Torque vrs HP

My Nissan has about a 20 HP/Ft-lb difference between its peak torque & HP numbers, torque being higher. In looking at gear ratios, the two choices (the two easy choices) each have their merits for where I am shifting on track. The torque curve is moderatly flat, but where it crosses the HP numbers, the HP has already peaked.

The shorter one puts me into the crest of the HP curve, but the falling (or fallen) part of the torque curve. The taller option puts me into the rising part of the HP curve and the falling part of the torque curve, but still moderately high on it.

I have never read or heard a good description of the difference between torque & HP in driveability and lap times so I am not sure if I should utilize the higher torque or the better portion of the HP curve. Most people go on about HP and never say what the torque is, but I know from driving diesels torque does play a big part.

Curios what peoples thoughts or experiences are.

Matt

Well . . . a higher wheel horsepower at a given speed will produce greater acceleration.

Now . . . when I'm picking a gear ratio, I graph the torque curve and modify it with the gear ratios (oversimplification, because it assumes equal friction losses at all gear ratios) to see what will work best. The best combination is whatever will give you the highest wheel torque up to a realistic top speed. I ignore anything over about 120MPH, because that is out of my range at any track I run. Again, it's an oversimplification - but it seems to work.

Put simply: Horse power = torque / second.

Ideally you want to be at peak torque coming out of a turn and be at peak horsepower going down a straight.

For best performance you want to always be above peak torque. I always shift at roughly peak horsepower +500 rpm. That way after I shift I'm still above peak torque. Its a decent rule of thumb.

I prefer my shift points to be right after I've exited a turn. I think that gives me the best accleration through the turn and a better starting speed for the straight.

MMiskoe,

I'm not sure what you are saying in your second paragraph, so if my response doesn't help at all perhaps you could try to state it differently.

Are you saying that you shift at specific points on track with both gearing? If so, don't do that. Shift based on your engine, not track location.

Here's my really simple way of looking at it. Torque is what you want when coming off a corner. It's what gets you going. HP is what you want going down the back straight cause it gets you top speed. The little truck motor in my 240 has good torque so I come off the corners well, but HP starts to fall off after 6000-6200 so I'm lagging a little at the end of the back straight at Road Atlanta.

I'd think you'd want to gear as low as you can go without hitting your rev limit too soon on the fastest part of the track. That way you stay in the meaty part as long as possible. I'm curious to see what some of the more experienced people say.

David

If you have a dyno chart, you can graph the HP and torque curves per gear. Graph them on top of each other, and see where the graph from one gear overlaps the graph from the next highest. That crossover point is your shift point. If they don't cross, then shift at redline.

I've got an excel spreadsheet that does it for me (PM me if you are interested).


Really, though, torque and HP are intertwined. HP is just the torque multiplied by the RPM (and then divided by 5250). So putting out 1000 ft-lbs at 1 RPM (0.19 HP) == putting out 10 ft-lbs at 100 RPM (0.19 HP). In other words, HP is how many times you can put out a given torque over a period of time.

That said, I think you want to head towards the max HP number. Why? Because the max HP is your best combination of torque AND RPM combined.

E.g. your best torque is at 1000 RPM, and is 100 ft-lbs (19 HP). (yes, your nissan is now a honda! lol!) You can spin your motor to 10000 RPM, and there it only makes 50 ft-lbs (but now makes 95 HP). So it's sorta making 20 ft-lbs 10000 times a minute instead of 100ft-lbs only 1000 times a minute.

So, to summarize (since I've confused), you should tune to be close to the crest of the HP curve, not the torque curve.

joe

Horespower is how fast you hit the wall.

Torque is how far you drag the wall with you.

For the sake of argument though, what about a car (like the audi diesels) that have roughly half the horse power that they do torque. Would you still tune to the peak HP curve if the torque had started to go away? I know from driving trucks that they have great pull at the lower part of the gear, then just fall off as the rev's increase since they don't make much in terms of power. Go to your next higher gear and get that grunt again.

My comment about the different ratios each having merit is in regard to where on track I'm shifting - ie not trying to upshift on the steep part of an uphill, or having to either upshift for a 100 yards or run higher RPM than I like.

Thanks for all the input.

Matt

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For the sake of argument though, what about a car (like the audi diesels) that have roughly half the horse power that they do torque. Would you still tune to the peak HP curve if the torque had started to go away? I know from driving trucks that they have great pull at the lower part of the gear, then just fall off as the rev's increase since they don't make much in terms of power. Go to your next higher gear and get that grunt again.
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I'd still map out the HP/torque curve and see where they overlap for each gear. The diesel may have a wildly different peak torque vs HP number - but since the torque happens so low in the rev range, the peaks will be close to each other. It is still the case that you want to be in the meaty HP point of the curve for acceleration.

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My comment about the different ratios each having merit is in regard to where on track I'm shifting - ie not trying to upshift on the steep part of an uphill, or having to either upshift for a 100 yards or run higher RPM than I like.
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Yep - I can understand that. I can run most tracks with a 4.6 rear end. However, at Summit Point, that 4.6 means I have to shift in the carosel (instead of right after), and that I also have to perform a downshift entering a turn that requires very little braking. So my braking is spread out a little more (to do the downshift) AND I can't just do a nice soft left-foot brake for the turn. So I'm going to try a 4.3 ratio.

joe

I would not try to compare it to a diesel because of how different diesel engines are.

