When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Crank horsepower is interesting from an engine design point of view, but it is NOT the amount of power available to the rider on the street, so is or academic versus practical interest.
I never claimed total control of all variables. That is not within my practical capability. I do not own the dyno, note the facility, nor can I control the weather, nor can I think in advance to warn a certified experienced mechanic to not over oil the air filter. I do the best I can. I think the results are clear enough despite the limitations.
Jim G
What actually is relevant for the rider is NOT the rwhp but acceleration rate (deceleration rate), possible top speed, and overall traction. These values are highly influenced by e.g. the surface/material of the road, moisture, the weight of the rider, the weight balance, tire pressure, rubber mixture etc. etc.
The rwhp is of no use to the rider if you cannot put that power on the street.
And as the rwhp is affected by too many variables the only reliable/practicable reference for 'horsepower comparison' is crank horsepower. Your dyno runs at least give proof of that!
". . . therefore you don't believe in the value of DOING dyno tests in the first place,"
That statement is PROOF that you make statements beyond your knowledge.
You say that with the same conviction that all of your statements have. Too bad you didn't take the time to read the very next post admitting my math error or did you? I use and swear by dyno tuning. Cliff at Piper Performance in Decatur does the tuning on my bike and I'm not the least bit ashamed of my little 88.
Replace the belt drive with a chain drive and you will also realize a gain in RWHP with no gain in crank HP. Replace the belt drive with a shaft drive and you will lose RWHP...
Yes of course. A chain is slightly more efficient than a belt, and a shaft drive is far less efficient (but easier to maintain ).
There were NO adjustments to tune after installation of the new tire. The tire made the difference.
HOW in The World does the gain in power mean "nothing"???
Jim G
You haven't proven anything Jim, only confirmed that my monkeying with wheel diameters you can manipulate the numbers which racers have done for years. You aren't creating any more horsepower by putting a smaller tire on, you're only making it slightly easier for the motorcycle to turn the dyno drum.
You haven't proven anything Jim, only confirmed that my monkeying with wheel diameters you can manipulate the numbers which racers have done for years. You aren't creating any more horsepower by putting a smaller tire on, you're only making it slightly easier for the motorcycle to turn the dyno drum.
First, a correction. I changed the diameter only insignificantly - 0.6 of one percent. That is not what changed the "reported" power. It was the WEIGHT difference that did that.
But, I am glad to see you got my point: I NEVER said I made any more REAL power. I said simply that by changing the moment of inertia of the rear wheel, I caused the Dynojet dyno to "report" more power. THAT is the point.
I realize I may not have been clear enough in my opening statement to my posting, so I have amended it to read:
"
This posting will show that changing to a lighter weight rear tire does indeed appear to increase the torque and power REPORTED by a Dynojet dyno, even when no actual engine power improvement has been made.
"
Hopefully, this clarifies that the use of a lighter tire with a lower moment of inertia causes less of the engine's developed crankshaft power to be diverted towards spinning up the rear tire during the dyno run (which is 100% acceleration on a Dynojet dyno), and that undiverted power is then reported as extra rwhp beyond what was previously shown with a heavier tire.
Of course, a lighter WHEEL would also cause such an "improvement" in reported power.
Therefore, when trying to compare the power output of supposedly identical engines on a Dynojet dyno, it is important to ensure that the combined moment of inertia for the entire drivetrain is also the same for the two bikes being compared. ie. If one of the bikes has a heavy aftermarket or optional HD wheel, or tire, expect there to be variances in the reported power and torque.
I'm drawn to the differences in AFR. I'm no expert, but atmospheric conditions(temp, humidity, & pressure) on the days dyno'd can also be adding/stealing torque & hp.
... Also 4-5 #'s of more air pressure in a tire will make 1-3 better numbers on a tune and How many runs have been made..
A bike up to Temp will do better than one that just barely warmed up also...
Lots of Variables when you going for peak numbers...
... Also 4-5 #'s of more air pressure in a tire will make 1-3 better numbers on a tune and How many runs have been made..
A bike up to Temp will do better than one that just barely warmed up also...
Lots of Variables when you going for peak numbers...
4 to 5 psi of air for a dyno run should only happen with a RACE bike. A street bike whose tire psi has been maintained at the correct level for that tire and that bike will not experience any improvement in reported power if the psi is raised. And in fact, raising the psi beyond normal street level, which is much higher than race level to begin with, risks causing the tire to "slip" on the dyno because the contact patch is suboptimal.
As for operating temperature, both of the dyno runs displayed were done with the bike still hot - on the "before run" right after the dyno tuning, and on the "after" run, fresh from being ridden there minutes beforehand (There was no wait - I was the first bike there after the rain stopped and the roads dried).
But yes, I am not able to control all the variables to the degree we'd like, but then neither is the typical dyno shop.
If you think the data I have presented is persuasive, use it. If you don't regard it as persuasive, ignore it. I like to get as much data as I practically can without spending more money than I have available to spend.
Dyno's are nothing but a computer synced to a mass that work's on a predetermined formula that needs to satisfy all makes and models of powered vehicles regardless of gear ratio's, tire size, weight etc. it only knows that what ever the device used to wind up the drum, it will only see (or store) the rate of acceleration (time) to determine or calculate the amount of force that was used to rotate the drum to predetermined rpm's to make it's calculation. A computer only knows what it is programmed to know. Crank HP is just that, changing anything beyond the crank does not change HP ratings of the source (motor) but does change rate of acceleration and is not an effective tool of predicting a HP rating at the crank. However in theory a larger mass will absorb more force than a smaller mass which in theory will take more Hp from the source. What the dynojet is programmed to see is the amount of power that the rear wheel is exerting onto the drum and transforming that to Lehman terms so we have some type of idea of the final output of the device driving the drum. Also it does not matter how much crank HP you have if you cannot transfer it to the ground...... I like rating my power by the seat of my pants, if it scares the **** out of me its fast...
Last edited by 98hotrodfatboy; Oct 12, 2014 at 08:28 PM.
7 Surprising Harley-Davidson Products that Are Not Motorcycles
Slideshow: The bar-and-shield logo shows up on far more than motorcycles, some of the company's most unexpected products have nothing to do with riding.
Slideshow: From the troubled AMF years to modern misfires, these bikes earned reputations for reliability issues, questionable engineering, or disappointing performance.
Crazy Bunderbike Build Looks Amazing, But Is It Impossible to Ride?
Slideshow: The Swiss custom shop has taken a Harley Softail and stretched it into something so long and low that it looks closer to a rolling sculpture than a conventional motorcycle.
Engraved Rebellion: Inside Bundnerbike's Glam Rock II
Slideshow: A standard cruiser becomes an intricate metal canvas in the hands of a Swiss custom house known for pushing Harley-Davidson platforms far beyond their factory brief.
Slideshow: Harley-Davidson's challenges aren't abstract; they show up in dropping shipments, shrinking dealer traffic, and strategic decisions that aren't yet translating into growth.
... Also 4-5 #'s of more air pressure in a tire will make 1-3 better numbers on a tune and How many runs have been made.. 
