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I wrote this for another forum so I hope it answers some of your questions. This is in regards to inertia dynos. Load dynos are different. Larger tires have more mass to accelerate so they will show less horsepower.
I have seen a lot of posts from members unsatisfied with dyno numbers and dynos in general so I thought I would write up my experiences with them and what I have seen.
Some background first. I was involved very early with chassis dynos for motorcycles and automobiles. I have well over 10,000 runs over 10 years of running. I have dynoed up to 60 cars in one day at events.
I also have experience with engine dynos. I have been involved with several successful racing endeavors in NASCAR, BAJA, Road Racing, Bonneville and NHRA.
I was directly involved in dyno testing for all three NASCAR divisions.
I no longer am involved in any way, shape or form in vehicle testing, racing or tuning. I do not represent any manufacturer or sanctioning body of any kind. I work as a Maintenance Manager at a chemical plant in Las Vegas.
This is not a “Why dyno’s are better than the Track” thread. If you are not going to look at this with open eyes then you might want to go to the track because nothing here I say will convince you, nor will I try. All I will do is explain what I know and have observed firsthand from my experience. I don’t want this to become a “Track vs. Dyno” thread and will not entertain that direction.
That out of the way, I will explain what I know and what my experiences have been.
This post will be broken up into 4 parts:
Theory, Dyno construction, Dyno practices and Common Dyno questions.
There are two main ways to measure horsepower or torque. Depending on which one you measure, the other is calculated from the first.
Load has been used since the very first for measuring torque. If you apply a load to an engine and stop it from accelerating you can measure thetorque available required to hold speed (or RPM) at that point. This is how engine dynos do it. They hold the engine at a specific point and read a load cell. Using the formula: HP=TQ*RPM/5252 you arrive at a horsepower number; horsepower is not actually measured… its calculated.
Inertia testing and the need for Chassis Dyno testing came later. Some more background as I know it:
The first commercial inertia chassis dyno that I know of was created for the purpose of tuning motorcycles, specifically vacuum slide carburetors. Since they are dependant on rate of acceleration and work off of vacuum they proved to be very hard to tune under steady state load.
Street testing was very difficult due to the speeds involved and ever increasing police presence. The inertia dyno idea was born. If you take a bike out and time a run in the 1 to 1 gear you can put it on a rotating drum and add mass until you get the same run time. If you can accurately measure time, mass of the drum and its circumference you have all the components needed to measure horsepower.
Force = Mass x Acceleration
If you know the mass, time of the run and change in acceleration you can measure horsepower. But F=MxA is not the end of the equation. Force has to be converted into HP by the equation HP= Force x Distance/ Time then divide by 550 (hp is the force required to move 550 lbs 1 foot in 1 second or 330,000 feet per minute).If you plot this against time you have a power curve.
A requirement to do this type of testing is accurate measurement of time. This is where the computer comes in. Inertia was only a theory until we had a way to measure time accurately.
So if you know the distance the drum travels in 1 revolution (circumference), the mass equivalent being rotated (since the drum rotates the mass rotated is not the mass static) and time you have all the parts to measure horsepower.
Notice that RPM and torque are not mentioned. You do not need either to measure horsepower. But to calculate torque you need RPM. An inductive pick up is used to measure RPM. From there you enter RPM into the equation and you get torque. Torque is part of the process as the force on the dyno drum is torque but it is not engine torque but wheel torque. Wheel torque in first gear can be in the thousands of pounds at the wheels due to gear ratio. This torque is measured indirectly as work in the equation work=force x distance.
Because we are measuring acceleration we don't need a brake to hold a steady RPM or speed. We just "Let 'er eat" or go full throttle. As long as there are no physical restrictions in the linkage this is a very repeatable state.
That’s it. No other measurements are needed for inertia. But how do we compare results in Las Vegas to results in Indy or North Carolina? Enter the correction factor.
The correction factor uses a standardized set of conditions to compare a run at different conditions.
If you look at the standards we are concerned with they are:
SAE= 77 degrees 29.234 inhg 0% humidity
STD or STP = 60 degrees 29.92 inhg 0% humidity
If you make a run under those conditions you will see a correction factor of 1.00. This is a multiplier. As conditions improve or degrade you will see the multiplier add or subtract power from all your numbers.
This is what makes comparing vehicles in varying conditions possible. This is the same as Density Altitude correction that drag racers use for the most part.
So to summarize, if you can time the run, know the mass equivalent and circumference of the drum then you can measure horsepower. Simplicity usually equals repeatability.
A quick aside about repeatability vs. accuracy:
The experiment theory is basically measuring your variable against your control. The more you control, the more accurate your results. But as you get to smaller and smaller measurements it gets harder to repeat your results.
If I measure a bore and get 4.065 and then measure it again I may get 4.067. If I measure only to inches the results repeat at 4. But is that good enough? No engine builder will agree. But if my measuring equipment is a pair of calipers and not a bore mic what will my results be?
