#LT

sorry if this is a stupid question.

will i dyno more rwhp and rwtq with 4.10 gears compared to 3.42? if so, can someone explain the physics behind why? thank you.

lt

turbotko

Nope, you will probably lose a little power on the dyno due to the increased friction caused by the increased gear multiplication. It'll sure run better though.....and that's all that matters

#LT

thanks for the response. could you explain to me why it wouldn't dyno more hp? i'm sure you're right, but the dyno is measured at the wheels and doesn't the dynojet measure the hp by how fast the wheels can turn the drum on the dyno or how easily the drum is moved.

MachinistOne

The dynojet is an inertia dyno, it measures hp by sampling how long it takes to accelerate the 5400lb rollers...then based of a tach signal it will extrapilate a torque number.

Increased friction in the gearset requires more HP to turn, besides that it is a new gearset and will free up a little hp as it wears in.

Capn Pete

I've never agreed with the theory that it takes more power to turn lower gear ratios ..... what would be the point of running them in the first place?? Unless I have my head really far up my a$$, and never learned a thing in physics class, is there not this nice little thing called "mechanical advantage" that you gain by running lower gear ratios?? Mechanical advantage makes work easier to perform. If something is easier, it should require less input to get the same output. But again, maybe it's just not that simple???

Here's my take on the situation (until somebody can offer a better answer, I'm still skeptical of the more friction theory):

There has been so much discussion surrounding lower ratios killing dyno #'s, perhaps there's some truth to it. But what about this "theory": if you run your car with 3.42's in 3rd or 4th (auto/manual) to give you the 1:1 ratio (consistancy) through the full RPM band from idle to redline (~6000 RPM) then you will be spinning the dyno up to ~134 mph. Now put in 4.10's, run in the 1:1 gear, and rev to ~6000 RPM, now you're only spinning the dyno to ~112 mph. Because the dyno is just doing some sort of computer calculation, perhaps it has less data to use by only spinning to 112 mph than it would have spinning to 134 mph??

OR

Because of the increased mechanical advantage of the gears, the engine really doesn't have to work as hard to spin the dyno. If the engine isn't truly working as hard as it may have to with a higher ratio, does the dyno see this "lack of effort" as less HP?

FWIW, I dyno'd my car, w/A4 & 4.10 gears. Except for the lid & Flowmaster (aka "Chokemaster") catback, the car was bone stock. It put down 289 HP / 307 TQ. Given that automatics rob a lot of HP, and low ratios apparently kill dyno #'s, it doesn't look out of line for a car that has also run 13.0 @ 107 mph? I figure with a 6-speed and ~3.42 gears, the car would probably pick up ~15 - 20 HP on the dyno (if you figure that 289 RWHP is an 18% loss from 350 FWHP, and 310 is about 12% off 350). But really, 289 doesn't seem out of line for an A4. I'm not really convinced that the gears were affecting the #'s much in the first place??

AL SS590 M6

Torque is measured in foot pounds. That's moving weight a certain distance.
Horsepower is moving weight a certain distance in a certain amount of time.
Changin the gear ratio of a vehicle does not in any way change the amount of power that the motor puts out. It changes the poundage vs distance vs time to do the work. i.e. going from 3.42 to a 4.10 the 4.10 will pull the weight easier and quicker but to a shorter distance. So the numbers will come out very close to the 3.42s.

Capn Pete

^ ^ so are you saying this is a myth and not a fact?? I've always been a skeptic anyways.