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fasttech / TECH / CYLINDER HEADS / fasttech HEADHEAD MASTER Pay attention as Stu offers up a masterclass in what you need to know about cylinder heads. related to its ability to pump air almost everywhere and yet retains will develop its peak power (see in great volumes, and the limiting sensible fuel economy, driveability Fast Ford issues 245 and 246 factor to how much air can be and idle characteristics? for more in-depth camshaft and pumped is related to the pathway I would say that over 90 per cent timing information). Don’t forget into and out of the pump. of us, myself included, want the that the airfl ow capability of the In the case of an engine this is latter, so before you take a Dremel head and the velocity of the everythingeverything fromfrom thethe airair fi lterlter toto to your cylinder head I suggest you airfl ow itself are all factors that the next few thethe inletinlet valve,valve, andand converselyconversely grab a coffee and absorb a little are taken into account when OVER months I intend everythingeverything fromfrom thethe exhaustexhaust valvevalve more information on what does designing camshafts, so be aware to cover some of the more old- toto thethe exhaustexhaust backback box,box, butbut todaytoday andand doesdoes notnot workwork inin thethe worldworld ofof that modifying the head too much school yet fundamental tuning wewe areare dealingdealing withwith thethe cylindercylinder cylindercylinder hheadead portingporting andand design.design. can also mean the camshaft is no processes that some modern headhead andand itsits associatedassociated items.items. Let’sLet’s looklook atat thethe fundamentals.fundamentals. longer suitable. tuners and magazines seem to So,So, ifif wewe increaseincrease thatthat airair volumevolume have forgotten all about, since the pumpingpumping abilityability thenthen wewe increaseincrease PORTPORT FLOWFLOW VSVS AIRFLOW advent of electronic tuning trickery. ourour potentialpotential powerpower output.output. SimpleSimple MIXTUREXTURE VELOCVELOCITYTY REQUREQUIREMENTS I’d like to start this series with isn’tisn’t it?it? WeWe establishedestablished earlierearlier thatthat anan NaturallyNaturally the fl ow requirementrequirement the good old-fashioned cylinder ActuallyActually no,no, sadlysadly itit isn’t.isn’t. engine’sengine’s abilityability ttoo fi llll itsits of an engine increasesincreases head and the various modifi cations AlthoughAlthough youyou wouldwould nono doubtdoubt as its revsrevs increase.increase. ForFor we can make to it. Any good tuner instantlyinstantly thinkthink thatthat thisthis mmeanseans example,example, anan engineengine will tell you that the cylinder head biggerbigger portsports andand itself is the heart of your engine and without a good head on your block you will never get good power no matter what else you do to it, and conversely you will also nevernever getget goodgood ddriveabilityriveability oror goodgood digestingdigesting 3030 fuelfuel economyeconomy either,either, asas thesethese areare lblb ofof airair atat 30003000 allall aspectsaspects mainlymainly dictateddictated byby thethe rpmrpm willwill trytry toto digestdigest 6060 lblb ooff airair cylindercylinder head.head. TheThe cylindercylinder headhead biggerbigger camscams atat 60006000 rpm,rpm, allall thingsthings beingbeing equal.equal. andand itsits associatedassociated iitemstems reallyreally dodo mustmust bebe better,better, inin cylinderscylinders isis thethe OneOne thingthing manymany peoplepeople forgetforget isis makemake oror breakbreak thethe torquetorque curvecurve thethe realreal worldworld thatthat limitinglimiting factorfactor thatthat thethe amountamount ooff timetime availableavailable and dictate the potential power isn’tisn’t thethe casecase becausebecause dictatingdictating whatwhat itsits uultimateltimate toto actuallyactually fi llll thethe cylinderscylinders withwith output of your engine, so let’s variousvarious dynamicsdynamics startstart toto powerpower outputoutput willwill be.be. InIn thisthis air/fuelair/fuel mixturemixture decreasesdecreases BEFORE go back to basics as always and taketake theirtheir partpart inin thethe proceedingsproceedings normally-aspiratednormally-aspirated (not(not turboturbo proportionatelyproportionately toto rpm.rpm. ItIt isis quitequite STARTING… examine why exactly the cylinder andand asas always,always, thingsthings areare notnot asas