Dyno Experts, Please Stand Up...

[Wayne 962]

In this particular thread, we see a whole bunch of dyno tests, some at the wheels, and some on the bench. Unfortunately, my expertise with dynos is somewhat limited. I'd be interested in hearing (and I'm sure so would other people) from people who know a bit more about the subject.

Specific questions that come to mind are:

- Torque and HP is shown on the graphs. In college, we learned that HP is is a function of torque. Why no linear relation here?

- Some engines are dyno'ed at the wheels, and some at the flywheel. Is there a tried and true formula for calculating the losses in-between. Are they a function of HP (do they increase with the size of the engine), or are they a static number (like 20-30 HP, regarless of the total HP number). Do they vary with RPM (probably would).

- How do different dynos compare to each other? What about operating conditions? How do you calibrate these machines? Are all dynos relatively equal, or do they vary results? If so, then how much? 5%,10%, 80%?

- What are the statistical error bars for achieving numbers? How are these runs affected by temperature, air pressure, operators?

- Can you compare numbers if you use different brands & octane gasoline?

- How about different sized wheels? Surely a smaller wheel would produce a larger mechanical advantage for the car, yet reduce top speed. Can this skew results?

Gosh didn't realize that I had so many questions. Hopefully, the combined knowledge of this group can work together to create something of a dyno FAQ. Maybe one exists somewhere already...

-Wayne

[Wayne 962]

Here indeed is some semi-useful info:

http://www.superflow.com/support/support-engdyno-faq.htm

http://www.ontrackdigital.com/generic.html?pid=1

http://www.dynomight.ca/calendar/faq.shtml

http://www.land-and-sea.com/dynamometer/faq-dynos.htm

-Wayne

[beepbeep]

Most modern dynos are of "drive on" dynamic-type...you drive your car onto big rollers and accellerate @ full throttle to rev limit. Then you declutch and let rollers spin down freely (this is logged too). Big fans are pushing air onto the engine during the process and air temp/humidity is noted.

Accelleration of rollers (which are pretty massive) is logged by computer...by derivating accelleration (which is change in roller rotational speed per second), you can calculate torque at wheels, and torque at wheels multiplied by wheels RPM 0= power at certain RPM. If you know exact gearing ratio, you can calculate engine RPM.

Dynoing is usually done at "straight-trough" 1:1 gear (usually 4:th in 5-geared cars).

Now the funky stuff...by measuring time it takes for rolers to stop when you press the clutch, you can calculate driveline drag and thus calcualte driveline losses...and in that way estimate engine power @ flywheel.

Basic stuff.

[Oliver911]

From speaking with the guys at Dynojet, a 5rwHp difference on a 70rwHp motor between identical Dynojet dynamometers is possible. So between 5% and 10% is reasonable. Now if you factor in other manufacturers, this number may increase. I know of no way to calibrate these dynos so the difference in performance between dynos probably lies in the tolerances in the manufacturing of the rollers. What is so beneficial about these dynos is not their ability to record maximum torque and power numbers, rather their ability to aid in tuning and show trends in performance under somewhat "real-world" conditions. Environmental variables are taken into account automatically, I do not think tire diameter is a variable as it affects how much power is getting to the road is. Look at this user manual: http://montana.dynojet.com/production/car/cuser.pdf

Hope I answered some of your questions
Dante Oliverio

[CamB]

There was a thread a few weeks ago where Richard Nelson had had his car run on a dyno which looked to be the same model as the one I had my car run on.

I have a 901 gearbox and showed 11-12% transmission loss. Richard has a 915 and showed something like 14%. This makes intuitive sense as the 915 is bigger/heavier etc. These were (apparently) measured as beepbeep suggests (the car was left to run down front max revs, which drew another line on the graph).

I would expect front engine cars to have more still.

http://www.pelicanparts.com/cgi-bin/vb/showthread.php?s=&threadid=61178&highlight=dyno+AND+bosch

http://www.pelicanparts.com/cgi-bin/vb/showthread.php?s=&threadid=61152&highlight=dyno+AND+911t

[stevelb]

Tim:

HP and torque are always equal at 5252 rpm.

HP=torqe x rpm/5252


Dyno's measure torque not horsepower. Horsepower is a mathematical derivative of torque and is a measure of the rate at which work is done.

HP is defined as 33,000 ft-lb (force) per minute

one crank rotation is equal to 2pi or 6.2832

[c2turbo]

Hey Wayne I have started a site to collect dyno and performance runs if you are intrested

www.c2turbo.com

[widebody911]

is that

HP = Torque * (RPM / 5252)

or

HP = (Torque * RPM) / 5252

Originally posted by stevelb
HP and torque are always equal at 5252 rpm.
HP=torqe x rpm/5252

[Wayne 962]

It doesn't matter. Multiplication and division are not associative. You end up with the same result either way.

Unless you were joking...



-Wayne

[Streckfu's951]

Originally posted by widebody911
is that

HP = Torque * (RPM / 5252)

or

HP = (Torque * RPM) / 5252

HP= (TQ * RPM)/5252

[donporfi]

Dynos measure only torque and speed.
In the past there were only brake dynos, and dynos were referred to as brakes.
Now there are dynos that can motor the engine, this to measure friction losses or simulate different road conditions. This motoring can be done with hydrostatic machines, electric motors (DC and AC).
There is a big difference on engine dynos and chassis dynos.

