Engine tun­ing tra­di­tion­ally con­cen­trates on rais­ing peak power, but in truth it’s torque – ro­ta­tional force – that does most of the hard work, ar­gues Chris Hor­ton. Photos by Porsche and the au­thor

911 Porsche World - - Contents -

Tech man, Chris Hor­ton, and the the­ory and ben­e­fits of the torque curve

The head­line ben­e­fit of rais­ing any engine’s swept vol­ume, as we’ve sug­gested else­where in this anal­y­sis of Hartech’s en­larged M96 and M97 units, is an in­crease in power. It’s what mag­a­zine road-tests, tun­ing fea­tures and Top Gear pre­sen­ters have banged on about for years. (With ar­guably one no­table ex­cep­tion; see be­low.) But to Barry Hart – and, in truth, to the rest of us, if we did but know it, and had not be­come fix­ated on mere bhp – the most im­por­tant gain is a com­pa­ra­ble in­crease in torque. And, if you and/or your engine builder have done your sums, a mod­est but no less use­ful re­duc­tion in the engine speed at which both curves sub­se­quently peak.

‘Porsche sports cars have gear­ing that most own­ers will never ex­ploit to peak revs – and cer­tainly not in the higher gear ra­tios,’ says Barry. ‘Tun­ing a given engine to raise the revs for max­i­mum power looks im­pres­sive on pa­per, but of­ten fails to pro­vide the real-world per­for­mance in­crease that the graph im­plies. But even a mod­est in­crease in ca­pac­ity will usu­ally re­sult in much bet­ter torque, and so the eas­ier, faster ac­cel­er­a­tion that will also suit both typ­i­cal road users and oc­ca­sional track­day driv­ers far more than the re­sults of purely ex­ter­nal mod­i­fi­ca­tions alone. And a mod­est in­crease in ca­pac­ity will of­ten re­sult in the engine be­ing less stressed than it is by con­ven­tional tun­ing.’

It’s all about what Barry calls the rev-drop area. ‘Your engine’s max­i­mum brake horse­power is ef­fec­tively the same in any gear – and sim­ply de­ter­mines its po­ten­tial to do dif­fer­ent things, de­pend­ing on the way in which it trans­mits the out­put. But torque is not quite the same thing. You have the most torque at the wheels in the low­est gears – which is why when driv­ing on mud or ice you need to stay in as high a gear as pos­si­ble, in or­der to re­duce the chances of wheel­spin – and as you shift up­wards the torque re­duces in in­verse pro­por­tion to the ra­tio. Torque in sixth gear – again at the driven wheels, of course – is typ­i­cally be­tween four and five times less than it is in first.

‘When you change up through the gears, you in­evitably ex­change that torque for ro­ta­tional speed. To put it an­other way: in­crease wheel speed, so that the car can travel faster, and you re­duce the avail­able torque by the same pro­por­tion. This is why your car nat­u­rally ac­cel­er­ates faster in the lower gears, and less quickly in the higher ra­tios. And why, when you come to climb a gra­di­ent, you have to change down in or­der to main­tain your speed.

‘When you change gear at the rev limit the engine speed nat­u­rally drops to the level that is ef­fec­tively de­ter­mined by the next higher ra­tio. So if you are driv­ing to max­i­mum revs be­fore chang­ing gear you cre­ate a “rev drop” – or more likely a series of rev drops – in which the ve­hi­cle has to ac­cel­er­ate back up to the rev limit be­fore you shift up again. And this rev drop varies in size de­pend­ing on which gear you are in. It’s usu­ally at its great­est be­tween first and sec­ond gear, and at its low­est be­tween fifth and sixth. Es­sen­tially, the fastest over­all ac­cel­er­a­tion will be achieved by hav­ing the great­est av­er­age torque in that rev-drop area.

‘Typ­i­cally, tun­ing an engine to rev more quickly will usu­ally pro­duce more power at or close to max­i­mum revs – and in so do­ing place more stress upon it, of course. But it will also tend to pro­duce lower torque at lower revs, and lower av­er­age torque within that cru­cial rev-drop area. So its ac­cel­er­a­tion will nec­es­sar­ily be com­pro­mised. You can, of course, ex­ploit ben­e­fi­cially higher bhp at higher revs – even though it re­duces av­er­age torque – if you can also change all the gear ra­tios and the fi­nal-drive ra­tio to suit. But if, as is al­most in­evitable, you are stuck with the ra­tios you al­ready have, then in­creas­ing av­er­age torque in the rev-drop area that you are also stuck with will usu­ally in­crease per­for­mance by a greater mar­gin.

