What are the REAL advantages and disadvantages of each "type" of Dynamometer System? 

Inertial or Braked?

While there are many different types of Dynamometer out there,  as far as we are concerned, to keep things simple, only two are relevant.

Most of this page applies to both the ENGINE DYNO and also the CHASSIS DYNAMOMETER as  both be either a "braked" type of Dynamometer or an "inertia" or "inertial" Dynamometer. 

 In some cases they may be both at the same time!
 

Advantages and disadvantages of a modern computerised braked dynamometer system:

• They / the operator can vary the load so that the engine can be held at constant speeds while such things as ignition and fuelling can be adjusted to optimum values at different steady state throttle openings and rpm's - extremely useful!  Easier to fully set up a new engine management or carb system quickly and accurately.  For this reason alone these types of Dynamometers are preferable for setting up and tuning road cars and moderately tuned vehicles where you are not just looking for "ultimate" power.
   

• Unfortunately most braked dynamometers seem to use dual rollers on each wheel.  These "trap" the tyre, cause slippage, heat, HP losses that vary, and can cause tyre problems. A single drum per wheel of 16 inch diameter or greater is far superior, as in the picture above.

• Can be dangerous to certain very high performance, boosted, nitrous injected, or tuned two stroke type engines as they are already on the edge of detonation and heat damage.  This mostly depends on the experience of the operator.  Trying to hold a 450bhp turbocharged nitrous injected bike at steady full power is a recipe for an engine meltdown!  In the real world its not a problem because they would be going at say 170mph in 8 seconds and then shut off!  The correct ignition timing and fuel mixture for these types of vehicles is generally not suitable for continuous steady state non accelerating conditions.
   

• Because of the fact that they CAN be calibrated, and in fact need to be calibrated from time to time, means that they can never be totally accurate. And that an owner or operator can "ahem" re-calibrate it themselves!   Never trust a tuner to give you reliable figures!   Most are honest but some are not!  Its only too easy to make the results "look good" if you see what I mean...
   

• Because of the fact that, the rollers are "wrong" in layout, and size on most of these, and because they use a dc voltage signal from a load cell connected to the "brake" to determine roller torque (torque x rpm or speed gives them the BHP) which in itself has a level of Hysterisis, and because this low level DC output  is subject to electrical noise & variable resistance due to temperatures, errors from the analogue to digital conversion process etc, they can NEVER be completely accurate.  Or in fact very repeatable. (unless the whole process is done digitally, which I have yet to see.)

Advantages and disadvantages of a modern computerised inertial or inertia dynamometer system:

• 1st a word of warning...  Because the principal of an inertia dyno is so simple there are rather a lot of really badly designed systems out there.  Its hard to know which ones are good well designed systems, and which are practically useless.  For a few clues click HERE
   

• There is NO calibration ever required or possible on a good properly designed (not all are!) inertial dynamometer system.   My own systems for example used a free running solid steel turned drum of 402mm in diameter x 562mm wide.  All of the dynos I produced used this exact same drum.  The inertial values were calculated mathematically from the density of the steel and the technical drawings, and integrated directly into the software.  There are simply no external user parameters that could be changed to alter the "calibration".  There simply is no need.   So there can be no errors, or calibration requirements.  If a healthy car / bike reads 56bhp on one dyno, then it will read this exact same figure on all of the others!  Its just maths.   Unlike braked systems there is no no DC voltages, nothing can change, no variations due to Hysterisis etc.   Companies such as DynoJet do the same thing.   In fact I took my own motorcycle to ten different DynoJet dynamometers in the UK over a period of a couple of weeks, and it / they read between 135 and 136 BHP on all of them, as well as on my own dynamometer systems.  Nice to know your maths is the same as theres is!  You simply cannot do this with any  normal braked dynamometer systems,  as they vary quite wildly due to the above and one or two other minor things like they all require Calibrating!
   

• They are dynamic testing dynamometers only!  This means that when you open the throttle the drum always accelerates.  It is not possible to hold a steady "under load" rpm.  So making changes to fuel and ignition maps is much more difficult. It is less good at this kind of thing than a braked dynamometer is.  If your main job is to set up fuel systems across the range, for e.g., rather than absolute accuracy and repeatability you are better off using a braked dynamometer.
   

• For deadly accurate back to back power runs when testing things like different lubricants, plug gap changes, different ignition coils, tiny jetting or fuelling changes, then they are superb!  Its even possible to easily and repeatable see the effect of turning the lights on during a run!  Try that on a braked dyno and the small difference is lost in the "noise" of the system.  So for ultimate performance tuning, bikes, drag racing, etc they are preferable. 

For example...

On this bike below!  It was mine, but it does not like braked dynos (or rather having its rpm held at a constant speed) due to the sheer amount of nitrous being used in two stages.  It detonates a little after being held at a constant 6,000 rpm for a short period.  We were trying to get a fix on the mixture at this point, but this takes a good few seconds to stabilize.  In real life it does not see 6000rpm for more than a second on the strip.  So the heat build up on plugs / valves etc does not get time to start it detonating!   After 6000rpm - no problem. We know this because a knock sensor and a data logger tells me so...  Retarding ignition more at this point, or adding a touch more fuel stops this happening.  But the bike is then measurably slower!    So for some situations a dynamic run climbing through the RPM's is very important.  Whilst some braked dynos can also do rpm "climbing" runs, it defeats the object of using one  because then you may as well use a nominally more accurate inertial dyno as you can't check the mixture at any given RPM any more!

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