# Horsepower Ratio?

#1
06-17-11, 08:39 PM
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Horsepower Ratio?

Is acceleration a direct proportion to horsepower, as in three times as much horsepower is 3 times as fast acceleration? Also, is acceleration a direct proportion to gearing, as in a 1 to 3 ratio would take three times as long to accelerate as a 1 to 1 gear ratio?

#2
06-17-11, 09:05 PM
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No, and no. There are many factors to consider in order to figure these things.

#3
06-18-11, 07:21 AM
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What else is there to consider? I am looking at without the factor of wind resistance if that helps.

#4
06-18-11, 11:55 PM
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Stroke of the engine, airflow, timing setting and control, load, temp, drive ratio, etc... You can't just take horsepower and figure out what anything else will be by the one known factor alone.

What you're asking is like asking, "I have a 5.987, so what is X?" when you have no formula to work from. Acceleration is measured in a unit of distance per unit of time squared. (ex: meters per second per second). Horsepower doesn't convert to that.

#5
06-19-11, 07:26 AM
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Is there a formula to work with? I do have a control 5hp engine and an 8hp that i would like to put in.

#6
06-19-11, 08:49 AM
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Originally Posted by thedancingman43
Is acceleration a direct proportion to horsepower, as in three times as much horsepower is 3 times as fast acceleration? Also, is acceleration a direct proportion to gearing, as in a 1 to 3 ratio would take three times as long to accelerate as a 1 to 1 gear ratio?
An engine produces torque over an rpm band. Torque starts out very low then increases to a point then attenuates. Horsepower is the product of torque and engine speed (expressed in radians per second) divided by 550 ft-lbf/sec-hp. So if an engine is turning 6000 rpms and putting out 300 ft-lbf of torque, its hp is 6000*2*pi*300/(60*550) or 343 hp. In general hp is given as peak hp. Torque is provided as peak torque. But hp is defined as the product of both at a given speed. In general the peak hp is at the highest engine speed whereas maximum torque is at a speed considerably less than maximum.

If you want to accelerate at 3X your current rate, you would have to increase the torque by 3X, not the hp by 3X. The force that accelerates the vehicle comes from the tires in contact with the pavement. That force is directly proportional to the torque at the axle which is proportional to the torque provided by the engine. The torque at the axle is proportional to the overall gear ratio. The force at the ground is equal to the torque at the axle divided by the distance between the axle center and the ground. But do not forget that the greater the torque, the greater the friction losses in the gearing, etc are.

So if you lower your gear ratios (to achieve greater acceleration) you'll have to supply more gear changes to achieve the same desired speed within a given time due to the fact you are then limited by maximum engine rpm.

Clear as mud, huh?

#7
06-19-11, 11:36 AM
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Thankfully, I am a math minded person so that makes perfect sense. so now here's my question. I am replacing a 5 hp 4-stroke hemi-style engine (OHV) from a go kart with a 8 hp flathead engine out of a snowblower. What is the likely result?

#8
06-19-11, 01:46 PM
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Originally Posted by thedancingman43
Thankfully, I am a math minded person so that makes perfect sense. so now here's my question. I am replacing a 5 hp 4-stroke hemi-style engine (OHV) from a go kart with a 8 hp flathead engine out of a snowblower. What is the likely result?
The likely result?
You will have a go kart that blows snow. Have a good one. Geo

#9
06-19-11, 03:55 PM
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Well, some people need to get back on topic here* cough* cough Geo.

#10
06-20-11, 09:47 AM
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Assuming the 8 hp motor has more torque than the 5 hp, your end result is that you should accellerate a little faster with the 8 HP. Top end speed will be the same as both motors max at 3600 rpm. If you want a numerical answer as to the exact difference, I have no idea. I seem to remember a rough rule of thumb that applied to piston powered airplanes, that if you wanted to take a 200 hp, 200 MPH aircraft and get 400 MPH out of it, you needed 2000 hp, or a factor of 10 times the power to double the speed.

#11
06-20-11, 10:20 AM
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From what I've read, the hemi engine will rev lower, and the flathead will rev higher. True? Also, the airplane engine would also have to deal with aerodynamic drag, and at the speeds I'm dealing with, aerodynamics are of a minimal concern.

#12
06-20-11, 10:33 AM
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No.

There is no formula for anyone to go by and give you a definitive answer without knowing nearly every aspect of both engines and the equipment it is going on. It's a just a gokart.

If you put an 8 hp in place of a 5hp, obviously you will have more power available. Knowing the nature of these engines, and assuming the 8hp engine is suitable for this application, it should accelerate a bit quicker but the kart probably won't go any faster. The 8 horse will require a different mounting plate, a different clutch, possibly a different throttle cable and may have clearance issues. The carburetion may not be suited for the bouncing around on a cart either. The flathead engine is also a step backwards in technology and may not perform quite as well as the OHV in all aspects. Something to consider.

#13
06-20-11, 08:19 PM
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Originally Posted by thedancingman43
Thankfully, I am a math minded person so that makes perfect sense. so now here's my question. I am replacing a 5 hp 4-stroke hemi-style engine (OHV) from a go kart with a 8 hp flathead engine out of a snowblower. What is the likely result?
you'll likely hurt yourself , just kidding. It's safe to conclude that the 8 hp will put out more torque than 5 hp at most RPMS so you'll accelerate faster. By changing gear ratio you should be able to attain higher max speed but I think you then accelerate slower since torque to wheel is reduced by change in ratio. I did a bunch of study on this way back. Torque curve is actually measured on an inertial dyno by measuring the acceleration of a heavy wheel as engine is run from 0 to max rpms.

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