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Discussion Starter #1
Today at work it was me against 4 other guys with a discussion on the chasis dyno. I was telling them that whether the car has 3.55's or 4.10's or even 4.30's the dyno should read the same between a certain RPM say like 4000-6500. Maybe I'm wrong but none of them are into cars and I can't see 4000 rpm with 3.55's and 4000 rpm with 4.10's would be any different because the motor is at the same rpm...

Am I just that stupid or am I missing something?

I can see maybe it revs faster with 4.30's but I'm totally lost now.
 

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If you are talking about the torque output at the rear wheels then yes gearing will make a difference, if you are talking about horsepower then no it will make no difference because HP is the result of torque/RPM.
 

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the hp will not be different. hp is derived from calculating drum speed vs time.
 

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Discussion Starter #4
If you are talking about the torque output at the rear wheels then yes gearing will make a difference, if you are talking about horsepower then no it will make no difference because HP is the result of torque/RPM.
We were talking about hp so I'm right but in away we are both right and wrong. Ok if hp is the result of torque/rpm and the torque would change why doesn't the HP change? Anyone got any dyno sheets?
 

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[edit] History of the unit

The development of the steam engine provided a reason to compare the output of horses with that of the engines that could replace them. In 1702, Thomas Savery wrote in The Miner's Friend: "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…" The idea was later used by James Watt to help market his improved steam engine. He had previously agreed to take royalties of one third of the savings in coal from the older Newcomen steam engines.[3] This royalty scheme did not work with customers who did not have existing steam engines but used horses instead. Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet in radius, therefore the horse travelled 2.4 × 2π × 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds. So:
This was rounded to an even 33,000 ft·lbf/min.[4]
Others[who?] recount that Watt determined that a pony could lift an average 220 lbf (0.98 kN) 100 ft (30 m) per minute over a four-hour working shift. Watt then judged a horse was 50% more powerful than a pony and thus arrived at the 33,000 ft·lbf/min figure.[citation needed]
Engineering in History recounts that John Smeaton initially estimated that a horse could produce 22,916 foot-pounds per minute. John Desaguliers increased that to 27,500 foot-pounds per minute. "Watt found by experiment in 1782 that a 'brewery horse' was able to produce 32,400 foot-pounds per minute." James Watt and Matthew Boulton standardized that figure at 33,000 the next year.[5]
Most observers familiar with horses and their capabilities estimate that Watt was either a bit optimistic or intended to underpromise and overdeliver; few horses can maintain that effort for long. Regardless, comparison with a horse proved to be an enduring marketing tool.[citation needed]
A healthy human can produce about 1.2 hp briefly (see orders of magnitude) and sustain about 0.1 hp indefinitely; trained athletes can manage up to about 2.5 hp briefly[6] and 0.3 hp for a period of several hours.
[edit] Horsepower from a horse

R. D. Stevenson and R. J. Wassersug published an article in Nature 364, 195-195 (15 July 1993) calculating the upper limit to an animal's power output. The peak power over a few seconds has been measured to be as high as 14.9 hp. However, for longer periods, an average horse produces less than one horsepower.
[edit] Current definitions

The following definitions have been widely used:
Mechanical horsepower
hp(I)≡ 33,000 ft-lbf/min = 550 ft·lbf/s
= 745.6999 W
Metric horsepower
hp(M)≡ 75 kgf·m/s
≡ 735.49875 W
Electrical horsepower
hp(E)≡ 746 WBoiler horsepower
hp(S)≡ 33,475 BTU/h = 9,809.5 W
Hydraulic horsepower= flow rate (US gal/min) × pressure ([[lbf/in2|psi]]) × 7/12,000
= 550 ft·lbf/s
= 745.6999 W

In certain situations it is necessary to distinguish between the various definitions of horsepower and thus a suffix is added: hp(I) for mechanical (or imperial) horsepower, hp(M) for metric horsepower, hp(S) for boiler (or steam) horsepower and hp(E) for electrical horsepower.
Hydraulic horsepower is equivalent to mechanical horsepower. The formula given above is for conversion to mechanical horsepower from the factors acting on a hydraulic system.
[edit] Mechanical horsepower

Assuming the third CGPM (1901, CR 70) definition of standard gravity, gn=9.80665 m/s2, is used to define the pound-force as well as the kilogram force, and the international avoirdupois pound (1959), one mechanical horsepower is:
1 HP≡ 33,000 ft·lbf/minby definition= 550 ft·lbf/ssince1 min = 60 s= 550×0.3048×0.45359237 m·kgf/s since1 ft= 0.3048 m and= 76.0402249068 kgf·m/s1 lb= 0.45359237 kg= 76.0402249068×9.80665 kg·m2/s3g= 9.80665 m/s2= 745.69987158227022 Wsince1 W≡ 1 J/s = 1 N·m/s = 1 (kg·m/s2)·(m/s)Or given that 1 hp = 550 ft·lbf/s, 1 ft = 0.3048 m, 1 lbf ≈ 4.448 N, 1 J = 1 N·m, 1 W = 1 J/s: 1 hp = 746 W
 

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We were talking about hp so I'm right but in away we are both right and wrong. Ok if hp is the result of torque/rpm and the torque would change why doesn't the HP change?
The HP wouldn't change because now the drum speed (RPM) has changed. HP is just a number, (torque x RPM) / 5252. With the same torque, HP goes down if RPM also goes down.

dammit, got tree'd.
 

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I was told the higher the ratio the lower the HP number on the rollers.
 

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although it is not supposed to change the numbers, it will a small amount. Take a dirt car with 6.xx gears in a nine inch, then install 4.89 set in it, you will see about a 10hp swing + on that car. Most of the time going from a 3.08 rear gear in a mustang to a 3.73 will net about a 4-6 rear wheel hp loss, EFI or carb'd on our dynojet (man trans, IAT/ ECT/ oil temp same)
 

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dam Richard, you have a lot of time on your hands, lol
 

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Discussion Starter #10
Thank you Richard G.
 

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...Most of the time going from a 3.08 rear gear in a mustang to a 3.73 will net about a 4-6 rear wheel hp loss, ...
OK, I got lost on that. If you go from 3.08 to 3.73, you're multiplying the torque going to the ground (the engine's spinning faster at the same MPH), so why isn't that a (apparent) HP increase?
 

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OK, I got lost on that. If you go from 3.08 to 3.73, you're multiplying the torque going to the ground (the engine's spinning faster at the same MPH), so why isn't that a (apparent) HP increase?
Because HP is just a calculated number. Simply put, it's the amount of work done in a certain amount of time. HP = (torque x RPM) / 5252. More torque, but takes longer to do the same work = same horsepower.
 
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