Dyno question

[AC sports]

If I made 185 rwbhp on a chassis dyno yesterday with a 4.1:1 rear end.
How much will I make today with a 4.3:1 rear end? No other changes.

[Stan Weiss]

185 rwbhp

Stan

[Charliesauto]

185 rwbhp

Stan

100% correct

[AC sports]

So the chassis dyno will still show 185 hp regardless of the final drive ratio?

[Stan Weiss]

So the chassis dyno will still show 185 hp regardless of the final drive ratio?

I don't own a chassis dyno and have not looked at their software, but have to believe it is like mine. Rear gear ratios of 4.1, 4.3, 5.38 Note Rate rev per sec increases with change of ratio also note MPH decreases with ratio change.

Stan

__Weight_(or_Req._Inertia)_=_1000.0#____Radius_=_12.0_Inches____Rear_Gear_Ratio_=_4.1

____________________________Rear_____Aero_____Rolling____________Rear_W__Accele___Time___Rate
__RPM______MPH___Velocity___Wheel___dynamic___Resist.__Elapsed____Horse__ration__Differ_Rev_Per
__________________ft/sec___Torque__Drag_-_HP____HP______Time______Power__in_G's__ential___Sec

____0.0____.000_____.000____0.000______.000____.000____.0000_______0.00__0.0000__0.0000_____0.0
__500.0___8.707___12.771___135.54______.000____.000____.7143______12.90___.5557__0.7143___700.0
_1000.0__17.415___25.541___135.54______.000____.000___1.4286______25.81___.5557__0.7143___700.0
_1500.0__26.122___38.312___135.54______.000____.000___2.1429______38.71___.5557__0.7143___700.0
_2000.0__34.829___51.083___135.54______.000____.000___2.8571______51.61___.5557__0.7143___700.0
_2500.0__43.536___63.854___135.54______.000____.000___3.5714______64.52___.5557__0.7143___700.0
_3000.0__52.244___76.624___135.54______.000____.000___4.2857______77.42___.5557__0.7143___700.0
_3500.0__60.951___89.395___135.54______.000____.000___5.0000______90.32___.5557__0.7143___700.0
_4000.0__69.658__102.166___135.54______.000____.000___5.7143_____103.23___.5557__0.7143___700.0
_4500.0__78.366__114.936___135.54______.000____.000___6.4286_____116.13___.5557__0.7143___700.0
_5000.0__87.073__127.707___135.54______.000____.000___7.1429_____129.03___.5557__0.7143___700.0
_5500.0__95.780__140.478___135.54______.000____.000___7.8571_____141.94___.5557__0.7143___700.0
_6000.0_104.488__153.248___135.54______.000____.000___8.5714_____154.84___.5557__0.7143___700.0
_6500.0_113.195__166.019___135.54______.000____.000___9.2857_____167.74___.5557__0.7143___700.0
_7000.0_121.902__178.790___135.54______.000____.000__10.0000_____180.65___.5557__0.7143___700.0
_7500.0_130.609__191.561___135.54______.000____.000__10.7143_____193.55___.5557__0.7143___700.0


__Weight_(or_Req._Inertia)_=_1000.0#____Radius_=_12.0_Inches____Rear_Gear_Ratio_=_4.3

____________________________Rear_____Aero_____Rolling____________Rear_W__Accele___Time___Rate
__RPM______MPH___Velocity___Wheel___dynamic___Resist.__Elapsed____Horse__ration__Differ_Rev_Per
__________________ft/sec___Torque__Drag_-_HP____HP______Time______Power__in_G's__ential___Sec

