Dyno ques. regarding gear & tire diameter
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From: Los Lunas, NM
Dyno ques. regarding gear & tire diameter
What's up every1,
I have a few questions on my mind that you guys might be able to provide some insight upon. Regarding dyno's. My last dyno was done with a 4th gear pull for all 4 runs. What is the difference from doing it in 4th gear versus 5th gear? What kinda "hp & tq" reading difference? Last time I did the dyno, I had 17" rims on, this time around when I get to doing another dyno after I finish a project on the car, I will be running 20" rims. Will these heavier rears hurt the hp & tq rating in any way? Thanks for any input every1, take it easy.
--Justin
I have a few questions on my mind that you guys might be able to provide some insight upon. Regarding dyno's. My last dyno was done with a 4th gear pull for all 4 runs. What is the difference from doing it in 4th gear versus 5th gear? What kinda "hp & tq" reading difference? Last time I did the dyno, I had 17" rims on, this time around when I get to doing another dyno after I finish a project on the car, I will be running 20" rims. Will these heavier rears hurt the hp & tq rating in any way? Thanks for any input every1, take it easy.
--Justin
Both the gear you run in and the size of the wheels will have an impact on your #s.
The 20's will most like kill your hp and tq output. Spinning a wheel that large and heavy eats up a lot of power.
The 20's will most like kill your hp and tq output. Spinning a wheel that large and heavy eats up a lot of power.
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So technically, the higher you go in gears, the less hp you will see correct? So a 4th gear pull versus a 5th gear pull should yield higher numbers right? Hmm if 5th is 1:1 then I would rather take the hit of lower numbers and know what my real hp is around. I would imagine 4th gear getting more hp #'s because of power multiplication on the positive side, rather than say 6th gear and the negative side because of being underdriven. Thanks for the input Zivman!
Another thing now is, since the 20's will kill hp & tq, what if I get the FD ratio to go from 3.5 to 4.083. I wonder how the power ratings will differ, ie. more or less since the 4.083 would technically overdrive thus mulitplying power. Is this where "true" hp & tq ratings can get fishy?
--Justin
Another thing now is, since the 20's will kill hp & tq, what if I get the FD ratio to go from 3.5 to 4.083. I wonder how the power ratings will differ, ie. more or less since the 4.083 would technically overdrive thus mulitplying power. Is this where "true" hp & tq ratings can get fishy?
--Justin
i get freaked out about the same ****, dude. i have the base 17's right now. i don't know if i should keep them, or get lighter ones. does the diameter matter even if the weight is the same? i guess so, cause of that whole rotatin mass bs. can u possibly go too light with rims?
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A couple things:
1. Your gear ratios should be taken into account when you do the dyno run. You won't see a torque multiplication when running in a lower gear vs a higher gear on the dyno. I think most people try to dyno in a 1:1 gear to minimize any variation in the geartrain. Sometimes a 1:1 ratio basically locks the gears and therefore you arent spinning them all (that's how it would be in a planetary arrangement, anyway). I doubt our gearbox would work the same way, but I still don't think the 1:1 is critical.
If you have a previous dyno, I would take your next readings in the same gear so you will have less variables in your comparison.
The heavier wheels will definitely lower the HP and Torque, but will have less of an effect if you dyno in a higher gear. The reason for this is because the added weight of the tires only affects the dyno due to the angular acceleration of the wheels (the rate at which their rotational speed increases). This is because you are not actually moving the wheels, just spinning them in place. So, if you were to dyno in first gear, your wheels would go from stationary to full speed in a very short time and hence have a large angular acceleration, whereas a pull in fifth gear would take much longer and the angular acceleration would be much less. So from only a dyno run, you won't see the true effect of the heavier wheels. Depending on how long the dyno run takes, you could run into a case where your dyno numbers look very good compared to the actual effect (since you won't be moving the mass of the wheels and since the acceleration on the dyno is less than it would be in reality - like maybe a 1st or 2nd gear acceleration run). On the other hand, maybe the dyno spins up so fast you have a larger than normal reduction in power shown on the dyno. In any case, the effect of the wheel is probably only a couple HP on the dyno anyway.
Again, if you really want to know how your mods are doing, you should run with the same wheels you used before, although it is probably not a huge deal either way.
TheBigShow,
I don't think you can go too "light" with rims, but you maybe can go too "weak". Not sure how your roads are, but where I live they are usually pretty good, so I went with the lightest wheels I could easily afford for my race wheels (also I won't be using them that much, so the chances of damage are less).
-D'oh!
1. Your gear ratios should be taken into account when you do the dyno run. You won't see a torque multiplication when running in a lower gear vs a higher gear on the dyno. I think most people try to dyno in a 1:1 gear to minimize any variation in the geartrain. Sometimes a 1:1 ratio basically locks the gears and therefore you arent spinning them all (that's how it would be in a planetary arrangement, anyway). I doubt our gearbox would work the same way, but I still don't think the 1:1 is critical.
If you have a previous dyno, I would take your next readings in the same gear so you will have less variables in your comparison.
