Just watched a video of the new 335i pulling on a dyno. They dyno'd in 3rd...what I thought to be standard, then they dyno'ed it in 4th. The final number came out to 276HP and 299ft/lbs...however, it makes me wonder which numbers they used, the pull in 3rd or in 4th. Will the 4th gear pull give better or worse numbers or does it rely on the gearing of the car?
Dynoing in 4th?
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It all depends on whether or not the dyno was properly setup. The standard dyno gear is 1 to 1 or as close as possible. In M6 GTO's that would be 4th gear. If you dyno in a different gear, the dyno needs to be calibrated for that to correct the numbers.
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It does not work like that...the numbers will be close no matter what gear you make the run in, but 1:1 is the most efficient and should show the best numbers. Some vehicles with very high rear gears we dyno in 3rd gear to keep from ending up at 160+ mph at the end of the run which on stock drivetrains could lead to a driveshaft bending and coming loose/flying out...gotzaGTO said:
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I could see how you could dyno in any gear provided that you entered a gear ratio into the dyno computer so that it can compensate. For example, if you switched to a gear that had twice the leverage, then you would put down twice the angular force (torque). The dyno computer could then divide your torque in half to compensate.machinistone said:
The reason I could see you wanting to do a 1:1 is for efficiency (i.e., minimize drivetrain loss).
If the gear ratio is not entered into the dyno computer to compensate then I don't see how the dyno could interpret the torque applied by 1st gear as being equal to the torque applied by 6th gear. First gear would certainly spin the drum faster. If I'm wrong, then someone please enlighten me.
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Because I have a dynojet - based off the tach signal it automatically shows you what your final ratio is. This is why dynojet numbers are consistant across the country with many different shops using them - no chance for operator error.
A dynojet caculates HP based on accelerating a known mass (5400lbs) over time, then based off the tach signal it will extrapilate a torque curve.
A dynojet caculates HP based on accelerating a known mass (5400lbs) over time, then based off the tach signal it will extrapilate a torque curve.
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So would it be appropriate to assume that the dynojet is able to calculate your leverage (approximate gear ratio) because it knows how fast the engine is turning and it knows how fast the drum is turning? In other words, the drum would be turning a lot slower at redline in first gear (~50mph) than it would at redline in say 4th gear (~130mph). However, in both cases, the engine was turning the same speed (6500rpm). That information could be used to calculate an approximate gear ratio, which could then be applied to the force that was exerted on the drum. Since fourth gear is approximately 1:1, and first gear would accelerate the drum a heck of a lot faster, you could take the force applied in first gear and multiply by 50/130 to get back to the same force applied in fourth. I'm speculating hear, so feel free to correct me if I'm way off base. It just seems that from a physics perspective, the computer would have to somehow understand what kind of leverage the car had or else each gear would give different dyno results.machinistone said:
EDIT:
LOL, I just re-read your post and saw that.machinistone said:
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I agree- on a Dynojet, my numbers were almost identical in 3rd and 4th. However- a buddy has a "dynocom" dyno and I get higher numbers in 4th than I do in 3rd, odd. But the 3rd gear run on the Dynocom is comparable to my numbers on the dynojet. Go figure..
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