Dyno Figures

Hi GB and Wayne-o,

It's not uncommon for the Kw and torque figures quoted for a motor to be calculated without any ancillaries (Alternator, power steering pump, air con or even viscous fan) connected while the motor is run up on the test bed dyno.

Hang a couple of G/boxes (Main & Transfer) a couple of diffs, a couple of drive shafts and all the extra's as above and it's not hard to see where all the power and torque has gone, before it gets to the rear wheels. (All those bearings and all that friction).

My thoughts are; It's not often that more than about 65-70% of developed power gets to the wheels.

Disco.

There is no rule of thumb that really applies.
About 30% for a 4wd in 2wd.
Have seen some figures that suggest a good road car 2wd might be 20%.
One of the 4wd magazines did a test on a 4wd dyno running a 4wd in 2wd and 4wd, was about 30% loss in 2wd and 40% loss in 4wd, due to the extra (front) diff losses.
As disco says, some car companies produce rated power figures at the flywheel without ancillaries connected. The japanese were famous for this.
Most European cars have a DIN rated power figure that includes some ancillaries.
So I suppose the dyno people measure something that has a little more real world relevance, power at the back wheel.
But then it is really only for comparison between before and after tuning and cars of the same model and spec.
A car with a top gear which is not 1:1 , or different diff ratio, or different tyre size will show a different power figure at the rear wheel, even with the same engine. Air temp on the day makes a difference.
Does the car get you to where you want to go, be happy :o)

Wayne

If you loose 30% off the motors figures you add 42% to the driving wheel figures to get back to the motors figures.

PeterD

Hi All,
Just to clarify a point; there is NO constant figure that can be used to calculate ENGINE Horsepower/Kw from the dyno figure at the rear WHEELS.
Similarly NO constant to determine Power at the WHEELS given the stated ENGINE power.

There are too many variables: ie, power steering, Alternator load, aircon on/aircon off, state of tune/condition of engine, gearboxes and diffs, etc.

The best you can do is to rely on handbook for engine power and use Dyno to see what you get at the wheels.
Two similar vehicles may well return differing results on the same Dyno.

Trust this clarfies the situation.

Disco.

Gday Robin
One would need a "tapley decelerometer" to do the job.
Murray

Could do Murray - however i have 2 other approaches , first I wrote some software which plots acceleration from speedo pulses on a Patrol - but it only worked under windows 98 , so now I record the acceleration on my GPS in 1 sec increments and work from there - however this showed that my $1350 Unichip did nothing much so maybe it was a dumb idea.

Robin Miller

Robin
You better have that "chip" with the fish tonight.