Lower gear ratios increase torque, that's why trucks have lower ratios so they can pill heavier loads.

So shouldn't lower gears read higher ration on a Dyno?

R~

 

Lower rear gears don't make your engine more powerful at any given RPM; they just allow you to reach higher RPM quicker. If you change from a 3.08 to a 4.11 gear, and you go a 1/2 second quicker E.T., you're still using the same engine, right?. No increase in torque or horsepower. If you have, say, 400 hp at 5000 rpm, it doesn't change no matter how fast (or slow) you accelerate.

 

I think you are just wrong about this.

Lower gears 4.56 as opposed to 3.08 multiples torque.

So why don't you take off in 3rd gear instead of 1st gear, you have the same engine.

Ralph

 

.....you CAN take off in 3rd gear......and when you reach 3000 rpm, your engine will register the same HP as when you reach 3000 rpm taking off in 1st gear. 3rd gear just takes longer to get there. Rear gears cannot, and do not add HP to your engine.

 

They add torque I believe.

R~

 

Higher numeric gears do not add or increase the torque of the engine. Higher numberic gears multiply the torque of the engine by a higher ratio to put more torque to the wheels at the same engine rpm. They do this by reducing the speed of the rear wheels by the same ratio.

Higher numeric gears do not increase the HP at the rear wheels.

The hp and torque numbers calculated by the dyno are put on a graph with the X axis being the engine rpm, not the wheel rpm. This means the higher torque at the wheel due to the higher numeric gears is ratioed back to engine rpm and it comes out the same as the lower numeric gears (or at least very close to the same).

 

"Higher numeric gears do not add or increase the torque of the engine.
True but it does increase power to the wheels.
The only way to increase engine power is to modify the engine.
But we are talking about a chassis Dyno. and power to the ground.

 

THIS IS NOT CORRECT.

This has always been a topic which causes a lot of angst.
so lets get the context right to clear this up.

On one hand we have power and torque measurements available at the flywheel [engine power and torque vs rpm],
and on the other hand,
we have power and torque available at the rear wheels (as can be measured by a chassis dyno using a load cell and roller speed sensor)

In rotational systems, mechanical power is the product of the torque T and angular velocity ω,

P = T.ω

P is calculated as watts, where T is measured in Newton metres and ω measured in radians per second.

On a dyno, when you change diff ratio from lets say 3.08 to 3.7, there will of course be NO change in the engine power or torque output at the flywheel. It is the same engine, so how can it change?

There will be, however, a difference in the relative speed of the rear wheels at any given engine rpm. This affects the angular velocity component of calculations at the rear wheels, as measured on the dyno. The 3.70 diff gear will rotate the rear tyre at a slower angular velocity than a 3.08 diff gear at the same engine rpm. In fact, the gearing change will reduce rear wheel velocity by 20% at the same given engine rpm.

What we will see also, is an increase in the rear wheel torque applied to the dyno roller. Funnily enough, negating the frictional loses and heat energy conversion which would occur due to different sized gears in the diff, we would see a a 20% increase in rear wheel torque.

The net affect on power is NO change at any given engine rpm.
when you multiply the 20% increase in torque at a given rpm, by the 20% reduction in angular velocity, the result will be the same power is produced at each rpm measurement point.

So to summarise.

Changing the diff gears, or performing a dyno run in 3rd gear vs 4th gear, will NOT affect the torque output or power calculated at the flywheel.

It will NOT affect the power output at the rear wheels either!

It will however, affect the torque output at the rear wheels, and that is exactly why accelleration rate increases with for example, a 3.7 diff gear over the 3.08 diff gear. (assuming you get traction )

my 0.02c worth of thought.

 

No, gears DO NOT increase the power to the wheels. If you have 300hp at the engine then you have 300hp minus the drivetrain losses at the wheels, no matter what gear ratio you use.

Stop using torque and power interchangably in your arguements. They are not the same thing. Torque is a twisting force and power is the work that the torque will do. This misuse is showing you don't really even have a grasp on these basics and should really spend some time with Google or a good book before trying to argue about something you don't really understand.

 

People get confused when they look at dyno tests. Engines produce power. But dynos measure torque, and use that and engine RPM to calculate HP. HP is a mathematical function of torque.

