Here are the HP numbers measured at the crank and at a rear wheel (one wheel):
TRX500--22.0 HP at crank
13.8 rear wheel
TRX450--23.9 HP at crank
14.9 rear wheel
TRX400--21.0 at crank
13.1 rear wheel
TRX350--20.6 at crank
TRX250--15.6 at crank
These numbers are from Honda and I don't know why they are measuring the HP at the rear wheel at a single wheel.

Why would the 450 have more hp then the 500?

Something doesn't look right, the TRX500 (Rubicon) only has 1.4 hp more than my TRX 350 (Rancher)? The Rubicon has 170cc advantage!

:(

I know it looks strange. I am waiting for a reply to a letter inquiring about torque figures and an answer as to why the HP #'s are what they are. I don't know if you can "de-tune" an engine that much and why you would do that. I don't understand the determination at one rear wheel. What about the other 3 wheels? How do you account for the HP being used by them?

I will defer to the Honda engineers for a true answer; thinking about the question myself, here are some thoughts.

As to the 500 producing a lower horsepower peak than the 450, the 500 may have valve timing producing a broader torque curve and lower HP peak, a design choice of the manufacturer.

As to the other three wheels, horsepower is work per unit time; 550 foot-pounds/second (I think) equals one horsepower. Since only the measured wheel is loaded, it's the only one doing any appreciable work; therefore, a valid horsepower reading can be obtained at that point. Dynos generally use a water brake measuring the total torque produced by the vehicle under test, converting that figure to horsepower at rpm. The only work done at the unloaded (spinning) wheels, after they accelerate to measurement speed, is the work overcoming the friction of the bearings in the drivetrain (and, yes, air resistance, if one wants to get technical).

Now, if all the wheels were connected to the water brake, would the horsepower reading change? Only if the four-wheel connection is more efficient in transferring power to the water brake. For comparison purposes, measurement at one wheel is probably sufficiently accurate and repeatable.

My guesses only.

Tree Farmer

Wow, that makes the SP500 HO's 38 hp look REAL good. I figured my Ruby was making at least 30 hp, what gives? Come on Honda! This isn't rocket science. The HP rating should be on every ATV/Bike sold. You can find out the hp rating on every car! Why can't the manufacturers get this straight? Is it because the Ruby really only makes 22 hp and the SP really makes 38 hp? I guess Honda wouldn't sell many then, huh. I'm a little irratated, I hope those numbers are wrong.

Thinking further--13.8 horsepower at the rear wheel, right?

O.K. If we drill a DEEP hole, tie a LONG rope onto a 550-pound weight and drop it down the hole, then, rev up your Rubicon and take a dally (you've been around rodeos enough to know what a "dally" is!) around a rear wheel; at what rate will the 550-pound weight rise? Let me think--I know! 13.8 feet per second, right? True, if one horsepower is 550 ft-lb/second.

Good! Now, let's drill three more holes, at the other three wheels, dropping roped 550-pound weights down each.

Now, revving up the Rubicon once more, dally all the ropes simultaneously on the four spinning wheels; at what rate do the weights rise? If the weights rise at 3.45 feet per second, the Rubicon still produces 13.8 horsepower at the wheels.

Of course, actually drilling holes and weighting ropes for these hypothetical experiments remain impractical, given gearing considerations and other logistical problems. However, the concept illustrates the validity of measuring horsepower at only one driven wheel.

Tree Farmer

Bis, the horsepower of your ATV is printed on the Manufacturer's Certificate of Origin. See? There it is, "H.P. (SAE)," along with "Shipping Weight," and "NO. CYLS." However, I don't know how this horsepower figure is obtained. SAE measured horsepower at the flywheel? Rear wheel(s)? "Taxable" horsepower, determined by formula? Net horsepower (i.e., with alternator, water pump, etc., operating)? Gross horsepower (i.e., external cooling and electrical power)?

I don't know!

Question, to the ATV Industry: HOW ARE THE HORSEPOWER FIGURES PRINTED ON MANUFACTURERS' CERTIFICATES OF ORIGIN DETERMINED?

Tree Farmer

Coming from the Personnel Watercraft world, everybody new the HP of a given watercraft. This info was in brochures & available on the internet. I believe this is true for snowmobiles. My question is why doesn't the ATV manufacturers publish this info in their literature or on their web sites?

Take a look at DynoJet's HP charts on their site.
A Honda 450S tops of at 11.9 HP.
Where as a Sportsman 500 tops off at 18.8 HP.(nowhere near 38)
Manufactures "pump-up" the HP numbers too sell machines. Look at the aftermarket pipe makers. They claim a 5HP gain on a 4-stroke with pipe and re-jet. Ya right!!!! Maybe they mean .5HP..... Who would buy a loud pipe that only adds .5HP? No one.

Check out all of the dyno runs on GT Thunder's website. Very interesting info.

I don't mean too ruffle any more feathers. But a Sportsman HO is "supposed" to put out 20% more HP. That only brings total HP too 22.5. Not 38.....

Once again. I am only reporting the facts.