84 85911 & PORSCHE WORLD911 & PORSCHE WORLD

BIG-BOREENGINES

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

The headline benefit of raisingany engine’s swept volume,as we’ve suggested elsewherein this analysis of Hartech’senlarged M96 and M97 units,

is an increase in power. It’s what magazineroad-tests, tuning features and Top Gearpresenters have banged on about for years.(With arguably one notable exception; seebelow.) But to Barry Hart – and, in truth, to therest of us, if we did but know it, and had notbecome fixated on mere bhp – the mostimportant gain is a comparable increase intorque. And, if you and/or your engine builderhave done your sums, a modest but no lessuseful reduction in the engine speed at whichboth 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 therevs for maximum power looks impressive onpaper, but often fails to provide the real-worldperformance increase that the graph implies.But even a modest increase in capacity willusually result in much better torque, and sothe easier, faster acceleration that will alsosuit both typical road users and occasionaltrackday drivers far more than the results ofpurely external modifications alone. And amodest increase in capacity will often resultin the engine being less stressed than it is byconventional tuning.’It’s all about what Barry calls the rev-drop

area. ‘Your engine’s maximum brakehorsepower is effectively the same in anygear – and simply determines its potential todo different things, depending on the way in

which it transmits the output. But torque is notquite the same thing. You have the mosttorque at the wheels in the lowest gears –which is why when driving on mud or ice youneed to stay in as high a gear as possible,in order to reduce the chances of wheelspin –and as you shift upwards the torque reducesin inverse proportion to the ratio. Torque insixth gear – again at the driven wheels, ofcourse – is typically between four and fivetimes less than it is in first.‘When you change up through the gears,

you inevitably exchange that torque forrotational speed. To put it another way:increase wheel speed, so that the car cantravel faster, and you reduce the availabletorque by the same proportion. This is whyyour car naturally accelerates faster in thelower gears, and less quickly in the higherratios. And why, when you come to climb agradient, you have to change down in orderto maintain your speed.‘When you change gear at the rev limit the

engine speed naturally drops to the level thatis effectively determined by the next higherratio. So if you are driving to maximum revsbefore changing gear you create a “rev drop”– or more likely a series of rev drops – inwhich the vehicle has to accelerate back up tothe rev limit before you shift up again. And thisrev drop varies in size depending on whichgear you are in. It’s usually at its greatestbetween first and second gear, and at itslowest between fifth and sixth. Essentially,the fastest overall acceleration will beachieved by having the greatest averagetorque in that rev-drop area.‘Typically, tuning an engine to rev more

quickly will usually produce more power at orclose to maximum revs – and in so doingplace more stress upon it, of course. But itwill also tend to produce lower torque at lowerrevs, and lower average torque within thatcrucial rev-drop area. So its acceleration willnecessarily be compromised. You can, ofcourse, exploit beneficially higher bhp athigher revs – even though it reduces averagetorque – if you can also change all the gearratios and the final-drive ratio to suit. But if,as is almost inevitable, you are stuck withthe ratios you already have, then increasingaverage torque in the rev-drop area that youare also stuck with will usually increaseperformance by a greater margin.‘The reason it is difficult to create extra

torque at higher revs is purely because asthe revs go up the period during which theinlet and exhaust valves are open – to allowfuel and/or air in, and exhaust gases out –reduces by the inverse proportion of theengine speed, until there is simply insufficienttime for the engine to breathe effectively.It’s like a world-class athlete who, howeverpowerful his muscles might be, just cannotget sufficient air into or out of his lungs quicklyenough. Trying to improve breathing at thevery highest revs at which the engine isalready struggling is not easy, and bringsfew rewards. By way of contrast, at lower revs– when there is more breathing time available– the valves are open for longer, and as aresult they can handle more air flow andcreate more torque.‘So although increasing capacity might or