‘The rea­son it is dif­fi­cult to cre­ate ex­tra torque at higher revs is purely be­cause as the revs go up the pe­riod dur­ing which the in­let and ex­haust valves are open – to al­low fuel and/or air in, and ex­haust gases out – re­duces by the in­verse pro­por­tion of the engine speed, un­til there is sim­ply in­suf­fi­cient time for the engine to breathe ef­fec­tively. It’s like a world-class ath­lete who, how­ever pow­er­ful his mus­cles might be, just can­not get suf­fi­cient air into or out of his lungs quickly enough. Try­ing to im­prove breath­ing at the very high­est revs at which the engine is al­ready strug­gling is not easy, and brings few re­wards. By way of con­trast, at lower revs – when there is more breath­ing time avail­able – the valves are open for longer, and as a re­sult they can han­dle more air flow and cre­ate more torque.

‘So al­though in­creas­ing ca­pac­ity might or might not in­crease max­i­mum bhp at max­i­mum revs – or, at least, by not as much

as some ex­pect – it al­ways cre­ates ex­actly what makes a car ac­cel­er­ate faster. That is to say, more mid-range torque in that cru­cial rev-drop area, and that al­lows the driver to go faster while revving the engine more mod­estly. This brings im­proved longevity, of course, and the bet­ter mid-range re­sponse – as a re­sult of in­creased torque – makes it less im­por­tant to be con­stantly chang­ing gear in or­der to drive quickly.

‘You could be sur­prised to learn that whereas an in­crease in ca­pac­ity of, say, eight per cent, from 3.6 litres to 3.9, might in­crease bhp by around the same per­cent­age – and at peak revs when the breath­ing limit is reached it could be even less – in the rev-drop area, where there is more time when the valves are open, the ex­tra ca­pac­ity can al­low in more air and ex­pel more ex­haust, and in­crease torque by up to 15 per cent. With that kind of im­prove­ment comes the sort of in­crease in ac­cel­er­a­tion that you might have pre­vi­ously ex­pected from a car with the same engine tuned for a 15 per cent in­crease in bhp at higher revs – which is ac­tu­ally ex­tremely dif­fi­cult to achieve. And cer­tainly not for any­thing like the same cost.’

All well and good. But just what is this mys­te­ri­ous and so fre­quently mis­un­der­stood torque? And why is it so of­ten con­fused with power? To ex­plain that is go­ing to re­quire some Gcse-level Physics, but try to stick with us here, be­cause once you have grasped the ba­sics of this fas­ci­nat­ing sub­ject you will never feel quite the same again about the sim­ple brake horse­power – or more cor­rectly, per­haps, the mun­dane kilo­watt.

In its very sim­plest terms, it’s all about force. In physics, a force is said to do work if, when act­ing, there is a dis­place­ment of the point of ap­pli­ca­tion of the force in the same di­rec­tion as that force. If you push your car with a force of, say, 50 new­tons*, but the hand­brake is on, you will have ex­pended en­ergy, cer­tainly, but per­haps sur­pris­ingly you will have done no work. Re­lease the hand­brake, how­ever, and push the car a dis­tance of 50 me­tres (on level ground; we need to keep this as sim­ple as pos­si­ble, with­out grav­ity cloud­ing the is­sue) then the work done – that is to say, the force mul­ti­plied by the dis­tance – will be 2500 joules. Well done; have a well-earned rest. From this, it fol­lows fairly log­i­cally that power can be de­fined as the rate of do­ing work. Let’s say that you now ask a friend to help you push the car, and thereby dou­ble the force to 100 new­tons. This will en­able you to halve the force with which each of you has to push – or else both to push with the same force as be­fore, and to halve the time the process takes. (So-called ‘wind re­sis­tance’ is not re­ally rel­e­vant at this low speed, al­though un­for­tu­nately it re­mark­ably soon be­comes so.)

So far, so straight­for­ward. But car en­thu­si­asts (and car jour­nal­ists and even car man­u­fac­tur­ers, all of whom ought to know far bet­ter) don’t talk about new­tons or joules, but about horse­power (or, more con­fus­ingly still, brake horse­power) and torque. Or torques, as Jeremy Clark­son iron­i­cally but ac­tu­ally quite per­cep­tively and help­fully puts it. The ac­cepted wis­dom be­ing that the more you have of both the bet­ter. (And the ‘torques’ thing an ob­vi­ous jibe at the fact that few peo­ple re­ally un­der­stand the con­cept in any case.) But horse­power is, in truth, an ar­chaic term, dat­ing from as far back as the 18th cen­tury, when Scot­tish en­gi­neer James Watt needed a way of com­par­ing the out­put of early steam en­gines with the ca­pa­bil­i­ties of the draft horses they were grad­u­ally but in­ex­orably re­plac­ing. In fact, the cor­rect SI mea­sure­ment of power is to­day the watt, named af­ter that same en­gi­neer.

