We have found that there is a wide variation in power output reported by different rolling roads

There are many reasons for this – here are just a few:
1. Variable loading and acceleration rates.
2. Varying effectiveness of cooling systems
3. Different diameter rollers
4. Different surface on rollers
5. Compensation factor applied
6. Operation of rollers

1. Most rolling roads control the acceleration of the rollers to a fixed rate. The slower the acceleration then the higher the effect on heat soak of intercoolers. This can reduce power. With a high acceleration rate then turbo spool can be affected resulting in a lower boost level than would normally be achieved on the road. Different gearing throughout model years will affect run up speed on some rollers. Consider that on the road the acceleration rate of the car will change relative to torque output. On the rollers it is held at a constant rate regardless of output.

2. There are no rolling roads that can properly simulate the airflow normally achieved on the open road through the intercooler, radiator and induction system. Proper cooling simulation would require a moving floor and variable speed fans (as found in professional wind tunnels). Some rolling roads do a better job than others, using high-powered centrifugal fans. Others just have a single axial flow low-pressure unit. On occasion this is just aiming at the intercooler and not actually producing any significant flow through it. Very few rolling roads cater for top-mounted intercoolers. Proper flow through the intercooler is critical in order to simulate normal running conditions a 10 degree increase in charge temperature can cause a considerable loss in power The airflow can also affect the MAF signal on cars fitted with an induction kit, this can lead to totally wrong air fuel ratios existing during the test and knock/detonation may be experienced.

3. The diameter of the rollers and their distance apart will have an effect on the heat generated at the tyre interface. This requires power. Some rolling roads will calculate the rolling losses during the run down phase of the power run. This however only measures the drag on the tyres and transmission under zero engine load conditions. No rolling road software can calculate the increased loss occurring when the tyres are fully loaded. This is dependant on the individual tyre size, profile, construction and compound. Some rolling roads don't even try to measure the rundown losses ! The DynoDynamics for example, when running in 'shoot' mode just uses a lookup figure from a table to estimate the losses ! Simply pump your tyres up to 50 psi and find yourself some bhp. It has been known for some rollers to produce 25bhp gains just from warming up the transmission and tyres over a few runs. Mike Wood from Prodrive experienced this recently when testing a WR1, the bhp increased by 20bhp from the first run to the final run, nothing was changed !

4. Surface finish of the rollers can also affect the power loss due to friction losses and tyre deformation. Smooth rollers are prone to allowing tyre slip on high output cars. This can either cause the bhp figure to rise or fall depending on the how the engine rpm is calculated. If rpm is calculated via the roller speed monitor then slip will generally cause measured bhp to fall, however as the engine torque reduces at higher rpm the rollers eventually “catch up” with the tyres and get an inertia “kick” as they synchronise speed. This normally shows as a blip of 20-50bhp in the power graph close to maximum rpm. If the engine rpm is read directly from the engine then slip will show as a considerable increase in displayed bhp.
This is due to rolling roads measuring torque x engine rpm. If the software thinks the same torque is produced at the higher (slip) rpm then the power figure is multiplied in direct proportion to the percentage of tyre slip occurring.

5. Most rolling roads “correct” the measured bhp to a din standard for atmospheric pressure and temperature (and some don’t bother!). The purpose of this correction factor is to allow for example a comparison between a five deg C winter day power run and a 25deg C summer day power run. The cooler air in winter being denser will (all else equal) produce more power than the 25 degree summer air. The din calculation compensates for the different air density and corrects all results to reflect what would have been achieved on a standard temperature and pressure day. This correction works well on normally aspirated cars but is inappropriate to turbo charged cars.The rollers which do not compensate are likely to read high results in winter and low results in summer. The din calculation looks at only air inlet temperature. There are however, two temperatures that affect a turbo cars output – air inlet temperature at the filter intake and air temperature after intercooler (charge temp) The air temperature at the filter does not affect the turbo car in the same way as it does in a non-turbo car. This is due to the turbochargers ability to partially compensate by spinning faster and still compressing the same amount of air regardless of temperature. There is however, a price to pay for this in increased exhaust gas back pressure and higher turbo discharge temperature which is dependant on density recovery in the intercooler.
The air temperature after the intercooler will normally be higher on a rolling road due to the reduced cooling airflow. Depending on the ECU this may or may not be compensated for in the fuel and ignition maps. Power output will be reduced as a result. It is not uncommon to see a 40 degree C rise in charge temperature on a rolling road run. This is not compensated for within the rolling road software.
Some ECU’s have there own internal compensations to adjust boost pressure in relation to atmospheric pressure and temperature. This also is not considered in the rolling road din correction factor and the two can either work together or against each other.

