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Why Do Rolling Road Power Figures Vary From Place To Place?

We have all been there or read about it. You do a run on dyno X, a run on dyno Y and a run on dyno Z, and they all differ. This leads to people naturally preferring to use the one that reads highest – it’s an ego booster. However, it’s worth knowing what actually affects a dyno runs readings and why there is a procedure that MUST be followed, but quite often is not.

Rolling Road power figures

Dyno Correction Factors. The Biggest Problem.

Why do we need correction factors? Why not just tell us the power measured? Well, here is the problem. If you bring your car in on a nice crisp spring morning at 8am and we run it up first thing, the intake air temps are a chilly 6 deg C and she makes 150bhp. Perfect. You then go away, and come back at mid-day after doing nothing at all to the car, the suns out and the air temperatures are now 20c. Guess what... it makes 142bhp. What’s wrong? Is it broken? No. It’s just breathing warmer air, which is less dense. This is why we need correction factors. Those corrections re-align the figures to a standard temperature. Now, no matter what the temperature, the car will always read the same power.

But there is another small problem...

On some days there is also a higher or lower atmospheric pressure too!  (Look at a barometer) So when the pressure is high your engine will make more power and the dyno shows your 150 BHP engine is now 158 BHP.  So we now need another formula that corrects the measured power for atmospheric pressure variations too! This correction factor ensures that your engine will now read 150 BHP regardless of both air temperature, and atmospheric pressures. Finally there is humidity, which also adjusts power a little as the water molecules in the air replace oxygen so usually cost you power. (Some engines gain power though due to combustion efficiency)

Rolling Road power figures

To ensure all the correction factors are correctly applied, the above air temp, atmospheric pressure and humidity information must be entered into the dyno console before EVERY RUN.

This is rarely done and thus, the figures produced are simply wrong. The operator may have simply forgotten, but the output is still wrong. (Our system features an automated weather station that updates itself every run for that very reason)

Engine cooling airflow.

On the road we seem to average between 30 & 70 mph day to day, and that is the minimum airflow our vehicles are designed to operate within. Radiators, oil coolers, intercoolers, even air-boxes are all designed to have this much airflow forced at them. Next time you’re doing 50mph, stick your arm out of the window and ask yourself, is the dyno fan at tuner X providing that sort of pressure as a minimum? Very few are. Secondly, is it fresh cold ambient outside air being brought in, or just recycled workshop air just getting hotter and hotter as its forced through the hot engine parts and then slowly recycled around the room until it goes back through again, hotter than last time? This affects power. (Rising air intake temps should show this)

Cell Extraction Volume.

As mentioned above – if air is not removed from the cell at the same or higher rate than it’s sent through your engine, it will increase in temperature. Furthermore, any crankcase breathers etc that run to atmosphere, or exhaust leaks not being dealt with by exhaust extraction will be polluting that air with not only deadly, but power sapping fumes. The accepted standard for a high power dyno cell is to change the air in the cell 700 times per hour. That is a windy room! Was your last dyno run done in a cold windy room? Our ventilation system changes the air in the cell 900 times per hour. That’s once every 4 seconds! Our cell very rarely rises above the temperature of the car-park outside and when it does, the most we have seen is a 5 deg Celsius raise.

Exhaust Extraction Efficiency.

A very big problem. How many times have you stood in a dyno cell choking on the fumes from the exhaust? It is worth remembering that whilst the fumes are very lethal to humans, they are also going to do nothing but drop an engines power output if it’s also being forced to breathe them. (Lack of oxygen)

Rolling Road power figures

Tie Down Force And Tyre Pressures. Yet More Problems.

Did you know that every click of the tie down ratchet after “optimal” will cost you power? The harder we dig the tyre into the rollers, the harder they are to move and that costs us BHP. To make matters worse, if it’s too loose it can read higher power. You can’t win.

Tyre pressures? Want more power? Over-inflate the tyres.


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