 

The problem is not understanding that torque and HP are not the same same thing.

Engines don't really produce power, the mechanics in the engine produce a twisting force.

Inertia wheel dyno's don't measure torque or hp. They measure how fast the drum accelerates and they measure the engine rpm and they use that data to calculate the "rear wheel" torque and hp vs engine rpm curve.

 

Second Dyno run started its recording after your motor pasted it's peak torque. That's why the number recorded was so low.

 

I am not confused about Horsepower and Torque.

I'm using the term POWER as a generic term. Power = Force
I have not been talking about horsepower, just torque.
To me the phrase "power to the wheels" means power. Since I was not talking about horsepower the only thing left was torque.
Torque can be multiplied horsepower can not.

I hope that clears up the verbage.

Ralph

 

I noticed that my torque curve was on the decline before the recording.

Ed the Dyno opperator should of caught that and reset where the recording started.

Since horsepower is calculated from torque, Then all the numbers are suspect.

Thanks for pointing that out hugie82.

Ralph

 

Oh, OK. Strange way to define power when discussing an engine but whatever. I usually use the physics definition of power which is the rate energy is used or to measure the rate at which work can be performed. I consider torque to be a measure of force used to do work, not a measure of power.

 

Ya, typical. You try to deal with someone who doesn't seem to understand and then get called a physics teacher, and not in a good way. Next will come the arguement that the definition of power is just "book learning" and that what the car does happens in the real world (where, apparently, physics doesn't apply).

What does the first curve the computer plots on the screen have to do with anything?

I'd also really like to hear the explanation of how your dyno measures hp. Is there a new "hp pickup" available to purchase I haven't heard about?

 

Hmm, you make your post about, and I paraphrase, "your dyno can measure whatever you want but mine measures horsepower". I'm not sure who your comment was directed at, but since I was the only one that posted a comment about the dyno not measuring hp but rather calculating hp, it sure seemed directed at me.

Thank you for giving a nice quote that proves my point.

On another note, the person quoted goes on about F=ma and that HP=torque x rpm/5250 and then says that the Dynojet doesn't use this and that it calculates hp directly in the manner he is describing. The funny thing is that he doesn't actually describe how it directly calculates hp. Not at all. Not one bit. The whole paragraph didn't give me any enlightenment whatsoever.

Overall, a few vague paragraphs about how the dyno "works" that was directed at a lay person who has little understanding behind the operation really isn't much use to prove anything.

I did notice that Bristol Dyno is apparently closed and selling off their dyno's. Going out of business?

 

My only experience was with a Clayton water brake chassis dyno. Horsepower was determined differently on that style of dyno. It only gave a torque reading and horsepower was factored mathematically from that reading.

It appears that an apples-to-apples comparison cannot be made between the two types of dynos as they each calculate their numbers differently. That's where my confusion came about.

Carry on.

 

Work/time is the same as force x distance / time. HP = torque x rpm/5250. Notice the torque is force x distance and rpm is per minute or the divide by time. 5250 is to properly convert between the different units. A dyno that calculates hp = work/time is first figuring out the torque and then using the standard torque x rpm/5250 formula or some very close relative of this formula. There is no magic in the inertia dyno that bypasses this calculation and the inertia dyno certainly does not only calculate hp without calculating torque first.

The inertia dyno measures the acceration of the drum and then calculates the force needed to accelerate it. That water brake dyno likely had one or more load cells a certain distance from the crank center line. It would measure the force directly and multiply by the distance from the crank center line to get the torque. So, they both calculate the torque first, just in different ways. Then, they both calculate the hp using the torque.

There is a fairly simple reason why the inertia dyno can't display torque if the engine rpm reading isn't picked up. The dyno calculates the torque at the drum shaft. It can then convert this to a hp curve no problem since hp is hp no matter what gear ratios are involved. The hp at the drum shaft is the hp at the engine minus the losses between the engine and the drum even and it can still be calculated without knowing the engine rpm. However, the dyno can't convert or ratio the torque to reflect a different speed (ie the engine speed) if it didn't get a proper engine rpm reading since the torque varies by the gear ratios involved.

 

So what effect did it have when my max torque had already peaked before the Dyno started recording my info?

Ralph