The Truth About Carmaker Engine Power Claims
That engine power figure you read in the specs: Where does it really come from? How is it determined? Is there any guarantee?
And now - with the clock on the clubhouse wall nudging ‘jihad o’clock’ let us spin the bullshit shredder to ‘puree’ and turn this well-intentioned (but still bullshit) question into the dung-based smoothie it so deserves to be derided for:
“I am a retired automotive teacher which I did for 35 years, covering Cert 2, 3 and 4 in the automotive trade. There is one aspect that you do not mention when comparing engine power and torque. Some manufacturers measure this at the flywheel and some at the rear wheels. This can vastly different due to power used for the drive train.” - Frank
Well, Frank, I hate to be the bearer of bad tidings, but that’s utter bullshit. All official carmaker engine power determinations are measured at the engine output shaft - on an engine dyno. Not on a chassis dyno. There have been dozens of different engineering standards for doing this - I’ll draw you a map in just a sec. But first…
THE URBAN MYTH
This ‘at the wheels’ fallacy is a favourite - I note - historically, among apologists. I get this more from Isuzu Ute apologists, who, for the past several models, needed a plausible excuse to justify the comparatively lame performance of the previous model. This was the best they could come up with.
Broadly, 15 per cent of the engine’s peak power is lost to friction, getting to to the wheels from the crank. So 300 kilowatts/horsepower (whatever) at the crank is about 250 at the wheels (ballpark estimate). But manufacturers all measure power at the crank.
WHY CARMAKERS USE CRANKSHAFT POWER
One good reason for doing this is: It’s a bigger number - so the marketing department approves. Another cracker of a reason is that if manual and auto transmissions are available, you’d get different power figures at the wheels - inconveniently. Despite having the same engine.
WHAT POWER IS
Power is typically measured in either imperial horsepower or kilowatts. One imperial horsepower is 746 watts, or 0.746 kilowatts. Simple. It’s also 550 foot-pounds of torque per second.
One kilowatt is 1000 Newton-metres per second. Or one kilonewton-metre per second.
For more on this, watch the video (right) or check out this report >>
If you want to know more about how this affects performance, check out power versus torque for peak performance >>
If you’ve ever seen the Germans quote their power figures in PS (which is an abbreviation of the German word for ‘horsepower’ - pferdestarke - that’s a thing called metric horsepower. It’s 735 watts, or about 98.6 per cent of an imperial horsepower. So multiply PS by 0.986 and you get normal horsepower. (I said 'divide' instead of multiply in the video - my bad for winging it. Sorry.)
That’s how the units work - but the standards are important too. We engineers use standards so that everybody does the test the same way. It’s like a recipe. So we all bake the same cake. And the most common recipe is the SAE standard - for Society of Automotive Engineers - and there’s about 10 flavours of the SAE standard.
BRAKE HORSEPOWER. Back in muscle car days, people talked about BHP - or brake horsepower. That was the SAE standard for gross power at the time. Basically it tested the engine without the ancillaries - like the alternator, radiator fan and water pump. Sometimes they used headers instead of the stock exhaust manifolds.
BHP went away in the early 1970s and was replaced by SAE standard J1349 - which measured net power by including all of the power-consuming accessories like the water pump, etc., and also the emissions controls, exhaust system and air cleaner.
People say emissions control killed power production in the 1970s - partly true - but it’s also because they changed the power measurement standard, and the figures dropped.
SAE CERTIFIED POWER. In 2005 they tightened up the measurement standard again, with a thing called ‘SAE Certified Power’ which basically meant if a carmaker wanted to claim SAE Certification that they had to do it in an accredited lab, witnessed by an accredited third party. Some engines went up in power as a result and some went down. Go figure.
Visit the official
SAE Certified Power website >>
They also tightened the rules to prevent carmakers cooking the books by running the test engine low on oil to reduce internal losses. And the fuel used was also controlled.
Basically, manufacturers can’t even cook the books on power production by feeding the engine nice cold air - because correction factors are mandated for pressure, humidity and temperature to standardise the results everywhere from Sweden to Dubai.
There’s a GERMAN STANDARD for power as well - it’s called DIN (D-I-N for German Institute for Standardisation) - basically very similar to the SAE net power standard. Again, it’s measured at the crank. All the ancillaries are connected. Power is expressed in PS.
There’s an outdated CUNA STANDARD from Italy, which mandated the use of all accessories essential to the engine’s running (like the water pump) but allowed others (like the alternator and radiator fan) to be omitted.
There’s a bunch of EUROPEAN STANDARDS as well - ECE and EEC standards - that might as well be reflections of the German DIN standard, with ‘hair and makeup’ differences only. There are ISO standards and Japanese standards as well - all measured at the crank. Minor technical differences only.
So, in short, if you are a carmaker, there have been 100 different recipes for baking the cake of engine power. But they all involve dining at the crankshaft. None of them involve determining engine power by measuring it at the wheels.
Wikipedia has an excellent entry on the secret life of horsepower determination >>
POWER AT THE WHEELS
In practise, you can measure engine power by parking your car on a chassis dyno, measuring the power at the wheels and reverse-engineering the power at the crank by fudging the 15 per cent transmission loss backwards, upstream.
That’s how they do it (right).
This is good for doing aftermarket tuning - because it means you don’t have to pull the engine out every time you change something, just to test the power. Because that would be iteratively nauseating, I’m sure you agree.
Carmakers universally quote power at the crank. None quote power at the wheels. All power figures are determined by standardised tests - and there is some minor variation in the standards, but not enough to make a substantial difference to users.
What gobsmacks me here is that a guy can teach automotive whatever for 35 years and not even know this. (Because: ‘Straya!) Christ knows how many young minds he might have poisoned with this bullshit claim, however unwittingly, in that time.
So - good safety tip: Should you wish to apologise for the crap engine performance of your car, you cannot do this by claiming it’s measured at the wheels and everyone else’s isn’t so yours is just as big. Unless of course you are speaking with uninformed idiots.