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Torque and acceleration

Joined
30 November 2000
Messages
59
Location
Prescott, AZ, USA
I’d like some help from you gearheads out there. Let’s start with a quote from a tuning book

“When it comes to moving a car down the road, horsepower is useful in creating a maximum top speed. As the frictional loads of the machinery, the resistance of the road, and the drag of the wind all pile up against the engine’s output, it is horsepower that does the work. However, overcoming the inertia of a car and accelerating it up to that speed is a function of torque, not horsepower.”

Now let’s look at some data regarding 0-150 miles per hour acceleration. According to Car and Driver (Aug., 1998), an NSX took 35.9 sec. and 5191 feet, and a C5 Corvette took 37.8 sec. and 5616 feet. The Corvette was 8% heavier, but had 56% more torque than the NSX’s 224 lb-ft. Both cars were six-speed and presumably had similar amounts of drag. If torque is indeed the motivator during acceleration, how could the NSX beat the Corvette?
 
Very simply answer - the gear ratio of the NSX's transmission is so different from the Corvette's transmission, that the *net* torque at the rear wheels favors the NSX.

In any transmission, input-outpit HP is constant (except for mechanical losses) but input-output torgue is multiplied by the gear ratio.
 
Originally posted by AndyVecsey:
Very simply answer - the gear ratio of the NSX's transmission is so different from the Corvette's transmission, that the *net* torque at the rear wheels favors the NSX.

Preach on, brother.

Acceleration is a function of torque at the wheels. Torque at the wheels equals torque at the crank times gearing. The NSX, with its VTEC variable valve timing, allows the torque curve (torque at the crank) to stay relatively flat up to much higher RPMs. This allows the NSX to stay in a lower gear where conventional engines (e.g. Corvette) must upshift to stay in the torque band. While the NSX loses by having less torque, it gains through the gearing. (This is a long paragraph that says the same thing as Hal did in six words.
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Here is some good reading on why torque matters, and why horsepower matters: http://www.g-speed.com/pbh/torque-and-hp.html

[This message has been edited by nsxtasy (edited 25 October 2002).]
 
I'm having trouble with the "magic gearbox" explanation (the one in which most NSX owners would like to change gear ratios!). If I recall correctly, the Corvette has more torque at 1800 rpms than the NSX has at peak, and it is very hard to believe that it has less rear wheel torque in any gear, at any speed, than an NSX. How about the Porsche 911 Carrera, which the NSX also beat. It has 34 lb-ft more torque. Is the Porsche also suffering from poorly matched gearing? Frankly, I find that hard to believe.
 
Changing gearing doesn't make a huge actual difference (although it can make a larger perceived difference). Usually, it has the effect of improving acceleration at some speeds while hurting it at others.

It's not that the Corvette or the 911 have POOR gearing; it's that their engines don't have the wide powerband of the NSX. So they must upshift into a higher gear when their engines reach perhaps 6000 RPM or 6500 RPM. As a result, they lose performance after the upshift due to the higher gearing, even though they might have more torque. Remember, more torque with poorer gearing can equal poorer acceleration.

Originally posted by Sven:
If I recall correctly, the Corvette has more torque at 1800 rpms than the NSX has at peak, and it is very hard to believe that it has less rear wheel torque in any gear, at any speed, than an NSX.

Even without having the Corvette's performance profile in front of me, I can assure you that, for example, a Corvette in fourth gear will most certainly have less rear wheel torque in fourth gear than the NSX has in second gear.

The ratios of adjacent gears are typically around a 4-3 ratio, which means that two gears apart the ratio becomes 16-9. That means that for a given point on the rev band, a car will have little more than half as much torque at the wheels in fourth gear as it has in second.

I am not saying that the Corvette is necessarily in fourth gear at a speed (81 mph) where the NSX is still in second, but I'm sure it must upshift to fourth at a much lower speed than the NSX (114 mph). Whether that is because optimal shifts are at redline and its redline is lower, or optimal shifts are before redline, I have no idea.

I'm sure the same thing is true of the 911.

[This message has been edited by nsxtasy (edited 25 October 2002).]
 
Originally posted by Sven:
I'm having trouble with the "magic gearbox" explanation (the one in which most NSX owners would like to change gear ratios!). If I recall correctly, the Corvette has more torque at 1800 rpms than the NSX has at peak, and it is very hard to believe that it has less rear wheel torque in any gear, at any speed, than an NSX. How about the Porsche 911 Carrera, which the NSX also beat. It has 34 lb-ft more torque. Is the Porsche also suffering from poorly matched gearing? Frankly, I find that hard to believe.

Your using peak numbers for torque… not average numbers. 100, 75, 50, 25=250 (62.5)average 75,75,75,75=300 (75) average! and in the case of the NSX rev limit before shifting; add another 75

Your also using crank torque, not rear wheel. Advantage NSX (they need to move to this standard)

Not to mention traction, or the relationship between weight and torque. Advantage NSX
 
The torque curve can be derived from the HP curve and vice-versa, so suggesting that one is more important than the other is stupid, to put it bluntly. Either of those curves is much more indicative of a car's acceleration performance envelope than the peak HP or TQ numbers, so if you're comparing those...

As for the NSX/C5 comparison you cited, the tested NSX was undoubtedly a better-than-typical performing specimen (as noted in the article) so that single data point isn't of much use for the present analysis.

[This message has been edited by Number9 (edited 25 October 2002).]
 
Originally posted by Number9:
the tested NSX was undoubtedly a better-than-typical performing specimen

Primarily because the typical 3.2-liter NSX is an NSX-T, whereas they tested the lighter NSX coupe for that article. Perfectly legitimate.

