Dynamometer (or “Dyno”) testing After reading hundreds of pages on the internet (some of which were interesting and informative and some scarily inaccurate or damn right lies) I decided to gather some facts together, “facts” I hear you say, yes facts i.e. they have been verified by reputable chassis and engine dyno owners/operators. Before we go any further we need to understand what we are measuring as the term BHP is a number derived from two others, Torque and RPM For years engineers, engine builders and then motor manufacturers relied on direct results to sell their products however humans like to quantify everything so there arose the need for a single number to indicate performance (James Watt was using a De-Pony brake load on his engines years before) Torque is the engineering standard (the tendancy for "force" to rotate around and axis) but this does not give any indication of engine speed so BHP was born And that’s all BHP is it’s the ability of an engine to deliver a specific torque at a certain rpm theres even a formula BHP = (Torque x rpm)/5252 That’s great so now we have a single number that allows us to say that engine A is better than engine B…….NO WE DON’T A single number gives no indication what so ever of the drivability of the car in relation to specific environments, for example. Engine A, gives out 500hp but only between 5500 and 6000 rpm, the rest of the time it’s a flat as a pancake. Engine B, gives out 500hp between 5500 and 6000 BUT between 4000 and 6500 it doesn’t drop below 400hp Engine A is great to brag about Engine B wins the race Dyno types 1. The chassis dyno (the most common type) This is a set of rollers (single or double) usually sunk into a concrete floor that allows a cars drive wheels to sit either on top of or in-between, the theory being that when the engine turns the wheels it in turn drives the rollers, if a known load were applied to the rollers then the “force” to turn them could be calculated. 2. Hub dynos This is a setup whereby the drive wheels are removed and a force sensing device is connected To each hub, these devices are then electronically linked to a central computer. Once again the theory is the same as above; if a known load is applied then the “force” to turn them can be calculated. 3. Engine dyno This is the serious end of the market, setups 1 and 2 above are designed to measure the power with the engine in situe, the engine dyno measures the engine power with it out of the car the theory being that this removes external influences, allows the engine to be situated in a controlled environment and returns true, credible engine “power” Each type has its own strengths and weaknesses The Chassis and hub dynos are quick, convenient and negate the need to remove the engine from the car, they are however susceptible to a number of factors that can render the results inaccurate (these I will cover later on) The Engine dyno is as accurate as you can get however the process is time consuming and expensive ranging from several hundreds to thousands of pounds depending on what is required Purpose of dyno testing To gain a measure of power output in order to ensure that build, fuelling, mechanical and electrical timing are optimum. Without dyno testing performance modification is a guessing game. Scope of “dyno” testing A dyno is a “comparative” measuring device, it is next to useless when used out of context in the pub or on a forum, this is due to many factors, the two most common are RWHP versus FWHP and dyno operators….I will discuss these later on. Aside from component choice, the internal combustion engine is susceptible to a number of external factors that can influence power such as fuel, temperature, pressure, humidity and skill/integrity of the operator. All of these should be considered when testing is commenced, a engine dyno will commonly be in an environmentally controlled room where temperature, pressure and humidity can be controlled or recorded Chassis and hub dynos will vary, some I have been to are, and some are not, either way the operator must still record temperature and pressure for the purpose of “correcting” the final result. General Acuracy Outside of the motor manufacturers, there is no enforced legislation that insists that aftermarket engine testing dynos (types 1, 2 or 3) are calibrated or inspected; operator training levels range from expert to amature, accuracy relies on the professional honesty of the owner/operator. Its worthwhile reiterating that a dyno is a comparative tool and as such when considering all things, runs by the same car on different days can be several % out. Runs by the same car on different dynos can be 10% out (witnessed 1st hand) Coast down This is a phase (on a chassis dyno) whereby the operator will at peak power, drop the car into neutral and coast to a stop, the computer can then calculate the resistance and add a figure to the results, this coast down can be the cause of a number of errors if done wrong. The effect of atmospherics Atmospheric conditions can play a big role in affecting engine performance For example On an NA engine of 3 litre, the difference in power output between a day where pressure is low to a high pressure day can be 10 HP The same engine will see a power drop when the humidity is high due to displaced air. Raise the temperature by a few degrees and power will fall once again due to displaced air Take a warm summers day just before or after rain and you may have a mixture of all three, you aint never going to break records