I know that sometime in the late 60’s or early 70’s US car manufacturers changed how they measure the rated horsepower of an engine. For instance, back in 1965 one of the engines available in the Mustang was rated at 225 HP, and then again in the late 80’s the Mustang had another engine available also rated at 225 HP. However, the late 80’s version actually produced more power, the numbers are the same only because the rating method changed somewhere along the way.
Anyone have any details on this? What’s the difference between the two methods, why the switch, and how do the two methods compare? That is, what would 225 HP in 1965 terms equate to today?
I think that geepee might have provided part of the answer. I seem to remember reading somewhere (sorry, i don’t have a cite for this; maybe someone who knows more can confirm or deny) that the way of measuring horsepower was changed from indicated horsepower (the power produced within the engine’s cylinders) to brake horsepower (the power available at the shaft of an engine). This latter was thought to be a more reliable measure, as it gives the amount of power that is actually transferred from the engine to the running gear of the car. And because of friction etc. involved in the transfer of power down the line, brake horsepower is always going to be less than indicated horsepower.
To throw another twist into it there another common power measurement: wheel horsepower. Naturally, the power available at the wheels is less than that available at the crank.
It stands for British Horse Power. I don’t know if that is different from any other kind, but when I took Physics (thank you Mr. Housek for all the entertainment) it was 550 foot pounds of force per second for one horsepower. Meaning you can continue to move 550 pounds the distance of one foot in one second. Starting up is another matter, torque, a subject I never really understood (involving angular momentum, and I faked my answer on the final on it, and Mr. Housek probably figured that out). I do understand that if you are a real car buff, the torque rating is more studly than the h.p. rating as torque goes to ability to accelerate.
Actually, I think brake horsepower is measured at the drive wheels, by putting the vehicle on set of rollers which resist the wheels and measure the force. Thus, it includes losses from the driveline, exhaust system, etc.
The older method measured the horsepower of the engine at the flywheel, often without any accessories attached (water pumps, alternators, etc).
The rule of thumb is that pre-1972 cars had published horsepower ratings 16-18% higher than post-1972 cars.
I may have to dispute your dates a little bit, Sam. My 1968 Cadillac Eldorado’s horsepower was brake horsepower. So, at least some cars were measured using the “new” method back then.
BHP, as commonly used, stands for brake horsepower. This is measured with a device called a dynomometer (a dyno for short). The dyno ‘brakes’ the power source by providing a resistance, which allows you to calculate the bhp of the power source. BHP can be measured either at the crankshaft of an engine or at the wheels of the car. As far as I know, all car manufacturers publish bhp numbers measured at the crankshaft. This way, they get to inflate the numbers a little bit because they can ignore losses in the drivetrain.
IHP, indicated horsepower, is generally calculated from the physical specifications of an engine. This number is always overly optimistic because it ignores not only losses in the drivetrain, but losses due to accessories (like waterpump, AC, alternator, etc) as well. I believe that pre-1972 cars either used this calculation, or sinmply measured horsepower without those accesories attached (as Sam Stone noted).
So, the current BHP measurements are more accurate than the older IHP numbers, but still far from totally accurate. SAE horsepower is an even more standardized number that compares the horsepower of engines at certain, specific conditions. Power can vary depending on ambient temperature, humidity, barometric pressure, etc. So SAE hp attempts to compensate by specifying uniform testing conditions. I don’t know what the actual conditions are however.
Now, the tricky one: Torque. Torque, to put it simply, measures the ‘twisting force’ an engine produces. Horsepower measures the power (big surprise!), which in a car engine is torque times RPM (and some unit conversions as well). So horsepower is really based on torque. If you know how much torque an engine produces at a given RPM, you know the horsepower. Engines vary wildly in their characteristics with respect to hp and torque. Small, high-tech engines (like Honda V-TEC) tend to produce low torque. But they can rev to high RPMs, which means they still have decent peak horsepower ratings. Bigger engines (like old American V-8s) tend to have a lot of torque, but they can’t rev as high. So their peak horsepower isn’t as much different from the small engines as you might think.
The simplisitic rule-of-thumb is that lots of torque gives fast accleration, and lots of HP gives a high top speed. Of course, there is also gearing to consider. . . Basically, the whole thing is pretty complicated and the horsepower numbers you hear quoted by auto makers are just a small part of the story.
Adding to this, the performance of a car is also determined by how usable that horsepower is. Two engines with the same maximim horsepower rating can behave totally different on the track and street.
For example, those huge big-block V-8’s were great at producing gobs of power at low RPM. Unfortunately, they produced more power than the tires and suspension could transmit to the road, so much of it was wasted. If you go back and look at 1/4 mile times of some of the old muscle cars, you’ll find cars with 350 hp running as fast as the monsters with 450 and more, because that extra 100 horsepower was simply unusable, unless your goal was to make clouds of white smoke.
On the other end of the spectrum are those cars with small, high-revving engines that can only create horsepower at very high RPM’s. Put one of these on a track against a V-8 with tons of low-rpm torque and it may actually be faster. However, the car will not feel anywhere near as powerful in day-to-day driving, because that horsepower is simply not usable in stop-and-go traffic. Remember that the next time you’re thinking of buying some car with tiny four-cylinder engine that makes a lot of horsepower by strapping twin turbos and an intercooler to it.