[This message has been edited by nsxtasy (edited 25 October 2002).]
 
Originally posted by nsxtasy:
Primarily because the typical 3.2-liter NSX is an NSX-T, whereas they tested the lighter NSX coupe for that article. Perfectly legitimate.

[This message has been edited by nsxtasy (edited 25 October 2002).]

Those particular NSX numbers were also better than all the published Zanardi numbers, which is lighter still, so that car was clearly an outlier in terms of engine performance - in addition to being lighter than a T.
 
No, I think the published Zanardi numbers may have been the ones that are flawed. For example, the 3.2-liter NSX-T generally tests at 4.7 to 4.9 seconds 0-60. Due to its lighter weight, I would expect the Zanardi to be closer to 4.5.
 
Originally posted by nsxtasy:
No, I think the published Zanardi numbers may have been the ones that are flawed. For example, the 3.2-liter NSX-T generally tests at 4.7 to 4.9 seconds 0-60. Due to its lighter weight, I would expect the Zanardi to be closer to 4.5.

Well, if you think the data are suspect, there's not much left to argue about! Except that I prefer great taste to less filling!!!
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Just one point. I'd be interested in seeing the actual shift points (in MPH) for each car. I would be surprised to see that the Vette is shifting to a given gear at lower speeds than the NSX. In fact, I anticipate the opposite. "Muscle" cars tend to spread the gears out and rely on massive torque to pull them. The NSX also spreads them out, but because it has a torque curve of modest height but considerable breadth. In the end both are geared tall, but I’d expect the Vette to be taller (assuming the same number of gears).
 
Originally posted by nsxtasy:
No, I think the published Zanardi numbers may have been the ones that are flawed. For example, the 3.2-liter NSX-T generally tests at 4.7 to 4.9 seconds 0-60. Due to its lighter weight, I would expect the Zanardi to be closer to 4.5.

This reminds me of a test done by Car and Driver in 2001. They tested a NSX-T and came up with 4.5s in 0-60 and 12.9s in 1/4. Has any other testers came up with numbers close to that?
 
Originally posted by Zuerst:
This reminds me of a test done by Car and Driver in 2001. They tested a NSX-T and came up with 4.5s in 0-60 and 12.9s in 1/4. Has any other testers came up with numbers close to that?

If I recall correctly, C&D has come up with the 4.5 sec 0-60 and sub 13 sec 1/4 mile results with 2 different test cars featured in different issues. One of the tests was on a silver car and the other was on a yellow car. I dont remember which was a coupe or T.
 
Originally posted by Number9:
The torque curve can be derived from the HP curve and vice-versa, so suggesting that one is more important than the other is stupid, to put it bluntly. Either of those curves is much more indicative of a car's acceleration performance envelope than the peak HP or TQ numbers, so if you're comparing those...

As for the NSX/C5 comparison you cited, the tested NSX was undoubtedly a better-than-typical performing specimen (as noted in the article) so that single data point isn't of much use for the present analysis.

[This message has been edited by Number9 (edited 25 October 2002).]
A couple good points here. An areospace engineer tells me that what is actually important is the horsepower that an engine is putting out at any point in time. In full throttle acceleration runs, you remain in high horsepower ranges which is the oly reason an NSX or S2000 can stay on the road with high torgue vehicles.
Torque is most important for getting you up there in first gear and for around town driving. That's why you have to run your car reasonably hard from a stoplight to keep up with the old foggie in the Caddie doing a regular start.

These acceleration numbers also raise a question discussed recently on another thread about whether Acura has understated the horsepower rating. Both the performance figures and difficulty of reconciling 3.2 vs 3.0 HP given the higher dispacement, addition of headers and six more years of technolgy make the answer look like they did.
For my 2 cents worth on gearing, I would guess speed in gear for the Vette is extremely close to the NSX.
 
That's not right. When knowledgeable people suggest a preference for high HP over high torque, they are implicitly comparing the case where the compared engines have one of those respective characteristics and not both. In that case, take the high HP engine because you can gear down to get torque at the wheels. In the high torque case, it would implicitly be making the big torque only at low RPM (and not at high RPM, or it would have big HP number too, which contradicts the original premise).
 
Originally posted by sjs:
Just one point. I'd be interested in seeing the actual shift points (in MPH) for each car. I would be surprised to see that the Vette is shifting to a given gear at lower speeds than the NSX. In fact, I anticipate the opposite. "Muscle" cars tend to spread the gears out and rely on massive torque to pull them. The NSX also spreads them out, but because it has a torque curve of modest height but considerable breadth. In the end both are geared tall, but I’d expect the Vette to be taller (assuming the same number of gears).


I was going to note the same thing. I owned a 99' Vette, and the shift points in MPH are pretty close (1-2 and 2-3). So, the RW-torque (MPH) band is actually pretty close on the two cars, which is why they have similar 0-60 times.


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01' NSX-T Silverstone
02' Chevy Avalanche 1500
00' Suzuki Hayabusa GSX1300
02' Honda VTX-1800
02' Aprilia Falco SL1000
 
Originally posted by JohnC:
I owned a 99' Vette, and the shift points in MPH are pretty close (1-2 and 2-3). So, the RW-torque (MPH) band is actually pretty close on the two cars, which is why they have similar 0-60 times.

If the road speeds while shifting are the same, but the Vette shifts at a lower RPM point, then the NSX still has a gearing advantage (which would explain the similarity in RW-torque despite the advantage in engine torque of the Vette).
 
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