might not increase maximum bhp atmaximum revs – or, at least, by not as much

as some expect – it always creates exactlywhat makes a car accelerate faster. That isto say, more mid-range torque in that crucialrev-drop area, and that allows the driver togo faster while revving the engine moremodestly. This brings improved longevity,of course, and the better mid-range response– as a result of increased torque – makes itless important to be constantly changing gearin order to drive quickly.‘You could be surprised to learn that

whereas an increase in capacity of, say, eightper cent, from 3.6 litres to 3.9, might increasebhp by around the same percentage – and atpeak revs when the breathing limit is reachedit could be even less – in the rev-drop area,where there is more time when the valves areopen, the extra capacity can allow in more airand expel more exhaust, and increase torqueby up to 15 per cent. With that kind ofimprovement comes the sort of increase inacceleration that you might have previouslyexpected from a car with the same enginetuned for a 15 per cent increase in bhp athigher revs – which is actually extremelydifficult to achieve. And certainly not foranything like the same cost.’All well and good. But just what is this

mysterious and so frequently misunderstoodtorque? And why is it so often confused withpower? To explain that is going to requiresome GCSE-level Physics, but try to stickwith us here, because once you have graspedthe basics of this fascinating subject you willnever feel quite the same again about thesimple 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 workif, when acting, there is a displacement of thepoint of application of the force in the samedirection as that force. If you push your carwith a force of, say, 50 newtons*, but thehandbrake is on, you will have expendedenergy, certainly, but perhaps surprisinglyyou will have done no work. Release thehandbrake, however, and push the car adistance of 50 metres (on level ground;we need to keep this as simple as possible,without gravity clouding the issue) then thework done – that is to say, the force multipliedby the distance – will be 2500 joules. Welldone; have a well-earned rest. From this,it follows fairly logically that power can be

defined as the rate of doing work. Let’s saythat you now ask a friend to help you pushthe car, and thereby double the force to 100newtons. This will enable you to halve theforce with which each of you has to push –or else both to push with the same force asbefore, and to halve the time the processtakes. (So-called ‘wind resistance’ is notreally relevant at this low speed, althoughunfortunately it remarkably soon becomes so.)So far, so straightforward. But car

enthusiasts (and car journalists and even carmanufacturers, all of whom ought to know farbetter) don’t talk about newtons or joules, butabout horsepower (or, more confusingly still,brake horsepower) and torque. Or torques,as Jeremy Clarkson ironically but actuallyquite perceptively and helpfully puts it. Theaccepted wisdom being that the more youhave of both the better. (And the ‘torques’thing an obvious jibe at the fact that fewpeople really understand the concept in anycase.) But horsepower is, in truth, an archaic

term, dating from as far back as the 18thcentury, when Scottish engineer James Wattneeded a way of comparing the output ofearly steam engines with the capabilities ofthe draft horses they were gradually butinexorably replacing. In fact, the correct SImeasurement of power is today the watt,named after that same engineer.Either way, this particular problem is further

compounded by all the different ‘types’ ofhorsepower there are: mechanical (alsoknown as imperial); metric; electrical;hydraulic; boiler; shaft; drawbar. See what wemean? For the purposes of this exercise,however, we shall stick to watts or, since inautomotive terms those are rather small(one metric horsepower is equivalent toaround 735.5 watts), the now increasinglywidely used kilowatts. (One kilowatt is equalto 1000 watts.) The arguably equally archaicbrake horsepower is also an imperial unit: ameasure of the force that needs to be applied

to the engine’s crankshaft in order literally tobrake it, or in other words to stop it rotating.In this context it is of genuine value only whenmeasuring and comparing power outputson an engine dynamometer connecteddirectly to the crankshaft.The concept of torque is on the face of it

only marginally less perplexing for the manyof us without a long background inmechanical engineering – and that includesthis writer. Think of it as rotational force,however, and it begins to make rather moresense. Plainly your engine does not propelthe car in the same simplistic way as you andyour mate straining against the rear bumper,your thigh muscles burning with theunaccustomed effort. Instead it applies, viathe gearbox, the differential and not least thewheels and tyres, a surprisingly widelyvariable rotational force to the road surface.(‘Horsepower’ – or perhaps just power, iekilowatts – is torque applied over time.) Aswe have seen, the more of that force you

