Contents -1. Ensuring You Get The Right Horsepower Figures
1. Rolling Roads - Dynamomenters - Ensuring You Get The Right Horsepower Figures
going to the rolling road make sure your vehicle is in good mechanical
order - ao no oil or water leaks, and tyre pressures that are correct.
When putting the car on the rollers it should be secured then test run to set the rollers for the road speed using a gear as near to 1 to 1 as possible usually fourth. Then program in air temperature baromic pressure and the method of losses you are going to use. If you are tuning the car for more power always use the same rollers before and after so no matter how the rolling road is set up, it should show if you've gained or lost power.
Below we list different ways a rolling road can be set up to give different readings at the wheels:
Running the car in a lower gear
say third instead of fourth - this will give a higher wheel bhp reading.
High tyre pressures
Less friction will give higher bhp at the wheels.
Setting up the rollers for hotter air temperature and lower baromic pressure
This will give a higher correction factor, and thus affect the calculated flywhel bhp reading.
Adjustment for drivetrain losses.
There are three ways to calculate losses. Percentages, run-down and fixed. A TVR S6 standard 4.0 litre has a maximum power of 342 bhp @ 6500 rpm, with a wheel bhp 302 bhp so we know the losses are 40 bhp. Run-down losses are 30 bhp which we can say would give 332 bhp at the fly wheel. Percentage losses using the same 302 bhp at the wheels figure plus 15% gives us a near-enough 347 bhp at the flywheel - but use 20% and this will give a reading of 362 bhp at the flywheel.
Which is where we have a problem. If you build an engine with more bhp say 360 bhp at the wheels, and the dyno is set up correctly using the percentage method at 15 % this will yield a flywheel bhp of 414 bhp. All well and good. Now, if you add 20 % this will read 432 bhp, which in our opinion is a nonsense because cross-checking this reading against the known 40 bhp of drivetrain losses gives a flywheel bhp that would in fact under 400 bhp!
Graphs that give separate bhp and torque with no other information this can be confusing to you as a customer and lack of information. The best graphs should have bhp and torque scales on both sides of the graph, and also show flywheel and torque crossing at around 5252 rpm, with a third graph line to show with wheel bhp. The chart should also state any variable factors set by the operator, so you know the losses dialled in using with air temperature and baromic pressure corrections. All the above can be programed into most rolling roads as can the method of losses.
Remember, its the wheel BHP that counts - so if you see somewhere in print or web dyno figures that look too good to be true, use the information we've just given you in this section to draw your own conclusions.. They just might be....
2. Rolling Roads - Dynamometers - Inertia or Steady State?It has bothered us for some time why wheel horsepower figures on some rolling roads cannot be replicated on others by a big margin. To explain how this can be so, it is worthwhile bearing in mind that there are two main types of rolling road dynamometers in use today, each of which has its own unique set of operatonal characteristics.
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