Water Brake TQ Arm

[Dave W]

When you run the water brake dyno and there is a 1' torque arm mounted to the brake and the engine produces 500 lbs of tq would there be 500 lbs on the load cell at the end of the arm or is there some kind of formula for this that gets converted?

When building a water brake how do you know what size would be sufficient ?

Is there a Calculation for matching the brake to the load cell?
A 19" brake will produce more drag at 3000 rpm than a 10" brake and would pull on the cell harder and therefore give different readings. Thanks Dave

[DaveMcLain]

If the engine can produce a 500lb load at the end of a 12 inch long bar it's making 500lbs/ft torque at the back of the crankshaft,no special formula is needed.

I'm not sure how to calculate the capacity of the water brake for a given size but a 13 inch absorber seems to be about right for testing up to about 900 horsepower and around 800lbs/ft at 5500rpm.

The size of the brake will no matter when it comes to producing x amount of force on the arm, the engine and the amount of torque it makes will determine that. What seems odd about the dyno is how the throttle does not control the rpm, the load does, the throttle controls the amount of torque.

With that said, a larger brake will have a higher torque capacity than a smaller brake so it can test an engine with a larger amount of torque at a given rpm. If the absorber can't hold the engine at an rpm or hold it down to a certain controlled amount of acceleration, say 3-600rpm/sec then it either needs to be larger or possibly have a larger supply of water.

Hope this is helpful.

[Dave W]

Yes everything will be helpful on this project.

So back to the TQ arm, If you were to put a scale under the arm it would say 500 lbs.

Will the outer case of the brake rotate 360* if there were no hoses or fittings attached, like the outer race of a bearing.

Where is the vent line connected on the brake.

I checked out L&S and they were very nice people. There prices aren't to bad I guess, but out of my budget. Thanks Dave

[DaveMcLain]

If you would remove the stops, fittings etc from my brake it would rotate around with the shaft and rotor 360 degrees. The torque arm keeps the absorber located upright. The vent is located at about the 2:00 position, the fill port is at 12:00 both of them are about 1 inch out from the bearing on the rear of the absorber.

If I put an engine on the dyno that makes 500lbs/ft at say 5500rpm. Then I open the throttle and the load valve(thus putting water into the brake at a given rate from water supply, through valve, through brake and out the outlet hose as hot water). I can go to wide open throttle and then by adjusting the amount of water flowing to the brake I can make the rpm go up or down or hold wherever I want. Then if I adjust the water flow so that the engine is running at 5500rpm steady state the torque arm will read 500lbs pressure if it is 12 inches long. If it's 2 ft long it'll read 250lbs/ft. 6 inches long, 1000lbs pressure on the scale.

Then I can take my 500lbs/ft at 5500rpm and figure horsepower 500x5500/5252= 523.6 horsepower!

You can see then how if the engine makes the same amount of torque at a higher rpm it makes more horsepower. Or less torque at a higher rpm and the same horsepower. And also why horsepower tends to climb as long as the torque is not dropping at a faster rate than the rpm is going up.

[Dave W]

So if you smack er wide open and load the brake with say a 1.5" hose at 50 psi wide open you could stop the engine from reaching say 9000 rpm until you backed off on the load valve letting less water in which would show less TQ.

The discharge hose is smaller and does this act as a restriction

I noticed that some of these dyno brakes have a transducer in the brake. Is this just to pick up rpm or does this thing tell the load cell that there is a certain amount of pressure in the brake.

You guy's talk about 600rpm sec. I know its a time measurement but don't fully understand it. Thanks again Dave

[DaveMcLain]

Could I stop it from reaching 9000rpm until I backed off of the load? Yes. I would have to reduce the load being supplied by the absorber to allow the engine which is being held to accelerate. With the load reduced it would then pick up speed till it reached an rpm where again it's torque production would match that of the load supplied by the absorber.

The drain hose is smaller than the fill hose but the absorber can drain itself very quickly and thus react quickly too.

Same is in the opposite direction, if we had the engine running wide open at lets say, 7500rpm. Without backing off of the throttle we could reduce the rpm of the engine by increasing the amount of water flowing to the brake. Most likely the torque output of the engine would increase with the dropping rpm but when the torque and the load become balanced rpm would again become stable.

Where it can be a bit odd is when you are loading the engine way below the torque peak where adding a little load can make the rpm really drop.

600rpm/sec what that means is simply the controlled acceleration that's being used on the dyno pull. Most of the time I use 300rpm/sec in my testing. Accelleration pulls produce more conservative numbers than steady state "step" testing but I think it produces more complete data in that you have a sample at each rpm from the start to the end of the pull instead of at steps where you actually hold the engine at a given rpm for a second or two.

Acceleration pulls produce lower numbers because besides having the absorber's load the engine also has to accelerate it's own inertia and that takes away from the amount of torque it can feed to the absorber.

[Tony]

[DaveMcLain]

No, that's the OD of the absorber that I have. Earlier in the other thread on this same topic I posted the dimensions of the absorber and the rotor. It's not as large as you would think and the water supply is not very complicated either. This water brake is great about header clearance, I've never had much trouble in that area except for 180 degree crossover circle track headers.