have, the faster you can cover a givendistance. And, crucially, the faster you will beable to accelerate the car, to overcome itsinertia and get it rolling, even if only to walkingpace. 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 thoroughlyconfusing. Here in the UK we havedeterminedly hung on to all manner ofabsurdly old-fashioned terms, but the SI unitis the newton metre (all lower case), usuallyabbreviated to N m or Nm, or in other wordsa force of one newton applied at a distance ofone metre from the point of rotation. (Andfrom this it follows that torque at the roadsurface is also determined by the car’sgearing, or the mechanical advantage thatconfers. Even the diameter of the wheels andthus the size of the tyres has an effect, asBarry Hart discovered to his cost when herealised that two tyres of nominally the same

* The ‘units’ usedthroughout sciencecan be baffling, notleast because there

have evolved somany ways of

expressing the samething. Pounds or

kilograms? Miles orkilometres? Gallons

or litres? The newton(lower-case ‘n’) is theInternational System

of Units- (or SI-)derived unit of force.It is named after the

English polymathSir Isaac Newton

(1642–1726) for hiswork on classical

mechanics, includinghis famous laws of

motion. One newtonis the force needed toaccelerate a mass ofone kilogram at the

rate of one metre persecond squared.

Other units of forceare the dyne, the

kilogram-force, thepound-force and thepoundal. The joule,

named after anotherEnglish physicist,

James Prescott Joule(1818–1889), is anSI unit of energy,

defined as thattransferred to an

object when a forceof one newton acts

upon it in thedirection of its

motion through adistance of one

metre. If you want toknow more about all

of this, then pouryourself a strongdrink and settle

down with Wikipediaon your laptop...

The concept of torque ison the face of it only

marginally less perplexing”“996Turbo – here inCabriolet form witha factory hard-top –had an impressiveenough 420bhp at6000rpm, but whatreally did thebusiness was itstorque ‘curve’,essentially a flat,560Nm plateaufrom 2700rpm to4600rpm. Evenwith a supposedly‘lazy’ Tiptronictransmission – andperhaps because ofit – 0–62mphwas easily andconsistentlyachievable in thefactory’s claimed4.3 seconds, and0–100mph in 9.5.Power is one thing;torque – and theability to use it –quite another

The Carrera GT’s V10offered 612bhp at

8000rpm, and a hefty590Nm at 5750rpm,but ‘clean’ standingstarts were not easyto achieve – and its205mph top speed isby today’s standardshardly remarkable

87911 & PORSCHE WORLD86 911 & PORSCHE WORLD

size can vary in diameter by up to 10 per cent– or in other words by roughly the sameamount as the percentage gain in torque thatcan 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 carmoving – and then to keep it moving againstinertia, gravity and inevitably that windresistance we talked about – is governedmore than anything else by the torque itgenerates. That’s what really does thebusiness, pushing (and/or pulling) you downthe road; past that on-the-limit truck on achallenging two-lane highway. The moretorque you have, the more flexible andresponsive the car will feel, and – generallyspeaking – the easier and the more relaxing itwill be to drive for a given throttle position.Both power and torque are – in automotive

terms, anyway – generally expressed atspecific 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 topof the rev range. Which is all very well forracing or perhaps trackday work, but sincefew people – out on the public road, anyway –routinely explore even half of their engine’sfull potential, raising power has relatively littleeffect in terms of everyday performance.Quite the opposite, in fact, if as a result theengine becomes less tractable; more peaky,as another old term puts it. Even modestlyincreased torque, however – a natural by-

product of increased cylinder capacity; think ofit as 10 of you pushing against the bumperrather than just you, or even you and yourmate, rather than you alone trying to push10 times harder – makes a huge differenceto the way the vehicle behaves in everydaycircumstances. Potentially to its efficiency andthus fuel consumption, as well. The moretorque you have, the less difference it makeswhat gear you are in when you wish toaccelerate – and the more naturallyresponsive the car will be in the higher gears.A good example in Porsche terms is surely

