Correct. We're talking steady state.

As force is increased, so is friction:

http://www.school-for-champions.com/...n_equation.htm
https://en.wikipedia.org/wiki/Friction

 

There is one more part to the puzzle. Most, not all, but most chassis dynos read the horsepower through measuring the acceleration of the drum the tires are turning. The quicker you accelerate the drum, the more horsepower is made. This is an inertia dyno. They are measuring the work being done, IE: horsepower.

When you increase the horsepower, you increase the rate of acceleration of all the rotating parts from the crank to the tires and everything in between. The quicker you try to accelerate this mass, the more power it takes. Being the dyno is measuring the rate of acceleration, all of the inertial parasitic losses are part of the final measurement. Of course the frictional drag will be part of the parasitic losses and will account for part of the losses in the horsepower.

Just to answer the question before it comes up, inertia dynos do not measure torque. They calculate torque back to the crank, if you hook up the rpm pickup. The parasitic losses are already figured in the torque because it is calculated from the horsepower measurement at the rear wheel.

 

Ok now that is an explanation I can see some logic in. So since torque has to increase in order to show an increase in power at the same RPM then the additional torque being produced by the engine creates more force on the meshing gears thereby increasing the drag. I can believe that.
Now is the increase in HP consumption going to be linear?
Not all of the losses are due to meshing gears in a TH350. In a manual transmission, more likely.
The gears are going to remain separated by a layer of oil so the additional drag is gong to be created by hydraulic drag. This as you pointed out will be affected by type of oil and presumably temperature and viscosity as well.
The constant HP consumed by a transmission would of course have to be at a specific RPM. Obviously at 1 RPM that could be generated by hand you're not even generating oil pressure by operating an oil pump in the TH350, that would certainly make it easier to rotate.

 

Another key is understanding that losses are neither a fixed amount nor fixed percentage- they rise exponentially (non-linear) with RPM and torque applied.

IOW- the losses with low load and high RPM are far less than high load and high RPM. The inverse is also true.

The statements that quote a fixed loss irrespective of any other conditions are incorrect.

 

I remember reading about how inertia (chassis) dynos calculate torque and do not measure it. Since torque converters have torque multiplication would this then lead to erroneously high torque calculation for the engine in automatic transmissions?
Since the chassis dyno uses acceleration to compute power then it would show a power increase if the drive train components were lighter wouldn't it? Smaller torque converter, lighter wheels and tires, aluminum drive shafts all that would show a HP increase even though one does not exist?

 

An exponential rise is what I would expect with rpm and I can see how additional torque would also increase drag.
Now the additional torque being applied is based on the assumption that additional force is being required to turn the drive train, like on a chassis dyno. However on the road at 60 mph the force required to maintain that speed does not change given a 500 HP engine or a 200HP engine, so in that instance the power being consumed by the drive train would remain constant.
So having more power doesn't nessecarily mean the trans is using more HP just that in computing Max HP output using the % rule achieves more accurate results if we know what % to apply.

 

Hydrodynamics and aerodymanics have some elements that are similar. Think of the gears and pumps in the trans fluid as your hand moving thru the air....the air is pretty easy to move thru at 5 miles per hour, try moving your hand thru the air at 65 miles per hour. This is the type of drag the gears-pumps and vanes your trans "feels" as speeds increase.

You also have increases in bearing drag that ad to the overall losses. There may be some other losses but these are the first two that come to mind.

Kevin

 

Correct.

 

it's all fluid dynamics I guess, so yeah very similar.

 

I'm guessing this is where the fixed HP loss numbers come from. It was computed for some steady state condition in normal operation and not for computing max HP numbers on a chassis dyno.

 

Not to drag this out but just to clarify for myself.
Since there are numerous variables involved in chassis dyno readings they really have no accurate correlation to the HP output of the engine do they. It can only give me an estimate of HP to the ground and that can only be used as a benchmark for changes on the engine or drive train for that specific vehicle.

A 1970 vette with a 350 gross HP engine using a TH 400 may only put down 227.5 HP on a chassis dyno if it needs to spin to 5700 RPM to generate that 350 HP and that creates a parasitic loss of 35% due to the rpm and additional mass of the TH 400.
Whereas a 1978 vette with 300 gross HP and a shorter cam using a TH350 may put down 225 HP on a chassis dyno because it only needed to spin to 5200 RPM to generate it's 300 HP and only had a loss of 25% through the TH 350.
Anybody ever seen a chart or mathematical equation to compute losses due torque an/or RPM for a given transmission? Lol probably not.

 

'Having' more power is irrelevant, it's the actual power being transmitted through the drivetrain at the moment of measurement that counts.

Neither a 200HP or 500HP engine are developing that amount of energy when the vehicle is at a constant highway speed, a mere fraction in fact.

 

Your first paragraph - basically yes.

Your second paragraph - No.
Dyno runs should be accomplished by starting at approx. 1500rpm and going full throttle to redline (or what ever RPM you are comfortable with.)

