HP vs Torque

[Guest Carlissimo]

Grumpy, once again you are confusing the issue with facts.

[Guest Anonymous]

Grunp,I thought you must be on vacation in Hawaii.....LOL

[Phantom]

The article on the ubermensch site is very good. Pretty plain english and the math isn't quite third semester calculus. It pretty well supports what everyone has put together in so many words.

[Pop N Wood]

Grump, you da man. But that first link contradicts itself.

 

The link first says:

 

Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. ... horsepower has nothing to do with what a driver *feels*.

 

But then goes on to say

 

Because (to quote a friend), "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*.

 

That last statement is the definition of horsepower! This guy is making the same mistake. He needs to start thinking about torque "at the rear wheels". His own water wheel example points out the fallacy of his own conclusions.

 

Horsepower is a very real entity. Dynos compute HP indirectly from torque only because that is the most convenient way of doing so. Everyone talks about the "5252" number as though it were some magic constant pulled out of the air. It is simply a unit conversion from torque in ft-lbs to power expressed in HP. Give the power rating in kilowatts (as our Aussie friends do) and the 5252 becomes something else.

 

The second link starts to go through Newton's equation and relates torque to acceleration. This guy even correctly makes the statement:

 

the only thing that changes a car's acceleration is the torque at the wheels, which is the result of torque from the engine

 

But what he doesn't do is convert the equation relating force and torque at the rear wheels to engine torque. He needs to change that last statement to read:

 

the only thing that changes a car's acceleration is the torque at the wheels, which is the result of torque from the engine MULTIPLIED BY THE GEARING RATIO

 

I stand by my earlier statement. Whoever has the most horsepower between the shift points wins.

[grumpyvette]

you guys are making this harder than it needs to be...

look here..

 

http://www.prestage.com/carmath/dynochart.asp

 

http://www.prestage.com/carmath/calc_ETMPH.asp

 

http://users.erols.com/srweiss/calcrgr.htm

 

http://users.erols.com/srweiss/calcrpm.htm

 

http://users.erols.com/srweiss/calcafhp.htm

 

play with these calculators, the secret to building a fast car is simply to get an excelent power to weight ratio then gear the car to stay in the engines most effective rpm range during any high performance use, and to get the suspension to transfer that power to the ground.

 

 

http://www.wallaceracing.com/reargear.htm

 

http://www.chris-longhurst.com/carbibles/suspension_bible.html

 

lets say you want a nice drivable car that will kick butt on 90% of the cars on the road, your (Z) weights 2950 with you in it and you have installed a 700r4 transmission ( a manual trans would be a better choice)

well with that weight and gearing youll want to have about a 140mph top speed MAX (BECAUSE THERES NO PLACE TO USE ANYMORE SPEED), if your tires are 25" tall , useing the calculators we see youll need about 360-375 rear wheel hp, and a 4.11 rear gear give you a 2900rpm 75mph cruise rpm in overdrive, now that would mean that youll need to make your hp in the 2700-6500rpm range, and most of it in the upper 1/2 of that rpm range, and youll need about 435hp-470hp at the fly wheel. thats very easy to do with a 383 -406 sbc, provided your cylinder heads, cam and compression ratio are matched to be most effective in the 4500rpm-6500rpm ranges

look over choices #14.#15.#16,#17,#71#19,#39,#44,#54,#70,#92,#93

on this site!

http://www.ryanscarpage.50megs.com/combos3.html

 

then figure youll not really need nearly that much hp if you satisfied with a car that will beat most corvettes,mustangs ,ETC, and youll soon realize its not all that hard to build a fast (Z) because your starting with a huge weight advantage over most of the competeing cars

 

heres a few engine combos I know work

11:1 cpr 383

CRANE #119661 roller cam RETARD 6 DEGREES)

afr 195cc heads

edelbrock super vic intake

850cfm carb

1 3/4" full length headers

 

11:1 cpr 383

crane #114681 flat tappet cam

pocket ported 220cc motown heads

edelbrock super vic intake

850cfm carb

1 3/4" full length headers

http://shop.theengineshop.com/dr/v2/ec_MAIN.Entry10?V1=979018&PN=1&SP=10034&xid=42186

http://www.theengineshop.com/newstuff2.shtml

[pparaska]

Having driven an NA L28 for 17 years and now stepping into a V-8 again I can tell you which I'm going to prefer - the one with the higher HP & torque that is across the broader band. I want to come off the line easily without having to rev the engine way up. I want the car to respond well in whatever gear I happen to be in without having to move it down 2 or 3 gears to find the sweet spot, maybe down one gear if someone really gets me going. And I want to run the quarter in about 3.2 - is that asking too much?