____0.0____.000_____.000____0.000______.000____.000____.0000_______0.00__0.0000__0.0000_____0.0
__500.0___8.302___12.177___135.54______.000____.000____.6494______12.90___.5828__0.6494___770.0
_1000.0__16.605___24.353___135.54______.000____.000___1.2987______25.81___.5828__0.6494___770.0
_1500.0__24.907___36.530___135.54______.000____.000___1.9481______38.71___.5828__0.6494___770.0
_2000.0__33.209___48.707___135.54______.000____.000___2.5974______51.62___.5828__0.6494___770.0
_2500.0__41.512___60.884___135.54______.000____.000___3.2468______64.52___.5828__0.6494___770.0
_3000.0__49.814___73.060___135.54______.000____.000___3.8961______77.42___.5828__0.6494___770.0
_3500.0__58.116___85.237___135.54______.000____.000___4.5455______90.33___.5828__0.6494___770.0
_4000.0__66.418___97.414___135.54______.000____.000___5.1948_____103.23___.5828__0.6494___770.0
_4500.0__74.721__109.590___135.54______.000____.000___5.8442_____116.14___.5828__0.6494___770.0
_5000.0__83.023__121.767___135.54______.000____.000___6.4935_____129.04___.5828__0.6494___770.0
_5500.0__91.325__133.944___135.54______.000____.000___7.1429_____141.94___.5828__0.6494___770.0
_6000.0__99.628__146.121___135.54______.000____.000___7.7922_____154.85___.5828__0.6494___770.0
_6500.0_107.930__158.297___135.54______.000____.000___8.4416_____167.75___.5828__0.6494___770.0
_7000.0_116.232__170.474___135.54______.000____.000___9.0909_____180.66___.5828__0.6494___770.0
_7500.0_124.535__182.651___135.54______.000____.000___9.7403_____193.56___.5828__0.6494___770.0


__Weight_(or_Req._Inertia)_=_1000.0#____Radius_=_12.0_Inches____Rear_Gear_Ratio_=_5.38

____________________________Rear_____Aero_____Rolling____________Rear_W__Accele___Time___Rate
__RPM______MPH___Velocity___Wheel___dynamic___Resist.__Elapsed____Horse__ration__Differ_Rev_Per
__________________ft/sec___Torque__Drag_-_HP____HP______Time______Power__in_G's__ential___Sec

____0.0____.000_____.000____0.000______.000____.000____.0000_______0.00__0.0000__0.0000_____0.0
__500.0___6.636____9.732___135.50______.000____.000____.4149______12.90___.7290__0.4149__1205.0
_1000.0__13.271___19.465___135.50______.000____.000____.8299______25.80___.7290__0.4149__1205.0
_1500.0__19.907___29.197___135.50______.000____.000___1.2448______38.70___.7290__0.4149__1205.0
_2000.0__26.543___38.929___135.50______.000____.000___1.6598______51.60___.7290__0.4149__1205.0
_2500.0__33.178___48.662___135.50______.000____.000___2.0747______64.50___.7290__0.4149__1205.0
_3000.0__39.814___58.394___135.50______.000____.000___2.4896______77.40___.7290__0.4149__1205.0
_3500.0__46.450___68.126___135.50______.000____.000___2.9046______90.30___.7290__0.4149__1205.0
_4000.0__53.085___77.859___135.50______.000____.000___3.3195_____103.20___.7290__0.4149__1205.0
_4500.0__59.721___87.591___135.50______.000____.000___3.7344_____116.10___.7290__0.4149__1205.0
_5000.0__66.357___97.323___135.50______.000____.000___4.1494_____129.00___.7290__0.4149__1205.0
_5500.0__72.992__107.056___135.50______.000____.000___4.5643_____141.90___.7290__0.4149__1205.0
_6000.0__79.628__116.788___135.50______.000____.000___4.9793_____154.80___.7290__0.4149__1205.0
_6500.0__86.264__126.520___135.50______.000____.000___5.3942_____167.70___.7290__0.4149__1205.0
_7000.0__92.899__136.252___135.50______.000____.000___5.8091_____180.60___.7290__0.4149__1205.0
_7500.0__99.535__145.985___135.50______.000____.000___6.2241_____193.50___.7290__0.4149__1205.0

[lefty o]

pretty sure it will give a different hp reading with a different rear gear in the car. they give different readings with transmissions in different gears, changing the rear gear is no different.

