How much do rear end gears affect dyno #'s?
 

If we take one vehicle, and the only thing we change are rear end gears, what would the differences be in their rwhp numbers?

Everything would be the same; evertyhing except for the gears.

Which would dyno higher? 2.73's or 3.42's or 3.73's or 4.10's? And what difference would there be between them?

If this has been discussed before, I guess I dont know how to use the search feature as well as I should.

Well easy thing to do with a stick is to do a run in 3rd gear and compare that to your 4th gear pull. That will effectively show the difference in acceleration rates on RWHP. The faster you accelerate the more inertia losses you will see. It will show you the difference between 3.23's and 4.10's..... so say you have a 3.23 gear and then do a pull in 3rd, that will basically give you the acceleration rate of a 4.20 gear. The only little thing here would be the higher frictional losses in the 3rd gear run, but it will give you a good example.

Bret

when i added a 12 bolt w/4.11's i lost 21hp from my 10 bolt w/ 3.42's. gears alone would be around 6-10 hp maybe?

Does anyone know of a formula or standard for the effects of a gear swap without changing the rear end?

my car went from 430/409 with stock rear to 423/404 w/ a 12 bolt with 3.73s

i think that throw ya back in your seat feeling is worth a few #'s on the dyno....

from my car back when i was doing the mikronite gear testing. only changes were the gears into the same 10 bolt. nothing else.

3.23 to 4.10
http://www.stealthram.com/familypics...nprocessed.jpg

4.10 to 3.73
http://www.stealthram.com/familypics...rprocessed.jpg

So, .. you lost power with the lower gears?

I could follow it better if the font color matched the graph :)

um, yes. lost power with higher numerical gears. it takes less energy to turn them.

and the colors do match the graphs. the hp lines are the same color, the tq is the lighter shade of the same color. except for the green. it came out black for the tq.

seeings how it takes less energy to turn the gears .. I would have thought the hp/tq numbers to rise with a lower gear set.. hmmm

I'm certain torque would increase with a lower gear. HP was my main interest. Explaining the loss of torque will be difficult.

Interesting graphs no doubt!

hp is a function of torque.

it has to do with load on the engine with the gearing. my wonderful 10 speed bike comparison:

from a stop, take off in 1st gear on the bike. it's real easy to get going, but you pedal your ass off to keep up top speed.

from a stop, take off in 10th gear. really hard to get moving, but easy to pedal at top speed.

so, you make more torque from a lower numerical gear because the motor has to work harder, producing more torque. and horsepower is a calculation of torque. and the opposite happens when you have a higher numerical gear. less load means less energy needed to move an object.

now, you are actually wasting the energy produced in the low rpms with the lower numerical gear (2.73) because it takes more effort to get moving. then the opposite happens with higher numerical gears (4.10). you waste energy in the upper rpms. if you could do a dyno run in third gear from 0 - top speed with both gears for comparison, you'd see the trade off. as with most dynos, you only get to see from the middle of the rpm to the top rpms.

this is why you get better 60ft and lower ETs with lower numerical gears, but you get a lower mph.

Just to throw in a little more confusion, I read a comparison test of the Fidanza aluminum flywheel to a stock steel unit. The hp up top increased by 7 but it increased by 20 in the lower RPM's. This is apparent hp from the dyno.

You confuse me here. The 1st gear in your bike example is like 2.73s, right. It is, indeed, easy to get going in 1st gear on a bike, but then you say it takes more energy to start moving with 2.73s, basically. This is contradictory. Seemingly the waste of energy would be in the upper rpms for numerically lower gears. In the bike example, you're pedaling your ass off to keep up top speed. With numerically higher gears, 4.10s for example, an energy loss would have to occur in the lower rpms where it is harder to get the larger gears going. In the upper rpms it would be more efficient. Again in the bike example, top speed is easy to maintain in 10th gear. We all know this. You can turn the pedals much slower and still be going just as fast as the guy next to you pedaling as hard as he can in 1st gear.

One more thing has me stumped. How can you travel a set distance(1/4 mile) in a faster time(lower ET) moving slower(less MPH)? Velocity=Distance/Time. The Distance remains the same(1/4 mile). If your time decreases(ET) your velocity(MPH) must increase. It's an inverse relationship. This much I know for sure. It seems to me a numerically lower gear(2.73) would give you lower 60 ft. because the engine does not have to work as hard to get it moving. Less rotational inertia. 4.10s would be better in the upper rpms. I don't know if it would mean lower ETs.

Not tryin to bust your balls..just wanna get my facts straight.

You have the gear thing a little backwards. The 2.73's are harder to get started with, not easier. The engine has less mechanical advantage over the weight of the car because of less torque multiplication. Changing the gears to 4.10 gives it more multiplication, allows it to get into its powerband quicker, but actually makes the engine do more work (revolutions anyway). That is where the drawback occurs at higher rpm's. You can run out of useable engine rpm.
If it helps think of the gears as pullies. If you have rigging set up with 3 pullies (or 2.73) the weight is harder to lift than if you had 4 pullies (or 4.10). The drawback would be you would have to pull 25% more rope to lift the weight the same distance but it would be easier to move.

As for the dragstrip, your velocity over time is better. Because you are accelertaing faster you cover the distance in a shorter period, so you have LESS time to accelerate. Your car can run 13.0 in the 1/4, and it can get quicker(maybe 12.7) but lose 1-2 mph, happens all the time. You are keeping the distance the same. Now if you keep the TIME (13.0) the same your end speed will most likely be better. This of course is if you have repeatable circumstances and conditions (traction, etc).
One of my old cars had crappy factory shift points. When I would let it shift itself at the track it would run about .4 slower but with a few more mph. When I held the rpm's it would be quicker but at a lower speed (it had less time to accererate).

OK I was wrong. Velocity=Distance/Time is a linear equation only. Acceleration has an effect. I understand that. What I don't understand is why numerically lower gears would give better 60 ft. and lower ETs. Can someone explain this to me please?

Think of a high stall torque converter. If you have a stall of 4000 rpms, that means the converter only allows to car to start moving when the engine hits 4000 rpms. Thus, allowing you to get your engine into it's more powerfull powerband before the energy is spent (wasted) getting the car moving. The 4.10 gears require you to rev the engine higher to get the car moving. Thus allowing you to get into your more appropriate engine powerband for a harder launch. Atleast, I think this is how it theoretically works give or take a few circumstances which someone has probably encountered.

Not sure of all the tech. reasons why but on a dyno your #'s go down with steeper gears. I still put down 452rwhp with my 12 bolt and 4.30 gears. If I had a stock rear with 3.42 gears it would have been in the 470rwhp range