I have to know

I was looking at the specs on a 2.5L 150-175-200 motors and all of the specs are the same, Dip, bore, stroke weight the only specified difference was HP.

How can all of the specs be the same identically but have 3 different HP ratings?

Can someone please explain?

Intake manifold variances, porting, timing, fuel amount variances, reed valves..........

Most mfg's, whether it be marine or automotive use the same block for a number of different engines - Merc is typical.

As you note the 150/175/200 all use the 2.5L block - the 135/150 also shared the 2.0L block, over the years the 40/50/60 shared a block as did the 75/90, 15/9.9, 6/8/9.9, 9.8/7.5, 18/25 - on and on.

So it may "seem" like you can easily convert one to the other - and in fact you can "easily" but not "cheaply" in most cases.

Sometimes it's nothing more than a few jets in the carb, sometimes it's new carbs and sometimes you have to change the entire intake/exhaust.

In any given (group) of motors the "on boat" performance difference is minimal at best.

General "rule of thumb" - you need to increase the horsepower by 50% to see a 15% top speed increase on any given hull. Such that if you are getting say 40 mph from a 150, to get to get 45'ish mph you need to move up to 225 horses.

Taking that same 150 and "up tuning" it to it's maximum - 200 horses on the 2.5L block would be an increase of 33% in raw horsepower - and yes, it would give you 10% better top end speed (and only top end, the power curve remains the same because the displacement never changed), but the change could cost you quite a few hundred bucks and without staring at a gps reading out the speed I don't know a person alive that can distinguish between 40 and 44 mph on the water.

I'm starting to get off on a tangent

So to give you a simple answer, it doesn't matter what the displacement of a motor is - by metering or restricting (or forcing - such as turbo and supercharged motors) how much gas gets into the cylinder and controlling the amount of exhaust that gets out you can "tune" it to a particular horsepower...

I was looking at the specs on a 2.5L 150-175-200 motors. How can all of the specs be the same identically but have 3 different HP ratings?

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The blocks as somewhat the same ,the differences are in the exhaust and intake port timing/height, sometimes the exhaust divider plate and tuner and the carbs due to internal metering is differences also gear ratios in lower unit.

General "rule of thumb" - you need to increase the horsepower by 50% to see a 15% top speed increase on any given hull. Such that if you are getting say 40 mph from a 150, to get to get 45'ish mph you need to move up to 225 horses.

Click to expand...

Thats not correct for a performance hull as you can take a 18ft Basscat with a 150 XR6 that runs 62 mph gps and install a 200 hp and it will run 73mph gps and rpm stayed the same but gear ratio changed and prop went up 2 pitchs to a 27 and thats also a 11mph increase.. Just rerigged this last week You calculations are not taking in account for gear ratios and prop pitch changes.

That is why I stated "general rule of thumb". The shape of the hull, degree of deadrise, hull factor, length to beam ratio, overall weight of the hull, balance about center of boyancy etc all factors in how a particular hull will perform with a given motor.

However, for the "average" production boat (Tracker, Lund, Bayliner, etc) which has a hull factor of about 150, the required boost in horsepower to achieve a 15% speed increase is about 50%.

A high performance Bass boat, with a hull factor of 190 will achieve a higher speed percentage increase for a given horsepower percentage increase - however, that hull requires significantly more horsepower (in a displacement to horsepower ratio) to achieve a given speed as compared with a lower hull factor boat or a flat bottomed boat. The primary difference is that the boats with the lower hull factor would become unstable and possibly cartwheel because they do have a much lower "maximum" hull speed (visualize a hydrofoil that exceeds the speed where it can stick to the water).

An 18 foot Lund fishing boat has a max safe speed of the low 50 mph range equipped with a 150. From Lunds own advertising when equipped with a 115 horse (minimum recommendation) it posts speeds of almost 40 mph. Bumping up to a 150 only gets you an extra 10'ish mph - so in this case almost 50% hp increase gets you about 25% speed increase on their "performance" hull. On another model from the same builder going from 75 horses (@35 mph) to 125 horses (@43 mph) the 66% horsepower increase results in a little over 22% speed increase (which factored would be a 50%/16.6% ratio).

So unless we analyze a specific hull a rule of thumb answer is generally good enough when talking in general terms to give a realistic expectation of what a given horsepower increase will do for the "average" hull...