Dyno Numbers?

[ASTAR]

Ok, so I know that there is a difference in flywheel HP and rear wheel HP but my question is, do larger tires affect the rear wheel numbers on a dyno? I know that there is a lot more resistance with a larger tire on the road but is it marginal on the dyno? The reason I ask is I am going to the Nor-Cal Winter Nationals Dyno Day and was thinking of bring a smaller pair of tires for the rear than my 37's

 

[KLockliear]

They can. The more mass you have to turn, and the farther it is from the centerline of the axle, the more HP it takes to accelerate it.



So, a wider rim/tire of the same diameter means more inertia at the same distance = less HP to the ground. Same width rim/tire with a larger diameter means more inertia at a greater distance = less HP to the ground.

 

[Cummins4Life]

I lost 30hp going to 36's on my 01 from 285's.

 

[jetenginedoctor]

Not only are there greater losses due to inertia with larger, heavier wheels and tires, there is also a great deal lost in rolling resistance. If you were to test on my dyno, I would encourage you to test with some old OEM stocker wheels/tires as they always give better results than a set of Swampers, etc.



It really depends on how much power you make, and what kind of dyno you test on. If you test on one of mine, traction won't be a problem. Anyone else's dyno, you might have trouble with wheel-spin.

 

[StLouisMO]

I have documented proof of the effects of larger tires. I put my truck on the dyno on 11/7/2004. See my sig for the size tires I'm running. Stock (no chip) my truck did a max HP of only 251. 9 HP and 456. 1 Torque. Plugging in my TS Performance Ramifier brought the numbers up to 331. 3 HP and 615. 5 Torque on my 2nd run.



I learned my lesson... ... I will definitely be bringing the stock tires with me next time I dyno.

 

[JGK]

While the larger tires seem to reduce the HP readings on the dyno according to the previous posts, one has to ask himself what the goal is of doing dyno runs. If you're simply out to post the best numbers you can, regardless of how your configuration represents your daily drive, I'm thinkin' there must be a handful of tricks and tweaks you can do to accomplish that (tire size and wheel weight, obviously, push the plate fully forward on a 12-valve, take off the air filter for max air flow, take the fan off to eliminate that engine drag, etc. ).



On the other hand, if you're trying to see what your BOMBs are doing, keeping everything else, including tires, the same each time eliminates those variables.



Just depends on what you're trying to prove!



Time for another glass of chianti...

-Jay

 

[SEmerson]

So does that mean a duely will post lower #'s then a 2500?

 

[BPonci]

Why would you want to change tires for a dyno run only? You would think you want to know what you have all the time.

 

[ASTAR]

Why would you want to change tires for a dyno run only? You would think you want to know what you have all the time.

Well for one, the high number is cooler , and for two, I'd know what kind of power I could expect with a gear change to closer to stock configuration...

Ideally, I'd Love to see a w/37" tires run and a stock tires run, just to see what kind of number loss there actually would be.

 

[jetenginedoctor]

The biggest differences we see are when a guy tests with big rock-crawling tires versus road tires. Tires suitable for highway use/RV pulling have excellent sidewall stiffness, and deform much less than the off-road tires do. This reduces the sidewall "wrinkle" seen as the truck is tested. It takes a lot of power to push that wrinkle around the tire at 100 mph or so, and a smaller wrinkle takes a smaller amount of HP to push.



With regard to the difference in moment of inertia between different wheel and tire combos. . . . Customers who get a really thorough test session usually get better figures than a cheap-o quick dyno test customer. We can figure out what the drivetrain inertia is and enter that value into the dyno. That essentially negates the inertia losses everywhere between the flywheel and the rollers. If this value is set too high, the HP and torque readings will read on the graph higher than what they really are. If this value is set too low, the HP and TQ readings read on the graph lower than what they are. This is because the dyno recognises that there is a certain amount of HP "stored" within the spinning parts of the truck. Its easier to show than describe. . . . ask me about this if you're ever on my dyno and I'll give you a demonstration of what I mean.

 

[ASTAR]

Its easier to show than describe. . . . ask me about this if you're ever on my dyno and I'll give you a demonstration of what I mean.

Wishin i was in TX right now