Horsepower vs Torque????

[jenavet]

I am all confused on this one....my 406 was dynoed at 590 horsepower @ 6500 rpm and 528 lbs of torque @ 5000 rpm....the car ran 9.89 @135 mph all day long.

I called comp cams and the guy spent about 20 minutes using all my stats and suggested a different cam that he said was a good step up in power...i pulled the motor,had the cam installed and re-dynoed the motor,without any other changes......i was severley disappointed with the new cams performance on the dyno...it made 550 horsepower @ 5500 rpm and 565 lbs of torque @ 4500 rpm

i was kinda ****ed off,and was going to put the old cam back in,but decided to put some passes on the new cam.....the car went 9.69 @ 137.98 mph!!!

what the hell?? 40 less horsepower but the car picked up 2 tenths and almost 3 mph...i dont understand.. it did make 37 more lbs of torque...so obviously the torque must have some relationship with et........this has got me nervous because my spare motor i had put on the dyno again the other day with some minor changes...it made 650 horsepower @ 7500 rpm...but only made 535 lbs of torque @ 5500 rpm...does anyone understand the relationship between horsepower and torque?? cuz i dont!!

 

[MaineShark]

Horsepower is just torque, applied over a length of time.

What moves your car isn't the peak numbers. It's the whole powerband. If you have the actual dyno charts, you may notice a wider powerband with the new cam than with the old one. What you can do at one particular point is not really critical. The average that you can do over the whole range of RPM's that you actually use is what gets the job done.

Now, if you have a larger number of gears, you can keep changing gears to keep the engine operating in a more narrow range, so you can concentrate more on making that particular area strong.

In other words, if you typically use the 3000-6000 rpm range, you want the average power over that whole range to be as high as possible. Your previous engine may have had a higher peak, but it probably had a lower average.

Joe

 

[jenavet]

joe
I am still confused...but then again,it dosent take much to mess me up...i launch the car at 4800 rpm and shift the car at 7200 rpm[per comp cams]......if i take an average over that rpm band...the slower motor made an average of 540 horse and 498 lbs of torque........the faster motor had an average of 525 horse and 485 lbs of torque.......so over the rpm band that i use...the faster motor made 15 LESS horse and 13 LESS lbs of torque...really weird.

 

[Brett]

Haha, wow, I was just going to post basically the exact same thing.

There is still something I can't seem to get straight about horsepower VS torque.

Horsepower, as I have researched, is top speed, while torque is acceleration.

So, how come horsepower always seems to be the primary statistic of a vehicle's performance? You always see the horsepower listings of a vehicle, but not its torque numbers. When someone dyno's, it's always about what rear-wheel horsepower they put out. It seems to me that torque, especially over a broad powerband, is what everyone should be concerned about. But usually you don't even see the torque stats of a vehicle unless you do some deeper research. Why is that? Even in trucks, since torque is pulling power as well, it seems torque should be the only stat anyone cares about.

Also, since torque is acceleration, how do the low displacement import vehicles go as fast as they do when they have such low torque numbers? They have peaky horsepower and low torque, so what is the deal? How do they do it?

 

[Ken]

This one is as old as the internal combustion engine, at least. I'm sure it's gonna turn out to be a long thread too, because everybody's got their opinions. :L

I think this subject will require input from a professional, i.e., an engineer-type, and one who is adept in teaching adults, especially those whose ideas are already firmly implanted.

I'll see what I can dig up for you all.

 

[grumpyvette]

we have all heard it, " you need massive low rpm tq" "you need a screaming high rpm hp peak" well heres some info,
More in-depth description:
http://www.revsearch.com/dynamometer/torque_vs_horsepower.html
http://www.dynacam.com/Product/Torque_vs__Horsepower/torque_vs__horsepower.html
http://vettenet.org/torquehp.html
http://auto.howstuffworks.com/horsepower4.htm
http://homepage.mac.com/dgiessel/engine/hpvstq.html

first thing to keep in mind is that theres no such "thing" as horsepower, horsepower is a mathmatical formula for the RATE at which TORQUE can be applied the formula for hp is (tq x rpm/5252=hp
example
450 ft lbs of torque at 3000rpm=257hp
450 ft lbs of torque at 6000rpm=514hp
because the torque at the higher rpm useing gearing can be applied faster
here read this
http://www.69mustang.com/hp_torque.htm

http://www.ubermensch.org/Cars/Technical/hp-tq/

http://vette.ohioracing.com/hp.html

where most guys go wrong is in not correctly matching the cars stall speed and gearing to the cars tq curve, if you mod the engine for increased high rpm performance but fail to also match the stall speed and gearing to that higher rpm tq curve much of the potential improvement is wasted.
example

in the close to stock engine above, the engine should be geared to stay in the 3500rpm-5000rpm range for max acceleration (lower in the rpm range if mileage is a big factor)

in the moded engine above the rpm range moved to 4000rpm-6500rpm requireing differant rear gears and slightly higher stall speeds to gain max acceleration in the same car,
you should readily see that a trans that shifts at 5000rpm will work in the first example but would waste most of the power curve in the second example,where shifting at 6500rpm under full power acelleration would make more sence.
a 3.08 rear gear and 700r4 trans matches the first example well but it would take a swap to a 3.73-4.11 gear to allow the engine in the second example to keep its most effective power band matching that second power curve well.

links youll need to figure out correct rear gear ratios

http://www.miata.net/garage/tirecalc.html

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

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

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

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

http://www.prestage.com/Car+Math/Ge...io/default.aspx

http://www.geocities.com/z28esser/speed.html

http://server3003.freeyellow.com/gparts/speedo.htm

http://www.pontiacracing.net/trannyratios.htm

http://www.tciauto.com/tech_info/gear_ratios.htm

 

[Ken]

There ya go - someone who knows what he's talking about; an expert. Hell, he's been building engines for forty years! Thanks Grump!

