Kind of a noob question, but does gearing have any effect on rwhp and torque? for example if a car puts down 350rwhp with a 3.15 rearend how would that same car compare with 3.90's.
Thanks in advance
Luke

 

Noob question...gearing has nothing to do with hp, the 3.90s would allow you to move off the line quicker but you give up top end

 

Yes. Even more if you throw a high stall into the mix. As far as your example goes I'd say 10-15rwhp loss on average depending if it's a A4.

 

It does effect chassis dyno readings. As gear ratio increases numerically, horsepower drops. Not a bunch but some. From a 3.15 to a 3.9 you may see a 15 HP decrease.

 

Inertial losses increase as the rate of acceleration increases. IE, a car will dyno LESS in 3rd gear than in 4th, as you are rotating everything quicker, including the drum.

While the change will be small, a car will dyno less with a 4.10 gear than it will with a 3.42 gear, all other things being equal.

 

I don't think RWHP changes at all with a change in gears. Torque will change, and so will RPM, so HP remains the same.

That is different, you are changing the powerpath through the transmission. Usually 4th is a direct drive where 3rd is not, so 3rd will have more drive losses. If you are in the same gear at the tranny, but change the rear end ratio, I don't see how RWHP would change.

Rick

 

If torque changes, then, by definition, horsepower changes. Your rwhp will go down and torque up with a numerically higher gear change. Crank horsepower is unaffected.

 

Rear wheel dynos measure power by measuring acceleration of a given load at a given speed. More torque at a slower speed can be exactly the same HP as less torque at a higher speed.

Back to the original question, per Newton's physics, the quicker you accelerate a disc (flywheel, driveshaft, wheels, rotors), the more centrifugal losses there are. I could have explained why 15 years ago when I was taking my machine vibrations class, but now I simply remember to bottom line.

Drivetrain losses are a different matter.

 

quite correct

about a 2 to 3 % drop on the dyno.

but your car will be quicker in the 1/4 mile. And that goes to show ya that dyno slips don't win races.

 

There's a lot of guessing going on here... please put your slide rulers away. It doesn't matter what your common sense is telling you... there is a drop on the dyno when changing to a more radical rear axle ratio.

Real world experience would show you this.

 

 

Going From 3:15 To 3:42 You Will Lose About 10 Rwhp . So I Think Going To 3:90 Gears You Would Lose More . I Put 3:90s In My Car But I Have Not Had It On The Dyno Since Then 3:15 To 3:90s

 

No, HP=torque x RPM / 5252, (torque is measured, HP is calculated).

So if torque goes up and RPM goes down, HP remains the same.

 

As mentioned above, you're neglecting to figure in a higher parasitic loss. Physics tells us that the quicker you accelerate something, the more power it will consume. 4.10 gears accelerate the axles, wheels, tires, dyno drums, etc. more quickly than do 3.15 gears. Therefore more power is "lost" to accelerating those parts at a more rapid rate and less will be measured at the wheel. Your car will still accelerate more quickly because of the gained mechanical advantage.

It's been proven time and again on dyno's everywhere so if you still don't believe it, I don't know what to tell you.

 

FACT: On a drum roller dynomometer you will decrease "calculated" rwhp when moving to a higher gear ratio (3.15 to 3.90). In the real world this will increase your reaching speeds but decrease your total top speeds.

 

OK, agreed, but it depends on what type of dyno we are talking about. For the "usual" ones with the large inertia roller, then measured HP will change. If is the type that clamps on the wheels and allows a reading at any steady-state RPM (sorry, forget what that type is called), then measured HP will not change.

 

rwhp = hp - drive train loss

(>gear ratio) = (> drive train loss) = (< rwhp)

The "precise" term should be an increase in drive train loss with a net effect of a lower indicated rwhp/tq.

 

I agree, parasitic loss. You turn the wheels more with a lower gear ratio per engine rotation which eats more power via gear multiplication. The whole point in a gear swap is to make it useful by putting the power where you need it to accelerate the car quicker. Overgearing will not only cause more of a loss in performance but can make traction problems and make a car actually slower in the 1/4 (for example, if you redline 4th and need to shift to 5th before the traps) not to mention the higher cruising RPM on the highway.

For this reason I always stay conservative with gear ratio swaps. So I decided to go to 3.73's instead of 3.90's in my M6. If I were going to race it more I'd go 3.90, and if I had an auto strictly for racing, I'd go even lower (higher numerically).

But in some cases lower gears arent always better. Alot of it depends on where and how much HP and torque an engine makes. Lower gears are great if you have a high revver(to get you into the powerband quicker), but if you have a car that makes tons of bottom end and you drive it everyday, lower gears can make the car harder to drive/control and could hurt the mileage too. Lots of things to think about for gears, its not just gears=quicker. Depends on the setup, driver, weight and the intended usage. Older cars need more gear to get the weight moving (like an old Chevelle) but as light as a Corvette is, drastic gear changes are something I'd be weary of unless it was strictly a drag racer, IMO.

 

I completely agree with your conclusions, but you have come to it by the wrong means. The lower the gearing (higher the numerical gear), the slower the axles, wheels, tires, etc will turn at a given RPM. In gearing, the higher the rotational speed, lower torque is transferred. We go to higher numerical gearing, to get lower rotational speed, to get higher torque at the wheels. This is completely evident, since the engine now has to run higher RPM's just to do the same speed.