The ultimate ‘street’ engine would be one that creates enough torque to max out the road-holding capability of the tires and suspension, and holds it there throughout the RPM range. That’s why those exotic supercars with the big turbocharged or supercharged engines are so thrilling to drive. Their max horsepower may not be higher than a much cheaper vehicle, but they can make that horsepower at any speed, and they have big fat sticky tires to transfer it to the road and exotic suspensions to keep the car under control even while it’s at the very limits of adhesion to the road.
Road & Track has a pretty good explanation of the various standardized measures of auto horsepower–see the first article at the link below. And the carmakers switched to SAE Net hp in 1971. The SAI gross number did not take into account accessory drives such as for water pump, oil pump, distributor, etc., and gave a higher hp rating.
To complicate the picture further I believe the auto makers undervalued the true HP numbers on muscle cars. I had heard this was for insurance purposes (the idea being lower advertised HP numbers meant lower insurance rates for the drivers). Could be an Urban Legend though.
Well…yes and no. I’ve been smacked down on this subject a here in the past, but I still believe for the vast majority of “muscle cars” that the horsepower figures are nothing more than numbers made up to fuel testosterone-crazed adolescent wet dreams of steel. I cannot count the number of times I’ve heard someone with their sad old 350 Chevy tell people how it came stock with “400 hp”, and yet has an apparant rear-wheel horsepower of around 180.
That having been said…there were indeed a few limited runs of very high powered cars out there, that truly had insanely high outputs. Not the sort of thing that pulls up next to you at a light on Friday night with the heads bobbing and the stereo thumping*, but mostly kept and maintained by collectors and true hot-rod enthusiasts.
[sub]* I realized on previewing that this makes me sound like the cranky old woman I am…[/sub]
I don’t recall any published power output for the 350cid Chevrolet motor over 375hp.
Inflated claims on the part of car owners are much more common than those on the part of the manufacturers. In some cases, manufacturers have actually deflated hp claims. Witness the 1968 Oldsmobile W-30 455, which claimed 400hp gross when installed in the Hurst/Olds. It was actually closer to 440 or so but was deflated to fall in line with GM corporate policy forbidding 400+hp engines in mid-size cars. Other muscle cars were similarly deflated for similar reasons and, supposedly, to scam insurance companies that scammed their customers by basing premiums on mfrss horsepower numbers.
Another classic was the 1989 20th Anniversary Trans-Am. It was rated at 250hp so that it would not violate the GM rule that no F-body shall have more power than the almighty Corvette (255hp that year). Pontiac insiders claim the first production engine made 301hp in dynanometer tests.
Interesting info about underrated hp on older American cars here!
Other instances I’ve heard about: Newer cars (the engines being largely computer controlled) can supposedly run on almost any octane gas. They will advance/retard the ignition timing to compensate for the gas as needed. So, you put in 87 octane, and the ignition is retarded to reduce pinging and you get slightly less power. You put in 93 octane and the ignition is advanced and you get slightly more power. Now, a car manufacturer can only advertise the higher number if they note that the car requires high-octane gasoline. So on sports cars, you get the high-octane requirement and the higher horsepower figures. But on regular cars you see the lower figures. In any case, the difference would be almost negligible for a regular family car, so there would be no point to spending extra for the high octane. On the other hand, sports cars may really require the high-octane in order to run properly (pinging can be very hard on the engine).
In Japan, there is a horsepower limitation on all cars of about 275 horsepower (205 KW is the measurement they use). I’ve tried to find some more info, but I wasn’t getting much on Google for some reason. Supposedly, this limit is voluntary and applies only to the published figures. In other words, a car could produce more power but they would be able to advertise more. I don’t know if this is really accurate information. What happens if a manufacturer breaks this limit? I do know that there are a large number of Japanese sports cars that have the exact same published horsepower numbers. . . Anyone have more info on this?
I don’t have any info on the rule etc., but the Acura NSX 3.2 has 290hp (at least in America) according to the company website; interestingly, the Lexus LS430 has the same horepower rating.
Of course, it is entirely possible that these companies detune their engines somewhat for the domestic market - after all, how fast can the Japanese drive in those traffic jams? Some time ago (and maybe even now - i don’t know) Ferrari produced a 2.0 litre car exclusively for the Italian market to get around the extremely high taxes imposed on any car with a larger capacity engine in that country.
Of course, this doesn’t address the question of what the rule is regarding the importation of high-powered cars to Japan. Quite a few wealthy Japanese get around in high-powered Euro machines.
Anthracite: I think you’re confusing the claims of car owners with the claims of car manufacturers. Many car engines came in a number of output levels. The Chevy 350 was available in versions that made anywhere from 180hp to 375 hp. The 454 was even more extreme - versions of that engine differ in output by more than 250 hp.
Of course, just about any young kid who owns a 60’s muscle car will claim that his version of the engine was the most powerful, and of course there are lots of cars out there with aftermarket engine badges.
Also, you have to remember that many of these cars that have been around since the 60’s have gone through at least one major engine rebuild, and in some cases multiple rebuilds or even outright engine changes. And often those high-output engines got replaced with generic OEM rebuilds.
I did a pretty extensive study of the claims of muscle car manufacturers for a thread last year, and by and large they matched the average of being about 16-18% higher than the same engine would be rated today. So a 350 HP engine in 1967 might be rated at 300 HP or so today. Not that big a difference. I posted a whole list of 1/4 mile times as measured by Road and Track and Car and Driver when those cars were new, and htey matched what you’d expect from that level of horsepower.
Yet another reason why the ‘apparent horsepower’ might be a lot lower than the rated claim is that tire and suspension technology on those old cars just can’t make use of the horsepower.