Thers no way you can replicate road conditions on the Dyno, ie wind resistance..
Fact.
As for the AFR reading, simple Q I hope H20RBO can answer, not one of his muckers.
Read below, taken from Andy Forrest,
There are obvious advantages and disadvantages to having your engine tuned on a bench dyno. However, the same can be said for rolling roads.
Both are mapped at fixed rpm intervals. Whilst this allows an approximate setting, real life conditions can not be achieved because in reality the engine is accelerating at a dynamic state. This means that the engine load is transient and not performing at a constant steady rate. A speed derived (constant state) map will be slower on the road than a map optimized to the cars actual acceleration rate (although the rolling road derived map may well produce bigger numbers on the rollers). A car optimized on the rollers will only run well "on rollers" and ironically not on the road. Rolling roads and engine dynos are great to get the general shape of the fuel and ignition maps. This will not provide a final working map.
All race cars are usually mapped on an engine dyno, or some may even use a rolling road and a wind tunnel, to get as close to road conditions as possible. However, all race teams use track testing for their final working maps. Actually, top teams throw a lot of resources into track testing. This is one reason why we tune our turbo cars on the road, mimicking real life temperature and conditions. However, I have developed all my maps on a rolling road. If the car is new it will require a completely different approach. The advantage to my particular approach is that the maps are tailored the to suit the particular needs of the car. I have no individual preference for either rolling road or dyno and I have used both methods as well as different manufactures and throughout the UK and in Europe.
Rolling road power figures can vary for the following reasons:
Variable loading and acceleration rates
Varying effectiveness of cooling systems
Different diameter rollers
Different surface on rollers
Compensation factor applied
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.
2. There are no rolling roads that can properly simulate the airflow normally achieved on the open road through the intercooler and radiator. 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. producing flow through it. 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.
3. The diameter of the rollers and there distance apart will have an effect on the heat generated at the tyre interface. This requires power. A rolling road will calculate the rolling losses during the run down phase of the power run. This however only measures the drag on the tyres under zero engine load conditions. No rolling road software can calculate the increased loss occurring when the tyres are fully loaded. This is dependent on the individual tyre size, profile, construction and compound.
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 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.
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 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.
Therefore, a degree of caution should be taken when mapping solely on the rollers. Also if the car is getting checked on the road, after it has been mapped on the rolling road, then the figures achieved on the rolling road will be different. I personally think the best option is to get the general fueling curve on the rolling road and get the 80% of the mapping on the rolling road then getting the map finished on the road. A final run should be done after. So as you can see this is very time consuming to be done for every car and requires 2 dyno sessions not one. Unfortunately, all rolling roads can be played with and the results can be tampered with. so even the before and after rolling road figures can be of very little significance in some circumstances.
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 and it starts to become pointless to compare results between cars.