rajahFD

GtoRx7

iTrader: (5)

So, what your saying is a car with the most engine torque will get off the line faster? I think you mean the car with the most Wheel Torque will launch the hardest if both are the same weight. And as we all know, high wheel torque can be made from many combinations.

Orr89rocz

mainly wheel tq..cuz what gets to the ground is what counts...lol

but yeah, more low end torque you can push to the tires on launch the better off your gonna be and the faster ur gonna be.

i should have added this to what i said earlier..i was hinting at it but didnt state it right. same weight cars and gearing, the one with most torque hittin the ground will be faster of the line and probly during the whole run. thats pretty obvious tho, but shows torque force is very important

jimlab

Well, horsepower doesn't really "take over". More torque at any rpm equals more horsepower, so you can look at it as his engine simply making more "high end torque" than yours does. Having a gearing advantage and a higher redline for more wheel speed in each gear doesn't hurt either.

I think people prefer to quote peak horsepower numbers because A) they're usually higher than peak torque numbers, especially with pint-sized engines, and B) most people don't have the first clue how many lb-ft. of torque their engine produces or how their torque curve applies to drivability or performance. For those type of people, torque is somehow low-rpm only and something that only applies to trucks and tractors.

Maximum acceleration (not speed) in any gear is at peak torque... not peak horsepower.

jimlab

Thanks for letting us know that you don't know or understand the math and physics involved. Next.

Orr89rocz

yeah thats it.... hp takin over is just high rpm torque being greater...

jimlab

Yes.

Like dropping the clutch at 6,000+ rpm?

ericgrau

iTrader: (1)

Thanks to those who chimed in after I stopped posting. Someone brought up fatigue, so I'll post on that and then stop again... unless there are sensible questions that other people don't answer (not nitpicking/ignorant/contempt-motivated criticism).

I had an engineering class with a large section on fatigue. Fatigue does break the tranny in the long run. Spinning faster reduces fatigue life linearly (rpms). Higher stresses reduce fatigue life exponentially (torque). Conversely, a small increase in tranny strength allows a large (exponential) increase in rpms, but only a small (linear) increase in torque.

Behind the tranny, engine torque & rpms don't need to be distinguished. A low rpm / high torque engine becomes the same as a high torque / low rpm engine b/c the tranny trades rpm for torque. Behind the tranny, horsepower (torque & rpm combined) determines what breaks. High torque engines do NOT need beefier axles, differentials, etc. Likewise the back end of the tranny doesn't need to be beefier either.

rosey

iTrader: (1)

WRONG! More horsepower means more work is being done and maximum acceleration will happen when the horsepower is the greatest.

Horsepower is a funtion of torque...(torque*rpm)/5252=horsepower. Easy.

88IntegraLS

Sorry man, Labreck is right, as usual.

F=M*A , or seen as the rotational equivalent, T=I*A where I = rotational moment of inertia and A = rotational acceleration. Clearly, acceleration directly correlates with torque.

Horsepower is defined as the rate at which work is done. The faster the car is going, the less it has to accelerate to accomplish the same rate of doing work given negligable frictional losses.

In other words, if you downshift at a given road speed, you can accelerate faster because of gearing *edit* assuming equal torque output, (I almost owned myself, lol!). The rate at which the engine is doing work, defined as Force * Distance, will also increase, but the fact remains, at least according to the widely accepted laws of physics, that acceleration directly correlates with force, not power.

rosey

iTrader: (1)

The fact of the matter is "horsepower" is torque... horsepower doesn't truly exist, its just a math equation - (torque*rpm)/5252=horsepower. Having torque at a higher RPM would mean that the torque is being applied more times per minute and the car would accelerate faster then if it had the same amount of torque at a lower rpm.

If I had an engine that made 500ft-lbs of torque at 2000rpm and 400ft-lbs of torque at 6000 rpm you can plug both points of the graph into the forumla and find out that...

(500ft-lbs*2000rpm)/5252 = ~190hp

(400ft-lbs*6000rpm)/5252 = ~457hp

That engine (assume gearing is constant) would accelerate a car faster at 6000rpm even though it is making more torque at 2000rpm.


Take another example, something we can maybe relate to....

A 86-88 NA rx7 makes 146 HP @ 6500 RPM and 138 Ft/lb @ 3500 RPM. Put it in 2nd gear at 2000rpm and punch it until its 7000rpm redline...all in 2nd gear. You'll notice while its torque is peaking at 3500rpm, the car accelerates fastest at 6500 RPM.

razorback

whats harder on a trans/rearend...


400 ftlbs of torque...or a 6krpm clutch drop?

ie, constant or sudden?

