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Re: Horsepwer/torque gain with rear gear change ?
[Re: 70Cuda383]
#1270315
07/19/12 07:31 PM
07/19/12 07:31 PM
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Joined: Jun 2006
Posts: 5,161
CT
GTX MATT
master
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master
Joined: Jun 2006
Posts: 5,161
CT
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Gears multiply torque. Torque x RPM / 5252 = Horsepower.
The dyno measures torque. The gear, as well as multiplying torque, is dividing RPM by the same amount. So horsepower will remain the same no matter what the gear.
I've always assumed that since the dyno measures the torque and it knows the rpm of the tires it just takes the torque measurement and then reduced it to a 1:1 ratio.
Remember that even in direct drive the differential is multiplying torque and the height of the tires is dividing the torque. I'd be interested to hear from someone who knows for sure how the dyno is figuring it out.
Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street
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Re: Horsepwer/torque gain with rear gear change ?
[Re: GTX MATT]
#1270317
07/19/12 09:02 PM
07/19/12 09:02 PM
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Joined: Mar 2007
Posts: 3,300
Northern Indiana
Dunnuck Racing
master
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master
Joined: Mar 2007
Posts: 3,300
Northern Indiana
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Quote:
Gears multiply torque. Torque x RPM / 5252 = Horsepower.
The dyno measures torque. The gear, as well as multiplying torque, is dividing RPM by the same amount. So horsepower will remain the same no matter what the gear.
I've always assumed that since the dyno measures the torque and it knows the rpm of the tires it just takes the torque measurement and then reduced it to a 1:1 ratio.
Remember that even in direct drive the differential is multiplying torque and the height of the tires is dividing the torque. I'd be interested to hear from someone who knows for sure how the dyno is figuring it out.
Chassis dynos are good for comparing changes while tuning. The horsepower numbers they give are usually not very accurate,but are useful if you have a good dyno and operator to keep things repeatable.
Different dyno manufacturers have different ways of "figuring" what the numbers are. I have some pretty good paperwork comparing some of them. There is one brand of chassis dyno (would have to look up which one) that came up with a formula by back figuring based on what a vehichles manufacturer claimed the flywheel horsepower was.
If you ask some of them how they calculate things,some will tell you and others will tell you it is proprietary.
There are some that use a generic percentage for drivetrain losses too. We know some vehichles are more efficient then others though.
Back to the OP's questions though,tire and gear changes can change the numbers the dyno shows. The engine makes the same power. You can change tire pressure,or tighten the tie downs down more and see a difference due to effective gear ratio.
Get a good operator,use it for tuning changes,and you will be fine.
If you want real flywheel numbers,stick to an engine dyno.
Keith
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Re: Horsepwer/torque gain with rear gear change ?
[Re: DPelletier]
#1270318
07/19/12 09:03 PM
07/19/12 09:03 PM
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Joined: Sep 2003
Posts: 1,688
Marlboro, NY, USA
Rick_Ehrenberg
top fuel
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top fuel
Joined: Sep 2003
Posts: 1,688
Marlboro, NY, USA
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Quote:
...and of course, the correct answer is zero.
Dave
For sure!...But the cheapo inertia type dynos (w/o a brake, e.g., DynoJet) may show some variation. EG: if you swap to an aluminum flywheel, the engine obviously makes no additional HP, but the D.J. will show a gain. Yet, the car will, in all likelihood, be quicker in the quarter mile. So an inertia dyno can be useful -- think of it more like a stationary drag strip than a true dynamometer.
If that car were mine, I'd sure leave the 3.23s...or go 2.94:1! Just my two cents...
Rick
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Re: Horsepwer/torque gain with rear gear change ?
[Re: Rick_Ehrenberg]
#1270322
07/19/12 10:14 PM
07/19/12 10:14 PM
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Joined: Nov 2005
Posts: 3,456
Fly Over States
PHJ426
master
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master
Joined: Nov 2005
Posts: 3,456
Fly Over States
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Quote:
Quote:
...and of course, the correct answer is zero.
Dave
For sure!...But the cheapo inertia type dynos (w/o a brake, e.g., DynoJet) may show some variation. EG: if you swap to an aluminum flywheel, the engine obviously makes no additional HP, but the D.J. will show a gain. Yet, the car will, in all likelihood, be quicker in the quarter mile. So an inertia dyno can be useful -- think of it more like a stationary drag strip than a true dynamometer.
If that car were mine, I'd sure leave the 3.23s...or go 2.94:1! Just my two cents...
Rick
I dumped the 4:30's in my 72 Roadrunner and went with a 2.94 suregrip unit.....
Did my RB stroker have a HP loss?........
Not at all it just increased my cars ability to be driven past the 1/4 mile. There is a big world out there no need to travel it 1/4 mile at a time.....
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Re: Horsepwer/torque gain with rear gear change ?
[Re: Mopardude440]
#1270324
07/19/12 10:53 PM
07/19/12 10:53 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
Engine stops making power around 4,500
You have a 440 with a .484 cam that stops making power at 4,500 rpm? And it supposedly made 510 horsepower on an engine dyno?
Something not adding up here.
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Re: Horsepwer/torque gain with rear gear change ?
[Re: Mopardude440]
#1270325
07/19/12 10:58 PM
07/19/12 10:58 PM
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Joined: Sep 2003
Posts: 1,688
Marlboro, NY, USA
Rick_Ehrenberg
top fuel
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top fuel
Joined: Sep 2003
Posts: 1,688
Marlboro, NY, USA
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Quote:
was curious how much gain i could expect changing rear gears to 3.55 or 3.91 i have 3.23 now 69 roadrunner 4-speed 28in tire 440 510 hp/ 540 tq flywheel 310 hp/ 415 tq rear wheel 4400, redline thought would have done better but dyno crew says my rear gear is killing it, 
Might be interesting to see what the 1/4-mile trap MPH is, then plug the numbers into an online calculator. As Larry Shepard used to say: "Drag strip dyno".
Rick
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