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Confused on HP, RPM, gear ratio, etc...

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Old 10-16-2009, 02:14 PM
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Default Confused on HP, RPM, gear ratio, etc...

Guys -

Can someone explain this to me?

Let's say I have a Spectre 30 w/ Twin 300 Mercury Promaxes (I do). I spin 15 X 32 Merc Cleavers (3 blades) at 6200 RPM's. Top speed is normally 94-96 MPH. In the fall when it's cool out and I have no weight in the boat, it will pull 97-99 MPH. I think the gear ratio is 1.52 (Gear ratio could be 1.62, but I swear I read 1.52 off the case).

The new 300XS also spins 6200 RPM's. I'm guessing it has the same gear ratio? So, if I put the same props on the 300XS's, is there any reason my boat would be any faster with the 300XS's?

Reason I ask, and I've asked this before, is I'm going to have to replace my motors at some point. I have heard the 300X's will pull 6700 RPM's and run quite a bit faster (105+ MPH) on the Spectres.

I'm just confused on why the 300XS's would be faster than my Promax's, assuming the same gear ratio and prop and RPM.

Thanks.

Mike Sufka
St. Cloud, MN
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Old 10-16-2009, 02:48 PM
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Who said the Xs's would be faster. Same props at the same height, same rpm, same lower unit ratio....same speed. If your Pro max's are a little tired, and you are over trimming to get rpms, the fresh motors that are stronger may allow you to run lower trim and less slip and may net a different speed. The X's are a whole different animal. They make more HP. and turn more rpms doing it.

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Old 10-16-2009, 03:02 PM
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Same RPM, same gear ratio, same boat = same mph.
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Old 10-16-2009, 03:38 PM
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Did I read something wrong? You said same RPM but you went from talking about 6200 to 6700. 500RPM would give you the 7-8MPH speed boost you mention.
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Old 10-16-2009, 03:46 PM
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Quote:
Originally Posted by Boss460 View Post
Did I read something wrong? You said same RPM but you went from talking about 6200 to 6700. 500RPM would give you the 7-8MPH speed boost you mention.

No.....

300 Promax = 6200 RPM
300X = 6700 RPM
300XS = 6200 RPM

I'm trying to determine why the 300XS would be faster than my old Promax.

Mike
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Old 10-16-2009, 04:12 PM
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Oh then the only part of the equation missing is prop slip. As someone pointed out it might change based on trim angles, drive height etc. Prop slip changing is the only thing that would make it faster if everything else is the same.
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Old 10-16-2009, 04:35 PM
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300xs has more torque, and could pull a bigger prop at same rpm, so a 15x33 or 15x34 at same rpm, giving more speed. Same prop+ same rpm= same MPH

If you get XS and same prop, I'll bet you pull more RPM and may hit limiters at 6350
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Old 10-16-2009, 06:10 PM
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300xs has more torque, and could pull a bigger prop at same rpm, so a 15x33 or 15x34 at same rpm, giving more speed. Same prop+ same rpm= same MPH

If you get XS and same prop, I'll bet you pull more RPM and may hit limiters at 6350
Is the limiter for the 300XS 6350?
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Old 10-17-2009, 09:45 AM
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The 300xs will not be any faster than your 300 promax unless of course the promax's are tired. They will however give you at least twice the fuel economy and smart craft.

The rev limiters on your 300 promax should be 6350, your beepers should beep around 6250 but the motors wont retard until you get to 6350....at least thats my understanding.

300x make around 330HP with a rev limiter of I think 6800, your 300 promax new is between 290-300. You are correct in that your boat would pick up about 7-8 mph with a 300x setup.
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Old 10-17-2009, 09:48 AM
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Oh yeah, I am pretty sure you have 1.62 gears in your promax's.
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