Megan Header Dyno Results - Toyota Yaris Forums

Fastnx · 01-10-2007, 02:20 PM

Right, but it is important to know what final drive is and how a transmission works.

According to chinocharles, the engine's power goes through 4th gear, the speed is multiplied by 0.9xx and that's it. This is not correct.

The power goes through the clutch to the input shaft, then through a gearset to the output shaft. This gearset is different for every gear. The output shaft then travels through the differential, which reduces the engine's speed once again. Most final drives are in the 4:1 range.

So if your first gear ratio is 3:1 and the final drive ratio is 4:1, the wheels are actually travelling 1/12 the speed of the engine.

If your fourth gear is 0.9:1 and the final drive is 4:1, the engine is still travelling 3.6 times faster than the wheels.

The assumption that you have to dyno in the gear closest to 1:1 is false, since the power is going through a final drive, the wheel speed will never be close to the engine speed. If your dyno guy is telling you this, he is wrong. Just because you own and operate a dyno does not mean you know the science behind how a dyno works.

At a dyno I went to, the operator did not even know the fundamental relationship between horsepower, torque and RPM which is HP=(Tq*RPM)/5252.

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01-10-2007, 02:20 PM	#11
Fastnx Drives: Nissan NX2000 Join Date: Nov 2006 Location: Canada Posts: 12	Right, but it is important to know what final drive is and how a transmission works. According to chinocharles, the engine's power goes through 4th gear, the speed is multiplied by 0.9xx and that's it. This is not correct. The power goes through the clutch to the input shaft, then through a gearset to the output shaft. This gearset is different for every gear. The output shaft then travels through the differential, which reduces the engine's speed once again. Most final drives are in the 4:1 range. So if your first gear ratio is 3:1 and the final drive ratio is 4:1, the wheels are actually travelling 1/12 the speed of the engine. If your fourth gear is 0.9:1 and the final drive is 4:1, the engine is still travelling 3.6 times faster than the wheels. The assumption that you have to dyno in the gear closest to 1:1 is false, since the power is going through a final drive, the wheel speed will never be close to the engine speed. If your dyno guy is telling you this, he is wrong. Just because you own and operate a dyno does not mean you know the science behind how a dyno works. At a dyno I went to, the operator did not even know the fundamental relationship between horsepower, torque and RPM which is HP=(Tq*RPM)/5252.