BarkingMad

Nearly registered under a different name before asking that question!

What I am really wanting to know is how does a dyno cope with measuring vehicles with different gear ratios...

Logically if you are running a car with longer gears the speed will be higher but the power less?

dowser

Two stages; power run and coast down. The coast down measures the transmission loss - the calcualtion then gives you flywheel power.

PAW's will always be different - gear has an impact as you mentioned, but so does tyre pressure...if you want impressive paw figures, pump the buggers up

Richard

Edited to add: but also be aware the rolling roads are flawed; a road with poor cooling may still be OK for a car running standard boost, but a mod'ed car has generally been taken to the limit of what it's intercooler can do.....dumping it on a rolling road with one crappy fan pushing air under the car instead of onto the i/c can give the knock correction systems a pretty good workout

[Edited by dowser - 11/22/2002 6:21:03 AM]

BarkingMad

Ok, thanks for that Richard... makes sense!

Edited to ask: Does pumping the tyres up really make that much of a difference?

[Edited by BarkingMad - 11/22/2002 8:59:34 AM]

nom

There's twice as many contact patches as on-road (instead of base only there's sort of front & back), and the 'pincer' effect of the rollers means, especially with high power cars, a lot of energy can go into distorting the tyres around this shape. Especially 'not good' because the friction is different on the coast measurement for friction loss - the losses are lower on the coast than the run. So the reported BHP reading will be lower.
High pressure tyres minimise the effects of the 'twin roller'.

hypoluxa

The ‘coast down’ loss is not a real transmission loss. During the coastdown the engine is no longer connected to the drivetrain. Think about it. This measurement is irrelevant as their is no input to be lost!

Transmission losses will rise as engine power rises, although their are a few components in the transmission that have fixed losses and won't increase in proportion to the power increasing, so a higher powered car will have slightly lower transmission losses than a lower powered car. Gearing also plays a big factor in these calculations, the longer the gear the higher this ‘coastdown loss’ will be and consequently the more impressive your flywheel figure will be.

Some Dynos let you programme in the % transmission loss. Now this is a very easy way of getting big flywheel figures, simply input a higher %. True transmission losses are between 20 & 25% at the very most.

Other inaccuracies/variables include tyre slip/wheelspin - as nom mentions. Remember you are ‘driving’ on polished steel, not tarmac! also the tyre size and pressure. Tyres can make up to half the losses alone. Temperature correction can also give you some pretty strange results, if the temperature is measured next to the engine than the ‘ambient’ temps will obviously be much higher than what they really are. If the HP figures are then adjusted back to a standard or mean temp you will have have a higher BHP figure.

In other words they are rather unreliable.

BarkingMad

Thanks!

nom

Jase - could you drop me an email? Not sure if you're getting stuff from me

hypoluxa

Sorry Tom - haven't been able to access by mail (yahoo) since mid last week. Pain in the **** to say the least...

Down pipe will be with you Thursday. Might have gone a little overboard with the bubblewrap

Pavlo

Seeing as I used to work on them, I just thought I would point out how a dyno works in general terms.

For a "braking" dyno, think of a wheel (or disc, drum, whatever) and then apply a braking force to the wheel using a water brake, air brake, magnetic brake, friction brake etc. The braking effect will create a torque, which you want to measure.

So if you pivot the whole unit so it's free to rotate (a degree or 2 will be plenty) and restrain it with a load measuring device, such as a spring balance or electronic load cell, you can measure the reaction at known offset from the pivot point.

That will give you torque, speed you can measure on a car (ie ignition pickup on rolling road) or directly on the Dyno shaft (magnetic/optical sensor).

SPeed and torque give you all you need to know power output wise.

Other things to consider are what to do with all the power you absorb inthe brake, so air or liquid cooling of some sort is required. THen add a whole host of sensors and means of controlling it all.

Other method is the innertia Dyno, which uses the unit under test (engine, motor, vehicle etc) to accelerate a heavy rotating mass. The actual innertia can be measured by spinning up the mass and observing it's decelleration. Once this is known, measuring the acceleration whilst being driven will give you a measure of the torque involved, speed measured same as any other dyno.

Dno results are usually normallised so results can be compared from different days and ambient conditions, often reffered to as the correction factor.

All sorts of clever things can be done in software of computer controller dynos, such as road/race simulation including gear changes. Once you have fine and rapid control of the throttle, you can look at part throttle and throttle response etc.

Paul

nom

Thanks Jase