Dynamometer Facts and Fictions

Contents -

1. Ensuring You Get The Right Horsepower Figures

2. Inertia or Steady State?

3. True / Effective HP vs Dyno Jet HP

1. Rolling Roads -  Dynamomenters - Ensuring You Get The Right Horsepower Figures

Before 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!

Power graphs
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.

The first type is the 'Steady State', where the engine is set to work against either a water brake, or an electrically induced eddy current. These rolling roads measure torque at the wheels and convert it into  Wheel Horsepower.

Then there is the so-called 'Inertia' type (such as the Dyno Jet), where horsepower at the wheels is calculated from the rate of acceleration, with the torque figure being then converted from this. Compared to the steady state dynamometer, experience in the field indicates that the inertia type will give a lot higher BHP reading - sometimes by as much as 20 per cent.

As an aside, since the inertia type is measuring a load that is transient as opposed to constant, anything which is done to the internals of the engine to make it more responsive, will show up as an increase in power. Even though modifications such as the fitting of a lightened flywheel or a reduction in the weight of reciprocating or rotating masses within an engine may have no effect upon maximum constant horsepower in themselves.

3. True / Effective HP vs Dyno Jet HP

Good old Wikipedia gives a more succinct definition (though the page which originally held the relevant paragraphs appears to have been edited recently, but a copy of the missing key text can still be found on http://dictionary.sensagent.com), and we've extracted this here to save you having to wade through their whole entry on this topic -  though for people who want to get into more detail, it can be found HERE

--Sensagent/Wikipedia extract begins--

True / Effective horsepower (chassis dynamometer)

True Horsepower (THP), Effective Horsepower (EHP) and wheel horsepower (whp) are the brake horsepower converted to useful work. In the case of a road vehicle this is the power actually turned into forward motion as measured on a chassis dynamometer. Power available at the road is generally 10% to 20% less than the engine's actual bhp crankshaft rating due to vehicle related parasitic losses, much of which is due to the vehicle's rubber tires rather than true transmission losses. Aside from adding simple dyno parasitic drag, there are no additional power adding factors and power is corrected using appropriate atmospheric correction factors.

Dynojet horsepower / DJHP (chassis dynamometer)

Dynojet horsepower is a marketing hp scale that was created to read higher than True / Effective horsepower. They were successful in getting the magazines to accept the inflated numbers and are now the de facto standard for retail magazines. In the 100 djhp area, djhp is about 15% higher than True and in the 200 djhp, it's 18% to 20% higher. Most chassis dyno results that are published by magazines are djhp, causing confusion, as there is True hp, djhp and "estimated djhp". Commonly, the wide range of power sometimes reported is due to the error of djhp vs "estimated djhp" vs. the actual, unmodified True hp.DJHP is not "RWP" or "wheel power", it has its own factored up hp scale.Dynojet hp is always an inertia loaded sweep test.

Front-wheel drive cars (provided a transverse engine layout is used) suffer slightly lower coastdown losses due to the absence of the beveled crown and pinion gears used to change the drive direction in the back axle of a RWD car.

See Also
--Sensagent/Wikipedia extract ends--



Site content, images and technical information  provided on this site - unless otherwise noted  - are copyright RND Engineereing 2010. Reproduction of such content without permission prohibited. Specific engineering design changes to the AJP Speed Six  and other engines detailed upon this site, are the intellectual copyright of RND Engineering

Stumbled on this page through a search engine? Go to home page