Info Need on Dyno Types and Differences.
 

Info Need on Dyno Types and Differences.
Hi Guys,
I am doing my first Dyno after a rebuild with modifications.
I was not satisfied with the work and I requested the mechanic to run the car on a Dyno so we can see the actual engine performances so there is no misunderstandings.
Is there just one type of Dyno and are there any differences in the types of measurements. I want to be able to measure apples to apples when all is done. I look forward to your advice.
Thanks

Dynamometer – Dyno for short is defined by The Oxford English Dictionary as an instrument which measures the power output of an engine.

This definition is where all the fun begins – firstly and quite importantly Engine Power just can’t be measured – it can only be calculated from Torque and Speed normally lbsft and rpm.

If you want to measure the absolute power of an engine for an acceptance test then you will need a very well calibrated dyno and will also need to compensate for temperature, atmospheric pressure and humidity all which make it difficult to make comparisons between different test systems and even different days.

There is an ASME Code PTC 17 which defines the methods used for testing Reciprocating Internal Combustion Engines. I am sure there are also SAE Codes but I am unfamiliar with these.

If you are going to test the engine on a conventional Water or Eddy current brake you are likely to get an uncertainty of up to 3% depending on atmospheric conditions and I would think this is quite acceptable. (Gas Turbine test codes allow up to 5%). These types of dyno usually have a reasonable torque measuring system and within a few constraints are relatively repeatable.

The most modern engine dynos use AC motors to absorb and drive, the accuracy of this type of system can be better than 1%. They have very accurate and high cost torque and speed measurment along with Key Phasors which allow torsional vibrations to be measured. They are usually only used by F1, Indycar and the like due to operating cost.

If you are going to make measurements using a Chassis Dyno then absolute values and comparisons become increasingly difficult.

If you use an Inertia type dyno it will be incapable of holding load and all power figures will be derived from the rate at which the drum is accelerated and the formulae used are likely to differ with machines from different manufacturers.

Some chassis dynos use eddy current retarders and can hold load for short periods of time – the time is limited as few of chassis dynos have cooling systems to prevent the brake from overheating so it can only absorb for relatively short times. This type of chassis dyno commonly has a crude torque measuring system.

The issue with all these machines will be the condition of the bearings supporting the drum can have a significant influence on accuracy depending on the level of maintenance.

The condition and state of the torque measuring system is also a source of uncertainty. The strain gauge instrumentation used on Eddy Current dynos is often quite poor quality and has a ‘CAL’ button on the panel which is of little value. I have never seen any ‘Rolling Road’ with a valid Calibration Certificate.

I believe that chassis dynos can only provide ball park horsepower figures but they are useful for looking at differences and analysing the effect of changes.

Comparing differences between different dynos is not a great idea and can be very misleading.

I have to say that I wouldn’t accept a contract to build an engine with a specific performance guarantee if were going to be measured on a chassis dyno, I would insist it were tested on a well set up and maintained engine dyno and preferably using a water brake. (Kahn or similar)

With all the above said...They establish a base line for you and work from there.
I know out on the West Coast we had a thing called Dyno day...You will usually always get slightly a different number every time you run.
Lots a variables - Even more on a different day (O.A.T. / Bar press. and so on)

I used to use a road nearby for "dyno" work.
It was very simple...a known length of road...a stop watch...and run the car through the direct drive gear from X RPM to Y RPM.
Any improvement in time was on the good side.
And..if you had a buddy that also did the same run..on the same day..etc....you had a scale to work with (advertised HP and weight ) of his car.
My buddy had a Hemi Cuda at the time...and my 67 Ford was about 3 seconds quicker on the test strip (427 high rise with Crane cam).
Bob

I would agree with this concept but if you fit a good quality 3 axis Accelerometer on a rigid mount you will measure peak acceleration rtaher than the average and get a result that will give better discrimination of differences.

The latest devices have a cabaility of entering weight and approximating horsepower.

I don't think the 911 has any direct gear which also needs a bit more calculation but still quite useful.

I think you can get an accelerometer 'app' for an I Phone but I am not sure if they double integrate distance from a GPS signal and this could be quite inaccurate.

Chris:
Ok, what about something a bit more basic for it's figure of merit; like combustion analysis?

It seems like actually measuring the pressure-volume curve over the crank rotation would give a direct reading of bmep and thus an accurate HP calculation.

What I've never had answered is wither bmep is the same off load as with a load. If they are equal, all that's needed is a pressure probe.

tadd

I dont think that will work..,Say you are mapping A/F ratio through the RPM range..given load ETC, I think DYNO is the only way to go

There are only two types of Dynos
One for improving engines
One for selling engines

Chris...at the time I was testing the car....even 1 axis accelerometers were in the future...LOL
I know that "seat of the pants" and timing through distance is very rough...but it gave me the info I needed at the time.
BTW...at the end of a racing season...we had found the 427 Ford High Rise to need 5 different heat ranges and 3 different makes of spark plugs to ge tthe best results.
Crazy ...yes....but when you are searching for that last 5/100 of a second...the only answer.
When I sold the car...the notebook went with it....the new owner though I was crazy...and threw it away...poor him...never won a single class race as long as he owned the car.
Lesson to be learned here...go with what works....and if the previous owner gives you his "secret tuning" book....at least save it...you never know.
Bob

You wanted apples to apples. Now you can't get that because you don't have the "before" test at a known good calibrated dyno. You might have expected more of a butt dyno change and now wondering how much increase you actually have. I think we need more info. E.g. car type and what changes were installed. One thing a dyno can be good for is checking the afr at WOT, this is even suspect as calibration of sensors or sensor types are in question. If your mods were supposed to provide an additional 100hp, then that is another situation. That's my 2 cents or 1.5.

Tadd I don't think BMEP on load and off load are the same as when a dyno controller removes load the engine will race and over rev unless the throttle is backed way off.

I would agree that stop watch and time is a simple way of making comparisons and very cost effective.

I have been spoilt as I have always been able to 'borrow' instrumentation from my workplace.

I also agree that looking for 10HP is entirely different to looking for 100.

NY930,

Not sure exactly what you mean when you say you are going for a dyno... I have worked with a dyno shop for over 20 years now and can say that going for a dyno can mean lots of different things. Just taking a car or engine to a dyno shop and making a pull is not getting much bang for the buck. The key to going to a dyno session is to tune what you have to make the best possible power. I have seen "well tuned" motors see a 10% increase in power under the curve when tuned on a dyno. What is tuned? That is a better question, it is more than A/F ratio ignition timing it is anything you can adjust on the motor while at the dyno session.

By example if your ignition timing is set too low you lose power, if too high you lose power and possibly hurt parts. Too little timing shows as low power output across the range, too much kills power quickly if it drives the motor into detonation.

Too small or restrictive exhaust may start off well but will strangle the motor as RPM come up.

The Air / Fuel ration too rich or too lean is a problem, setting the A/F to "THE" number is another trap I see on the dyno, each motor has its own "best power" A/F ratio as measured by an A/F meter based on combustion efficiency and 20 plus other variables.

What I am getting at here is a dyno is a tool used to get the most from what you have. If you go to a dyno shop and do not tune the engine, making one change at a time, it is a waste of time.

Now, there is some damned good advice!!! :) :)

+Now, there is some damned good advice!!! :) :)

Best,
Grady