Low End vs High End torque

[KarateBoy]

i know that torque plays a big role in acclerating but what is better low end 2K-3k revs or high end 4K-6K. I would think that high end because then you are using your peak Hp and Peak torque- or something near it. Or is this just a matter of preference. Also how can you increase the torque of a car?

[crisis]

Originally Posted by KarateBoy

i know that torque plays a big role in acclerating but what is better low end 2K-3k revs or high end 4K-6K. I would think that high end because then you are using your peak Hp and Peak torque- or something near it. Or is this just a matter of preference. Also how can you increase the torque of a car?

Depends on the application. Low end torque gets you off the mark quicker but high end torque would probably pull higher ks. Off road driving I would definitely prefer low torque where the motor didnt have to be stressed to maintain momentum through sand and over hills etc. Also driveabilty around town is helped by lower torque. Henk will be able to inform you of the advantages (yes I said it) of diesels in this regard. The length of intake runners has an effect of where the torque is maximised but it will take one of our more technically informed members to explain the finer details.

[KarateBoy]

i understand the point of off roading but what about "sport car" wise like a supra, skyline, 350z, etc... would high end torque be better there and when you're in 1st i doesn't take long for a car to rev up from my experiences. But i want to hear more about this so lets keep the good replies coming

[Matra et Alpine]

Originally Posted by KarateBoy

i understand the point of off roading but what about "sport car" wise like a supra, skyline, 350z, etc... would high end torque be better there and when you're in 1st i doesn't take long for a car to rev up from my experiences. But i want to hear more about this so lets keep the good replies coming

It's generally accepted that a torque curve leading the power curve is better.
The where you want torque is about drivability for speed.
WRC are now playing with 4-speed boxes because they have managed to increase the torque across the range enough to save them a grea. The theory is avoiding a gear change will make stage times quicker as the driver won't change gears as often.
Because of the way torque ss developed/measured, I don't think you can expand torque beyond the upper power band, but I'll need to think about that overnight to remember why
Simply put you can always use a gearbox and clutch to 'exchange' power for torque. An extreme example of this was the 12 geared bikes raced on the isle of Man TT in the 60s. These had a tiny usable power band and next to no torqu, so riders used gears to keep the engine in that narrow band.
On the example you quote of quick spin up in first then you could utilise a higher first gear if the engine had enough torque. My father had a BMW 325i for a while and as at the time he was in his 60s he found it too harsh and difficult to control acceleration in 1st and got into the habit of driving off in 2nd then changing to 4th. Drove 10s of thousands of miles in that car for a couple of years and I don't think he ever used 1st or 3rd and there was not a single grumble form it
Of course if you start modifying gearboxes to get the best acceleration then you're likely going to lose some of the flexibility in the original gera rations shoen by the manufacturer

[henk4]

Originally Posted by crisis

Depends on the application. Low end torque gets you off the mark quicker but high end torque would probably pull higher ks. Off road driving I would definitely prefer low torque where the motor didnt have to be stressed to maintain momentum through sand and over hills etc. Also driveabilty around town is helped by lower torque. Henk will be able to inform you of the advantages (yes I said it) of diesels in this regard. The length of intake runners has an effect of where the torque is maximised but it will take one of our more technically informed members to explain the finer details.

Too much honour here, and beware the converts have a habit of becoming the real fanatics.

But anyway yes, the diesel will provide you with low end torque, but everything is relative because power development normally stops above 4000-4500 revs so the actually usuable rev range is not so big. When going up the gears I hardly ever rev above 2300-2400 as full torque sets in at around 1700 revs, although you need to rev a bit further from time to time to clean the engine internally. 150 kph comes with 3100 revs in 5th gear.
I just read the story about Matra's father's BMW and I could probably do the same. Idling (1000 revs) only will bring me into 3rd gear, by just feathering the clutch. I could probably do without 2nd and 4th gear.

[fpv_gtho]

Originally Posted by crisis

The length of intake runners has an effect of where the torque is maximised but it will take one of our more technically informed members to explain the finer details.

ive seen somewhere how it all works with the intake runners, its something like pressure waves reverberate throughout the intake runners and whether its a short or long runner determines where the torque comes on in the rev range. you can get the best of both worlds here with a variable inlet tract, or just a 2 stage system like what Ford implement on the Falcon's 6

[khooper]

But as far as low end and high end when talking about engine performance people are generally referring to the RPM range.

[Kozy]

Originally Posted by KarateBoy

Also how can you increase the torque of a car?

To increase torque is simply to increase the amount of air and fuel burnt with each stroke, this is in the realms of regualr engine tuning, i.e. getting as much air in the cylinder as possible.

What I think you possibly meant was how do you shift the torque curve up and down the rev range?

Lets take a simple 2 litre four pot engine. This engine has a rough sine wave torque curve, i.e. the peak is around the middle of the rev range, say 4.5k out of an 8k max. One thing to remember here is that while the engine speed varies greatly, the speed at which air can accelerate and change direction doesn't. This is largely about optimising the engine to work with the natural resonances within the air.

First, we want to shift the peak torque down the rev range, to about 2k. There are a number of ways to do this. The inlet manifold is a good starting point. Ideally, we will want to increase the manifold runner length. What this will serve to do is increase the time it takes for one pressure pulse to bounce off the closing inlet valve, back up the runner, bounce off the back of the plenum and return down the runner to the valve opening. With the stock runner length, the amount of time this movement took was synchronised perfectly with the valve opening at 4.5k. While this effect is in sync, the pressure pulse will raise air pressure as the valve opens, creating a small boost in volumetric efficiency.

