Why is it when you look at automotive engine Horsepower and Torque graphs, they almost always (or always) intersect at around 5200 RPM?
Does this apply to ALL gas engines that can reach that RPM level?
KnK
Well, I’ll be. I didn’t know this before, but How Stuff Works has a concise explanation:
And hats off to you, KnK, for asking that question. I’ve been looking at horsepower/torque curves most of my life and somehow never once noticed that phenomenon.
Well now, that’s interesting.
I knew that horsepower=(torque X rpm/5252), but I always thought it was “just because”.
D’oh! I guess I shoulda knew to look there first!
Thanks for the props Sofa King .
KnK
How about part 2 of the OP?
B]Does this apply to ALL gas engines that can reach that RPM level?**
KnK
Knk: Yes it applies to all engines.
The 5252 rpm crossing is just a mathematical coincidence that has to do with the units. The plots you’re looking at are in foot pounds, horsepower, and rpm. If they were plotted in watts, newton meters, and rpm then the curves would cross at a different number.
Also, they might not cross at that rpm if the person making the plot chose to slide the plotting scales up or down for some reason.
>I knew that horsepower=(torque X rpm/5252), but I always thought it was “just because”.
It IS “just because”. Race Bannon has it right, and the point is that it doesn’t say anything at all about engines. IF you used different units the graphs would cross somewhere else.
Hmm… I’ve never noticed it either. Check out some dyno graphs here:
http://www.ws6.com/mycar.htm
They all intersect at around 5300 rpm. Even with the different mods.
But what about nitrous or blower applications, same result?
Sure. In English units, horsepower=(torque X rpm/5252), where torque is in ft-lbs. By the definition of HP, when the speed = 5252 rpm, the horsepower is numerically equal to the torque. No application dependence whatsoever.
You’re missing the key point as to the relationship. It’s a function of the units of measurement being used. It has nothing to do with 2-stroke, 4-stroke, inline 4, V12, blown, not blown, nitrous, gasoline, hydrogen-powered, etc. I posted in excrutiatingly detail about 2 years ago here, but since anything older than a week doesn’t seem to exist here, look at Sofa King’s link.
It will be different if you use different units of measure. Such as inch-pounds, or hp versus Nm, etc.
Just because I like pi, I’d like to point out that the 0.10472 radians per second is just Two Pi (one whole revolution) divided by 60 seconds (one minute). That is, .10472 rad/sec is one RPM. So, the 5252 comes from 550 divided by (2pi/60), or 550x30/pi, which is 16500/pi.
On every dyno plot you’ll notice that the horsepower (HP) and Torque (T) curves intersect at 5,252 rpm. This is a consequence of the definition of horsepower:
HP = (rpm/5,252)*T
There is nothing intrinsically important about HP, at least in terms of defining engine characteristics or performance, as HP is wholly dependent on Torque & engine speed. An expression I heard a long time ago from a fellow powertrain engineer sums it up nicely: he said, “people BUY horsepower, but they DRIVE torque.”
I hope this helps!
No, it really does not.
It probably doesn’t help the OP…who hasn’t posted here in over ten years.
Another fact that may or may not help:
The two curves you see are not separate measurements. They’re just one measurement presented in two different ways. The dyno can only measure torque (and RPM)–that’s the raw data coming from the dyno. Power is inferred by taking torque times RPM (times a constant of 1/5252 in the case of customary units). Give me a graph of torque vs. RPM and I can produce a horsepower curve without any additional information.
Even for customary units, the curves will not necessarily intersect at 5252 RPM. That’s just the point where the two values are numerically equal. But graphs can have different scales for the different data sets. Usually ft-lb and horsepower are reasonably close numerically so the graph uses the same numbers, but this isn’t guaranteed.
Power equals torque times rotation rate, exactly. The 5252 number, once you include its units, is just a funny way of writing “1”. It’s no different than, say, 2.54 centimeters per inch.