Gears affecting rwhp?

Armon said:

Sorry if this is a stupid question, but:

Alright, obviously getting higher ratio gears doesn't affect how much hp your engine makes. However, if I remember right a dynojet dyno just measures how fast your tires can spin those big drums

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The dynojet measures how much power is put into the drums, not just how fast they spin. The rate at which they spin up is coupled with the rotational speed to determine the power input (and thus power output at the wheels...)

(for measuring rwhp. for torque you need engine rpm as well). So, if upgrading to 4.10 gears makes your car faster in any way, it would be safe to say that your tires would spin the dyno drums faster, meaning you have more rwhp. Either that, or the car just "feels" faster but really isn't.

Anyone have any proof/disproof, comments, etc?

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The drums spin up faster but they don't spin as fast overall. Think about the equation HP = T * RPM / 5252. If you change the gears from 3.27 to 4.10s, you bump instantaneous rear wheel torque by about 25% (4.10/3.27 ~=1.25) but you also slow rear wheel RPM by the same factor, resulting in a wash in the equation: the torque being higher by a percentage is offset by the wheel speed being lower by the same percentage, resulting in the same HP being produced.

I have never quite understood how they measure the difference in RWHP and RWTQ etc. etc., and also how they take into account different gear ratios. I'm interested in seeing this question answered too.

CManT1914 said:

I have never quite understood how they measure the difference in RWHP and RWTQ etc. etc., and also how they take into account different gear ratios. I'm interested in seeing this question answered too.

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The rwTQ number is pretty much always "fake". The DJ measures power at the wheels but instead of using rear wheel RPM to back-calculate wheel torque, it uses the engine RPM. Think about it: even in 4th gear (1:1), a stock 300 ft-lb (crank) GT will make 300 * 3.27 or 981 rear wheel ft-lbs (ignoring losses) at the engine's torque peak.

So the rwTQ number provided is a mongrel really, taking power at the wheels - power produced after losses in the drivetrain - and translating that back to a "happy, sensible" value using engine RPM... As long as it's consistently applied, it doesn't really matter in the end. My car makes 247 rwHP and something like 290 "ft-lbs" of torque, a figure arrived at by using rwHP and engine flywheel that thus does not equate to rw torque nor engine torque.

Make sense?

Good explanation. I remember last year someone here argued forever that he had 900+ rwtq.

goot points trinity.

To the original question, Yes the dyno will see less power to the rear wheels with a steeper gear.
Below coinsides with much of what trinity says.
http://www.shane.roberts.net/dynojet.htm

I used to have a dyno graph that showed pulls in 2nd, 3rd, and 4th gear and the curves were noticably higher each time. this is cause by the added losses through the drivetrain (both rotational & frictional) that are increased by gear ratio.

quick search: here it is. http://www.corral.net/forums/showthread.php?s=&threadid=356919

My car shows a 8HP difference between my street tires and my drag tires that are an inch taller.