 

Phantom, you're preaching to the choir, now that I'm reading this thread . One of the things that really irritates me about STREET driving my Z when it had an L6 and presently driving my Eclipse GSX (turbo 2 liter) - especially with A/C running is the almost complete lack of grunt at what I prefer for cruising rpm - no more than 3000 or so rpm!

 

Yes, call me a lazy, knuckle-dragging, 'merican redneck, but I like my car to be able to GO when I punch the go pedal at cruise. Lots of cubes is the easy way to do this. Can be done with a super or turbocharger if set up for it also. But a high winding NA motor that's dead below 4000 rpm is not such a fun street car, to me anyway.

 

Jim, Power = Torque * (some constant). If you say you need HP, then that's the same as saying you need torque. The issue is (like Mike K and Mike C point out) is the power (or torque) BAND - how flat is it and how high. And of course, how the car is geared/tired.

[Pop N Wood]

Power = Torque * (some constant). If you say you need HP, then that's the same as saying you need torque.

 

Beg to differ. The (some constant) you write above is not a constant, but RPM. You can always generate more rear wheel torque by gearing the engine down, but only if you have sufficient HORSEPOWER.

 

I did the math. Starting with F= MA you can compute the available acceleration from the engine torque as follows

 

Acceleration = [Engine Torque] * [Gear Ratio] / [Mass of Car * Tire Radius]

 

The gear ratio must include the tranny and rear end ratio. Since the weight of the car and tire radius are more or less constant, this equation says that for any given gear, the car's ability to accelerate will follow the engine torque curve. Thus the maximum acceleration in any given gear will be felt when the engine is spinning at the RPM corresponding to the torque peak. This is the argument everyone takes when claiming "torque rules" and torque is the only thing a driver "feels"

 

Unfortunately they don't take the analysis far enough. The gear ratio is simply the ratio of the engine RPM to the wheel RPM. Thus the equation can be written as

 

Acceleration = [Engine Torque] * [Engine RPM / Wheel RPM] / [Mass of Car * Tire Radius ]

 

But notice that [Engine Torque] * [Engine RPM] = Horsepower.

 

Also note that [Tire Radius * Wheel RPM] = vehicle speed, so the equation for acceleration can be written as

 

 

Acceleration = [Engine HP] / [Mass of Car * Vehicle Speed]

 

 

This equation says that at any given speed, the maximum acceleration that can be achieved is directly proportional to the available engine HP. Thus it is POWER that determines how quickly a car can accelerate, not engine torque.

 

The equation also says that to achieve the maximum possible level of acceleration in the 1/4 mile, you need to gear the car to always keep the engine spinning a the HP peak, NOT torque peak. Since hybrid-electric vehicles and continuously variable transmission have other problems, that means you have to approximate the final equation with a finite number of gear ratios. Thus if you had complete freedom of choice in setting your gear ratios, you would want to select your gears such that shifting from a point just on the high side of the HP peak will put the engine at an RPM just on the low side of the HP peak. That way you would keep the engine near max HP all the way down the track. Obviously the wider and flatter the horespower curve is, the less gears you need and the wider the spread of gear ratios you can tolerate. Conversely, if you do like most people and start with a JY transmission with a fixed set of gear ratios, then you had better build an engine with a broad enough HP curve to cover the spread.

 

The one other point I would like to make is the statement that for any given gear the maximum acceleration (i.e. max torque to the rear wheels) will be felt when the engine is spinning at the torque peak. The links Grumpy posted above interpreted this as meaning you want to drive around in the gear that keeps you near the engine torque peak, that this is where you will feel the maximum kick in the pants. This is incorrect. Lets say you are cruising with the engine revs right at the torque peak and floor it. If you were to downshift to a lower gear such that the engine revs up to near the HP peak and then floor it, you will register an even higher reading on the butt-o-meter.

 

No question. For maximum acceleration maximize horsepower, because the more HP you have the more rear wheel torque you can generate through the appropriate gearing.

 

Now getting back to what you are saying about building a car that is pleasant to drive. I am with you completely on this. I don't want 27 gears (I don't even want 6) and I don't want to go over 3000 RPM unless I feel like scaring my kids. That is why I will follow Grumpy's advice to me many posts ago. I am dreaming of eventually building a 350 with 400 - 425 HP peak HP. This should give me more car than I will ever be able to handle, yet allow me to pass 80% of the cars on the road without taking it out of overdrive.

[Z-Dreamer]

zzzzzzzzzzzzzzzzzzz... oh is it done yet?

