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Posting a reply to: RE: 175 Repower Options

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[QUOTE]RJ - 6/1/2011  3:16 PM

I generally am all for MAX HP too, but I do find it interesting performance with the different HP ratings.

Take the optimax 2.5L engines (135, 150 & 175)

175 - 2.5L, 1:87 Gear Ratio, 431 lbs, 5750 Max RPM
150 - 2.5L, 1:87 Gear Ratio, 431 lbs, 5750 Max RPM
135 - 2.5L, 2:00 Gear Ratio, 431 lbs, 5750 Max RPM

So lets assume we have an identical boat for all three engines:

By the Math, if you ran a perfectly propped 175 you would get X mph @ 5750 rpm, if you took that prop and put it on a 150 it would likely be a bit heavy and turn slightly lower RPM say it is 5500 (due to less HP), RPM X the Gear Ratio (1:87) would make the 150 X amount slower, I'm sure there is an exact formula, i just don't have it. Now if you took that same prop and put it on the 135, your MAX RPM may be similar to the 150 because of the lower gear ratio (2:00) but would be considerably slower.

OF course the idea with proper prop selection is to maximize the RPM for optimal performance so you would likely go a bit lighter for the 150 to get the RPM back. By the numbers you will not be able to equal the 175 but it will not be out of the picture. Now depending on the HP curves the lighter prop may actually help the 150 out of the hole if the ventilation is not perfected on the 175.  The idea of the lower 2:00 ratio on the  135 is to help it get up and going and is why basic 135 performance is acceptable, usually just slower than it big brothers. This is the very reason some of the variable pitch props perform so well at both low and high speeds. You just have to pay through the nose for them.

If prop selection was spot on and optimized for both the 150 & 175 by the numbers the 175 should always be slightly better performing than the 150. Math does not lie. But I don't think most of us have the time or resources to do that so we always end up in some type of comprimise.

It would be interesting to find some HP / Torque curves on some of these engines as usually the 150 & 175 share displacement and gear ratios.[/QUOTE]

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You are replying to:

Posted 6/1/2011 3:16 PM (#500909 - in reply to #500400)
Subject: RE: 175 Repower Options

I generally am all for MAX HP too, but I do find it interesting performance with the different HP ratings.

Take the optimax 2.5L engines (135, 150 & 175)

175 - 2.5L, 1:87 Gear Ratio, 431 lbs, 5750 Max RPM
150 - 2.5L, 1:87 Gear Ratio, 431 lbs, 5750 Max RPM
135 - 2.5L, 2:00 Gear Ratio, 431 lbs, 5750 Max RPM

So lets assume we have an identical boat for all three engines:

By the Math, if you ran a perfectly propped 175 you would get X mph @ 5750 rpm, if you took that prop and put it on a 150 it would likely be a bit heavy and turn slightly lower RPM say it is 5500 (due to less HP), RPM X the Gear Ratio (1:87) would make the 150 X amount slower, I'm sure there is an exact formula, i just don't have it. Now if you took that same prop and put it on the 135, your MAX RPM may be similar to the 150 because of the lower gear ratio (2:00) but would be considerably slower.

OF course the idea with proper prop selection is to maximize the RPM for optimal performance so you would likely go a bit lighter for the 150 to get the RPM back. By the numbers you will not be able to equal the 175 but it will not be out of the picture. Now depending on the HP curves the lighter prop may actually help the 150 out of the hole if the ventilation is not perfected on the 175. The idea of the lower 2:00 ratio on the 135 is to help it get up and going and is why basic 135 performance is acceptable, usually just slower than it big brothers. This is the very reason some of the variable pitch props perform so well at both low and high speeds. You just have to pay through the nose for them.

If prop selection was spot on and optimized for both the 150 & 175 by the numbers the 175 should always be slightly better performing than the 150. Math does not lie. But I don't think most of us have the time or resources to do that so we always end up in some type of comprimise.

It would be interesting to find some HP / Torque curves on some of these engines as usually the 150 & 175 share displacement and gear ratios.