Thrust engines' output are measured in terms of force. A turbojet will deliver a constant force from 0mph to infinity (neglecting ram effect)

Driveshaft engines' output are measured in terms of power. This is because the force they output is inversely proportional to the linear speed it is delivered at. In other words an internal combustions engine with a continuously variable transmission will delivery infinite force at 0mph, decreasing to zero force at infinity. Power is a way to show force per speed. Power is independant of wheel speed (but dependant on engine speed, which would be kept constant in the ideal case of a CVT).

 

"Why is this law of physics so hard to get across?

EJ"


ALLTHROTTLE&NOBOTTLE, great explanation, thanks.

EJ

 

Very interestng and informative thread to say the least!!! All throttleandnobottle your thesis on this topic really was mind blowing!!!

 

Alot to absorb from this thread. Another way of obtaining additional info in order to come to a conclusion is to take a poll of vettes with their times and their hp/ tq numbers and mph so you get some 'real world' data to go with the theoretical.

 

http://vettenet.org/torquehp.html

http://www.58fuelie.com/Caro-Vettes/hp.htm

http://www.ls1.com/handt.htm

 

No, HP is calculated from Torque and RPMs.

HP = (Torque * RPM) / 5252

 

at any specific RPM, you want as much torque as possible.
But if you had the choice, its better to have that torque at a higher RPM.

OTOH- dont decrease torque by 50% to get a 25% increase in RPM...

ALLTHROTTLE- is that by you? I've seen it posted (in its entirety) in more then a few other locations...

 

Only some of it

 

It's never just torque because any human can get on a bike and with enough theoretical gearing one can make 10,000 lb.ft. torque, but at very low rpm; that does no good. It's never just rpm either for obvious reasons.

It's the combination of torque and rpm, which is power. Power is the reservoir which one dips into to set up the proper axle rpm/vehicle speed and axle torque/tire-surface thrust combination. So a 500 hp (at high rpms) car that makes 380 lb.ft. torque (at mid to high rpms) may seem like a bummer for drag racing, but not if you gear it correctly.

If you lack the torque at the flywheel, just gear it steeply, ** that's what transmission is for.** As long as you have the high power there, you'll have great speed vs. shiftpoint relation and strong accel. across a wide range of speeds given good ratio spacing and adequate number of ratios.

This type of question is always really meaning: "Do I want to achieve the ** GIVEN ** power with an engine that has..."
A.) A low rpm limit, lots of torque. Like many turbo-diesel engines.
B.) A moderate rpm limited engine with a balance of peak torque (in lb.ft.), peak hp, and a reasonable torque spread over the rpm range.
C.) A highly strung engine with moderately low torque that's held to very high rpms.

Let's say you have to generate 400hp for 3 different applications. For a load carrying/transporting function such as a medium duty truck where extra long durability is desired, you'd like to get 400hp by low rpm bandwidth and high torque (well above 600 in lb.ft.). The slow rotational speeds will ensure extremely long bearing service life and lower frictional losses for maximum fuel efficiency that counts for fleet service. As long as the assembly can take the cylinder pressure and you have plenty of underhood space for a big motor then you'll be fine. In a super sedan, you are more limited with allowable engine dimensions so you have to stick with a more moderate displacement of an engine. This means you need more rpms to make 400hp than the truck example. Contemporary examples could put one at ~6L swept displacement, ~6k rpm limit, and probably ~400 lb.ft. torque peak that holds a good fraction of that to the rev limit. The 6k rpm limit ensures long service life and the generous torque at lower rpms allows for gearing enabling relaxed road manners/reasonable fuel consumption while achieving the acceleration requirements. Now if you have a track only application, and are not concerned about noise levels or rebuild cycles (# hours operation), then you have the green light to go very small in displacement and make the 400hp out of stratospheric rpms (permitted by proper cam, heads, inlet/exhaust geometry, rotating assembly geometry). The small motor will pay dividends in mass distribution and possible aerodynamic packaging benefits too which in turn will mean paybacks in potential driving dynamics with proper chassis setup and tuning. Finally torque on this application will sure be relatively small (maybe 220-270 lb.ft. range) but for a given overall rolling radius, it's the torque at the wheel that counts, right? So you make up for the lack of engine-out torque with transmission gearing. Even though high numerical gearing slows the vehicle speed for given engine rpms&gear, the engine rpm limit is very high! So the power ensures a good tire thrust and vehicle speed combo up to very high speeds.

To answer your question, one should never ask torque or horsepower. The question actually doesn't make sense since applied torque doesn't guarantee speed (in the cycle example given above) and in fact it doesn't even guarantee movement...think of applying torque to a peanut butter jar cap and if the cap is too tight, you'd apply torque but get no angular motion!

There was a letter by a GM powertrain engineer to an Automobile Mag. editor long, long ago mimicint these words. Csaba Csere (C&D cheif editor and M.I.T. grad) said the same thing: "Torque is friend, power is king" The friend part on a high torque motor generaly means one can achieve a certain accel. objective with lower numerical gearing (which has numerous durability/fuel economy/NVH benefits).

Conclusion: Thick torque band across a wide rpm is the best. Because we stated torque and rpm in the same sentence, power is implicit...you never get away from that when you mention both. Power encapsulates the two equally important components (torque and rpm) necessary for acceleration.

 

Holy crap!!!

You guys are taking the fun out of drag racing!!!!

Good information Steve. Where were you when I was picking a new gear ratio!!!!


Also can someone comment on peak torque in an A4 not being accuarte because of the spike from the converter?

 

Im going to have to agree with scissors here... In all reality, it honestly depends on how the engine makes the power, and how long you are racing for. For example, if you have an 8000 RPM redline, and when you shift, you are staying higher in the powerband HP is considerably more important. Take a Supra for example.. Most 450RWHP Vettes will beat a 800RWHP Supra in the 1/8. The Supra with low torque while the turbo spools. Once the Supra gets to higher RPM's where HP takes over the Supra starts pulling hard. Thats why you can see a Supra run 12's @ 125+ and a Vette run 11's at 115. So. to answer your question, it depends on setup, application, and task. A higher HP car will win a longer race (like a standing mile) everytime. In the 1/8 mile, (assuming = traction) the car with more torque will win. And the 1/4 is the fine balancing line where it all depends on setup. I hope that helps.... Because I think I just confused myself.

 

Oh yea... and quick note... Technically, horsepower is a function of torque and rpm's. As said before, one is derived from the other.

And a side note... It takes the same amount of force to move 1000 pounds one foot as it does to move 1 pound 1000 feet.

 

In my head I kept it simple...

"Torque gets you moving and HP keeps you going"

 

Okay I wont pretend that I understand most of what is being said here but ALLTHROTTLEANDNOBOTTLE seems to be all over the issue.

My simple question is this:

I was tuned using LS1 edit and had 384RWHP and 371RWTQ and ran my best of 11.98@116.49 MPH with a 1.82 60ft.

Now my new tune with same mods except I changed from a TRAP to Vararam and from a YANK ST2800 to a YANK SS3600 is 375 RWHP and 421 RWTQ.

I was pleased with the fact that I only had to give up 9 HP to get 50 ft lbs, BUT will this new tune cause me to run slower or faster???????????

Thanks

Cajun