6. The rolling road operator is able to affect the power result either up or down. If the car is not fully up to temperature, particularly the exhaust system and turbo then power will be down. Also if the car is run a number of times in short duration then heat soak can also adversely affect results. If the clutch is only partially depressed or is binding slightly during overrun then the drag figure will be increased, this will increase the measured power output.

The above just skims some of the issues with rolling roads but hopefully gives an insight into why figures can vary so much from roller to roller, from car to car and from day to day.
It is our experience that a 15% variation exists across the UK’s rolling road facilities.
Now add to this the fact that you can gain up to 10% extra power depending upon the fuel/additives used (even a 3ml per litre dose of NF can add 5%) and it starts to become pointless to compare results between cars.
The table below shows the variation that may be claimed/achieved for similar specs depending upon which rollers/temperature/pressure/fuel/tyres etc etc. 
98 Octane average result UK Rollers Variation Increased octane

Span of 15% across Rolling Roads (+/- 7.5% from average)
+ up to 10% additional power using NF/Methanol/Race Fuel

Optimax Average result --- UK Rollers Variation --- +Increased octane 
--------- 300 bhp -------------278-322bhp-------------352bhp
--------- 320 bhp -------------296-344bhp------------ 378bhp

--------- 340 bhp -------------314-346bhp-------------399bhp
--------- 360 bhp -------------333-378bhp-------------423bhp
----------380 bhp -------------351-409bhp-------------446bhp
--------- 400 bhp -------------370-430bhp-------------470bhp
--------- 450 bhp -------------417-483bhp-------------528bhp
----------520 bhp -------------481-560bhp-------------611bhp

Tuning, Road and Rollers

Engines tuned on bench dynos or rolling roads are generally mapped at fixed rpm intervals. Whilst this allows an approximate setting to be achieved, real life conditions are always occurring not at static rpm but at a rate of acceleration. A fixed speed derived map will be slower on the road than a map optimised to the cars actual acceleration rate (although the rolling road derived map may well produce bigger numbers on the rollers).
This is one reason why we tune our turbo cars on the road under real life temperature and conditions.

Prior to Rolling Roading your car

Due to the vast differences in dyno power run duration and Subaru specific TMIC cooling set ups, I would recommend that anyone with significant upgrades should run some decent octane booster (such as NF)
The reason for this it to retain the ECU safety margin under potentially heavier loading conditions.

If the car is not overloaded, then this addition alone will not net you any more BHP but it will minimise the risk of your ECU retarding the timing which may then take some time to recover to normal running on the road.

Newage cars and AVCR's have gear recognition control of boost, ie they will target higher wastegate settings in a lower gear to achieve the boost target. The rpm/mph rise rate on the rollers will be different to the rate achieved on the road so some cars may overboost whilst others fail to achieve target boost. This is neither a fault with the car or the rolling road, its just a typical condition.

Cars with underbonnet induction kits would be advised to run with the bonnet down, the reason for this is that a direct blast of air from the cooling fan onto the filter can disturb the airflow around the MAF sensor, this can cause the car to over or under fuel.

Finally, ensure your transmission is fully warmed up by giving the car a 10 min run on the road shortly before your run is due, also pump your tyres up to the recommended level (ask the operator) this is particularly important on Dyno Dynamics rolling roads as the transmission drag is not actually measured, just estimated.

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