a direct comparison between the eight-valve944 and the 16-valve 944S, both with 2.5-litre,four-cylinder engines. Peak power and torquefor the former is generally quoted as 163bhpat 5800rpm, and 205Nm at 3000rpm,respectively. In the ‘S’, peak power rose toan 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 tellsa very different story. The plain fact of thematter is that the ‘S’ has far less mid-rangeflexibility than the eight-valve car, and as aresult (or so believe most of us who haveexperienced them) can be incrediblyfrustrating to drive. You have to keep theengine on the boil by changing gear all thetime; rowing it along on the gear lever, toquote yet another old car-magazine cliché.Even the later 944 Turbo, with 250bhp and noless than 350Nm, suffers from a relative lackof torque until the blower is actually boosting,

and it is really only the naturally aspirated3.0-litre S2 (211bhp at 5800rpm, and 280Nmat 4000rpm) that puts a smile on your face themoment you floor the throttle.Perhaps having learned a lesson from this,

Porsche itself made much of the favourabletorque characteristics of the 996-model 911Turbo when it was launched in 1999, for the2000 model year. (And turbocharging hasfamously become an ‘easy’ route, in allmanner of engines, to not just improvedpower but crucially also to substantiallyimproved torque.) Maximum power – anot exactly unimpressive 414bhp – wasdeveloped at 6000rpm, but the engine’stour de force was in practice a plateau ofmuscular torque the size of South Africa’sTable Mountain, from as low as 2700rpm allthe way to 4600rpm. This concept of a broadand easily accessible torque spread wasfurther honed over the following years, thanksto techniques such as variable turbinegeometry, or VTG, with the result that the580bhp 991 Turbo ‘S’ has no less than750Nm from just 2250rpm, and this barelytails off at the 7200rpm red-line.What it meant – and still does, of course –

was that you could leave the transmission inalmost any gear, with the crankshaft rotatingat perhaps a leisurely 1500–2000rpm, and stilltake off like a guided missile whenever younailed the throttle. Which is a very neat trickif you can pull it off. And one so utterlyaddictive that you will surely repeat it at everyavailable opportunity. PW

Most road vehicles haveinternal combustionengines of one form oranother, but the steamlocomotive has at least

two external combustion engines, fed withsteam from a boiler. Instead of generatingenergy within each cylinder, the steamlocomotive creates the energy required tomove it by heating water by fire, most oftenusing coal, but sometimes oil.Each ‘engine’ on the locomotive typically

takes the form of a cast-iron cylinder blockwith an integral valve chest located abovethe cylinder. Some engines have twocylinders, some larger ones three. Thevalve employed may be of the slide typeor, on more powerful locomotives, a pistonvalve sliding to and fro, admitting andexhausting steam to and from each endof the cylinder in turn. (The steam enginescores a point over its internal-combustionrivals by being double-acting. Every pistonstroke counts.) Steam engineers flirted withpoppet valves – as in modern internal-combustion engines – but seemed alwaysto return to the trusty piston valve.Steam locomotives don’t have a gearbox

but they do have a ‘reverser’, which notonly controls the direction of travel (intheory the engine can travel as fast in ‘backgear’ as it can in forward gear) but also theamount of steam admitted to the cylinderduring each piston stroke. This can be asmuch as 75 per cent (steam is admitted forthe first three-quarters of the piston travel,

and then ‘cut off’ for the remaining quarter)to as little as none, in which case the loco isin ‘mid-gear’, ie not in forward or back gear.The work done by expanding steam in

