I am new to dyno results, Fly wheel vs RW, and so on and so on. I have a question that may be a stupid question, but I have to ask.

Will the RWHP be the same for a car that has 2.59 rear gears as it would be for the same car only changed out the 2.59's with 3.73's?

Keep in mind I'm talking about the same car in both cases. The only difference is the gear ratio in the rear end.

 

It should be the same. There might be a slightly different loss from the different gear ratios but it should not be much.

Good color choice for your vette. :cheers:


[Modified by AquaMetallic94LT1, 8:47 PM 8/19/2002]

 

Thanks,

So what changes? The RWTRQ? I mean why is there such a noticeable difference in performance between the 2 gear ratios?

 

The actual hp and torque of the motor will not change by changing the rear gear. The torque to the rear wheels will be higher with a numerically higher rear gear ratio in any given gear. The seat of the pants performance improvement is from the higher torque at the rear wheels.

 

the numbers shouldnt change, youll only feel the power better usually

 

Your rear gears are just like a pry bar. W hat I mean is that with the same amount of force(engine tq/hp) you can lift a "200" block easier with a 6' pry bar that with a 2' pry bar. Just like a car will move easier with a 4.0rear gear that with a 3.0 rear gear. It's just a mechanical leverage situation. You don't increase force, just amount of leverage. I hope I didn't confuse you.

 

It all makes perfect sence. I knew that you don't actually add HP/TQ to an engine by changeing out the gears. I guess my question is aimed more at the equipment (Dyno Tester).

For instants, I know that basically a 3.73 gear is transfering the power from the motor to the wheels easier than a 2.59 gear, therefore it is easier to move the car, which results in better performance.

But it would seem that would be noticeable on a dyno? Since I have never been near a dyno, I don't know what is involved, so I am taking a stab in the dark here. Do you have to input that info into the dyno? (Like the gear ratio) If so that would make more sence to me, on how the dyno would come up with relatively the same #'s with both gears. Otherwise, I don't understand how the dyno can not translate the actual performance difference of the car into either HP or TQ.

In other words, if you do input the gear ratio into the dyno, it looks like you could see how much HP/TQ you "feel" like you added to your car by lieing to the dyno. If I input to the dyno that I had 2.59 gears and actually had 3.73 gears, wouldn't the dyno think that I just had more HP or TQ?

 

my dyno doesnt care what is sitting on top of it....it only calculates the torque applied to it under load. it would be interesting to change gear ratios to see if it saved some power :)

 

with an inertia dyno (dynojet), gearing will affect measured output. but not for the reasons you think.

some background...

the dynojet measures work over time (power), directly. the software has a pickup for rpm. it knows the exact circumference of the rollers, and the engine rpm, therefore it knows the exact overall gear ratio.

power is strictly work over time, so with no rpm pickup, the dynojet will still output a power graph. it can only output a torque graph if it knows the overall gearing, meaning it must know the engine rpm. (this sounds backwards from what most people know about dynos, but it's absolutely true.)

this is the answer for how/why gearing changes don't fool the dyno.

now, the reason gear changes do make a difference...

the dynojet is an inertia dyno. (simplified,) it takes the starting kinetic energy (say, the mass of the drums turning at 40mph), the ending kinetic energy (the drum mass turning 130mph), and the time it took to achieve the difference. this is power.

so, if we add mass to the masses which are experiencing a change in inertial state (energy, rpm, however you want to call it), it will take longer to go from one rpm to another. this is because more power is being absorbed in changing the inertial energy of that extra mass.

the corollary (sort of) is that if you gear a car such that the rpm rate of gain is very slow (tall gears), more of the engine's power is going to change the speed of the rollers, and less to change the speed of the engine. so the dyno reads more power.

another example is revving the engine in neutral with a stock vs lightweight flywheel. floor it and the engine is making the most power it can make. how quickly it gains rpm depends on how much mass is being accelerated (change in inertia). so the engine isn't making more or less power, but more or less of it is being consumed in the internal inertia change.

yet another example of this phenomenon is to make a dyno pull through all the gears. each subsequent gear change shows a higher power curve than the previous gear.

for the gear ratio difference you state, the measurement difference on a dynojet would be significant. different cars seem to respond differently to this, probably dependent on driveline mass. more or less overall engine/driveline mass will affect how much power difference is measured with different gears.

my guess would be 5-10hp, since yours is an extreme example. i've seen ~20hp difference between 3rd and 4th gears, but that's a more dramatic difference in overall gearing. i've never seen more than 10rwhp difference just from gears.

lastly, obviously, this phenomenon does not mean the car gets slower!

 

Some great points there MSR :yesnod: . This is the same phenomenon that people experience with the "frisbee removal" mod on L98's. There is a power gain from it, but only under conditions when engine speed is changing. So you will see a difference in your 1/4 mile times, but it won't do a darn thing for your top speed. In the case of a dynojet, a mod of this type would show up as a few horsepower in lower gears, less of a gain in higher gears, as compared to a baseline run. Brake horsepower gains measured at a constant load/rpm on an engine dyno would be -0-. The gain is real, but is variable based on dynamic operating conditions.

I would be curious to see a dynojet run of a manual transmission car that was run through all gears, including overdrive. All the runs that I see always stop at the 1:1 gear. This data could provide some insight into inertial losses versus rear gear losses.

 

vince, i was tempted many times to do overdrive dyno runs, just to see the numbers, but the speeds involved really scared me. my imagination is too healthy!

the driveshaft broke on an old chevelle i was dynoing, once, at over 120mph. it's definitely a shock. one of the little needle rollers hit a parked car (15' away) hard enough to leave a dent, even after bouncing off the floor first. the floorpan of the chevelle looked like someone had shot a bowling ball through it. luckily, no one was hurt, even though it was a club event with ~50 spectators.

i've had other incidents, too, so i'm always safety-conscious with the dyno.

 

:eek: I never thought about that. Now that you mention it, I can only imagine what happens to a tire at redline in 6th gear. Wouldn't be pretty :lol: . So much for that experiment!