The heavier wheels will definitely lower the HP and Torque, but will have less of an effect if you dyno in a higher gear. The reason for this is because the added weight of the tires only affects the dyno due to the angular acceleration of the wheels (the rate at which their rotational speed increases). This is because you are not actually moving the wheels, just spinning them in place. So, if you were to dyno in first gear, your wheels would go from stationary to full speed in a very short time and hence have a large angular acceleration, whereas a pull in fifth gear would take much longer and the angular acceleration would be much less. So from only a dyno run, you won't see the true effect of the heavier wheels. Depending on how long the dyno run takes, you could run into a case where your dyno numbers look very good compared to the actual effect (since you won't be moving the mass of the wheels and since the acceleration on the dyno is less than it would be in reality - like maybe a 1st or 2nd gear acceleration run). On the other hand, maybe the dyno spins up so fast you have a larger than normal reduction in power shown on the dyno. In any case, the effect of the wheel is probably only a couple HP on the dyno anyway.
Again, if you really want to know how your mods are doing, you should run with the same wheels you used before, although it is probably not a huge deal either way.
TheBigShow,
I don't think you can go too "light" with rims, but you maybe can go too "weak". Not sure how your roads are, but where I live they are usually pretty good, so I went with the lightest wheels I could easily afford for my race wheels (also I won't be using them that much, so the chances of damage are less).
-D'oh!
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From: Arizona
Originally posted by D'oh
A couple things:
1. Your gear ratios should be taken into account when you do the dyno run. You won't see a torque multiplication when running in a lower gear vs a higher gear on the dyno. I think most people try to dyno in a 1:1 gear to minimize any variation in the geartrain. Sometimes a 1:1 ratio basically locks the gears and therefore you arent spinning them all (that's how it would be in a planetary arrangement, anyway). I doubt our gearbox would work the same way, but I still don't think the 1:1 is critical.
If you have a previous dyno, I would take your next readings in the same gear so you will have less variables in your comparison.
The heavier wheels will definitely lower the HP and Torque, but will have less of an effect if you dyno in a higher gear. The reason for this is because the added weight of the tires only affects the dyno due to the angular acceleration of the wheels (the rate at which their rotational speed increases). This is because you are not actually moving the wheels, just spinning them in place. So, if you were to dyno in first gear, your wheels would go from stationary to full speed in a very short time and hence have a large angular acceleration, whereas a pull in fifth gear would take much longer and the angular acceleration would be much less. So from only a dyno run, you won't see the true effect of the heavier wheels. Depending on how long the dyno run takes, you could run into a case where your dyno numbers look very good compared to the actual effect (since you won't be moving the mass of the wheels and since the acceleration on the dyno is less than it would be in reality - like maybe a 1st or 2nd gear acceleration run). On the other hand, maybe the dyno spins up so fast you have a larger than normal reduction in power shown on the dyno. In any case, the effect of the wheel is probably only a couple HP on the dyno anyway... -D'oh!
A couple things:
1. Your gear ratios should be taken into account when you do the dyno run. You won't see a torque multiplication when running in a lower gear vs a higher gear on the dyno. I think most people try to dyno in a 1:1 gear to minimize any variation in the geartrain. Sometimes a 1:1 ratio basically locks the gears and therefore you arent spinning them all (that's how it would be in a planetary arrangement, anyway). I doubt our gearbox would work the same way, but I still don't think the 1:1 is critical.
If you have a previous dyno, I would take your next readings in the same gear so you will have less variables in your comparison.
The heavier wheels will definitely lower the HP and Torque, but will have less of an effect if you dyno in a higher gear. The reason for this is because the added weight of the tires only affects the dyno due to the angular acceleration of the wheels (the rate at which their rotational speed increases). This is because you are not actually moving the wheels, just spinning them in place. So, if you were to dyno in first gear, your wheels would go from stationary to full speed in a very short time and hence have a large angular acceleration, whereas a pull in fifth gear would take much longer and the angular acceleration would be much less. So from only a dyno run, you won't see the true effect of the heavier wheels. Depending on how long the dyno run takes, you could run into a case where your dyno numbers look very good compared to the actual effect (since you won't be moving the mass of the wheels and since the acceleration on the dyno is less than it would be in reality - like maybe a 1st or 2nd gear acceleration run). On the other hand, maybe the dyno spins up so fast you have a larger than normal reduction in power shown on the dyno. In any case, the effect of the wheel is probably only a couple HP on the dyno anyway... -D'oh!
He is correct. The gearing will not significantly affect the results of a roller type intertia dyno. It will affect what is called tractive effort, but relatively few dynos can measure this (Dyno Dynamics and DynaPack can). With a roller type inertia dyno like a DynoJet, you are just accelerating a large drum of known mass. The rate of acceleration cooresponds to the output of your engine. Another way to think about gearing and dynos would be to picture jacking up your Z's rear end, putting it in gear, letting out the clutch and flooring it. The rate of acceleration isn't going to vary significantly depending on which gear you have the car in. This is because of how the load is being applied. An inertia dyno isn't too far removed from this analogy, as it has a much lower thermal load than other types of dynos.
One more thing regarding the wheel selection. As D'oh mentions it's angular acceleration that you are concerned with. So, looking at the weight of a wheel/tire combo isn't enough. You should consider that with a larger wheel, a significant percentage of the mass of the wheel is located farther out from center. So, a 17" wheel that weighs 25lbs would be easier to accelerate than a 20" wheel that weighs the same 25lbs. Just food for thought.
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