I have pondered this testing thing many times and now I think I have a good way to measure aerodynamic or engine performance/efficiency or rolling resistance mods:
First you need to know your cars exact basic data points like.
- Weigth with driver and fuel, more accurate data the better results will come
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Frontal area
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Cd and Crr values when you begin to do modifications.
Then its wise to make a basic fuel consumption chart with
Aerodynamic & rolling resistance calculator - EcoModder.com
There will be your basic data which you will use in your tests in the future. I have generally used just my fuel consumpion gauge, but when you go under 2.5 liters range or 100+ mpg the changes you see have huge effect to values and so many things effects those results that just by doing onroad testing its very hard to be able to see the changes effect to your car or fuel consumption, but I think I have a better way.
For example if you are doing tire testing you should do it at fairly low speeds where the rolling resistance has higher effect to fuel consumption. This can be done at many levels depending on your skills and aims and equiptment you have for testing purposes.
Tire testing
1. easy way.
- find "level" straigth road
- build a stopper under the gas pedal so your speed is limited to 40 km/h 24,8 MPH with 2nd or third gear on that straigth. So you driving with constant throttle CT
- Measure the average engine rpm on that road over some distance so put marks on the road. For rpm measurement its wise to have digital rpm data available
- you can also measure average time on that straigth with regular stopwatch but for that use you might need longer road to see effects, but then again the road does not necessary have to be "flat"
- Basicly this is constant throttle CT its max and you have limited the amount of fuel engine will inject. If speed rpm increses you have less resictance on your tires. From that time and also distance you can calculate the actual avg speed. You can now calculate the crr from your online tool data if your speed increases with same amount of fuel injected. What crr value gives the same fuel consumption.
Ofcourse the tires also effect aerodynamics and you can do the test with higher speeds to get also the aero effect visible but its more difficult. Generally you want to find a tire that has least rolling resistance and maybe test few with high speed if you can see the effects also there which is best. You use low gears because then you can see more clearly how much the rpm changes. 1% change is 1500 rpm is 1515 vs 1% @ 3500rpm is 3535 for example.
With this test you arent looking the effect to fuel consumption directly as you can use the calculator.
2. More scientific way
If you are able to measure the fuel injection amount at that is really injected via VAG com etc you can then check does the injection value stay the same if you have the stopper installed.
If it stays then you dont have any problems but sometimes it don`t depending on the cars design criterias etc.
In that case you need to alter your fuel consumption maps from the ecu this aint easy but propably can be done and I will test it next summer with my A8 project. So you limit fuel injected that under no conditions the engine will only inject that amount of fuel you want. So basicly you are limiting the max power to certain level like you do in flat out accelerations but this time for different purpose.
If you have that vag com cable you can measure thing like injecter fuel or the avg rmp or avg speed directly by logging data. This way its not so important to have straight level road and you can make the test lets say at 4 km long test route. So measure again still avg rpm and speed and that time. results are calculated with that same online fuel consumption tool.
aerodynamics testing
That was for tire testing but if you change the aerodynamics then you should use little bit higher speed something like 70 km/h 43,5 MPH to have bigger aero effect.
For this you want to use your 3rd or 4th gear for same purposes high rpm more accuracy on your results. You don`t want to test at higher speeds because aerodynamic drag will increase with so high steps that you wont see mods effect easily on the speed or rpm or the time travelled unless you have done major modifications but you want to see even those smaller mods effect to your car as 10 small good changes can have that 1-2% better FE. So every mod counts to overall results.
When calculating the results now just play with the Cd value to get your desired results. now you are propably wondering where you get those HP data point as the tool only gives them at 5 mph steps between 5-200 mph and 1 mph steps between 40-65 MPH. You need to fill those data points to exel and make some lines Where you pick up the speed and follow that to hp level. Someone with exel skills could make instructions I am not best at that :/.
You can use this method also for oil, tire pressures, injection timing or any other modifications etc to see which setup is best for your purpose. Ofcourse here you should do the ABA testing to rule out weather etc effects to results
Could this work in action?