 

Mark

[Guest Anonymous]

CHARGE (POTENTIAL ENERGY) - CONVERSION (BURN/EXPANSION) - ENERGY (FORCE/POWER) - PISTON (LINEAR FORCE/POWER) - ROD SMALL END (ROCKING FORCE=TORQUE) - ROD BIG END (RECPROCAL FORCE=TORQUE) - CRANKSHAFT (CONVERSION OF LINEAR ROCKING AND RECIPROCAL FORCE/POWER TO A USEFUL ROTATING MASS) - TORQUE (ENERGY AVAILABLE TO DO WORK AT THE FLYWHEEL END OF THE CRANKSHAFT).

 

CHEMICAL ENERGY IN - CHEMICAL/MECHANICAL CONVERSION - MECHANICAL ENERGY OUT.

 

In the first paragraph;

 

1. We see the available fuel burning/expanding in an enclosed chamber which forces the piston down. Is there horsepower here? No! Is there Torque here? No! Is there RPM here? No! There is only linear force/power here.

 

2. We next pass through the conrod. Is there horsepower here? By definition, Yes! Is there Torque here? Yes because there has been a directional change of force/power at the small end. Is there RPM here? At the small end No, but at the big end Yes!

 

3. Now in the crankshaft: Is there horsepower here? Yes, but in potential only! Is there Torque here? Yes, but in potential only. Is there RPM here? Yes (the engine is running).

 

4. Now we connect a load to the flywheel. Is there Horsepower here? Yes, the definition of horsepower is now a reality. Is there Torque here? Yes, the definition of Torque is now a reality. Is there RPM here? Yes, the definition of RPM is now a reality.

 

5. Where did the horsepower come from? When a load is applied to the spinning flywheel, Horsepower can then be measured as an energy "work force" (it is only useful as a factor of measure in calculating engine performance).

 

6. Where did the Torque come from? Measurable torque is the result of an applied load also. It came from the conversion of chemicals into linear energy/force/power and then conversion into a recripocating torque potential at the flywheel.

 

7. The RPM is self explanatory.

 

NOW TO THE CHASE!

 

1. What makes a car move? The process just discussed is passed through a series of gears to the rear wheels in such a manner as to cause the tires to rotate against the ground causing a forward motion against the entire load.

 

2. Is it horsepower or torque that moves the car? Horsepower is there, but only as a measure of the

"force/power" conversion to Torque. The prime mover of the car is Torque coming from the engine flywheel!

 

3. Why is Horsepower talked about more than Torque? Manufacturers agreed to use this value among themselves and with the public to compare their vehicles; And as noted before it is a harbinger of force/power value.

 

4. And what about the Torque? Actually, Torque, when applied to the driving wheels, becomes a continously variable mechanical function that depends on the transmission gear selected.

 

5. What causes accelleration? A combination of the force/power explained and the resultant Torque from that action. The two cannot be seperated if a complete explanation is to be given.

 

6. Should I increase Torque or Horsepower to improve accelleration? You cannot have more of one without the other except through gearing changes. The basic engine build fixes Horsepower at a peak measure and it's level is extreemly subject to RPM changes, but although the Torque has a fixed peak also, it does have a much more stable (level) so called "power band" which can be used to advantage with proper transmission gearing. The best RPM range can be physically selected through gearing to suit the load as you drive; The horsepower cannot do this but, follows the torque wherever it goes to support the Torque load with it's own means of Force/Power and Accelleration potential!

 

7. Should I look to Horsepower as a guideline for accelleration? It is the "Source" of that phenomena as

an constantly available potential and without it you cannot accellerate but, withought increasing the available Torque at any given RPM there will be no greater accelleration (remember that horsepower follows Torque around wherever it goes).

 

8. Should I look to Torque as a guideline for greater accelleration? Again.....Horsepower has the potential of accelleration in it but has to chase Torque and then Torque causes it's application!

 

 

TAKE TWO ASPIRIN AND CALL ME IN THE MORNING.....LOL

[Pop N Wood]

zzzzzzzzzzzzzzzzzzz... oh is it done yet?

 

Mark

 

LOL. I wondered if I was going to overdo it. But then again, your browser has a back button.

 

Just thinking out loud.

[Mikelly]

I'm with Grumpy. You guys are making this WAY to complicated!

 

Mike

[Kevin Shasteen]

.., heres a few engine combos I know work

11:1 cpr 383

CRANE #119661 roller cam RETARD 6 DEGREES)

afr 195cc heads

edelbrock super vic intake

850cfm carb

1 3/4" full length headers

 

11:1 cpr 383

crane #114681 flat tappet cam

pocket ported 220cc motown heads

edelbrock super vic intake

850cfm carb

1 3/4" full length headers

.., [end quote]

 

Grumpy,

 

What are the specs for those cam's: crane #114681 & #119661: and please dont forget the advertised/seat to seat spec....thankyaverymuch! 8)

 

Kevin,

(Yea,Still an Inliner)

[grumpyvette]

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=114681&B1=Display+Card

 

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=119661&B1=Display+Card