For driving performance you want your engine RPM above peak torque. The best performance you can get is by keeping the engine RPM between the peak torque and peak HP.

I wouldn't shift going up a steep uphill and its probably not worthwhile to shift for only 100 yards. Its very situation dependant on when you should adjust your shift points.

Even easier way to do it if you got some dyno time....

Pull some runs in each of your gears and have the dyno plot your accelleration vs rpm.
Then you can see where lower rpm in higher gear out accellerates higher rpm in lower gear.

Most dynos can read accelleration vs rpm in ft/sec.

jhooten, what you wrote is the funniest thing I have seen on here, thanks for the laugh.

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Horespower is how fast you hit the wall.

Torque is how far you drag the wall with you.
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:D :happy204: :035: :lol:

HP = (Torque x RPM)/5252

What makes your car go faster is horsepower and, as the formula implies, you can get it from torque or rpm. The diesel Audi has no revs but enough torque to pull tree stumps; F1 cars have no torque (due to displacement limitations) but spin at 18,000+. Both get the job done, they just take different approaches.

It's an interesting situation. Let's say you have two cars that are identical; they even have the same HP. The only difference is that one engine gets its power from high torque with a low redline, the other from the opposite, i.e. high rpm but moderate torque. The two cars have identical HP but will feel very different to the drivers: the high torque car will launch hard off the corner but seem to fade as the straight progresses; the hi rpm driver feels bogged down at the launch but feels power building as the motor winds up.

Gearing being equal, it's a question of where you want to pass/get passed: at the beginning of the straight, or at the end?

(The "5252" is just a constant which converts from metric/SI units.)

***HP = (Torque x RPM)/5252***

Torque = a Honda CRX

RPM = a Mazda RX-7



Torque leaves the black tire marks & when the black tire marks disappear HP has taken over. :023:

In SM we can't change gear ratios...(well you can but you have to change cars ;-) ) but I adjust my shift points based on maximizing the area under the curve. I made a spreadsheet for it that calculates the area between shift points and I interactively adjust the shift points based on getting the max area.

The method some of you guys use sounds good if your torque/HP curves are monotonic until redline...or at least for the usable part. But for my SM with the restrictor the torque dies after about 6200 RPM however the HP flattens out so there's usable power there. Now if you drive a VTEC Honda (I drive a GSR daily) you know both your torque and HP drop like Wylie Coyote off a mesa...so shifting until redline is probably the best anyway.

a car accellerates fastest when torque at the wheels is maximum. Horsepower is a somewhat confusing concept. If you know the torque curve of the engine and your gear ratios, you can always see at what point in the curve you will deliver the most torque to the wheels by comparing the output torques of the gears at the same vehicle speed. Say you have 1:1 4rth and a 1.25:1 3rd. Looking at it both ways, you have a 1.25 or a .80 ratio (.8 is the reciprocal of 1.25). In third, the engine will run 125% faster and multiply the engine torque available at that rpm 1.25x to the driveshaft, and would accelerate the car faster-but the engine torque is falling off quickly when you get up in revs. In this vehicle, if the 80% of the torque at 4800 rpm is greater than 100% of the torque at 6000, you should be in4rth (.8x6000=4800) In a VW it certainly is.
To keep it simple, I used 1.25(.8) for an example. In all cases, express the 2 ratios as a fraction and also as the reciprocal of that fraction to manipulate/compare output torques/rpm/gear/speed. The problem with using this info is that most of us don't have good torque curve info unless we go to a good dyno/operator.
If you do and analyze the info, you'll find that the old proscription is pretty close: unless your ratios are really shitty, you want to upshift at peak HP and your RPMs will drop to the area of peak torque. But if you have good data, you'll ussually find that despite your seat of the pants feelings, you're shifting way too late!
In any gear, the max acceleration is at the torque peak-which is at a pretty low rpm in most IT cars. phil

Best explanation I've heard. Shift when the torque at the wheels in the next gear matches the torque at the wheels in the current gear.

Thanks Phil,

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The torque curve is moderatly flat, but where it crosses the HP numbers, the HP has already peaked.
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Is it just me, or is something wrong here?

If I understand correctly you are saying that peak HP happens at a lower RPM than the peak Torque?
If so, something is wrong with the plot chart or your interpretation of it, or my interpretation of your statement.

IF Torque increases as RPM increases than HP will also increase. HP will continue to rise as RPMS rise as long at torque is constant or begins to fall quicker than RPMS increase.

As far as which is prefered Torque or HP....the more gears you have to choose from and the more constant the speed of the track is, the less important torque is.

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Horespower is how fast you hit the wall.

Torque is how far you drag the wall with you.
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Great analogy.
Electric version is volts and amps.

Ever notice how tq and hp curves are the same at a certain point?

Yep, function of the math. At 5252 rpms the Torque and HP numbers will be equal.

At 2600 rpms the torque will be approximately twice the HP. At 10500 the HP, twice the torque. etc.

Torque can get you out of trouble
Horsepower can get you into trouble