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Max RPMs on 502MPI Magnum

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Old 07-16-2019, 09:03 PM
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Default Max RPMs on 502MPI Magnum

I have a 1991 Baja 250 ES with a 502MPI Mag merc, bravo 1 drive

the engine info plate says Max RPM at WOT is 4800-5200

but Iíve yet to see it turn more than 4600rpms

ive tried different props; 3 blade 25p - 4blade 26p - 4 blade 24p

still. Nothing more than 4600rpm. I know itís not my gauges. Intake flap opens 100% with throttle wide open. Just canít seem to figure it out. If anyone has some information that could help me. It would be greatly appreciated!
PS. Was hoping to see 5000rpm out of the engine
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Old 07-16-2019, 09:58 PM
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If you’re certain your tach is right, you’re probably over-propped.

Directly from Mercury manuals-

Effects of Elevation and Weather on Performance
The following conditions lower engine performance and cannot be compensated by the engine fuel or electronic management systems:
• Above sea level elevations
• High temperature
• Low barometric pressure
• High humidity
These conditions above reduce air density to the engine, which in turn lowers the following:
• Boost pressure on supercharged engines
• Horsepower and torque throughout the RPM range
• Peak RPM
• Cranking compression
EXAMPLE: An engine run at an elevation of 8,000 feet will have over a 30% power loss while a loss of engine power on a hot and humid day could be as much as 14%. These losses apply to normally aspirated and supercharged engines.
Compensating for power robbing conditions:
• Switch to lower pitch propeller.
• Change gear ratio.
Some boat performance can be regained by dropping to a lower pitch propeller, but engine performance will still remain lower. In some cases, a gear ratio reduction may be more beneficial. To optimize engine performance, prop the engine to allow it to operate at or near the top end of the recommended maximum RPM range at wide‐open throttle with a normal boat load.
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Old 07-16-2019, 10:03 PM
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From Mercury Racing-

WHY DOES MY BOAT PERFORM DIFFERENTLY ON A HOT DAY VERSES A COOL EVENING?

It is a known fact that weather conditions exert a profound effect on power output of internal combustion engines. Therefore, established horsepower ratings refer to the power that the engine will produce at its rated rpm under a specific combination of weather conditions.

Corporations internationally have settled on adoption of I.S.O. (International Standards Organization) engine test standards, as set forth in I.S.O. 3046 standardizing the computation of horsepower from data obtained on the dynamometer, correcting all values to the power that the engine will produce at sea level, at 30% relative humidity at 70 deg.F (20 deg.C) temperature and a barometric pressure of 29.61 inches of mercury.
  • Summer Conditions of high temperature, low barometric pressure and high humidity all combine to reduce the engine power. This, in turn, is reflected in decreased boat speeds–as much as 2 or 3 miles-per-hour (3 or 5 Km per-hour) in some cases. Nothing will regain this speed for the boater, but the coming of cool, dry weather.
In pointing out the practical consequences of weather effects, an engine–running on a hot, humid summer day–may encounter a loss of as much as 14% of the horsepower it would produce on a dry, brisk spring or fall day. The horsepower, that any internal combustion engine produces, depends upon the density of the air that it consumes and, in turn, this density is dependent upon the temperature of the air, its barometric pressure and water vapor (or humidity) content.

Accompanying this weather-inspired loss of power is a second but more subtle loss. At rigging time in early spring, the engine was equipped with a propeller that allowed the engine to turn within its recommended rpm range at full throttle. With the coming of the summer weather and the consequent drop in available horsepower, this propeller will, in effect, become too large. Consequently, the engine operates at less than its recommended rpm.

Due to the horsepower/rpm characteristics of an engine, this will result in further loss of horsepower at the propeller with another decrease in boat speed. This secondary loss, however, can be regained by switching to a smaller pitch propeller that allows the engine to again run at recommended rpm.

For boaters to realize optimum engine performance under changing weather conditions, it is essential that the engine have the proper propeller to allow it to operate at or near the top end of the recommended maximum rpm range at wide-open-throttle with a normal boat load.

Not only does this allow the engine to develop full power, but equally important is the fact that the engine also will be operating in an rpm range that discourages damaging detonation. This, of course, enhances overall reliability and durability of the engine.



Find someone with a 23p Mirage+ and give it a try.

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Last edited by Speedracer29; 07-16-2019 at 10:19 PM.
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Old 07-16-2019, 11:10 PM
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WOT range for a 502mpi is 4600-5000 rpms. Rev limiter is 5150rpms.
Sound like you are over propped. Saying 3 blade xx pitch and 4 blade xx pitch means nothing if they are the wrong make of prop.
As said, try a 23 Mirage Plus and get a base line.
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Old 07-17-2019, 06:52 AM
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Originally Posted by Griff View Post
WOT range for a 502mpi is 4600-5000 rpms. Rev limiter is 5150rpms.
Sound like you are over propped. Saying 3 blade xx pitch and 4 blade xx pitch means nothing if they are the wrong make of prop.
As said, try a 23 Mirage Plus and get a base line.
I agree. Your 250 Baja is a lot of boat for the props you mentioned and only 415 HP. A 22p Bravo I (4 blade) or 23p Mirage+ (3 blade) like Griff mentioned will put you where you want to be.
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Old 07-17-2019, 06:58 AM
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As stated, likely over-propped. Easy to also verify the accuracy of your tach with an inductive pickup shop tach.
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Old 07-17-2019, 07:08 PM
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Originally Posted by Griff View Post
WOT range for a 502mpi is 4600-5000 rpms. Rev limiter is 5150rpms.
Sound like you are over propped. Saying 3 blade xx pitch and 4 blade xx pitch means nothing if they are the wrong make of prop.
As said, try a 23 Mirage Plus and get a base line.

the 3 blade 25p I have is a Mirage and it was awful compared to the 4 blades 24p and 26p were bravo 1 (the 26p was lab)

i went out this morning to test the bravo 1- 4 blade 24p again with just my self and 3/4 tank of gas (75ish gallons).
I was able to hit 4800 rpms. And the boat gets tour on plane very quick. But my top speed is still low in my opinion.

I was told told when I purchased the boat. It would hit 70mph but Iíve yet to see it hit 66. And I I know itís hot and climat does affect it. But with the 24p I was able to hit 64mph today

thoughts?
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Old 07-17-2019, 08:16 PM
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Over propped. The fact you lightened the load and got more just proves that theory.
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Old 07-18-2019, 06:41 AM
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Originally Posted by Tbenson95 View Post



the 3 blade 25p I have is a Mirage and it was awful compared to the 4 blades 24p and 26p were bravo 1 (the 26p was lab)

i went out this morning to test the bravo 1- 4 blade 24p again with just my self and 3/4 tank of gas (75ish gallons).
I was able to hit 4800 rpms. And the boat gets tour on plane very quick. But my top speed is still low in my opinion.

I was told told when I purchased the boat. It would hit 70mph but Iíve yet to see it hit 66. And I I know itís hot and climat does affect it. But with the 24p I was able to hit 64mph today

thoughts?
Unfortunately sellers overstate speed constantly.

64 mph sounds pretty right-on for a 1991 Baja 250 with 415hp, its not exactly known for being an efficient hull. 4800 rpm at full song is right where you need to be with that motor. Dropping 2" pitch might put you on 5150 limiters (assuming 400 rpm drop from going to 22p Bravo I prop).
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Old 07-18-2019, 07:13 AM
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It doesn't sound like you sea trialed the boat before the purchase if its not performing as stated.......if the 25 mirage is the + version it takes more power than you have to spin it correctly.
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