Gears do affect rwhp

blackfang

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Just some info I thought I would put out for you guys and gal.

A car having 3:27 rear end gears is placed on a DynoJet chassis dyno and
has a dyno performed. Then this same car is taken off the DynoJet
chassis dyno and has the rear end gears changed to 4:10. The car is then
placed back on the DynoJet chassis dyno and another dyno is performed.
Will the second dyno show a loss of horse power caused by the 3:27 to
4:10 gear change?

ANSWER:
Yes. This has been a long standing debate among car enthusiast, so I
recently emailed DynoJet and asked this exact question. Below is the
response I received.

"The 4:10 gear will show less horsepower than the 3:27. The reason is
due to rate of acceleration changes. The rate of acceleration is quicker
with the 4:10 because of torque multiplication being increased to the
rear wheel. The horsepower will show less because the targeted RPM is
met before the horsepower has a chance to overcome the rotational mass
(dyno, drive line, etc.) or moment of inertia in speed. Because the
speed is decreased and the RPM is met faster, the horsepower never has a
chance to catch up with itself, so to speak. The overall ratio of 1:1
will always produce the most horsepower on the chassis dyno. Having said
this, a similar problem can occur with horsepower loss when the rear
gear is too high. The horsepower is being absorbed in just trying to
keep the rotational mass spinning. Please keep in mind that your
engine's horsepower never changes but what gets to the dyno or drive
surface does.
 

Outlaw97GT

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Good write-up... :thumb:

Some how much HP loss will there be? any idea's...
 

NITROUSINJECTED

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id think that it would just move your horsepower numbers around not so much add hp
 

Lightning Struck

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hmmmm and most dyno shops ive talked too says gears dont have effect on hp numbers. :rollinglaugh:
 

91lxcoupe

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Makes perfect sense. The only reason dyno shops don't think gears effect hp ratings are because they dont effect hp at the flywheel. It technically changes rwhp but when you take your car to the dyno they load it up differently depending on ratios.
 

Downshift

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So by doing that would the torque be increased as the hp is decreased?
 

Adam

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Downshift said:
So by doing that would the torque be increased as the hp is decreased?

The torque multiplication is increased, not the actual torque. Your rear gears can only increase the multiplication factor of the cars torque output, not the actual torque number that the car produces. For example, you have a bolt thats stuck, you can't break it loose, so you get a longer breaker bar to break it loose, your not any stronger, your just using a tool to multiply the amount of torque that your arms can produce....same exact thing :)


And for the record Dynojets are all over the place when it comes to dyno numbers. A true load bearing dyno like dynapaks are much more accurate for tuning. As people spend more time involved with cars/dynos/racing etc they see how unimportant dyno numbers are, except in your internet sig..lol You would be surprised how much my car doesn't make. My 1st 12.92 pass was 247rwhp/271rwtq. Now i am around 260rwhp/285rwtq.
Adam
 

4.6snake

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you have to input what gears you have into the dyno before you run so the machine can compensate for it.
 

97gtconv.

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ditto, did that before and after from my 3.08 to 4.10's no difference
 

19mustang95

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Adam said:
The torque multiplication is increased, not the actual torque. Your rear gears can only increase the multiplication factor of the cars torque output, not the actual torque number that the car produces. For example, you have a bolt thats stuck, you can't break it loose, so you get a longer breaker bar to break it loose, your not any stronger, your just using a tool to multiply the amount of torque that your arms can produce....same exact thing :)
good analogy....and i love breaker bars, makes life so much easier. ;D
 

realitygt

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Adam said:
Downshift said:
So by doing that would the torque be increased as the hp is decreased?

The torque multiplication is increased, not the actual torque. Your rear gears can only increase the multiplication factor of the cars torque output, not the actual torque number that the car produces. For example, you have a bolt thats stuck, you can't break it loose, so you get a longer breaker bar to break it loose, your not any stronger, your just using a tool to multiply the amount of torque that your arms can produce....same exact thing :)


And for the record Dynojets are all over the place when it comes to dyno numbers. A true load bearing dyno like dynapaks are much more accurate for tuning. As people spend more time involved with cars/dynos/racing etc they see how unimportant dyno numbers are, except in your internet sig..lol You would be surprised how much my car doesn't make. My 1st 12.92 pass was 247rwhp/271rwtq. Now i am around 260rwhp/285rwtq.
Adam

wow, did you strip it and put massive slicks on stick rims? that's a sick time for just 247rwhp.
 

LaserRedSVT96

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then does that make my 3.73s the culprite for my lower than average dyno numbers? maybe i just need a tune. i dunno.
 

Venom351R

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LaserRedSVT96 said:
then does that make my 3.73s the culprite for my lower than average dyno numbers? maybe i just need a tune. i dunno.

no, that would have more to do w/ altitude, temp, plus every dyno is different due to those reasons and others.
 

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