The Quest For Accurate Engine Specs
Going over Nissan’s specifications for the 2011 Quest minivan, I came across these for the engine, on both the media and the consumer site:
260 horsepower @ 5,200 rpm
240 foot-pounds @ 4,800 rpm
Some non-Nissan sites provide slightly different numbers:
253 horsepower @ 5,200 rpm
236 foot-pounds @ 4,800 rpm
So perhaps Nissan recently found a couple more horsepower then rounded both figures up to the nearest five.
Both sets of numbers instantly struck me as impossible.
Torque is the amount of force exerted on the crankshaft at a given moment. Horsepower is torque multipled by rpm and then divided by an arbitrary number, 5252 in the case of SAE BHP and foot-pounds. Given this, at 5,200 rpm an engine must be producing a bit more foot-pounds than it does horsepower, specifically 262 in the first case, while the Nissan specs require that it be producing less than 240.
Why less than 240, and not 240? Because the torque curve cannot be flat all the way from the torque peak to the horsepower peak. At a certain point, an engine produces less and less torque as rpm increase. This is the torque peak. For a while after this point, rpm continue to increase faster than the torque falls off, so horsepower continues to increase. When torque output falls more than rpm increase, you’ve got the horsepower peak.
Most likely: the power peak is actually around 6,000 rpm, as with the similar V6 in the Murano. But why hasn’t anyone at Nissan or any of the initial reviewers realized this?
Michael Karesh owns and operates TrueDelta, an online source of automotive pricing and reliability data
Michael Karesh lives in West Bloomfield, Michigan, with his wife and three children. In 2003 he received a Ph.D. from the University of Chicago. While in Chicago he worked at the National Opinion Research Center, a leader in the field of survey research. For his doctoral thesis, he spent a year-and-a-half inside an automaker studying how and how well it understood consumers when developing new products. While pursuing the degree he taught consumer behavior and product development at Oakland University. Since 1999, he has contributed auto reviews to Epinions, where he is currently one of two people in charge of the autos section. Since earning the degree he has continued to care for his children (school, gymnastics, tae-kwan-do...) and write reviews for Epinions and, more recently, The Truth About Cars while developing TrueDelta, a vehicle reliability and price comparison site.
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Incidentally the 5252 number is not arbitrary, it is 5252.113... . and it is exactly 33000 divided by 2 pi. The 33000 comes from the definition of a horsepower, 33000 ft lb per minute. So hp=torque in ft lb*rpm*2*pi(to convert from revs to radians)/33000 Fancy not knowing that properly. That was high school science, thirty years ago.
So, the question is: Is there anybody responsible for making manufacturer's numbers accurate, do they have any teeth at all at enforcing any violations? Or can the manufacturers said anything they want? The first post mentioned manufacturers getting sued for inaccurate numbers. By whom? Class-action lawyers on behalf (ha!) of consumers? Attorney generals? Seems like the chance of this is pretty low, given how apparently prevalent these inaccuracies were. Thanks to Carve for the explanation, BTW. Better than any I've read on the subject. The quality of the commenters here on this site is amazing, not discounting the writers, of course!
I'm wondering why a vehicle as heavy as the pictured minivan (which must weigh over 5,000 lbs) isn't equipped to deliver torque at low RPMs. Seriously, I'm shuffling kids to school and soccer and grocery getting in my minivan. Most of the time I'm accelerating to and driving at less than 45 mph. I want the torque down low and I don't really need it up high. What gives?
Real men refer to "foot-pounds" while politically correct modern-day cubical dwelling metrosexuals use the term "pound-feet." That's all I got to say about that.