio of the prop to 2.33 from the 2.0 the 115 ran; slowing it down. That allowed you to turn a larger diameter prop slower and get more thrust for pushing heavy loads.

I bought the 115 so I never ran a 90 but do recall the differences. Long since sold that engine and current is a 3 cyl 90 which still uses the 2.33 but it's a whole different engine and is rated back up at 5500.

Mark

 

[Chris1956]

Your motor will have the 2::1 gear ratio, and distributor ignition. There is no rev limiter. You are a little overpropped, as was suggested. Hitting 50MPH with that power will require a light hull, with a hull shape suitable for that kind of speed, plus power trim. Even then it will be hard to achieve. Of course, exaggerating your speed will get you there, and I expect a lot of people do that.