Ei­ther way, this par­tic­u­lar prob­lem is fur­ther com­pounded by all the dif­fer­ent ‘types’ of horse­power there are: me­chan­i­cal (also known as im­pe­rial); met­ric; elec­tri­cal; hy­draulic; boiler; shaft; draw­bar. See what we mean? For the pur­poses of this ex­er­cise, how­ever, we shall stick to watts or, since in au­to­mo­tive terms those are rather small (one met­ric horse­power is equiv­a­lent to around 735.5 watts), the now in­creas­ingly widely used kilo­watts. (One kilo­watt is equal to 1000 watts.) The ar­guably equally ar­chaic brake horse­power is also an im­pe­rial unit: a mea­sure of the force that needs to be ap­plied to the engine’s crankshaft in or­der lit­er­ally to brake it, or in other words to stop it ro­tat­ing. In this con­text it is of gen­uine value only when mea­sur­ing and com­par­ing power out­puts on an engine dy­namome­ter con­nected di­rectly to the crankshaft.

The con­cept of torque is on the face of it only marginally less per­plex­ing for the many of us with­out a long back­ground in me­chan­i­cal en­gi­neer­ing – and that in­cludes this writer. Think of it as ro­ta­tional force, how­ever, and it be­gins to make rather more sense. Plainly your engine does not pro­pel the car in the same sim­plis­tic way as you and your mate strain­ing against the rear bumper, your thigh mus­cles burn­ing with the un­ac­cus­tomed ef­fort. In­stead it ap­plies, via the gearbox, the dif­fer­en­tial and not least the wheels and tyres, a sur­pris­ingly widely vari­able ro­ta­tional force to the road sur­face. (‘Horse­power’ – or per­haps just power, ie kilo­watts – is torque ap­plied over time.) As we have seen, the more of that force you have, the faster you can cover a given dis­tance. And, cru­cially, the faster you will be able to ac­cel­er­ate the car, to over­come its in­er­tia and get it rolling, even if only to walk­ing pace. Once you grasp that ba­sic con­cept, ev­ery­thing else starts fall­ing into po­si­tion.

As with power, there are many ways of ex­press­ing torque – most of them thor­oughly con­fus­ing. Here in the UK we have deter­minedly hung on to all man­ner of ab­surdly old-fash­ioned terms, but the SI unit is the new­ton me­tre (all lower case), usu­ally ab­bre­vi­ated to N m or Nm, or in other words a force of one new­ton ap­plied at a dis­tance of one me­tre from the point of ro­ta­tion. (And from this it fol­lows that torque at the road sur­face is also de­ter­mined by the car’s gear­ing, or the me­chan­i­cal ad­van­tage that con­fers. Even the di­am­e­ter of the wheels and thus the size of the tyres has an ef­fect, as Barry Hart dis­cov­ered to his cost when he re­alised that two tyres of nom­i­nally the same

The con­cept of torque is on the face of it only marginally less per­plex­ing

size can vary in di­am­e­ter by up to 10 per cent – or in other words by roughly the same amount as the per­cent­age gain in torque that can re­sult from in­creased engine ca­pac­ity.)

What it all boils down to is that, to some ex­tent re­gard­less of its ap­par­ent ‘power’ out­put, your engine’s abil­ity to get the car mov­ing – and then to keep it mov­ing against in­er­tia, grav­ity and in­evitably that wind re­sis­tance we talked about – is gov­erned more than any­thing else by the torque it gen­er­ates. That’s what re­ally does the busi­ness, push­ing (and/or pulling) you down the road; past that on-the-limit truck on a chal­leng­ing two-lane high­way. The more torque you have, the more flex­i­ble and re­spon­sive the car will feel, and – gen­er­ally speak­ing – the eas­ier and the more re­lax­ing it will be to drive for a given throt­tle po­si­tion.

Both power and torque are – in au­to­mo­tive terms, any­way – gen­er­ally ex­pressed at spe­cific crankshaft speeds. Max­i­mum power (the max­i­mum rate of do­ing work, re­mem­ber; and power is torque over – or di­vided by – time) tends nat­u­rally to oc­cur to­ward the top of the rev range. Which is all very well for rac­ing or per­haps track­day work, but since few peo­ple – out on the pub­lic road, any­way – rou­tinely ex­plore even half of their engine’s full po­ten­tial, rais­ing power has rel­a­tively lit­tle ef­fect in terms of ev­ery­day per­for­mance. Quite the op­po­site, in fact, if as a re­sult the engine be­comes less tractable; more peaky, as an­other old term puts it. Even mod­estly in­creased torque, how­ever – a nat­u­ral by- prod­uct of in­creased cylin­der ca­pac­ity; think of it as 10 of you push­ing against the bumper rather than just you, or even you and your mate, rather than you alone try­ing to push 10 times harder – makes a huge dif­fer­ence to the way the ve­hi­cle be­haves in ev­ery­day cir­cum­stances. Po­ten­tially to its ef­fi­ciency and thus fuel con­sump­tion, as well. The more torque you have, the less dif­fer­ence it makes what gear you are in when you wish to ac­cel­er­ate – and the more nat­u­rally re­spon­sive the car will be in the higher gears.