the cylinder is converted to motion alongthe track by a connecting-rod and a crankattached to one of the driving wheels (ordriving axles in the case of a cylinder insidethe locomotive’s frames). Power – typicallyexpressed in pounds of ‘tractive effort’ – is afunction of boiler pressure, cylinderdiameter and driving-wheel size. Higherboiler pressure: more force to drive thepistons. Big cylinders: able to accommodatemore steam. Small driving wheels: the workcarried out during one rotation moves thetrain a smaller distance than would be thecase with a big driving wheel. Freightengines that needed to move heavy trainsat low speeds had small drivers, whereasthe ‘racehorses’ like Flying Scotsman,designed to work faster and lighterpassenger expresses, had much largerones – effectively a higher final drive.So-called ‘mixed traffic’ locomotives had acompromise somewhere in between.Assuming full boiler pressure – on a

large, modern locomotive over 200 poundsper square inch (say around 1400kPaor 14 bar) – the driver has at his disposalmaximum power and torque at maximum(ie 75 per cent) cut-off. Thus when startingfrom rest judicious application of theregulator, which controls the flow of steamfrom the boiler to the cylinders, is called forin order to avoid wheelslip. ‘Traction control’

is the driver’s hand gripping the regulatorhandle, deftly reducing the flow of steamthrough the regulator valve. Large regulatoropenings and long cut-offs, though, arehugely wasteful: in car terms it would belike cruising at 60mph in second gear.The correct approach, once nicely on themove, is to reduce the cut-off so that steamis admitted to the cylinders for a shorterlength of the piston stroke; it takes onlya relative puff of steam to keep the trainmoving. And at the same time the regulatorcan be opened more widely; the reductionin torque due to the shorter cut-off reducesthe likelihood of wheelslip.Assuming sufficient traction is available

(and you are dealing with narrow steel‘tyres’ on possibly wet and greasy steelrails, remember), maximum acceleration iswith full regulator and 75 per cent cut-off;the car equivalent is a wide-open throttle infirst gear. Once up to speed, full regulatordelivers maximum power but at a shortcut-off a lower amount of torque. Think of itas being akin to your foot flat on the floor insixth or seventh gear. And, just as youchange down (for more torque) to climb ahill, so the engine driver lengthens thecut-off to achieve the same effect.With steam locomotives, then, there is

no ‘rev-drop effect’. They actually have acontinuously variable transmission, exceptthat the variability comes from the valvesthat control the admission of steam to thecylinders. And steam locomotives arecertainly not ‘automatics’. PW

BIG-BOREENGINES

Apogee of the water-cooled flat-six engineis surely – for thetime being, anyway –the 991 Turbo ‘S’,which thanks to itsingenious variableturbine geometry(above), first seenin a Porsche some15 years ago, cranksout a franklyastonishing 750Nmfrom as little as2250rpm

Published figures –and established logic– suggested that the16-valve 944S (farleft) should havebeen a much strongerperformer than theoriginal eight-valvecar, but you had torev it hard to accessthe extra torque, andon the road it was allrather disappointing.Likewise the 944Turbo (below), andit’s the 16-valve,naturally aspiratedS2, with its nearly3.0-litre engine, thatmany enthusiasts –us included –consider to be thebest of the breed

The author of thispiece, Peter Maynard,now owns a 2015Cayman GTS inplace of this 2013‘S’ model (below left),photographed atthe heritage GreatCentral Railwayin Loughborough,Leicestershire, wherehe both fires anddrives all kinds ofclassic British steamengines. That’s Peter– lucky chap – at thecontrols of 92220Evening Star, as itwas badged in 2015,as a tribute to the lastsuch locomotive builtby British Railways in1960, although it hassince been returnedto its ‘correct’ guise,92214 (below). Keyto any such engine’sefficient operation isthe so-called reverser(far left, bottom),which determinesthe percentage ofeach piston strokeduring which steamis admitted to thecylinders – hence thenumbers you can seeon the drum. Thinkof it as a crossbetween a Porscheengine’s variablevalve timing andits gearbox

POWER UNDER PRESSURESteam engines have huge power and torque from effectively zero revs, but how does that translate intoreality? Who better to explain than Cayman owner Peter Maynard, who has experience of many differenttypes, from diminutive 0-6-0s to massive 2-10-0 freight engines. Photos by Peter Robain and Chris Horton