[Stan Weiss]

pretty sure it will give a different hp reading with a different rear gear in the car. they give different readings with transmissions in different gears, changing the rear gear is no different.

Actually it is. You really want the trans ratio to be 1:1 as that is the most efficient. Also I am talking about manual trans where one does not have to deal with what the torque convertor does.

Stan

[lefty o]

pretty sure it will give a different hp reading with a different rear gear in the car. they give different readings with transmissions in different gears, changing the rear gear is no different.

Actually it is. You really want the trans ratio to be 1:1 as that is the most efficient. Also I am talking about manual trans where one does not have to deal with what the torque convertor does.

Stan

im talking manual trans too.

[autogear]

In theory, some rear axle ratios are going to be more efficient to turn than others. However, 4.11 and 4.30 are pretty easy to produce. If you had some weird ratio, it might show a slight decrease in power.

[dannobee]

They give different readings in different transmission gears because of the increased power it takes to drive the tranny in reduction. In 4th gear (or whatever is closest to 1:1), the tranny is as close to being a driveshaft as it can be. Pick any other gear and some of the work is overcoming frictional losses of gear reduction. Likewise, a lower rear gear (higher numerically) takes more power to turn because of friction, even though torque increases.

I was never big on chassis dynos because of the lack of repeatability, irrespective of the many other variables changing the hp/torque number (like the correction factors used by the manufacturer). When changing tire pressure or wheel alignment can affect the reading, it's less than ideal in my book.

Years ago, one of the sanctioning bodies impounded the top 5 cars in our class and subjected them to chassis dyno tests. Since a couple of us were familiar with the limitations of a chassis dyno, we were able to "skew" the outcome in our favor. FWIW

[Stan Weiss]

They give different readings in different transmission gears because of the increased power it takes to drive the tranny in reduction. In 4th gear (or whatever is closest to 1:1), the tranny is as close to being a driveshaft as it can be. Pick any other gear and some of the work is overcoming frictional losses of gear reduction. Likewise, a lower rear gear (higher numerically) takes more power to turn because of friction, even though torque increases.

I was never big on chassis dynos because of the lack of repeatability, irrespective of the many other variables changing the hp/torque number (like the correction factors used by the manufacturer). When changing tire pressure or wheel alignment can affect the reading, it's less than ideal in my book.

Years ago, one of the sanctioning bodies impounded the top 5 cars in our class and subjected them to chassis dyno tests. Since a couple of us were familiar with the limitations of a chassis dyno, we were able to "skew" the outcome in our favor. FWIW

Yes the tire dyno interface can be a problem and that is why the next logical step was to do away with it. The hub dyno.

Stan

[Dust Buster]

Chassis dynos usually reflect a loss of about 39% compared with an engine dyno. A different rear end could show different frictional losses.

For comparison purposes, the correction factor should be the same on the same dyno. Correction factors should really only consider atmospheric pressure and temperature.

There can be between 16% and 20% difference between inertia and eddy current chassis dynos.

[ptuomov]

Chassis dynos usually reflect a loss of about 39% compared with an engine dyno. A different rear end could show different frictional losses.

For comparison purposes, the correction factor should be the same on the same dyno. Correction factors should really only consider atmospheric pressure and temperature.

There can be between 16% and 20% difference between inertia and eddy current chassis dynos.

39% sounds really high to me.

What we’ve found is that for the particular car that I’m working with (928 S4), the following empirical formula seems to describe the manual transmission driveline losses pretty well in the 200-900 hp range:

Flywheel hp = 10 hp + rear wheel hp / 0.88

[Sparksalot]

Hypoid gears lose power by the rubbing involved in their engagement. The reason the lube stinks is it needs special ingredients to handle the rubbing between the pinion and ring gear. It is not a rolling contact such as between spur or helical gears. For any hypoid gearset increasing the ratio increases the rubbing and thus will loose efficiency. Thus, a higher ratio will deliver less power to the wheels, because there is more rubbing.

[Dust Buster]

39% sounds really high to me.

Here is a test between dynos: https://www.hotrod.com/articles/1106phr ... ine-dynos/