A good ex­am­ple in Porsche terms is surely a di­rect com­par­i­son be­tween the eight-valve 944 and the 16-valve 944S, both with 2.5-litre, four-cylin­der en­gines. Peak power and torque for the for­mer is gen­er­ally quoted as 163bhp at 5800rpm, and 205Nm at 3000rpm, re­spec­tively. In the ‘S’, peak power rose to an im­pres­sive-sound­ing 190bhp at 6000rpm, and max­i­mum torque to 230Nm at 4300rpm. On the face of it that should have made the ‘S’ a bit of a rock­et­ship, but the re­al­ity tells a very dif­fer­ent story. The plain fact of the mat­ter is that the ‘S’ has far less mid-range flex­i­bil­ity than the eight-valve car, and as a re­sult (or so be­lieve most of us who have ex­pe­ri­enced them) can be in­cred­i­bly frus­trat­ing to drive. You have to keep the engine on the boil by chang­ing gear all the time; row­ing it along on the gear lever, to quote yet an­other old car-mag­a­zine cliché. Even the later 944 Turbo, with 250bhp and no less than 350Nm, suf­fers from a rel­a­tive lack of torque un­til the blower is ac­tu­ally boost­ing, and it is re­ally only the nat­u­rally as­pi­rated 3.0-litre S2 (211bhp at 5800rpm, and 280Nm at 4000rpm) that puts a smile on your face the mo­ment you floor the throt­tle.

Per­haps hav­ing learned a les­son from this, Porsche it­self made much of the favourable torque char­ac­ter­is­tics of the 996-model 911 Turbo when it was launched in 1999, for the 2000 model year. (And tur­bocharg­ing has fa­mously be­come an ‘easy’ route, in all man­ner of en­gines, to not just im­proved power but cru­cially also to sub­stan­tially im­proved torque.) Max­i­mum power – a not ex­actly unim­pres­sive 414bhp – was de­vel­oped at 6000rpm, but the engine’s tour de force was in prac­tice a plateau of mus­cu­lar torque the size of South Africa’s Ta­ble Moun­tain, from as low as 2700rpm all the way to 4600rpm. This con­cept of a broad and eas­ily ac­ces­si­ble torque spread was fur­ther honed over the fol­low­ing years, thanks to tech­niques such as vari­able tur­bine geom­e­try, or VTG, with the re­sult that the 580bhp 991 Turbo ‘S’ has no less than 750Nm from just 2250rpm, and this barely tails off at the 7200rpm red-line.

What it meant – and still does, of course – was that you could leave the trans­mis­sion in al­most any gear, with the crankshaft ro­tat­ing at per­haps a leisurely 1500–2000rpm, and still take off like a guided mis­sile when­ever you nailed the throt­tle. Which is a very neat trick if you can pull it off. And one so ut­terly ad­dic­tive that you will surely re­peat it at ev­ery avail­able op­por­tu­nity.

996 Turbo – here in Cabri­o­let form with a fac­tory hard-top – had an im­pres­sive enough 420bhp at 6000rpm, but what re­ally did the busi­ness was its torque ‘curve’, es­sen­tially a flat, 560Nm plateau from 2700rpm to 4600rpm. Even with a sup­pos­edly ‘lazy’ Tip­tronic trans­mis­sion – and per­haps be­cause of it – 0–62mph was eas­ily and con­sis­tently achiev­able in the fac­tory’s claimed 4.3 sec­onds, and 0–100mph in 9.5. Power is one thing; torque – and the abil­ity to use it – quite an­other

The Car­rera GT’S V10 of­fered 612bhp at 8000rpm, and a hefty 590Nm at 5750rpm, but ‘clean’ stand­ing starts were not easy to achieve – and its 205mph top speed is by to­day’s stan­dards hardly re­mark­able

Apogee of the wa­ter­cooled flat-six engine is surely – for the time be­ing, any­way – the 991 Turbo ‘S’, which thanks to its in­ge­nious vari­able tur­bine geom­e­try (above), first seen in a Porsche some 15 years ago, cranks out a frankly as­ton­ish­ing 750Nm from as lit­tle as 2250rpm

Pub­lished fig­ures – and es­tab­lished logic – sug­gested that the 16-valve 944S (far left) should have been a much stronger per­former than the orig­i­nal eight-valve car, but you had to rev it hard to ac­cess the ex­tra torque, and on the road it was all rather dis­ap­point­ing. Like­wise the 944 Turbo (be­low), and it’s the 16-valve, nat­u­rally as­pi­rated S2, with its nearly 3.0-litre engine, that many en­thu­si­asts – us in­cluded – con­sider to be the best of the breed

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