I'm pasting this from another thread, that got into talking dyno errors/differences-
<<< just looked at dynodynamics website- I think I see why they show less power:
the two roller design dont meet up with the tire vertically- estimate 30 degrees before and after axle centerline- theres a triangle here that causes effective pressure on the roller(weight x tangent of angle) to be exaggerated, and although its divided by two contact points, the car appears to 'weigh' more by the tire deformation. Also the small diameter rollers will cause much greater tire deformation than the big dynojet rollers. its no wonder the tire hp readings are so low...
we have to put our cars in the US on a very similar looking dyno bi-annually for emissions testing: you stand in a little room to watch, and I was amazed at how much distortion occurred on my pickup- its got 275-60-15 tires, bed was empty, and contact at each roller was really mushing the tires- I hauled a bridgeport home that weighed nearly a full ton, and tires were not noticeably mushed at all.
In my opinion, the two roller design is a piece of
[email protected]#% way to measure power. tire diameter alone will affect contact angle enough that no 'percentage' factor could ever be used for correction. Looking at the dynojet pics shows single large radius roller centered under axle. Much more sensible, but still I feel the axle mounted could be the only accurate way to measure.
Next time somebody goes to a chassis dyno, try running tires at min and max recommended pressures- see how much hp you loose to tire deformation at lower pressure...adding a couple extra psi might get you more than that new camshaft : )
1 newton = 0.2248089 pound-force
I think by 'tractive effort' theyre referring to 'pull'.
just some quick figures:
Brenx said about 10000N
10000 x .2248089= about 2250 pounds pulling force
I'll guess about 28" tires, 28/12=2.333 ft dia /2= 1.166 ft radius
2250 x 1.166=2623 ft/lb torque
I'll guess 3.5:1 final drive ratio
2623/3.5= about 750 ft/lb at the driveshaft
I'll guess about a 2.4:1 low gear (aod)
750/2.4= about 312 ft/lb at the crankshaft
I'll assume torque peaked at around 3000 rpm
(torque[ft-lb] x RPM) divided by 5252= HP
(312 x 3000) / 5252= about 180 hp
(note actual hp at crank probably considerably more, as we backfigured from the tires- and that was after all the losses thru the drivetrain...probably 200+ hp motor in actuality)>>>
The first part, is just something i THINK might explain why the two roller dyno was showing 33% power losses versus the 15-20% on the single roller types...if i'm thinking right, tire pressure(and hold down strap pressure) will have a big effect on output, especially on the double roller type...but then someone mentioned getting a higher reading on the dynodynamics double roller...no idea how this could happen except gross calibration error. I hope someone out there will play with tire pressures on a dyno to see if this could explain the losses. I would have thought the dual wheels might have given more accurate reading, but then you mentioned having to strap it down really tight, so who knows...I believe the single roller dynojet uses heavyweighted rollers as 'flywheels', and just measures the time to accelerate them- if so, no way of really having calibration errors, but still tire deformation has to come into play here somewhere.
The second part, i was wondering if wheel power could be backfigured to the crankshaft, i think the math is right, but dunno- hopefully someone out there has numbers they can plug in, in the above format, to see if it sounds anywhere near correct...really dont matter, but was just 'wanting to know' if it could be worked out.
as for the 460, 500 hp should be reasonable if compression is decent(9.5/10:1), but I know some of the 'smog' low compression ones were rated VERY low on hp...still made decent torque, but peaked so low, kept hp figures looking silly for an engine of that size.
one other thought on the duallie- most people know better, but i have seen a couple around with one newer tire on the rear, or perhaps rotated from the front...a definite no-no with duallies as any diameter difference means one tire or the other has to slip due to the bigger one travels farther per rev than the smaller one... most everyone knows this, but still ive seen them in the tire stores buying one tire to replace a flat...hello friction...sorry for even mentioning this, but figured just in case...
someone please, next time at any chassis dyno, try a run at min recommended tire pressure, max recommended and let us know if this tire distortion is the problem or not...
Ps- are there a lot of duallies in Australia? I work for a company that makes the aluminum wheels for alcoa/ford, we've been running near 1000 a day for the last 4 years, just wonder where the heck they all go...rarely see them around here : )