SAE vs Dynojet dynamometer

[Fast4fun]

What's the difference between the SAE dyno used by racing organizations and the dynojet commonly found in bike shops?   Are the results very different?

[Super Dave]

Are the results different?  Yeah, the Dynojet reads, generalizing, 13-15% more "power" based on their equation that comes up with their figure. 

[Kurlon]

I don't think they use an SAE branded dyno, they use SAE's standardized correction factor to adjust the resulting readout to match what they'd expect the the run were made under 'ideal conditions', which I think are 60 degrees, 1 bar of pressure, and I don't remember what humidity.  Dynojets can display either 'raw' or SAE corrected output, along with a couple other standardized corrections if I remember correctly.

[123user]

Actually, there's a couple of different SAE specifications.   The latest, common one is SAE J607.  I haven't purchased the spec. from SAE yet ('cause it ain't free, ya know)  I believe this one (which is the same one used in Buell's press releases) is basically a "real" measurement of torque, @ standard temp. and pressure... but at the crankshaft, not at the rear tire.  This data would be taken using a load cell, measuring the force applied by a lever, converting it to a Moment (torque in ft-lbs).  Then multiplying it by RPM and dividing by 5252 to calculate HP. 

This is how Buell can claim 100hp from a stock XB12R and sell it to the consumer... because nobody but engineers know what the spec means!

Only a brillant madman could make sense of the formula that DynoJet uses to calculate "corrected HP"...  As far as I'm concerned its a crap number.  A reputable dyno manufacturer should tell you how their numbers are derived... DynoJet won't.  Its obvious to everyone paying attention that DynoJet has artificially inflated the number so bike shops can impress customers.  Thats good for business, but bad for being taken seriously.

[Garywc]

Dyno jets are all useless except to tune your bike to run smooth. as far as HP it will be different with each opreator, each dyno, weather ,rear tire, and a number of other factors. can be 10 hp +or - different everytime or could be dead on.

this is based on what has happend to me on the dyno over the last race season and the first round of this season at Daytona

[Super Dave]

This is how Buell can claim 100hp from a stock XB12R and sell it to the consumer... because nobody but engineers know what the spec means!

Well, don't call out just Buell on that.  That goes for Suzuki, Honda, Ford, BMW, etc.  Is it at the crank, SAE, SAE x Pi?

[123user]

Well, don't call out just Buell on that.  That goes for Suzuki, Honda, Ford, BMW, etc.  Is it at the crank, SAE, SAE x Pi?

SAE J607 should be a measurement of crankshaft HP... like I said, I haven't purchased the spec. from SAE, so I don't know if it includes all accessories running.

Sorry, but I mentioned Buell because they made a good example.  All are guilty!  The automotive manufactures are actually a little more honest-  remember the change in 1973, advertised HP had to include all accessories running?  That's how the same 350 chevy went from 300hp in 1972 to 180 in 1973.

A stock Buell will make about 80hp on a FactoryPro at the rear tire, which sounds about right for Buell's claim of 105 at the crank corrected.