I know for a fact that when we dyno our car, 4th gear will show a higher hp than 3rd gear. But when I asked around, I was told that even though the dyno show 10 or more hp on 4th gear, it is actually unrealistic hp (fake hp). Can someone explain to me what that means?

 

i actually got more hp in 3rd gear. that shows you that all dynos are calibrated different.

 

Hello friend I assume you are talking about an inertia dyno...

Here is one example of a 248 Dynojet and how it works, this will give you a better understanding of why changing gear ratios will yield different results.

A Dynojet 248 is an inertia dyno
A Dynojet 248 is an accelerometer and here is how it works (calculation to optain HP reading)...


Force = Mass X Acceleration

Mass is the known roller weight
Acceleration is a change in velocity over a period of time

Acceleration = Distance divided by Time

exp. known roller weight X a change in velocity of 50 mph over 7 seconds.

A dynojet can only measures power output with a change in velocity. It can only measure power constantly at Wide Open Throttle.

So with the above being said if you change the gear ratio that the car is ran in it will alter the amount of time that the car physically takes to accelerate. Thus changing the output as calculated by the dyno due to different acceleration rates.


To answer your question an inertia dyno is normally run with the car in a 1:1 ratio. The reason being is that there is no mechanical leverage or lack of with overdrive (maybe a little to over simplified) to change the rate of acceleration. If the car is run in another gear other than 1:1 (4th gear in most transmissions) then you will have skewed results.





Note: This does not hold true with all dynos just dynameters that use this inertia based calculation method.



Chris Macellaro

 

I was about 8 hp better in 4th than 3rd and 5 hp better in 4th than 5th.

 

In simple terms, Mr. Macellaro is trying to say that you get different results due to different amounts of load on the motor....

 

I kind of understand that to get the most accurate HP reading, you have to match the ratio as close as 1:1 as possible. But in plain words, what does the extra HP mean when tested on the "no so" 1:1 ration?

 

Here was a comparison of 4th gear and 5th gear on my S2000. 5th has more torque, 4th has more horsepower.



Tim