torque or horsepower
01-18-2013, 07:21 PM
#2
Horsepower is only a calculation of torque and speed.
For example you can make a lot of torque at low speed (diesel) or smaller torque but at a higher speed (gas) and make the same horsepower.
Boats are heavy and always under load so making good torque under 5000rpm is usually more important than making horsepower over 6000rpm for most boats.
If an engine can carry that torque through the rpm range nice and flat, you can have your torque and your horsepower which is why a well designed engine is important. Most car guys build want to build a high rpm motor (read: they want to put a pro stock cam in) with very little torque down low and they don't do well in boats.
Just my generalized opinion
For example you can make a lot of torque at low speed (diesel) or smaller torque but at a higher speed (gas) and make the same horsepower.
Boats are heavy and always under load so making good torque under 5000rpm is usually more important than making horsepower over 6000rpm for most boats.
If an engine can carry that torque through the rpm range nice and flat, you can have your torque and your horsepower which is why a well designed engine is important. Most car guys build want to build a high rpm motor (read: they want to put a pro stock cam in) with very little torque down low and they don't do well in boats.
Just my generalized opinion
01-18-2013, 09:12 PM
#3
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I make 1000 ft lbs of torque at 1 rpm with my ratched and a pipe on it, but that's not going to make my boat go very fast. Hp is king in a go fast boat. Torque is what makes a Bravo go kaboom. You still need enought torque relative to the size of your boat, to plane off and accelerate.
01-18-2013, 09:39 PM
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My all time favorite description of them is this:
"Speed is how fast you hit the wall
Torque is how far you move the wall"
That there is trademarked, not to be used
without written permission of Ricky Bobby, Inc
"Speed is how fast you hit the wall
Torque is how far you move the wall"
That there is trademarked, not to be used
without written permission of Ricky Bobby, Inc
01-18-2013, 09:52 PM
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Years ago when I worked at a SeaOx dealer I noticed that on a identical boat with a given Hp with outboard or an I/O the outboard boat was always faster on top speed, with the I/O carrying a load better. Also noticed that some times a 6000 rpm 140 outboard was faster than a 5500 rpm 150 (Johnsons where what we worked on at that dealer). I am sure that weight had most to do with that. I do agree that torque is very important, but you can give up a few Ft Lbs to gain a few higher rpm hp and still go faster. Or am I missing the point here?
I don't have coin for a big dollar drives, so I hope that building a higher rpm 454 over big inch low rpm engines will let my bravo's live a little easier life.
I don't have coin for a big dollar drives, so I hope that building a higher rpm 454 over big inch low rpm engines will let my bravo's live a little easier life.
01-19-2013, 06:33 AM
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Bingo.
"My neighbor drag races cars and gave me a deal on building the 454 in my 25' Baja and it now makes 650 HP's but is 5 MPH's slower and is now a pig. Why?"
Many people get confused when shoving a car motor in a go fast boat. Most cars have automatic transmissions w/multiple gears and a torque convertor that allows it to slip at a 100% rate while sitting at a light and stay at or close to the torque peak at all times. You drop the same hot rod motor in the 4500lb boat w/a 34" Bravo 1 on it an shove it in gear and see what happens @ 700 RPM. Or try to get it on plane.
If you could over lay some dyno sheets from the car guys and the boat guys what you'd see (on similar motors) are similar HP and torque #'s from mid range and up (say 3500 - 4000) but bellow that the boat guys would be building twice the torque twice as quick.
Your rig requires X# ft. lbs of torque to roll over on plane and if you have a steep cam w/a lot of "over lap" (where both intake and exhaust valves are open at the same time) your motor may never get to that point and you may never get to plane.
Huge difference and where the "GOOD" boat motor guys earn their money.
I knew a Checkmate dealer in Ohio that used to do all the testing on the new models and had some spooky fast boats. I mean box stock 25's that were well into the 100's
His secret was torque.At the time he had three motors he built at different power levels that he'd drop in the new models when they'd come out. One was 454 based and made 650 HP's which is damned impressive (I think w/a single 4) but also made 650 (as I recall) ft lbs of torque which is huge for that motor and was probably 100-150 more than most of the guys at the same HP level. He told me that was his secret. One of those times when a smart guy asks questions and then ****s up
Remember too, the heavier the rig, the more difference this will make.
BTW, fantastic post innerrage.
Last edited by Twin O/B Sonic; 01-19-2013 at 06:39 AM.
01-19-2013, 08:13 AM
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