Orr89rocz

i have taken engineering classes too, since i'm in mech. engring. but this makes no sense to me.


my 9 bolt rear broke on about 300+lbft tq at the wheels at about 2800 rpms during my launch at the track on slicks..... a newer condition posi in that rear should handle that all day long but eventually break like mine did.

a 10 bolt rear is even weaker and breaks routinely with mild powered cars..(around 350-400hp). stronger axles, better quality gears, diff support girdle, welding axle tubes, and solid pinion spacer greatly helps increase power holding potential. quite a few cars with 400whp and similar torque are launchin hard on slicks on beefed up rears. the trans is doing quite fine rated at the same power levels with teh rebuild kit.

rear ends under huge torque loads will break down, and better stronger stuff holds that power longer and better. why do you think super high powered GM fbodies upgrade to 9inch ford rears or 12 bolt chevy rears? cuz they are stronger to handle the insane torque load off the launch. because they arent launchin at peak hp,but around peak torque that the motor produces at the wheels...cuz it gets u off the line faster, cuz torque force gets you moving/accelerating

and transmissions suffer under torque too, just like you said. the whole driveline/chassis suffers under torque.


lol if you could fine me a motor making 500lb feet at 2grand and 400 at 6000, sign me up!!! that car would accelerate good the whole time cuz the torque band is so damn flat.

you should have taken a ride in my car...when the converter was stock 1200rpm stall. when i used to punch it in first from 1200rpms, the greatest seat of the pants acceration feel came in around 3000-3200rpms. right where my peak torque is..and the feelin died after that...as i reved to my peak hp at 4200rpms.. the car was accelerating greatest at its point of torque peak. it was obvious just by driving it. lol

shm21284

I am sorry Jim, but you are dead nuts wrong. I just spoke to the lead dynomometer professor at Kettering University and he concurs with my previous statements. He also added that the Japanese were the first to really realize that power determines the rate of acceleration of the vehicle, thus they built smaller engines and revved them higher to create the power. Additionally, he said that peak acceleration occurs at peak horsepower. Here's why:

Torque (Ft. Lbs) is a unit of energy and work. Horsepower is a rate at which energy is delivered, or the rate at which work is performed. An example is moving a car accross an intersection. It requires x amount of work to do it. But what if you want to do it in half the time? Then it requires that same amount of work, in half the time, hence torque (work) over time = Horsepower.

So, back to the moving accross the intersection example. I have a car with 500 ft lbs of torque at 5000 RPM. I gear the car with a 2:1 ratio. The wheels are producing 1000 FT Lbs of torque at 2500 RPM. Now I have a car that just lined up next to me that produces 500 Ftlbs of torque, but its doing it at 10000RPM. Gear ratio is the same 2:1. The wheels are then producing 500 Ftlbs of torque but at a much higher 5000 RPM. Which one can accelerate faster? The one that can produce the same amount of energy in half the time. Thats right, you guessed it! The one that makes more horsepower, which happens to be the one making 500 Ft LBs of torque at 10000 RPM, which is about 952 HP. The other car only makes 476 Hp.

So let's sum it up: horsepower is the rate at which work is applied. Torque is the amount of work possible. So, like I said before, you can do 2*x work in y time, or you can do x amount of work in 1/2 y time, its all the same, because at the end of that y time period, both examples are doing 2*x amount of work.

shm21284

I think you're the one who doen't have the first clue about physics and math involved with automobiles. Next.

shm21284

Well, you have one piece of the puzzle right. Yes, more torque at any RPM makes more horsepower, but in the case of the flat torque curved engine, the same torque at a higher RPM makes more horsepower, as well. Should I remind you of the equation, Jim? HP=Torque*RPM/5252. It's the fact that that torque, or energy, is being delivered in half the time, therefore, the work/time ratio (which is power) is higher. Therefore, in the same amount of time, twice the work is being done.

Actually, Jim, I tend to remember how much torque my engines produce because I use it as a reference curve to build an idea of how the power curve lays out over the RPM range.

Low RPM power, not low RPM torque, is still what is of concern when trying to accelerate a pickup truck with a trailer, a semi truck with a full load, a train, etc. Remember, acceleration depends on power alone, and "driveabiltiy" (a duty cycle which is usually spent in the lower RPM ranges usually not a Wide Open Throttle) is often commented to be better with a "higher torque" engine, i.e. a v8. The reason for this is that it produces say 80% of its peak torque at say 2000 RPM, therefore, its producing much more power at that RPM. This is better than a 4 cylinder, who is producing somewhere around 30-40% of its peak torque at 2000 RPM, which would cause very low "off the line" acceleration as the engine is forced to sweep past 1000, 2000, 3000, 4000 RPM. The 4 cylinder has much less horsepower at this RPM because of that equation I keep bringing up: HP=Tq*RPM/5252. (at any RPM lower than 5252, torque will always be greater than horsepower).

Wrong, Jim. See my post above. Maximum acceleration occurs at peak horsepwer not peak torque, because horsepower is the measure in which you deliver energy (torque) over a specific period of time (Acceleration is the change in speed over a specific period of time) If you disagree, argue with the Professor who runs the Dynomometer with a Ph.D.

