dyno experts...please help or explain!
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Melting Slicks
Joined: Sep 2000
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From: Carlsbad, CA CA
dyno experts...please help or explain!
why do auto car dyno in 3rd gear and manual cars dyno in 4th gear? Does it matter what gear you put it in? Will the results differ? Why?
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Re: dyno experts...please help or explain! (zelement)
To get your most accurate dyno reading you want to take out as much gear multiplication as possible. As such you want to get as close to a 1:1 gear ratio in the tranny. For most manual transmission cars this is 4th gear, for most auto's it's 3rd or 3rd is just as close as you're gonna get.
Technically the dyno can correct for it because it reads engine RPM and wheelspeed. However every time you put in some gear multiplication you increase your error, put two togethr like oh say the tranny and the differential and your error potential is multiplied. How much so? I've seen the same car make 4 passes on the dyno, 4th, 4th, 3rd, 4th. All the 4th gear runs were identical while the 3rd gear run was down 10 hp clear across the board. BTW, this was on an almost stock 93 Z-28
:cheers:
Technically the dyno can correct for it because it reads engine RPM and wheelspeed. However every time you put in some gear multiplication you increase your error, put two togethr like oh say the tranny and the differential and your error potential is multiplied. How much so? I've seen the same car make 4 passes on the dyno, 4th, 4th, 3rd, 4th. All the 4th gear runs were identical while the 3rd gear run was down 10 hp clear across the board. BTW, this was on an almost stock 93 Z-28
:cheers:
Pro
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From: Wilmington N.C.
Re: dyno experts...please help or explain! (zelement)
Nathan is right on the money------just try with the auto cars not to have it downshift on the dyno----with my GN it makes terribles tq. spikes--feels like it is going to pull right off the dyno :eek: :D
Geoff
Geoff
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Re: dyno experts...please help or explain! (zelement)
Actually, I just remembered that I have those dyno runs.
Notice the green one? That's 3rd gear instead of 4th. Funny thing is that it's lower, you would think that it would actually put more torque to the ground in 3rd and thus the numbers would be higher. All of these runs were made within 9 minutes of each other.
http://www.wku.edu/~nathan.plemons/geardemo.gif
gee, apparently the forum is screwed up again and won't let me imbed the image, just follow the link above.
[Modified by Nathan Plemons, 12:02 AM 11/27/2002]
Notice the green one? That's 3rd gear instead of 4th. Funny thing is that it's lower, you would think that it would actually put more torque to the ground in 3rd and thus the numbers would be higher. All of these runs were made within 9 minutes of each other.
http://www.wku.edu/~nathan.plemons/geardemo.gif
gee, apparently the forum is screwed up again and won't let me imbed the image, just follow the link above.
[Modified by Nathan Plemons, 12:02 AM 11/27/2002]
Thread Starter
Melting Slicks
Joined: Sep 2000
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From: Carlsbad, CA CA
Re: dyno experts...please help or explain! (Nathan Plemons)
That's what I thought, you want to dyno in the gear that is 1:1, but I was talking to some engineers and they explained that there is no such thing as 1:1. Because of the rear end ratio. It is true that the transmission ratio in 3rd gear for a 700R-4 is 1:1 but that power is mulitiplied through the rear end. For example, a car with auto 3.73 rear end would result in 1x3.73 torque mulitplication.
Since you don't input the rear end ratio on the dyno, how does the machine calculate this power.
And how does changing the rear end ratio affect hp and tq?
Since you don't input the rear end ratio on the dyno, how does the machine calculate this power.
And how does changing the rear end ratio affect hp and tq?
Pro
Joined: May 2001
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From: Springdale PA
Re: dyno experts...please help or explain! (zelement)
I'am not sure zelement but my understanding is if i go from this dumbazz 2.59 to a 3.54 or 3.73 my TQ #'s should go up. I think? Anyone?
ScottsLT1
ScottsLT1
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From: SE NY
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Re: dyno experts...please help or explain! (ScottsLT1)
You bet, torque would be up by 44% going from 2.59s to 3.73s; that's why you can launch harder with more gear.
Of course one downside is that instead of shifting into 2nd at 55mph you would then shift at 38mph, assuming a 6k RPM shift.
Of course one downside is that instead of shifting into 2nd at 55mph you would then shift at 38mph, assuming a 6k RPM shift.
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Re: dyno experts...please help or explain! (65Z01)
If you know RPM and you know the MPH you can figure out the effective gear ratio and then calculate torque. Have you ever seen a dyno run where it couldn't pickup an ignition trigger signal? It graphs the HP curve vs time instead of RPM. Since the dyno actually measures HP (a dynojet does anyway) it will still display this, but without the RPM reference it cannot properly calculate torque.
So, yes chaning the rear end gear will affect the amount of torque you put to the ground, but if the dyno can get all of it's proper readings it will compensate for them. BTW, most people who step up to 4.11 gears from 3.45's will dyno a few less horsepower. I think it's because these gears aren't quite as effieicent. Although the car is quicker because of the torque multiplication, there is actually more power lost in the rear.
So, yes chaning the rear end gear will affect the amount of torque you put to the ground, but if the dyno can get all of it's proper readings it will compensate for them. BTW, most people who step up to 4.11 gears from 3.45's will dyno a few less horsepower. I think it's because these gears aren't quite as effieicent. Although the car is quicker because of the torque multiplication, there is actually more power lost in the rear.
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From: Modesto CA
Re: dyno experts...please help or explain! (Nathan Plemons)
Have you ever seen a dyno run where it couldn't pickup an ignition trigger signal? It graphs the HP curve vs time instead of RPM. Since the dyno actually measures HP (a dynojet does anyway) it will still display this, but without the RPM reference it cannot properly calculate torque.
Thread Starter
Melting Slicks
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From: Carlsbad, CA CA
Re: dyno experts...please help or explain! (TIMSPEED)
These engineers also mentioned that the reason auto cars dyno in 3rd gear and manual car dyno in 4th gear is this. Since the dyno drums are so heavy, you will need to put it in an effective gear enough to spin the drum freely and yet without any slip, yet it has to be within a somewhat safe speed for the straps to hold in case the car slips off the drum. If you dyno in first gear, it is most likely that you will be spinning on the drums and therefore record a false reading, same with second gear. In 3rd gear for an auto you have the proper gear mutliplication to spin the drums safely and yet freely. They mentioned that if you dyno an automatic Honda civic you might have to do it in 2nd gear because it won't have enough power in fourth gear to even spin the heavy drums.
So, I am not sure what they are trying to say because when I asked if it would make a difference in power they said yes, but not really, because the dyno uses a set forumla (Torque= moment of interia X angular acceleration) to calcuate horsepower.
I am more confused after talking to these guys just as engineers always do.
Any engineers around here that want to join in?
[Modified by zelement, 1:01 PM 11/27/2002]
So, I am not sure what they are trying to say because when I asked if it would make a difference in power they said yes, but not really, because the dyno uses a set forumla (Torque= moment of interia X angular acceleration) to calcuate horsepower.
I am more confused after talking to these guys just as engineers always do.
Any engineers around here that want to join in?
[Modified by zelement, 1:01 PM 11/27/2002]