Your trying to compare a snapshot at a specific RPM. The dyno compares how quickly the drums spin up over the amount of time the run lasts.
Just because the 350hp ran to 5700rpm, that isn't a penalty compared to a car that only ran to 5200rpm. The 350hp car will have more hp/tq across the entire rpm band. It will get to 5200rpm quicker and continue to increase drum speed to 5700rpm. It may even get to 5700rpm in a shorter amount of time than the 300hp car takes to get to 5200rpm. As long as the drums are increasing in speed the HP registered will be higher.

Take your car, run it on a Dynojet chassis dyno and post the results.

 

Not to make this more confusing, but just need to point out that not all chassis dynos are inertia dynos (like DynoJet). The Mustang dyno is eddy current, which means you can measure power at a constant road speed. This removes the drivetrain rotating inertia from the equation and gives a more accurate value.

 

To the OP, both Mike Ward and Revi are correct. This is the same explanation given to me by the dyno operator. This discussion has come up numerous times on the forum. I have personally "run the numbers" on my engine and came up with the same results as generally computed.

My sb 427 dynoed 628 hp on the engine dyno. When installed in the car, it yields 490 rear wheel hp. This is with a TH400 auto trans. My math says that's a 22% loss. Others that have done the same test usually report 22-25% for auto transmissions and less (15-20%) for manuals......regardless of engine hp.

 

Not really. Most of the time when you are using the full horsepower of a vehicle, you are accelerating and so inertia is significant. Even in top speed trials such as at the Bonneville salt flats, most of the run is acceleration. Only when the acceleration slows, and the engine is straining to pull that last little bit of mph out, would a brake dyno give you a closer approximation to the way the power is being used.

Another reason the losses show so high on a inertia chassis dyno compared to an engine dyno is most engine dynos are brake dynos. Even though you can set most modern brake dynos up to do a sweep test, the sweep test are usually slow enough in their acceleration through the rpm's to keep the inertia from being a significant part of the results.

Gear ratio also is a large factor in results on a inertia chassis dyno. The lower the overall gear ratio to the tire (higher numerically) the shorter and quicker the test will be. Since the mass is accelerated at a faster rate, it will take more power to do so, and that will show up as a lower horsepower reading than if you use a higher overall gear ratio.

That being said, dynos should be used as a tool to measure the effects of the changes you have made to the engine, not to compare numbers from one dyno to another. Dynos can read quite differently from one to another. The procedures and setup the operator uses can skew the numbers as will the accessories or lack there of IE: air cleaners, water pumps, exhaust etc...

I always do testing on my dyno with everything as close to the same as possible every test session to try and give repeatable results. Tire pressure, placement of the tire on the drum, the engine and drive-train up to operating temps, etc... In this way it is a powerful tool in measuring the results of your work!

 

Sure I understand that the 350HP motor will produce more power and potentially more torque throughout the rpm range with the possible exception of low end torque. I was just comparing max output to max out put #s.
I am considering running my car on a dyno. This is what got me thinking about this and if the data received from a chassis dyno run will be of any value to me. If I can't correlate or convert it to an engine HP output then how can I use it to compare to other engines in the future unless it is installed in this chassis.
I can use it as a benchmark for this car and it's engine. Maybe that will be useful to me. I have to drive 180 miles to get it done so still undecided. I did have specific goals when building this engine and I would like to see if they were met. Without an engine dyno or a reliable method to convert chassis numbers to engine output numbers I may not be able to establish if I met those goals.
This information would be useful for choosing components for a 406 that I want to build in a year or two.

 

What I'm hearing you say is that the loss is a constant % regardless of power. What mike is saying is that it is greater as power increases, non-linear.
In your installation you only lost 22%.
In revi's installation he lost 28.44%. That is a significant difference.

In fact if you compare the two results they do not support either yours or mikes statements.
I'm not trying to be argumentative just pointing out that in revi's lower powered engine with his manual transmission he lost a greater percentage of power than you did with greater power through an automatic transmission.
Two dyno's is not a large enough sampling to see a trend of any sort but the two we have do not support the assertions made so far.
So installation is likely a big factor.
Nice thing is that in your case you know exactly what the loss is, and so does revi.

 

Your individual loss may vary a little due to resistance from a different exhaust system, rear end, tires, etc., than mine. But if you actually do the test, you will end up with the same results. The ballpark numbers run true 99.9% of the time.

 

Ok I get what you're saying. The HP loss is greater but the % loss remains constant. Sure I agree that is true mathematically speaking.
But mike said;
"Another key is understanding that losses are neither a fixed amount nor fixed percentage- they rise exponentially (non-linear) with RPM and torque."

So according to that statement power loss would rise % wise as well.
Your percentage of loss was less at a higher HP level than Revi's at a lower power level. Agreed?
Also it was stated that a manual transmission looses less power percent than an automatic. Again this did not prove to be true between your loss % wise on a th400 vs revi's on an M20 trans, 22% VS 28%.
As far as multiple dyno sessions is concerned, I'm not referring multiple sessions on one car, I'm referring to multiple dyno examples from different cars in order to get a larger sampling of engine dyno's VS chassis dynos of the same car for various transmission types.
In any case I have found out what I needed to know and I appreciate everyone's input.
Even Mike managed to not insult while teaching you'll never make je** of the year doing that mike, might wanna rephrase your responses to keep up appearances.