Since we’ve been talking more about European cars this year, we have been making frequent references to “DIN” power ratings. We wanted to be sure everybody is clear on what that means.
DIN is short for Deutsche Industrienorm (German industry standard), a standard issued by the German national institute for standardization, now called Deutsches Institut für Normung and abbreviated DIN. Among other things, the agency sets standards for how the horsepower and torque of automobile engines should be measured: Deutsche Industrienorm 70020.
As you might imagine, the standards of DIN 70020 are specific and very strict. Unlike the SAE gross standards used in the U.S. and Great Britain for many years, the DIN standard requires power to be measured with standard intake, exhaust, and accessory systems in place. DIN horsepower ratings, therefore, are comparable to the modern SAE net rating system, although they are usually reported in metric horsepower (sometimes referred to as Pferdestärke, the German word for horsepower, and abbreviated PS), rather than mechanical horsepower. One mechanical horsepower is about 745.7 watts, whereas one metric horsepower is about 735.5 watts; therefore, 1 PS equals 0.986 horsepower. Because of that and because DIN horsepower ratings are calculated with the engine in “as-installed” condition, they are always lower than gross ratings. (The peak engine speeds for DIN power and torque ratings are often lower, as well, reflecting the effects that mufflers, air cleaners, and accessories have on the engine’s power curve.)
Until the early 1970s, it was not uncommon for cars sold worldwide to have two power ratings — one DIN, one gross — largely as a concession to the salespeople. A Porsche 356 Super 90 engine, for example, was rated 90 hp DIN, but 102 hp SAE gross. Adding to the fun was the fact that, as we’ve previously discussed, SAE gross numbers sometimes bore very little relationship to actual output. This did NOT necessarily mean that the U.S. or British engines were more powerful than their European counterparts, simply that their ratings were figured differently.
So, before you write us to complain that we have maligned your favorite car by saying it had less power than advertised, please consider the source, and whether that source was quoting SAE gross, SAE net, or DIN figures.
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I got into a big argument with a guy (yes, online) about whether cars in the UK are rated at the wheels or the crank.
I said crank, but he claimed companies’ advertised ratings in bhp were at the wheels.
Do you know the answer?
Every other rating I’ve seen is for the engine, at the crank, with various test conditions (hence gross, net, or DIN). I’d prefer the published data were at the wheels, as it’d keep manufacturers more honest as to available power after drivetrain losses.
I have never seen any manufacturer officially list wheel horsepower; it’s always at the flywheel. (There were erroneous reports in 1971-72, when net ratings came into use, that net ratings were at the drive wheels, but that’s not correct.)
I think mostly it comes down to the fact that the numbers would be a lot less impressive. It would also illustrate the comparative efficiencies of different transmissions, which is something I suspect a lot of manufacturers would prefer to avoid if they could.
Although this makes sense to me (in GB or UK) after reading a few other articles, there’s one way that it doesn’t: I’ve never seen SAE used in British cars, & as far as I know, UK has normally used DIN for measuring torque & power. The units were usually lb ft & HP respectively (& I understood HP to be the same as bhp or PS, but am not pro at that). These could be converted by calculation into metric units Nm & kW, but the difference with SAE wasn’t simply units. Japanese cars didn’t use DIN – before 2000, their HP data was usually higher than British / European cars. Maybe that was using the old SAE standards, but I’m not sure.
There are three separate points here: British practice, units vs. methodology, and Japanese practice. First, the U.K. adopted DIN ratings after joining the EEC in 1972. Prior to that, you had a weird mixture of SAE gross, “net” (seldom clearly defined, but probably SAE gross recalculated with some allowance for accessories and mufflers/exhaust silencers), and DIN ratings. Even as late as 1975, some British-market cars still cited SAE gross numbers (bizarre, since even the U.S. had abandoned them by then), but by the late ’70s, DIN methodology prevailed. So, it depends somewhat when you’re talking about.
Second, the different rating standards (SAE, DIN, ISO, CUNA, JIS) are distinct from the units involved. The standards describe how the testing must be conducted (including what accessories must be attached) and parameters for calculation. (An important one is a correction factor for atmospheric conditions; each standard has correction factors for air temperature and pressure, but they differ somewhat.) Once you’ve calculated power and torque under a particular standard, you can convert the units from Imperial to metric or vice versa, but that doesn’t actually change the standard. The conversion is simple arithmetic, just as using a calculator to convert your thermometer’s Celsius readings to Fahrenheit doesn’t alter how your thermometer works.
That said, units are a big headache, in large part because mechanical horsepower and metric horsepower are ALMOST but not quite the same: one metric horsepower is about 0.9863 mechanical horsepower. So, it’s often a pain to know which a source is talking about. British publications are awful in this regard because they frequently insist on using “bhp” to describe both units. Some editors will also take it upon themselves to try to convert one to the other, not always correctly and usually without mentioning that they’ve done so. So, you might see some sources talking about a car with 150 PS DIN (150 mechanical horsepower) as 150 bhp and others describing it as 148 bhp. (A few will erroneously describe it as 146 bhp, which is what happens if you accidentally convert the units twice, or as 152 bhp, which is what happens if you convert in the wrong direction.)
Third, we have different regional practice. Generally, modern exporters quote output in whatever standard is in place in the local market. Japanese automakers use the JIS standards, which remained gross output until the mid-80s and then began a gradual and erratic shift to JIS net, today usually helpfully appended with values in kW as well as PS. However, in the U.K. and other EEC countries, Japanese automakers have used DIN ratings since at least 1980, although you still occasionally have the unit confusion mentioned above. I’m not sufficiently motivated to read through EU regulations to see if DIN ratings are a type approval requirement, but that would be my guess. In the U.S., California law explicitly requires automakers to advertise only SAE net ratings, so U.S.-market cars now all do so.
I hope that helps. This is a subject that often fall into the “needless confusion” category!
Can someone explain what din 6271 states? It is to do with reciprocating engine which i think calls for an engine of standard build to not exceed 10% of its rated bhp but im not entirely sure
I’m not that familiar with DIN standards, and I can only read a few words of German, but as best I can tell, it’s a supplementary standard related to DIN ISO 3046-1 and ISO 15550 (which govern how power, fuel consumption, and oil consumption are to be measured), establishing additional rules regarding allowable manufacturing tolerances. Any specific example of a given engine may be a little more or less powerful due to manufacturing differences, but for the DIN power rating to be valid, actual output can’t differ from the rating by more than a certain amount (assuming the standard reference conditions specified in the 3046-1 and 15550 standards, such as air temperature and humidity). For a more detailed answer, I’m afraid you’d need to get a German speaker to read the full text of the standard.