Dyno questions

[67coupe351w]

HI guys,

Ive never taken my car to a chassis dyno and I have a couple questions.

DO they specify a certian gear to be in when they take their readings?

Do they need to know what your rear end ratio is?

Do they need to know tire diameter?

I guess what im getting at is how it works exactly. If I understand it correctly the dyno is measuring torque at the ground and horsepower is back calculated through an equation something like hp = (RPM * torque) / 5252.

But what im wondering is you would get diffrent torque readings at the ground in the diffrent gears correct?

Can someone clear this up for me? Thanks!

 

[C0V3R]

HI guys,

Ive never taken my car to a chassis dyno and I have a couple questions.

DO they specify a certian gear to be in when they take their readings?

Usually in the gear that is closest to 1:1 (top in C4s and most toploaders, 4th in t5s etc)

Do they need to know what your rear end ratio is?

No

Do they need to know tire diameter?

No

I guess what im getting at is how it works exactly. If I understand it correctly the dyno is measuring torque at the ground and horsepower is back calculated through an equation something like hp = (RPM * torque) / 5252.

But what im wondering is you would get diffrent torque readings at the ground in the diffrent gears correct?

You dont really get torque but tractive effort. Yes gearing can affect this reading. Thats partly why they use the standard 1:1 drive gear as drivetrain losses can vary from gear to gear (eg your diff will lose more power at higher speeds than lower speeds due to more rotation, hence more internal friction)

This is also why dynos are best used as tuning tools (to show differences before and after making changes) rather than comparison tools as all these variables come into play between cars. The most important thing is how fast a car is between point A and B, not what number it puts out on the sheet.

 

[ratio411]

My only comment is that lower gears actually lower dyno numbers, while highway gears post higher numbers.
Why? I am not sure of the exact reason, just that it works that way.
And yes, they will accellerate your car normally through the gears. When they get to your 1 to 1 gear, they will level the rpms out for a moment, then they floor it. That is the dyno run.
Good luck
Dave

 

[C0V3R]

Your observations match what I said. 4.11s result in alot more driveline friction to turn the wheels at the same speed compared to 2.73s, hence lower HP numbers on the same motor.

 

[2bav8]

I had my car on the dyno once and did my first pull in 4th gear, and then tried 3rd gear a few minutes later after a cool down to see the difference. I got the exact same numbers.

 

[Ronstang]

Every dyno I have ever had a vehicle on asked for rear gear ratio as in input variable.

 

[1969mach1351]

When my brother went to a dyno they did two pulls in 4th and two in 3rd. Both 3rd gear pulls were about 8 horsepower more than 4th. I asked the guy why that was and he just said "Dunno, some cars just like 3rd gear."

 

[customperforman]

These guys all have the right idea. i will add that we have dyno'd the same car in 2nd, 3rd, 4th, and 5th and we only see ~3% difference in numbers until you get to the overdrive(s).

tires do affect the numbers you get, even though, its not factored in. the more tire you have, the more you lose through friction. weve seen a 50 hp loss going from street tires to wide slicks!

dynos measure the car's ability to overcome the inertia of heave drums. the rate of the acceleration of the drums is used by the computer software to calculate power.

something interesting that we've been playing around with is pinion angles. drag cars, especially, are gaining/losing significant power through changes in pinion angles.

Rick

 

[Ronstang]

are gaining/losing significant power through changes in pinion angles

This is exactly why I always argue that pinion angle needs to be checked when installing a non-stock drivetrain such as a T-5/Tremec in and older Mustang.....the increase in drivetrain angle you get by using commercially available x-members instead of doing the needed surgery can/will cost you power due to inefficiently large pinion angles if you actually shim the pinion to compensate for the increased engine/tranny angle. This is just another reason I always champion tunnel mods and custom homebuilt x-members to get the transmission in correctly.

 

[67coupe351w]

I would suppose the pinion angle would matter becasue the U-joint is less efficent when it is having to compensate for an angle vs when it is just straight.

So a rear end ratio is required?