Orr89rocz

wouldnt that be 500lbft at 10000rpms with the same 2:1 gear make it 1000lbft at 5000rpms, and your comparing that to 1000lbft at 2500rpms???

but for one, that motor will not be making peak torque at 10000rpms, while the other one could be at that rpm, so naturally second motor has a ton more torque than the other motor. more torque means u getoff the line faster... after that, maybe acceleration is based on work/time which is horsepower. thats why u launch your car at or alittle bit above your peak torque rpm...thats the maximum moving force u can apply off the line to the tires.

shm21284

more horsepower means "get off the line" faster. "Getting off the line" is acceleration; the rate of acceleration is determined by horsepower. I don't know how to be any more clear.

jimlab

Maybe you should go look it up.

jimlab

Well, hell, give me a chance to be all impressed and ****...

Apparently he's not aware that larger displacement engines were taxed at a higher rate in Europe and Japan, leading to smaller displacement engines that had to be turned faster to make more power, but whatever...

Then he's never seen an acceleration graph in g-forces (and neither have you). The rate of acceleration in each gear falls off long before peak horsepower occurs.

Sorry, we're not talking about the amount of work being done over time. We're talking about acceleration. I could be traveling at 60 mph for an hour and do a hell of a lot of work, but with zero acceleration taking place.

And work still has nothing to do with rate of acceleration... next.

jimlab

Then why don't you tell us how to calculate thrust at the contact patch and from that, rate of acceleration in g's...

jimlab

Arbitrary fantasy world examples aside, this is not true. Maximum acceleration (in g's) occurs at the torque peak, not the horsepower peak. If you don't believe me, do a little research.

Horsepower (assuming wide-open throttle) at the rear wheels remains basically constant at any given rpm regardless of gearing. An engine producing 400 lb-ft. @ 4,500 rpm at the flywheel with a 3:1 first gear, 2:1 second gear, and 1:1 third gear and a 4:1 ring and pinion produces the following horsepower at the wheels at 4,500 rpm in each gear...

First gear:
400 lb-ft. * 3 * 4 * 0.85 = ~4,080 lb-ft. at the axles
Axle speed = 4,500 rpm / 3 / 4 = 375 rpm
Horsepower = 4,080 * 375 / 5,252 = 291.3 hp

Second gear:
400 lb-ft. * 2 * 4 * 0.85 = ~2,720 lb-ft. at the axles
Axle speed = 4,500 rpm / 2 / 4 = 563 rpm
Horsepower = 2,720 * 563 / 5,252 = 291.6 hp

Third gear:
400 lb-ft. * 1 * 4 * 0.85 = ~1,360 lb-ft. at the axles
Axle speed = 4,500 rpm / 1 / 4 = 1,125 rpm
Horsepower = 1,360 * 1,125 / 5,252 = 291.3 hp

So if horsepower determines the rate of acceleration, why doesn't a car accelerate as quickly in 3rd gear as it does in 1st gear? Because TORQUE AT THE AXLES determines the rate of acceleration, and TORQUE AT THE AXLES decreases as gearing decreases. This also happens in each gear past the torque peak, unless the engine can maintain a relatively flat torque curve out to redline.

If you want a real world example, try a top speed run in a late-model Corvette. Past torque peak in 4th gear, the car feels "flat" and literally falls on its face. It's a long, slow pull to top speed in 5th gear because of increasing air and rolling resistance, but also because torque at the axles is dwindling rapidly. 6th gear is not considered an acceleration gear in a unmodified Corvette, because it cannot produce enough TORQUE AT THE AXLES to overcome aerodynamic and frictional losses at high speed.

Now swap the 3.42 differential in the Corvette for a set of 3.73s or 4.10s and make no other changes. Take it to a dyno and the measured horsepower will change very little, if at all. Take it out on the road and suddenly it accelerates much more quickly than it did before. What happened? It now produces more TORQUE AT THE AXLES in every gear, and TORQUE AT THE AXLES determines the rate of acceleration of the vehicle.

rosey

iTrader: (1)

So I hooked up a G meter in my mazda 3 (daily driver) which peaks in torque at 4500rpm and hp at 6500 rpm. I tried 2nd gear and 3rd gear and in 2nd gear did like .29G at 4500rpm and .22G at 6500rpm. 3rd was .18G at 4500rpm and .15G at 6500rpm.

Guess I was wrong...just made more sense to me that horsepower would make more of a difference because cars with small torqueless engines (like my rx7 when it was rotary powered) seemed to pull harder at high rpms. I guess I just wanted to get the chance to tell jimlab he was wrong and didn't do my research before hand....someday though, I will get my chance! haha

GtoRx7

iTrader: (5)

Totally correct. Now on a side note, I find it interesting how guys like nilhilantic are always wanting/ happy with a engine producing a sharp decrease in torque, past the peak torque to redline. I have always been one to try and make a engine delivering near peak torque all the time, very flat. And then dropping only slightly after peak torque, all the way to redline. I would think this is the best method, as this engine will accelerate the same, and hard, for its whole powerband. Is there any reason having flat torque is ever a bad thing?

SPOautos

Why dont you tell us how a quarter mile calculator that uses nothing but hp and weight is so accurate? Especially since it has no idea of the actual tq, gears, or anything else....nothing but the hp figure and the weight.

Especially since there is no math to support that hp creates acceleration

Stephen