By increasing the distance and thus time it takes to travel this distance within the runner, the pulses will become synchronised with the valve openings at lower rpms.

The next thing we can do with the manifold is decrease the throat width of the runners. This serves to speed up the velocity of the air mass within the runner. This air mass in turn generates more kinetic energy as it travels down the runner, and creates a ram effect as it is forced into the cylinder at a point where it would otherwise be full at atmospheric conditions.

The exhaust manifold runners can also be tuned in length for a similar effect. Longer runners will increase low range torque, which is why a 4-2-1 style header will yeild better low speed gains than a 4-1 type. What the ‘2’ in the 4-2-1 header is doing is effectively increasing the length of the primaries and thus increasing the pulse length.

We can also alter the valve timing to suit a lower torque peak. In the standard engine, the inlet valve will open a few degrees before TDC, and close a few degrees after BDC, and the exhaust valve will open a few degrees before BDC and close a few after TDC. To shift the peak torque lower in the revs, we will want to decrease the amount of time the valves are open, especially the period when both valves are open together, otherwise known as the duration of the camshafts. By opening the inlet valve later, i.e. closer to TDC than standard, we are reducing the possibility of the upwards stroke of the piston forcing exhaust gas into the inlet port, as at low speeds, this will easily overcome the pressure and velocity of the intake charge. By closing it earlier we are avoiding a similar scenario, where the upwards motion of the piston on compression forces the fresh intake charge back out of the cylinder and into the inlet port, albeit with the addition of considerable heat soak.

By opening the exhaust valve later, we can maximise the amount of time the high cylinder pressure is forcing the piston down before releasing the burnt gases out of the exhaust port. (It may or may not be obvious, but the exhaust gases largely exit the cylinder under their own pressure, not by the upwards motion of the piston.)

By closing it earlier, we are reducing the oppourtunity for inlet charge entering the cylinder during the valve overlap period to exit straight out the exhaust port without being burnt.

These are the simplest ways to shift torque down the rev range.

[Kozy]

To shift the peak torque up the rev range say to 6.5k, we want to use the same principals, but in reverse.

The manifold runners want to be short in length, this will decrease the time it takes one pressure pulse to reverberate back down to the valve, and so will be in sync with the valve opening at high rpms. Likewise, the throat widths of the runners will need to be wider. Whilst at low rpm we needed to boost the velocity of the inlet charge by restricting the throat width, at higher rpm, the inlet charge already holds significant veolcity, and so such a restriction would only suffocate the engine at high speeds. Thus a wider bore in the runners will allow the engine to breathe easier at high speeds, boost the upper range torque.

Valve timing will need to be changed also. In the low speed engine, we opened the inlet valve late, now we want to open it early. By opening it earlier, the air mass in the port is given more time to begin moving forward into the cylinder, and the pressure and velocity of the inlet charge is enough to overcome the residual exhaust gas pressure in the cylinder. If we opened the valve to late at this speed, the inlet charge would not have sufficient time to move into the cylinder, and we would lose VE.
By closing the valve later, we are utilising the high speed ram effect of the inlet charge to cram in more air particles than is theorically possible at atmospheric pressure, even as the piston begins to rise in the cylinder.

We will also shift exhaust valve opening forwards, this gives the cylinder time to ‘blow down’, whereby the the exhaust gases exit the cylinder under their own pressure. Most of the useful work is done on the power stroke by about 60°, and so we are sacrificing little by allowing the spent gas out early, however what we gain, is a reduction in pumping losses, generated when the piston has to physically force the remaining gases out of the cylinder. This is an increasing possibility as the engine speed increases, so by opening the valve early, we are ridding the cylinder of as much pressure as possible.

By closing the valve late, and leaving the inlet and exhaust valves with a long overlap in which they are both open, we are allowing the vacuum effect of the exhaust gas pulse travelling down the exhaust runner to ‘suck’ fresh inlet charge into the cylinder.

These modifications will serve to improve high end torque, at the sacrifice of low end.

Some systems, like variable inlet manifolds such as Toyota’s TVIS system allow two different optimisations, the same with variable valve timing systems such as Honda’s VTEC. By ultimately, even with these, the toque can only be optimised in two very narrow parts of the rev range, the rest remains a comprimise, albeit a slightly broadened one.

There are of course plenty of other ways to alter the engine characterstics in design, but these are the simplest modifications to an existing design.

Hope it is of some help to someone!

[Kozy]

Holy moly, just realised how OLD a thread this is! 2004?! Whatever possessed you to ressurect such an old post dude?

[Kitdy]

Kozy, that was very informative and nicely worded.

Props to you and a metaphorical +1.

[ruim20]

Thank you Kozy, very informative posts.

Just a question, would it be possible for an engine to have some kind of automated telescopic manifold runners in order to change the peak torque moment in the rev range?

[Knuto]

Originally Posted by ruim20

Thank you Kozy, very informative posts.

Just a question, would it be possible for an engine to have some kind of automated telescopic manifold runners in order to change the peak torque moment in the rev range?

Check out the Mazda 787: http://www.fd3s.net/787B/26b_3.jpg
Or the Honda F1 V12:http://www.khulsey.com/inomoto_honda...f1-engine.jpeg

[ruim20]

Originally Posted by Knuto

Check out the Mazda 787: http://www.fd3s.net/787B/26b_3.jpg
Or the Honda F1 V12:http://www.khulsey.com/inomoto_honda...f1-engine.jpeg

Thank you

Why don't road cars use this system?

Do they use variable valve timing systems instead of this?, could both systems be used together?

[Quiggs]

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