Dynamometer Test Procedure - see also Dyno Tuning Tips

see the pictorial and the movie at the end of this page

PLEASE COPY ONLY IF YOU GIVE CREDIT.  THANKS.

Maybe you're asking what's going to happen when you bring your vehicle in for its first dyno test.  Maybe you're looking for medical procedures and you came across this page by accident.  Either way, I'm going to tell you what's going to happen to your vehicle.

First, I will explain to you what is going to happen and exactly what a dynamometer test is and what it does to your vehicle.  Then you are going to sign a waiver stating that I explained everything to your satisfaction and you understand exactly what a dynamometer test is and what your risks are.  Risks?  What risks?  Unless I forget to chock your wheels and/or strap your car down, the only risk to your vehicle is the same that you encounter on the street every time that you floor the accelerator pedal and hold it there until the tachometer nears the redline.  Something on your vehicle might break.  Just because it happens while your vehicle is being tested doesn't mean that the test broke your vehicle.  Remember, the dynamometer at Bristol Dyno is of an inertia-type, not a brake-type.  Brake dynos apply force against your engine and drivetrain and measure how strongly your vehicle fights back.  The drum of an inertia dyno is sized to approximate the forces that a typical vehicle encounters when it accelerates itself down the road.  After learning about the dynamometer test, if you refuse to sign the waiver, I will refuse to test your vehicle.

I will then inspect the general condition of your vehicle.  If it appears that your vehicle is likely to experience a failure when the accelerator is floored (if your vehicle is a piece of junk), I may not test it.

If we both decide that your vehicle is going to be tested, I will strap your vehicle securely to the floor and place chocks before and after the non-drive wheels.  Securing a vehicle to the floor can prove to be very difficult in some cases, so if you happen to know of some good points to hook onto your car, please let me know - it will save a lot of time for the both of us.  If your tire pressure looks or feels low, I will measure it.  If it is low, I will inflate it to the manufacturers recommended pressure.  Keep in mind, tire pressure may have a significant effect on the test results, just like it has on a drag race.  My reasons for checking tire pressure are for safety, not for achieving the optimum test results.  If you want the optimum results, you must research the best tire pressure on your own, I will only make recommendations based on my own knowledge and experience.

The next step is to open the hood and place an inductive pickup on one of your spark plug wires (or one of your coil wires.)  This is how the computer will measure your engine's rpm.  Lately, some auto designers have made spark plug wires very difficult to access and have hidden them entirely in many cases.  If you own such a vehicle, help me out and tell me the easiest way to access a wire.  If a special tool is needed to remove a cover, be sure to bring it.  If you want to be really nice, remove any covers before you arrive or while I am strapping the car down.

At some point, I will enter your vehicle's description, your name, and some other parameters into the dyno computer.  Parameters include the redline on your vehicle's tachometer and the speed rating on the tires to provide limits for the test.  Once the speed rating is entered, the dyno will not exceed that speed during the test, and I will most certainly not enter a false speed rating so that you can get a test result at a higher speed.  If you want the test to exceed 150 mph, make sure that you have at least Z-rated tires.  Keep in mind the fact that many cars will easily exceed 150 mph in top gear if the vehicle is not fighting against any air resistance.

Your vehicle can be tested in one gear, or in all of its gears.  Theoretically, the results should be the same regardless of what gear is used because the increased torque in the lower gears is cancelled by the decreased speeds.  In the real world, there are always differences between the gears, and the differences can be explained by a number of factors.  Because of nasty things like friction and inertia, different gears always have different amounts of drag, and the greater the reduction or increase in the gearbox, the greater the drag.  A low gear may fly the engine through the rpm range so quickly that it never has a chance to build up any helpful intake and exhaust resonance that those header engineers spend so much time trying to achieve.  Fifth gear may be spinning your drivetrain so quickly that significant increases in gearbox drag are created.  Third and fourth gears are usually favorites for finding your actual horsepower, and testing all of the gears is a useful method for discovering problems in your drivetrain.  Many dynamometer operators go right for the gear that is the closest to a 1:1 ratio as it has the lowest drag in the gearbox and therefore the highest horsepower reading (usually) on a vehicle that is properly sized to be tested on this dyno.  Of course, many cars don't have true 1:1 gear ratios, and many cars are not the exact proper size that the dyno was designed for, so blindly going for the gear ratio that is closest to 1:1 is not always the right answer.  My dyno is sized for vehicles that are around 3000 - 3500 lb.  A significantly lighter vehicle, even though it may have the same horsepower as a larger vehicle, will probably have a final gear ratio that is higher.  Therefore it will seem to struggle a lot more than a heavier car.  In such cases, the lower gears, like third, may give the highest horsepower reading.

If you have an automatic transmission, things get a little trickier.  During the test, the transmission can not shift, or the test results will be ruined.  Therefore, an rpm (or speed) must be chosen so that when I stomp on the gas, the transmission will not downshift.  In a typical street vehicle, this is around 60 mph with the OD turned off.  If you can disable the downshift function, either by removing the linkage or by electronic means, please do so, as the test will be performed over a much wider rpm range and you will obtain more data.

