HP and TQ. How to understand a dyno chart.

[GenX]

I want to take a moment and explain how horse power (HP) and torque (Tq) relate to each other and hopefully help people understand what they see on a dyno chart.

PART 1 Torque.

First off, lets discuss the numbers you get from a chassis dyno. The two numbers everybody wants to see are HP and Tq. I am going to discuss Tq first.

The Tq number you get from a chassis dyno is a calculation of how much Tq you would see 1 ft from the center of the crankshaft and NOT the amount of Tq you have at your wheels. A measurement is taken at the tires and calculated back to the motor. The ACTUAL Tq you have at your tires is MUCH more than what the dyno chat says.

Engine Tq is multiplied by the gearing in your transmission and then again by your rear differential gearing. Tire diameter is also a factor in how much Tq you actually transfer to the street. Friction, drag and the mass (law of inertia) of all the drive line components are the final factors in the equation.

Here is an example of how to figure the Tq at the tire of the 2025 Hyundai Make-Be-Lieve 2.0T AMS Edition. While the numbers here are made up, they are close to real world numbers for the average auto. I just made them whole numbers for simplicity.

Engine peak Tq:
300 lbs @ 5500 RPMs

Transmission ratios:
1st 4:1
2nd 3:1
3rd 2:1
4th 1:1

Rear end gear:
4.00:1

Tire diameter:
24"

For 1st gear, you take 300 (Engine Tq) times 4 (1st gear ratio) times 4 (rear end ratio) which nets you 4800. The distance from the center of the axle to the tire contacting the ground is 12" so tire diameter (in this example) has
no effect on TQ at the tires. Subtract drive line losses (DLL) and what you have left is the Tq you have available at the ground. In this scenario, you have 4800 lbs of rotational force (minus DLL) pushing your car forward @ 5500 RPMs.

If you play around with this formula, you will find that the Tq at the tire changes with which gear you are in.

1st = 4800 lbs - DLL
2nd = 3600 lbs - DLL
3rd = 2400 lbs - DLL
4th = 1200 lbs -DLL

This is why you accelerate so much harder in 1st gear than you do in 4th gear.

Keep in mind that the Tq your motor makes changes throughout the RPM range. This means that the amount of force you have pushing your car forward is also changing. You will always accelerate hardest at your peak Tq in each gear and less as TQ drops away from your peak. Your acceleration curve will end up matching your Tq curve exactly.

So when looking at a dyno chart, keep in mind that the Tq curve represents exactly how your car will accelerate. A large jump in Tq at the beginning of the graph and Tq dropping rapidly as you approach red line is going to feel exactly like that on the street.

 

[GenX]

Part 2: Horse Power

After reading "part 1" I am sure you are asking yourself "why do I need to know HP?" As I explained, Your Tq curve dictates your acceleration curve, so why does HP matter?

The formula for figuring HP is Tq x RPM / 5252. So HP is a function of engine Tq and is directly related to RPM. As you can see from the formula, HP will always be lower than Tq below 5252 RPMs and above Tq at over 5252 RPMs. Here is a quick example using the formula: Our 2025 Hyundai Make-Be-Lieve 2.0T AMS Edition makes 300 lbs Tq @ 5500 RPMs. 300 x 5500 / 5252 = 314.16 HP @ 5500 RPMs.

The easiest way to explain HP is: Horse Power is a representation of what you can do with your engine's Tq using gearing. The higher the RPM you have substantial Tq, the more HP you have. Let me use an example to explain this.

Our 2025 Hyundai Make-Be-Lieve 2.0T AMS Edition makes 300 lbs peak Tq @ 5500 RPMs. The standard edition Make-Be-Lieve makes the same 300 lbs peak Tq @ 4125 RPMs but at 5500 RPMs it is making 150 lbs.

Using our gearing formula in Part 1, you will find that at 5500 RPMs, the AMS Edition is producing 4800 lbs of rotational force (314 HP) in 1st gear while the Standard Edition is producing only 2400 lbs of rotational force (157 HP). The Tq has dropped off so much that a gear change is needed to get the motor back into an effective RPM range. Switching to 2nd gear will reduce RPMs by 25% putting the Standard Edition's motor at 4125 RPMs and at its peak Tq of 300 lbs. The Standard edition is now putting out 3600 lbs of rotational force (235 HP) which is an improvement, but it is still less than the AMS Edition's 4800 lbs (314 HP).

As you can see, both cars cars are now at the same peak TQ (300 lbs), but the AMS Edition is at a higher RPM and can more effectively use gearing to increase acceleration.

Ideally, the Tq curve should be as flat as possible. This gives you the same acceleration rate at all RPMs in each gear. With a flat Tq curve, HP will continue to climb all the way to red line.

I hope this helps you understand dyno charts a bit more and how they relate to actually relate to how the car accelerates.

 

[WhiteStripes]

Wow, that was super helpful, thanks a ton. I've always known a flat torque curve was good but never knew exactly why. Awesome.