Ambient conditions affect the reading.
In order to compare readings from different dynos you need to add to your calculation formulae compensation for:
Altitude, humidity, intake air temperature, barometric pressure, exhaust back pressure etc.
There are different ways of testing: SAE, DIN, JIC etc. it depends what accessories the engine has when the test is done and how the test is done and how you correct for the a.m. conditions.
It depends of the quality of the dyno what reading error you get, most reading error is calculated considering the full scale reading, so if you test a small engine on a large dyno you can get a larger error.
The calibration of a dyno should be performed with certified weights, as in reality dynos are balances and have a theoretical lever arm to give a torque reading. The speed reading is obtained from a speed sensor usually a proximity sensor and a 60 toothed disc (similar to the sensors in the 80s Carreras or the crankfire ignitions).

There are dynos for different applications, a durability dyno is very different to a emmisions testing dyno (lab testing).
Regarding chassis dynos there are small roll dynos and large roll dynos. 12 inch rolls and rolls that can we up to 12 feet in diam. Larger rolls are preffered, on long runs the tires can get hot and even fail due to deflection ( small roll dynos use two parallel rolls to accomodate the tires, having two contact points with the tire).

Gasoline is a factor also, development labs in the auto industry use a certified gasoline to do their tests.

There are "el cheapo" dynos and "state of the art" dynos. In many cases the less expensive part of a test cell is the dyno, a well equiped test cell can cost even millions of dollars. All the auxiliary equipment can be very expensive.
There is no comparison on a workshop dyno and a lab dyno.

There is a very good book regarding engine testing, I am not sure of the title. I will post it tomorrow, in this book all these questions are answered

Industrial dynos are manufactured by companies like, Schenck Pegasus, GE, Siemens, Froude etc.
Workshop dynos are supplied by Mustang, Superflow, Dynojet, Bosh etc.

[UH1Charlie]

Last week Eurotech in Framingham, Ma. held a informative dyno session on an Dynapack Chassis Dynamometer. After a techinical presentation, a Q&A session followed. Then each car made two runs the newest car was a 2006 model with 6000 miles and the oldest my '77' Carrera at 96,000 miles. We could actually see and hear cars transition to detonation under load, the effect of Varioram induction kicking in, and different cam profiles on torque and horsepower curves. Very cool. Here's a link to Dynapack web site in New Zealand. Enjoy.

http://www.dynapack.com/products/

[Jeff Alton]

There is now way to determine driveline loss accurately. Even if you put an engine on an engine dyno and then put it in a car right away and put that car on the chassis dyno, you still would not know. You have two seperate dyno's calculating the hp and torque and that right there is too big of a variable. You also can't say "my engine puts out "X" number of HP according the manufacturer so when it puts "Y" amount to the wheels on this here dyno I calculate the loss to be "Z" %" Why, becuase you do not know what the HP and torgue of the motor really was, the factory number is just a number within a certain variance of what the tested engines produced.

Cheers

[porschetech]

This may help you.

http://germanmotorcars.com/Dyno_load%20vs%20inertia_1.htm

[H20911]

Many things are compensated for. As far as mech advantage of lower gears (higher final drive ratio). The dyno will comp for that because it will accel. faster but have topped out quik. So the time ran in the range will be less.
In other words a car that has (old chevy) 4.56 gears will not show better than a car with 3.73 gears all else being equal.

As side note I am not a fan of roll on dynos because they slip and you can come off if not strapped tight enough.

I have a nice Mpeg of a buick grand national that is smoking the tires on the roller so I know that the final numbers are off.

Here is mine on the rack. Did it run? No, because the rear bumper was off and the guy at the shop did not feel good at running wide open in 4th gear with out good cross strapping to hold it on.






I did not feel good about it either.
The next time if I can't bolt to the axle I won't go.

[Grady Clay]

Wayne,

I’ll work on a detailed post in the next few days. Too many docs tomorrow.

I have the basic, inexpensive “shop” engine dyno that I turned into a research instrument. This was in the analog days of ’69 to ’86. It was a big deal when I paid $600 for a single channel digital Type K thermocouple. It will live again with current digital technology and my long experience.

I was able to get better than 0.5% of reading repeatability. Around 1.0% of reading absolute accuracy. Less than 0.2% of reading detectable differences with statistical analysis. Of course I kept the readings in the top 25% of full scale.

All this took a lot of work controlling the engine and the measurement situation. I’ll post details this week. Anyone with something between the ears can do this. You don’t need a multi-$M test cell, just patients, control over the conditions and good procedures. You also need some reliable “mule” engines to perfect your test procedures.

While this is an engine dyno designed for 911s, chassis dynos are another good research and measurement tool. So is track testing. Each has its place. In combination you can really make performance improvements. The “Unfair Advantage”.


Thanks for starting this thread – much needed.

Best,
Grady

[Grady Clay]

Well, dumb 'ol me.

I think it is still worth revisiting. Let's bring it up to date.

Best,
Grady

[H20911]

oops.

[Wayne 962]

Yup, 2 books written since then!



-Wayne

[donporfi]

If somebody is still interested.
The title of the book:
Engine Testing Theory and Practice
Michael Plint and Anthony Martyr
Butterworth Heinemann Publisher