posted February 19, 2007 09:30 AM
why dynojet numbers are always wrong saw this over on another forum.
thought it was interesting and should share.
this was originally posted by MarcP from www.ducati.ms
i uploaded the PDF files for the article.
Quote:
Since we are in the post-Christmas slump I thought I would post something that would perk everyone up.
I found an interesting article in Sports Car International about Dynojet dynamometers. Take a look.
I have come back to motorcycling after a long absence and all of this reliance on dyno charts is new to me. It's pretty amusing watching the manufacturers make outlandish claims for their bikes (an R6 makes 133hp? In what universe?). I have read various anecdotes of that how Dynojets seem to give high readings but I never saw proof.
This article explains how things got to where they are, and the source is none other than the creator of the Dynojet himself. To summarize the long article, a Dynojet measures hp by using a weighted roller. Because the roller weight is known the amount of energy necessary to accelerate it can be calculated. From there it is a fairly simple matter to convert to horsepower.
What is missing from the equation is that in addition to overcoming the inertia of the weighted roller, an engine also has to overcome the weight of the pistons, crankshaft, flywheel, transmission, chain, wheel and tire. Obviously a Harley with a 240 tire has a lot more spinning weight and resists acceleration much more than say, an Aprilia RS 250.
The original Dynojet was indexed to a 1985 1200cc Yamaha V-Max. As a result a Dynojet will give high power readings for any bike with a lower spinning weight. Which is to say, nearly all of them. How convenient for Dynojet.
This also may explain why the Ducati twins consistently post lower horsepower figures than the I-4s. I found some figures for Ducs and the I-4s on the website of Factory Pro dynos. These use an electric motor to measure the engine power and claim it to be more accurate. The Factory Pro is also much more expensive which might explain why dealers buy Dynojet. (Or maybe dealers want to show artificially high figures too.) http://www.factorypro.com/dyno/true1.html
Look, I really don't care which bike makes the most horsepower. I ride a 749s and I'll be the first to admit that I can't use all the power on the street. Although IMHO I don't think that Valentino could either unless he wanted to seriously endanger his life, and he's too smart for that. Besides, he could kick my ass on a Vespa. As we enter yet another year of winter bike tests and endless comparisons, claims, and counterclaims I thought it would be fun to get this discussion going.
So here are the questions for all the motorcycle savants out there in Ducati land. Is the article BS? Does anyone know the engine and drivetrain weight/inertia difference between a 1985 Yamaha V-Max and a 999? The difference between a 999 and a GSXR? Why hasn't someone measured this in all these years? Are Factory Pro dynos truly more accurate?
posted March 10, 2007 11:42 PM
DJ numbers are not really "wrong", they're just not what we think they are. they are an estimation of the power closer to the crank. can't remember exactly what it is supposed to be estimating (power at the counter-sprocket?) but it isn't the power applied to the road under acceleration. perhaps they ARE trying to estimate power to the road, but under zero acceleration, using data collected from simulated acceleration. that would make more sense actually.
posted May 31, 2007 01:26 PM
I don't care what a dyno states, it's nothing more than a tool for tuning, and unfortunately "bragging rights" even though 1 Dynojet will state 145hp, and another Dynojet down the road will claim 154hp.
I saw one in a local shop put a ZX10 at 145 hp, and the owner's R1 at almost 160(both stock). After running my own R1 against a ZX10 and swapping bikes(both stock) I can tell you a ZX10 and R1 both make close to the same peak hp and not 15 -+.
I'll never get tired of riding bikes but I do tire of "Well, my ZGSXCBR1 dynoed at blah blah blah HP so it's faster, nah nah na nah nah!" Then we have the eventual posting of dyno charts leading to the "Internet Dyno Chart 500 Race"!
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Best Literbike: Performance Bikes, Superbike, BIKE, MCN, Road Racing World, and Motorcycle USA
posted August 07, 2007 12:35 PM
Edited By: Sticks_n_Stones on 7 Aug 2007 13:37
Car guys know that to be a fact, thats why they always take it with a grain of salt (sometimes a very large grain). I can't understand why the same dyno would show a 10R making less than an R1, but two diff dyno's will almost always give different numbers. And in regards to the V twin verse I-4's, I can buy it being off. My '02 TL1000R with PCIII, Yosh's, TRE and steeper gears (1 down 3 up) makes aprx. 130 RWHP, but from a roll has beat numerous R1's ('05 and '06's) and older GSXR's 1000's. I got pulled hard by a PCIII and slipon 06 GSXR 1000, though. Point is, those bikes make alot more HP on the dyno but it doesnt play out that way on the street. There's only one guarantee: If you pull away, you're faster!! Skill or torque or weight doesnt matter only the result. Ride your own ride, don't worry about the 'other guy' and enjoy it, because most bikes out there can beat another make/model/displacement in one or more category's. Learn its upside, compensate for its downside, and screw the 'stats racing'.
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posted February 19, 2007 09:30 AM
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