_ken

 

[Roadster Fan]

Here goes.....

my take on horsepower and torque. And yes Ken, I'm an engineer...albeit a construction engineer though. Please keep in mind, this is how I understand the relationship.

Horsepower is a force, while torque is force x distance. If you've used a pry-bar, picture the weight you apply to the bar (while prying something) is similar to horsepower. The torque would then be the weight x the length of the bar. So a 200# (~hp) man pushing down on the end of a 10' bar would be the same torque as a two 200# men pushing down on a 5' long bar.

How does this apply to cars and racing? Ok, let's use the pry-bar example. Say you've got your vette buddy over and you've been drinking half the day. Now that you've got him in a weakened state, you get him to agree to help you move that boulder on the back forty....embedded well into the ground to boot...to up near the house (for the wife of course). Grab the 10' pry-bar and a short piece of log to pry against and off you go. The initial prying of the boulder requires considerable bouncing and jumping by both of you to just dislodge and move the creature (large amount of torque..... 2 men at 200# times 10' = 4000# ft of drunk man-torque). Once you dislodge it, it becomes easier to roll. Now you only need to just push down rather than jump up and down on it, to move it. After you get it rolling, you notice that you can move the log further up the bar to keep it moving while pushing on it. Once you get that baby rolling you gotta be pretty quick moving that log in between prys so you don't lose that momentum. The required torque is diminishing as it gets rolling.....2 men at 7'= 400 x 7' = 2800# ft of still drunk man-torque. Pretty soon you realize that baby is moving along pretty well, so you decide to move the move the log to 5' up the bar. Before you know it the log is only a couple feet from the end you are prying on (2 men x 2' = 800# ft of tiring drunk man-torque) and that boulder is outa control speeding towards the house. Well....you get the picture. Reminds me, I should put some bigger brakes on my vette after I do the H/C package.

Getting back to cars and racing. Street light to street light and 1/4 mi runs, I would want high torque...as long as I could apply it to the ground. Nascar and high speed runs, I would want high horsepower as primary. The torque gets you moving and accelerating, while the hp takes over once it's up and running.


Hopes this helps,
Brett

 

[Imachad]

Wow!

Thanks Roadster Fan! That made alot of sense to ME at least. I love the bit about "Drunk man torque" ahhh, we've all been there....well i know i have

 

[jenavet]

whoa roadster fan....i didnt get it at first...but i had 4 or 5 beers and then i started to realize what you are saying...if i read the very last paragraph you wrote,then went back and read the story about rolling the rock around ,then it did make sense to me...which i think explains why the cam with the higher amount of torque gave my car the faster et.........but whats even more powerfull to me is that a girl can get you and a buddy to roll a giant rock around the yard...thats what i call girlpower.

 

[JohnZ]

Simply stated, torque is E.T, and horsepower is trap speed; what really matters in a street engine is "torque bandwidth" - the length of the torque curve and the area underneath it. The fancy magazines always bellow high-rpm dyno horsepower numbers, but never deal with driveability or torque at normal street rpm - how much time do you spend on the street at 6500 rpm and WOT? Horsepower alone is a lousy way to characterize a street engine, but it makes great magazine covers; torque is what matters for a daily-driven street engine. Been building them for 40 years.

 

[Ken]

Say you've got your vette buddy over and you've been drinking half the day. Now that you've got him in a weakened state, you get him to agree to help you move that boulder on the back forty...

Speaking from direct experience Brett? :L

Thanks.

_ken

 

[Roadster Fan]

You're welcome guys.

I had fun thinking up that visual at lunch

Ken- it's usually the wife I've got to loosen up first to get her to agree to a few of my ideas

Jenavet- yeah that girlpower is more addicting and powerful than any motor I've seen. Heck, it's toppled governments.

Imachad- glad someone could understand my explanation.

JohnZ- you'll be getting a pm from me about stopping out to see that grand garage of yours this summer :cool


Brett

 

[Ken]

Funny, but a new Volkswagon commercial gives a pretty good image of the torque/horsepower thing.

In the commercial, the VW salesman is riding shotgun and is telling the female driver that the car has a lot of low end torque. He tells her: Go ahead, pass that truck. Feel how it accelerates as you pass? That's torque doing the acceleration, horsepower does the rest.

I thought that was a pretty good, albeit simple, explanation of torque and horsepower, and how the terms relate.

_ken

 

[Rob]

Maybe this will help:

http://corvetteactioncenter.com/tech/hp_torque.html