If the transmission shifts during a test (either a MT or an AT) the test will be invalid.

If you're truly serious about your vehicle's power development, you can rent the dyno by the hour and tune it in between or during runs.  In this case, you may opt for a Dynotrac Load Control test.  Dynotrac allows the user to hold the vehicle's engine at a certain rpm or wheel speed by using the variable pressure brake on the dyno.  This option allows steady state load testing and can accurately mimic a brake-type dyno by recording the pressure on the brake.  Dynotrac is also a safe alternative to tuning the vehicle on the street when one wishes to tune throttle position and load maps.

You may also elect to have some air / fuel metering done during a run.  This information will be plotted on a graph just like your horsepower and torque, and you can see how the air / fuel ratio changes with your engine rpm.  In most cases, the meter samples air coming out of your tail pipe.  I can also replace one of your vehicle's O₂ sensors with the one in the air / fuel meter, or if you're really serious, you've made an extra threaded hole in your exhaust system that my O₂ sensor will fit into.  In most cases, the tail pipe method works great.  I tested a vehicle using both methods and plotted the results HERE.  As one can see, the results are almost identical.  At low rpm (below 2800 with this vehicle) there is a discrepancy, as the engine is not producing enough exhaust gas to displace the atmospheric air, so the reading is falsely lean.  The rpm at which this happens varies depending on the engine's displacement and the size of the exhaust.  Most users are not too concerned about the air / fuel ratio at low rpm anyway, as heat buildup and risk of damage to the engine is much lower.  If you still wish to replace one of your O₂ sensors with mine, you need to make sure that your vehicle will function properly with the original sensor disconnected.  If it won't, you have to bring an "oxygen sensor simulator" to plug into your wiring harness that will trick your vehicle's ECU into thinking that everything is working properly.  Look for them on the internet.

So now you'll need to decide what kinds of runs you want to use during your test.  You have your choice of gears or a Dynotrac Load Control test.  If you just want to know what your horsepower is, go for three fourth gear runs.  If you have a lighter car, like a 2200 lb. Miata, choose third gear.  Three separate runs will allow you to weed out any anomalies and it will prove the repeatability of the equipment.  It will also allow your vehicle to warm up, and if you so choose - cool down.  If you want to do a quick diagnostic test of your drivetrain and see the differences between your gears, try a run in third, fourth, and fifth gear.  If you suspect problems with your drivetrain, ask about a Negative Horsepower test.  If you're a racer, engine builder, or tuner with distinct goals in mind and you know how to achieve them, rent the dyno and take advantage of the Dynotrac Load Control test.  Be prepared to pay more for the Negative and Dynotrac tests and the air / fuel metering, because they take more time and cause more wear on the equipment.  See the PRICING page for more information.

Once the test is complete, the computer "corrects" the data.  Environmental conditions such as temperature and atmospheric pressure have a significant effect on the horsepower that an internal combustion engine produces.  Fortunately, this effect is well understood and easy for a computer to account for.  In order to make useful horsepower comparisons, most test results are tweaked so that it appears that the test was conducted at a specific temperature, pressure, and humidity.  In the United States, the most common correction factor is SAE J1349.  This standard makes it seem that the test was conducted at 77^oF, 29.23 in. of Hg pressure, and a humidity of zero.  If you want your horsepower corrected to a different standard, such as EEC (popular in Japan), DIN (popular in Germany), or "standard corrected," you must let me know before I print the results.  Remember, the standards use different atmospheric conditions and they will all return different horsepower values.  "Standard corrected" is popular with aftermarket manufacturers because it uses a higher pressure and a lower temperature (denser air), thus inflating the horsepower numbers.  If you want bragging rights over your peers, make sure all of you use the same standard.  You certainly don't want to be seen whipping out your calculators before you go street racing.

You will receive a printout of your vehicle's test results which will consist of a graph plotting horsepower and torque vs. engine rpm (or speed, if you choose.)  You will receive a graph plotting your air / fuel ratio vs. rpm if you elected to do such a run, and you may also receive a text printout in tabular form of the hp and torque vs. rpm or speed for each run if you wish.

 

The car is driven in and the drive wheels are parked on the dyno drums

The non-drive wheels are chocked

An inductive pickup is clipped onto a spark plug wire (or coil wire) and a ground wire is attached

The non-drive end is strapped with two or more straps

The drive end is strapped with two or more straps to prevent lateral movement on the dyno

The mother of all fans is used to keep the vehicle from overheating - NOT to simulate speed

CLICK HERE TO SEE A MOVIE OF A DYNO RUN FOR A HI-BANDWIDTH MODEM

CLICK HERE TO SEE A MOVIE OF A DYNO RUN FOR A DIAL-UP MODEM