Do rear end gears affect dyno numbers?

GTS 5.7 · 03-01-2007

'95 GT/5spd--did some work on the car,and put it on the dyno to check it out..we had some problems and didn't get to finish...

but best before we stopped was 280 RWHP and 300 RWTQ...I'm SURE I told him it had the stock 3.08 gears in it.I didn't even know they put a 2.73 in them til I was showing my son how to check what was in it.Then I got to checking the axle code and such and I DO have 2.73 gears

The question is-if the dyno numberswere done assuming 3.08 gears and they are really 2.73,then are the 280/300 numbers right?

If not how far off?

I assume the numbers are actually better,but how much?

P.S.-3.73s coming soon

Kyle F · 03-01-2007

You would think that by goign to a Lower gear like a 3.73 or 4.10 would actually improve your power numbers because you are multiplying your torque more. So, if you told him you had 3.08, but you had 2.73 you car would actually be makign more power than what was shown, but alas this line of thought has been debated on here before.

For some reason, and since I have never taken my car to a dyno yet, a lower gear makes less power. I am not usre if they enter in a gear correction or not (To calculate it back to a 1:1). THough, you would also think that the tranny gear you choose to run the dyno in would affect your numbers as well... as in you would thik that first would allow you to ramp up the rollers on the dyno quicker or move them against more force than say 4th.

So, can someone tell me if they correct it out? If so, then the answer to your question is if they were calculating it to a 3.08 gear and you actually have 2.73, you would be making more power than what was shown, but probably not a whole lot more.

**tripleblack** · 03-01-2007

Kyle, guys, at first I thought I knew the answer, but now I'm wondering.

At first blush, I'm thinking that the gearing shouldn't affect what the dyno reads as horsepower. Changing gears (all other things being equal) would certainly MOVE the curve (steeper for stronger gears, or with a longer time duration and flatter curve with numerically lower gears). Either way, the motor should generate the same power, just change the chart from a short, steep curve to a long, flatter curve, both ending in the same number.

But maybe there IS a multiplier going on in the dyno's brain.

I really don't know - but I bet someone on here does!

GTS 5.7 · 03-01-2007

I know he did the run in 4th gear for the 1:1 ratio,but the computer reading the force put on the rollers and it assuming (or programmed) I was running the 1:1 ratio through a 3.08 gear I THINK the computer would read it that there was LESS power coming from the engine than there actually was,since it (roller speed) didn't ramp up as quick and with as much force as it should have because it was actually running through the 2.73....???

By the way,just ordered the 3.73s--can't wait

More opinions puulleeeeze!!!

Stangster88 · 03-01-2007

depending on the dyno gearing will not have any effect, most dynos adjust for gearing automatically. All you need to due if you have an auto or stick find what gear is equal output of a 1/1 ratio usually 3rd or 4th gear depending on what tranny you have and how many speeds it may have. When doing a run gently get the car into either 3rd or 4th depending on what tranny you have. Once you have it in gear you will press the sampling button usually green (does not apply to every dyno). Once the button is pressed just open it up until you hit your desired rpm then let off. Let the car slow down to around 60 and engage the dyno brake. Do not ever touch your cars break while on the dyno. I repeat never touch your brake pedal once on the dyno. By doing this you may lock the tires with the dyno wheel spinning and you can possibly launch your car forward or off the dyno base if it is an elevated dyno.

90GT AOD · 03-01-2007

Seems to me that if the rear gear made a difference it wouldn't matter because you want to know what horsepower you're putting to the wheels. If you change your rear gear, the old horsepower number form the old gear doesn't matter, because the car performs differently with a new gear.

hairygrapes · 03-02-2007

No, expert on the subject, but I would think that gears would have to have some effect on the #'s. The rear end is multiplying the output of the motor to the dyno through the wheels. The reduction should allow a shorter gear to spin more dyno resistance with less effort, equating to a bigger number, no??

GTS 5.7 · 03-02-2007

Quote:

Originally Posted by hairygrapes

No, expert on the subject, but I would think that gears would have to have some effect on the #'s. The rear end is multiplying the output of the motor to the dyno through the wheels. The reduction should allow a shorter gear to spin more dyno resistance with less effort, equating to a bigger number, no??

What I thought-I'm DEFINETLY no expert,that's why I asked the question.....
But that's what I thought

maybe a little on RWHP,but I would think a good bit more on the torque numbers

Kyle F · 03-02-2007

Remember guys, the Lower rear gear (Higher Numerically) as with any gear ratio, muliplies the torque to the rear wheels, which is why lower gears accelerate the car at a higher rate.

Dynos do not, and you can not physicaly measure horse power. It must be calculated. Torque is measured then Horsepower is a calculation based on the ability of the motor to produce Torque at speed (RPM)

I think the equation is HP=(Torque*RPM)/5280. That is why all properly performed dyno test will produce a report where HP and Torque plots corss at 5280 (I may be wrong on this exact number, but this is in the ball park).

**EagleAutosports** · 03-02-2007

Right Kyle, but instead of 5280, it's 5252. And if I read this right, the gearing is entered into the dyno computer and is accounted for because the dyno reads the wheel speed, as well as other important variables.

Dynamometers Information on GlobalSpec

downsideup09 · 03-04-2007

no expert... but numerically higher gears probbaly produce a little less cause they have to spin faster which produces more friction which woul dreduce horsepower.... very simple but makes since in my head

Kyle F · 03-04-2007

Yes, that is correct... and also they are larger... thus more rotational weght that must be accelerated.

GTS 5.7 · 03-06-2007

I've been thinking about it...the Dyno measures what HP is delivered to the rear wheel-regardless of how it gets there.
True?

A 3.73 gear may transmit the power to the wheel quicker/easier,showing a faster "ramp up" on the chart than,say, a 2.73 gear but in the end,all that can be put to the wheel is all that can be put to the wheel.
Right?

So,my deduction is-you may get to your peak sooner,but the peak is still the peak

**tripleblack** · 03-06-2007

That was my initial opinion as well.

Some dynos use hydraulics to transmit information to the sensors - others even use air pressure...

But the logic makes sense. And the shape of the curve would follow.

Quote:

Originally Posted by GTS 5.7

I've been thinking about it...the Dyno measures what HP is delivered to the rear wheel-regardless of how it gets there.
True?

A 3.73 gear may transmit the power to the wheel quicker/easier,showing a faster "ramp up" on the chart than,say, a 2.73 gear but in the end,all that can be put to the wheel is all that can be put to the wheel.
Right?

So,my deduction is-you may get to your peak sooner,but the peak is still the peak

Kyle F · 03-07-2007

Quote:

Originally Posted by GTS 5.7

I've been thinking about it...the Dyno measures what HP is delivered to the rear wheel-regardless of how it gets there.
True?

A 3.73 gear may transmit the power to the wheel quicker/easier,showing a faster "ramp up" on the chart than,say, a 2.73 gear but in the end,all that can be put to the wheel is all that can be put to the wheel.
Right?

So,my deduction is-you may get to your peak sooner,but the peak is still the peak

False, it measures Torque put to the wheels then calculates horse power.

In any Mechanical System, gearing multiplies torque and sacrifices speed or increases speed and loses torque. A higher numerical gear will show more toque at the rear wheels, thats why a car with a 4.10 can accelerate quicker than the same car with 3.27's.