where should i prop based on dyno #'s and cam??
08-01-2005 | 06:57 AM
#12
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Joined: Mar 2002
Posts: 5,143
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From: Cajun Country
Re: where should i prop based on dyno #'s and cam??
Horsepower equals speed. Typically prop for peak HP and you will find the peak TQ is about 1200-1400 below the peak HP. The farther you can get away from the peak TQ the better, why?
Propping a motor to turn less rpm requires a higher pitch or larger diameter. The higher the pitch and the larger the diameter the greater the stern lift. This can often result in the stern feeling light, bow steering, and excessive prop slip. All of this will hurt the top end speed, handling, plane-ability, cruise speed, etc.
The smaller and lighter the boat the more important this is. A small 22' boat with a supercharger, 850hp and 850lb/ft of TQ may be a combination that will never run right due to the excessive TQ.
Take the same boat with a NA 850hp engine that turns 7500 rpm and you won't have a problem due to the TQ falling off at higher rpm's, however with the supercharged motor making the same hp at 5200 rpm you will have too much TQ and the boat may never run right.
It's important to work with your engine builder so you both understand what the goal of the motor is and it's application so everything works together. Make sure the motor is built to suit your boating style.
Matt
Propping a motor to turn less rpm requires a higher pitch or larger diameter. The higher the pitch and the larger the diameter the greater the stern lift. This can often result in the stern feeling light, bow steering, and excessive prop slip. All of this will hurt the top end speed, handling, plane-ability, cruise speed, etc.
The smaller and lighter the boat the more important this is. A small 22' boat with a supercharger, 850hp and 850lb/ft of TQ may be a combination that will never run right due to the excessive TQ.
Take the same boat with a NA 850hp engine that turns 7500 rpm and you won't have a problem due to the TQ falling off at higher rpm's, however with the supercharged motor making the same hp at 5200 rpm you will have too much TQ and the boat may never run right.
It's important to work with your engine builder so you both understand what the goal of the motor is and it's application so everything works together. Make sure the motor is built to suit your boating style.
Matt
08-01-2005 | 03:10 PM
#13
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Joined: May 2004
Posts: 422
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From: Long island
Re: where should i prop based on dyno #'s and cam??
Thanks Matt, i will contact you tommorrow when i have time. Assuming i need a 27", should we purchase a 26" and turn up to 27" or turn a 28" down to 27". Offcourse they will be labbed.
