Performance Tuner Magazine Article TECH : Dyno Discrepancies Transcribed Because of Image Quality

Performance Tuner Magazine Article TECH : Dyno Discrepancies Transcribed because of image quality Subtitle : Think of a Number Performance is a numbers game. But just how reliable are those numbers when they’re produced on a rolling-road? We take one car to five different chassis dynamometers on the same day to answer that question. Story and photos : Peter Knivett Rolling-road thrashes : don’t you just love them? Gathering a group of similar machines together at some willing rolling road always makes for pretty interesting reading – and particularly the owner’s reactions after what can all too often be a disappointing power run. The textbook line is usually ‘I’m gutted, I thought it would make over 300bhp,’ when their pride and joy has actually made just 187 bhp. On the face of it, who’d argue with this disappointed punter’s line of thinking? After all, they’ve probably spent thousands of pounds on all the right tuning goodies, plus they ‘had a mate on an internet forum who’s told me it must be really quick, because his brother said so, and he worked at Prodrive once, and he knows what he’s talking about…’ Month in, month out, you see this sort of tale repeated across all sectors of the performance tuning market, and it never fails to amuse, because it’s a classic case of putting the component-expenditure cart before the testing horse. Despite melting his credit card by splashing out on tuning goodies our disappointed punter forgot to spend just 50-75 pounds having his car tested on a rolling-road, which could well have made a crucial difference to its ultimate performance. We know, because we’ve seen the benefits it can have on many occasions. Without rolling-road testing all power claims are simply guesses, some of them not particularly educated ones at that, because real-world testing always produces surprises, some less welcome than others. Then there’s the other story that you frequently read from these rolling- road thrashes, usually with an owner saying, ‘I’m really pleased, it made 20bhp more than XXX’s rollers last month.’ He sounds pleased, but he shouldn’t be, particularly if he hasn’t tweaked the car since his last rolling-road run, because as we’ll show you, comparing the power figures from two different chassis dynos is a futile exercise that generally leads only to confusion and ultimately disappointment. Given that Performance Tuner is still a new title, we’ve the opportunity to advise you on how not only to avoid the mistakes others make, but also to arm you with some valuable information that will set you ahead of the opposition. Measuring power outputs and increases correctly is a classic case in point. How do you go about it, when should you do it, and what should you be looking for? Read on and we’ll be delighted to enlighten you. WHAT IS BHP AND TORQUE? First of all, what are we actually measuring on a rolling road? It’s surprising just how few people know how properly to define what brake horsepower or torque is, partly because there’s some maths required. But hold on, before you flick the page, it’s pretty simple stuff that’s worth knowing. To understand what brake horsepower actually is, first you need to establish what torque is. Torque is the measurement of how much leverage, or turning force, an engine produces at the point where the work is really needed, namely the radius between the centre line of the motor’s crankshaft, rotates the flywheel, and ultimately provides the force that drives the car. Torque is measured in terms of pounds per foot, or the metric equivalent, Newton metres, and the amount of torque an engine makes is pretty much governed by two main factors; it’s internal design and how efficiently it breathes. TORQUE – that’s the goal Altering the internal layout of an engine isn’t easy, so the simplest way of improving a motor’s torque output is to increase how much fuel/air mixture it burns. At its simplest, that’s what just about every single tuning part sold on the market today aims to do – by increasing the air flow through the motor and adding the correct amount of fuel, you’ll raise the torque output. Trouble is, torque is viewed by most car nuts as power’s boring cousin, which is wide of the mark, because it’s really more important than power. It’s torque – not power – that accelerates your car down the road. Don’t believe me? Well, chew on this, in 10 years of testing tuned car this writer can say that the best machines I’ve driven have always had almost as much torque as power – if not more. This is also the reason that turbodiesel cars general have such punchy in-gear acceleration times. Still doubtful? The 2.0-litre turbo motors used in World Rally Championship cars produce an often quoted 320bhp, but they also produce around 475 ft/lb of torque, because rallying is all about acceleration from low engine speeds between corners. Crucially, though, maximising torque is extremely important because power – brake horsepower – is a by product of torque WHAT IS BRAKE HORSEPOWER? When people talk about an engine ‘power’ they’re referring to how much brake horsepower the motor produces. This bhp is a unit of measurement that was invented in the 19th century to grade the power output of steam engines. And before anyone had cars, it made sense to compare the power of the new engines to the amount of work that a horse could do in one minute, which was calculated to be 33,000 lb/ft of torque – a combination of force, distance and time. Now for the maths bit. Because torque is ultimately measured at the engine’s flywheel, we need to calculate the circumference of the flywheel to work out the distance the flywheel has turned using good old pi – but here’s the simplified, cut-out-and keep equation: BRAKE HORSEPOWER = TORQUE x ENGINE SPEED IN RPM Clearly, engine speed has a large effect on bhp output, which means that increasing revolutions per minute is a surefire way of increasing power. Is that why a Honda Civic Type R produces 195bhp from 2.0L and revs to 8500 rpm? Could be ... but if that’s left you completely baffled, here’s a simple description in abstract terms – thick of bhp as the measure of how much work an engine can ultimate do, and torque as a measure of how quickly it can do it. Keep that in mind and you won’t go far wrong. HOW ARE BHP AND TORQUE MEASURED ON A ROLLING ROAD? A rolling-road (or chassis dynamometer) is a piece of equipment that enables an engine’s brake horsepower and torque to be measured under load, without having to go to the trouble of removing the power unit from the car. Chassis dynos use rollers to act as the interface between the tyres and the measuring apparatus, although these days so-called ‘hub’ dynos are also becoming popular where the measuring equipment is bolted directly to the driven axle of the vehicle. The engine’s power output is measured by calculating the force that it can generate against the rollers (or hub pickups), which are loaded to create a resistance by one of two methods. The most common is the eddy-current dyno, which uses opposing electromagnets to provide a variable roller loading, but an alternative is an inertia dyno, which simply uses the mass (in other words, the weight) of high-inertia drums to provide a consistent, measureable resistance. WHEEL, HUB OR FLYWHEEL FIGURES? Arguments have long raged about whether it’s best to measure the power figures from either the flywheel or the road wheel, and now we have hub dynos muddying the waters too. The problem arises because precious horses are shed between the engine’s flywheel and the road wheels, thanks to the power-sapping nature of a car’s transmission, it’s also compounded by the convention that the majority of manufactures (perhaps not surprisingly) publish their power figures as measured from the flywheel, and so thanks to the transmission losses wheel outputs tend to sound a touch weedy in comparison. Sadly, short of stripping the engine ou7t of the chassis and running it on a bench-test dyno there isn’t a definitive answer to easily calculating transmission losses. The latest eddy current dynos use a ‘coast down’ process that seems fairly accurately to calculate losses, but this process isn’t uniformly reliable – some makes of dyno I’ve used have produced some very odd flywheel-loss numbers. Given that, I’d always rely on power figures measured at the wheels, or hubs, because they’re not open to debate or doubt indeed, this is standard practice among US tuners. As a rough guide a rear wheel drive car will lose 10-15%, a front wheel drive car will lose 15-20 per cent, and a four wheel drive car can lose up to 25% in transmission losses, but the variables involved (gear design, transmission oil qualities and so on) are vast, so these sums aren’t an exact science, hence why the use of wheel figures is preferable every time. CORRECTION FACTORS The internal combustion engine is sensitive to variations in air pressure and ambient temperature, so these will alter the power outputs from one day, or even hour, to the next.

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