Dyno Question

[Dave22443]

This may be a stupid question, and I assumed I knew the answer pretty easily.  But the more I think about it and the more I talk to others about it, the more confused I get.  So here's the question:

How will rear end gear ratio effect your dyno numbers?

In other words, take a car with 4.11 rear, put it on the dyno, and lets say (for the sake of argument) that you read 300 HP and 350 torque.

Now, take the same car and make no other change except rear gear, to say... a 2.73.  Will this change the dyno readings?  And if so, in what way?  Do you get more effective horsepower with lower gears?  Or is the HP the same, just in a different speed?

Inquiring minds want to know! 

[Ghoste]

Think of the rear end ratio as a lever.  A longer lever produces more work over a shorter distance.  That is kind of oversimplifying it but if you change the units of measurement, you should get the idea.

[Dave22443]

 

[Big Sugar]

Think of the rear end ratio as a lever.  A longer lever produces more work over a shorter distance.  That is kind of oversimplifying it but if you change the units of measurement, you should get the idea.

YEAH LIKE HE'S 44 years old today !! Don't expect too much clarity !


Ron (Happy Birthday )

[andyf]

The dyno doesn't care what the gear ratio is in the car since the dyno calculates HP based on how quickly the car spins the weight of the inertia drum.

But, if you have the gear ratio way off in your rear end you might not be able to effectively transmit your engine power to the dyno drum.  The engine power will remain the same but you're dyno results might not reflect it properly.

So the answer is no, it doesn't matter unless you have it so wrong that it does matter.

[Ghoste]

I thought the dyno worked by placing a known load against the drum and then by knowing the amount of torque required to overcome said load, horsepower could be calculated?  By that it would mean that the rearend ratio would have an effect on it, no?

[Dave22443]

Well, thats kind of what I was wondering... 

[4402tuff4u]

I'm just guessing here and it's only my opinion, but given the basics in physics I'm going to say that the gear ratio will basically only affect the time at which the rear end will deliver the peak hp and torque in the car. In other words, the car's powertrain on the dyno will deliver "x" amount of hp and torque no matter what gear. The different gear ratio will only affect the time it will take for the power train to reach the peak.

It might also change the rpm that the peak hp and torque is reached at.