In our case the vehicle we are testing is the variable. When tuning a carbureted vehicle you can get 1 to 3 hp repeatability because there is little adjustment potential. Timing curve is fixed as well as fueling.
Enter the ECU or PCM. Now timing is variable as well as fueling. Depending on a myriad of variables you will see power change to best support what the ECU wants to accomplish. This is not always compatible with the highest horsepower! Now you have a whole new set of changes to your test subject. This means that 1 to 3 horsepower is the range you may see from run to run if not more as the computer adjusts and your load value changes due to temperature and other variables.
The higher degree of accuracy the less likely we can control all the variables and the less likely we are to repeat to a consistent number.
Just some things that can change a reading are: What gear you complete the run in, engine temp, trans temp, rear diff temp, atmospheric conditions, heat soak, state of tune, voltage, tire size, tire pressure, wheel weight…
The more things we control the more repeatable our numbers. Trying to repeat to 1 to 3 horsepower is unlikely due to the sheer number of variables. But if we can repeat within 3 to 5 horsepower we can measure a change above that number. More on this subject in Dyno Practices.
Load vs. Inertia
Why inertia? Because there are fewer variables and they are simpler to operate. They also reduce set up time and that means greater efficiency in testing. More cars in a day = More money for the shop. And not only that, it’s far less stressful on the car being run; this becomes especially important as the dyno pull count for a particular car increases.
Inertia needs fewer instruments to measure and they are not subject to calibration. You only need Mass, Circumference, a way of measuring time and a way of counting drum rotation.
There are no control loops to try and hold a vehicle at a steady state. This is a difficult thing to do as you must stabilize the RPM or speed before you can measure it.
Since both styles measure different values under different test conditions the results are not comparable. I have spent many years trying to make an engine dyno correlate to what the chassis dyno showed. I even wrote acceleration test profiles for the engine dyno that simulate what I saw on a chassis dyno run. I never had a match or even a ball park. Comparing Dynamic motion to Static motion just doesn’t compare easily.
Since inertia has fewer variables you will see higher repeatability. No one I know will say an engine dyno repeats within a couple horsepower without an extremely skilled operator. Since all you are doing on a chassis dyno is driving the car there is less need for the skills required of an engine dyno operator. You still need skills and I will cover that later.
Where load excels is in tuning fuel injection. Most EFI systems use tables for the ecu to look at for what fuel and timing values are needed based on load and vehicle conditions. You hold the vehicle at a preset rpm and adjust a/f and timing until you show the greatest torque output. Lather, rinse repeat and you have tuned a fuel and timing map. (This is greatly simplified but should give you the idea).
Inertia only hits so many places on the map so you will not be able to get to those other values. Since the drums only model a set amount of rotational inertia you may end up having a lighter load on the vehicle. If you set timing to this load you will find quite a bit of detonation on the street when real world conditions are added in.
The best dyno is one that has both worlds. Inertia for overall testing of the finished product and Load to get the maps right. You still only need inertia for test comparisons but you need load for tuning.
There are 2 types of chassis dynos:
Twin roller and single roller chassis dynos.
The twin roller chassis dyno was the original. Clayton is the first brand that comes to mind. In use the vehicle is driven over the rollers and the tire sits between the front and rear roller. It is essentially “pinched” between the rollers. The vehicle is run up to a certain RPM and a brake is applied to the rollers to stop the vehicle from accelerating. Two types of brakes are used. Water brake uses an impeller and water flow to provide a braking force against the drum. This is the choice for higher horsepower vehicles as it is hard to control the water flow needed for lower power braking. Eddy Current retarders are used in lower horsepower applications and work by current inducing a magnetic field to slow rotation (eddy current retarders are not my specialty so if I have that wrong understand that the principle is to provide a braking force).
Measurement is taken by a load cell. The load cell measures force from the rollers trying to slow down the tires. It measures this torque and calculates horsepower. Load cells need to be calibrated periodically but newer styles have a much greater ability to maintain calibration. My rule of thumb was to calibrate based on what info I could afford to lose. I calibrated weekly. Nothing is worse than testing and finding an anomaly and finding out that the load cell had drifted.
The twin roller has a problem in that by placing the tire between the two rollers it wants to climb the front roller. Standard practice is to tie down the vehicle in a vertical direction to place as much force as possible to combat this tendency. This places a greater load on the vehicle and can vary the power measurements considerably. It can also lead to premature tire failure as you have created two stress points at two small contact patches on the tire. This forces the internal belts to conform to the smaller radius and puts stresses in the tire carcass that it was never meant to comply with. Advances have been made in this arena but I have no practical experience in this area so I will leave that to those more knowledgeable than me.
To be continued...
Last edited by seanol; 01-30-2014 at 06:06 PM.