Something new re: Horsepower vs. Torque

[Ken]

The July issue of Hot Rod Magazine has an interesting e-mail from a former Chrysler engineer regarding the horsepower versus torque arguements that never seem to end.

An excellent example of the horsepower/torque confusion involved the long-ram intake manifold that was introduced in some of the '60 Chrysler products. This manifold was tuned for the torque peak, and the result, when installed in a car, was immediately sensed by the driver.

Engineering management was very impressed. But, before the project reached this point, there had been an intense battle between the management people of the Engine Laboratory and those of the Vehicle Performance Laboratory.

Those in the Engine Lab were dealing with dyno recordings, and those in the Performance Lab were dealing with calculations of vehicle performance. Those of the former were waving their dyno sheets and saying, "Look at those figures!" Those of the latter were waving computer outputs and saying, "Look at the calculated performance. This is a dog!"

The problem was that, though the car felt very "peppy" under normal driving, when it came down to a stop light drag race, that great low-speed torque could only be appreciated once, while in first gear. From there on, the engine was operating at speeds beyond the tuned speed.

Though they had lost the arguement, the Vehicle Performance Lab managed to bootleg a test at the proving grounds.

A drag race was arranged between two cars. One had the production long-branch intake and the other was fitted with the inline dual-quad manifold of the preceding year. They were otherwise identical.

As the computer program had predicted, the inline set-up ran off and hid from the long-branch.

A Former Chrysler Engineer​

via e-mail​

​

This person asked to remain nameless but witnessed the events he describes. There was a flurry of contradictory e-mail denouncing Marlan Davis' story. "Torque vs. Horsepower" in the Jan. '04 issue for its tendency to reveal that horsepower is actually more critical to acceleration than torque. Marlan fell short of making that controversial statement, but I won't. And for the naysayers who've parroted the "torque is what moves the car" theory, try this: Go to the dragstrip and shift at your torque peak, not after the horsepower peak.

It's gonna be interesting to see the responses to this post.

 

[Stallion]

I had a little trouble following the article.

"And for the naysayers who've parroted the "torque is what moves the car" theory, try this: Go to the dragstrip and shift at your torque peak, not after the horsepower peak."

So, is he trying to say that shifting at the horsepower peak should result in a better ET than torque peak? What's the technical explanation to that? :confused


And this brings up another drag racing question. Are you supposed to shift at the horsepower peak, or at the redline?

 

[1995 RedVette]

Seems to make very good sense with a good example. The L98 in the early C4's was a torque engine. I owned two of these cars, 86 iron head and 87 Aluminum Head. Both were great running cars. I could smoke the tires off of both of them most anytime I wanted, and stop light to stop light drags were almost hands down. I have now moved to a 95 LT1 car. I have the time slips to prove that the LT1 is much faster, however, it does give up some of the low end slam you in the seat satisfaction.

That said, I do apprecieate the newer cars better driving manners and the fact that I'm able to get more than 10,000 miles out of a set of rear tires (now I light the tires on purpose, rather than doing it half heartedly).

Brett

 

[Ken]

TR, peak horsepower and RPM redline are usually very closely related.

 

[Stallion]

Hmmm..not on my engine. I peak HP at 3000 RPM, and I redline at 5000...

 

[JohnZ]

TR, I don't have '78 engine specs handy, but your L-48 horsepower peak is more like 4800-5200 rpm, and your peak torque should be around 3000-3200.

 

[Stallion]

I didn't mean 3000, I mean 4000 RPM for peak HP.

185 HP @ 4000 RPM
280 ft lb @2400 RPM

 

[Edmond]

They also go with the lower RPM number if say the HP is equal at 3000 and 4000. I read that somewhere, think it was here.

 

[Stallion]

They also go with the lower RPM number if say the HP is equal at 3000 and 4000. I read that somewhere, think it was here.

:confused I don't understand

 

[Ken]

Just for the heck of it, let's review:

From the book, "Engine Management: Optimising Carurettors, fuel injection and ignition systems" by Dave Walker, Technical Editor, Cars & Car Conversions:

THE THEORY

You hear a lot of bar room talk about bhp, but how many of those discussions actually understand the numbers that they are quoting? One bar room expert will brag about his maximum bhp number, while another (claiming to be wiser) will tell you that what really matters is torque. They are talking about two different things. If you think of torque as the effort that your engine is making, then bhp is the actual work that gets done. If you struggle to lift a heavy weight, but fail in the attempt, you have expended a lot of effort. But by not moving the weight you have singularly failed to do any work!

A given torque figure will translate to a higher bhp number if the torque is produced at higher rpm. So, what exactly is bhp?

Before the invention of the internal combustion engine everyone used animals to provide motive power. The most popular working animal was the horse. When engines first appeared, people wanted to relate their power to a known value, i.e., the horse.

One engineer figured that a horse could move a 30,000 lb. load over a distance of one foot in one minute. Or, this mythical horse could move 1650 lb. a distance of 20 ft. In each case the work is exactly the same; force mutiplied by distance moved in one minute. This is our standard 1 bhp today.

Since the engine's torque was measured on a braking device, the power was called Brake Horsepower (bhp). The only engine value we can measure on a dyno or a rolling road is torque. The bhp is calculated from the torque reading and the engine rpm. The equation looks like this:

• bhp = 2 x Pi x torque (ft lbs) x rpm / 33,000

Since 2 x Pi is a constant this can be reduced to:

• bhp = torque x rpm / 5252

Therefore:

• torque = bhp x 5252 / rpm

If you have one value you can calculate the other, and both bhp and torque will be the same at 5252 rpm.

I am certainly not the wise one here, JohnZ is much more qualified for that position I'm sure, but I do try to enlighten, albeit with a hacked-up and clumsy approach.

 

[DRTH VTR]

That relationship is why some of those small displacement engines (eg Indy cars) produce so much horsepower. They turn 18,000 RPM! If torque remains flat, double the engine speed, and you double the horsepower.

 

[Edmond]

:confused I don't understand

I think it was something that if the power curve starts to flatten out, they'll take the lower RPM number.

I wish I could remember where I read that from so I could post it.

 

[Stallion]

Ah, I see what you mean.

 

[SG4206]

Look at the post by Grumpy that has the Super FLO dyno sheets. The dyno operator did not take this engine high enough;but If run to 6600 rpm , you would have seen that HP would have been around 375 ; which probably would make the best shift point.
RPM fall after the gear change, would be between 5000-5400 rpm. HP at this rpm is greater than or equal to the shift point.
As a general rule , best performance is attained shifting around 500 rpm past peak HP.
Shifting at peak HP lets the engine fall into a HP value less than when shifted; decreasing acceleration.
Shifting at peak torque ; well, just won't work for fastest ET.
There are always exceptions , but with the gear ratios in most of todays transmissions, 500-600 rpm past peak HP will work the best.



Normally , red line of an engine is determined by the mechanicial limitation of the valve train and the bearing loads at any given RPM. Doesn't have anything to do with rpm at peak power. Example , I have a SS/CS 331 SBC that makes peak hp at 7300 ; but red line is 8500 ; this is because of the light internal reciprocating weight of the bottom end and high valve spring pressure.