then. This is one reason why drag racers (amateur and professional) look at the weather I keep records of all of my runs against weather conditions at the time, and if the weather is against me I won’t get too hung up when I run a poor time. RWHP vs FWHP RWHP is “power” measured at the wheels this is a direct measurement of usable power FWHP is “power” either derived or measured (in the case of an engine dyno) that represents direct power output from the engine prior to any drive train losses (typically 15-18% on a rwd car) Drive train losses are made up of gearbox (gear loss and oil drag), diff (angular loss, gear loss and oil drag), driveshaft’s (angular change) and tyres (deformation and friction), most of these are frictional related with tyres being the greatest Confusion of the two often results in accidental or deliberate miss-quoting, for example if I chassis dyno my car and it yields 200 RWHP (which is “around” 240 FWHP) and I accidentally miss-quote the later figure as RWHP then those participating in the bar stool racing game will be deluded into thinking your engine is more powerful than it is. Don’t believe me well I have actually heard it, it goes something like this Frank “went to the dyno yesterday and we made 1000 HP” (it was a skyline of course) John “wow that was great, was that RWHP or FWHP?” Frank “Err…….RWHP, yep RWHP” (it so happens it was flywheel hp) John “Crickey that’s 1150 HP at the fly, you must be well pleased” Frank has now dug a hole for himself and cannot get out of it easily so he agrees; this also has the convenient side effect of making him look a legend in the process. If Franks ever asked to post up the graphs then in this instance photoshop is his best friend. Heres a few handy rules of thumb for a RWD car to allow you to spot some dyno bullshit Ask about the peak torque figure (where and how much) Ask about the horsepower/torque cross over point (hint, its always at the same point) To get a rough guess of FWHP take your RWHP, add 10hp then divide by 0.88, this will get you in the ball park. This power differential does not exist on an engine dyno as we are directly measuring crank horsepower with a load cell and exists to a lesser degree on a hub dyno as there are no tyre losses. Operator error or worse All dynos can suffer from operator error or manipulation the most common on the chassis or hub dyno being the miscalculation of a coast down figure after a power run, this is a process where the car is taken to full power and at peak the clutch is dipped and the car left to “coast” down to a point that the computer can calculate the differential losses, maybe I should clarify. Scenario 1 I take my car for a dyno run and during the coast down the operator lightly applies the brakes, computer sees larger than normal rolling resistance and hey presto your calculated flywheel figure is huge. Scenario 2 I take my car to a dyno and during the coast down figure the operator jumps out, the computer sees a fluctuation in load and it skews the flywheel figure, I have witnessed this 1st hand. Scenario 3 The operator does a coast down and then when the cars at a rest he ignores the figure and makes one up (30 hp) and adds it to the result. Once again, I have witnessed this 1st hand. Scenario 4 All, any or none of the above plus the operator puts in the wrong atmospheric values…. Once again, I have witnessed this 1st hand. Why would a dyno operator influence the figures, well it could be accidental or it could be that fact that a happy customer is likely to go straight onto www.VWcorsaxopug.com and discuss till the cows come home their 500 brake horsepower 1.6 bean can exhaust shopping trolley, after which a hoard of baseball capped McDonald employees will descend upon said dyno demanding similar results and paying £75 a pop per power run. So beware Figures If the dyno does not have a computer attached and is just a big dial, drive away, the results are of no use at all to anyone other than [people playing numbers games I once went to a highly recommended dyno near Leicester, it was a dismal dark wooden shed with a great big dial, all I got was a number and no understanding of where or when power was made…….totally and utterly useless. I also went to another dyno near Cambridge run by a well respected mini driver; once again it had a big dial with no ability to tell me where my engine was making power.....wasted trip. Participation Always try and get your hands dirty, some dynos (Wilcox etc) will not allow this due to (made up) health and safety issues, however many will allow you to get close and understand what is going on, after all their doing it to your car. My favourite dyno allows me to do the power runs. Look at your results IN CONTEXT, beware of numbers that look big, get to know and talk to the operator. Use the results to check tune not to publish in a pissing competition. The two chassis dynos I trust emphatically are Dave Walkers (EmeraldM3d.com) (Dave is an engine God), and Thurston engineering near MJP eastern auto, (Pete is a highly skilled, extremely knowledgeable and all round top bloke), neither has any interest in inflated figures and will tell it like it is, both will give you power at the wheels as well as power at the fly (calc) and both will reliably correct for atmospherics....plus both serve coffee and biscuits occasionally. And finally horsepower figures never ever won a race, you can troll all day long, giving it "HP this" and "Torque that" but at the end of the day pink slips talk and bullsh1t walks.