oror supercharged)supercharged) form,form, thethe staggeringstaggering whenwhen youyou tthinkhink aboutabout head is so important to our engine. simplesimple asas theythey fi rstrst appearappear toto be.be. cylinderscylinders areare fi lledlled byby thethe pressurepressure itit — atat 50005000 rpmrpm wewe onlyonly havehave The fi rst rule of working on differentialdifferential thatthat isis createdcreated whenwhen anan averageaverage ofof sixsix thousandthsthousandths ofof cars and using tools of any kind is don’t ever skimp on AIR WAYS POWER OR thethe pistonpiston movesmoves downdown thethe a secondsecond toto getget thethe airair iintonto thethe decent protection. Goggles, To describe what exactly an DRIVEABILTY? borebore againstagainst anan openopen inletinlet valvevalve cylindercylinder beforebefore thethe valvevalve closescloses gloves, ear defenders, masks and a set of overalls engine is I always fi nd it best to Big brash ports and cams, while (barring(barring aanyny rramam oror shockshock wavewave again.again. OnceOnce thethe engine’sengine’s airflairfl owow should be in your garage. compare it to an air pump. In often allowing the best top-end tuningtuning effects).effects). TheThe actualactual cylindercylinder demanddemand outstripsoutstrips thethe aabilitybility ofof thethe Use them. When using power tools, reality, all complexities aside, that power fi gure, will invariable fi lling effi ciency is measured and headhead toto deliverdeliver increasedincreased airflairfl ow,ow, protective gear is essential is exactly what it is — it draws air damage our low-end driveability, expressed in terms of volumetric VEVE tailstails off,off, asas doesdoes peakpeak power.power. — grinders and welders can make a real mess of your in, develops power with it after the fuel economy and torque. Is that effi ciency (VE). AsAs a quickquick example,example, ifif a headhead cancan soft skin and bone if you get it wrong. addition of fuel, and then expels it the result you wanted? A dog of an The timing events of the valves fl owow 3030 lblb ofof airair perper minuteminute aandnd tthehe Never work under a car again. This pumping cycle happens engine that goes OK up near the opening and closing is the primary engine at 3000 rpm requires 30 lb without supporting it using Words: Stewart Sanderson axle stands. A car falling on very quickly indeed, but the fact limiter but nowhere else, or did you factor determining the rev range of air per minute it will make 100 you is not something you’ll be remains that the power-producing actually want a good overall engine in which the engine will achieve per cent volumetric effi ciency here laughing about down the pub. capability of the engine is directly that goes better than standard its peak VE and where the engine and possibly its peak torque, but be 0 1 3 0 MARCH 2007 FAST FORD FAST FORD MARCH 2007 0 1 3 1 fasttech / TECH / CYLINDER HEADS / fasttech to continue fi lling the cylinder, direction and fl ow backwards as SQUISH EFFECT cramming in the air, sometimes the inlet valve is open at this point Something I have seen badly A large, inert volume of air in the inlet is slow to provide a signal Having massive inlet ports can actually lose you to the MAP sensor/airfl ow meter, giving lazy throttle response power, as it slows down speed of the airfl ow above 100 per cent VE as we allowing sometimes less pressure modifi ed many times over the actually compress some air due to in the cylinder than the exhaust years is the squish area of the the piston moving upwards and the port (a gas will always fl ow to the cylinder head. It has often been gas still travelling downwards when point of least resistance). This is destroyed by someone who quite the intake valve closes. not at all helpful as it dilutes our possibly doesn’t even know that it A good example of inertia is incoming fresh mix. This effect is exists, let alone what its purpose when we spin a heavy wheel, usually far worse at low rpm where was. Squish is the term used to maybe on a car or push bike. Once fl ow rates and velocity are at their describe the area of the cylinder we have let go after our spinning lowest due to obvious factors. head that comes into very close movement, the wheel takes no It’s worth bearing in mind that proximity to the piston at top dead more energy to continue spinning, this particular issue is normally centre (TDC). so it should have just stopped made far worse with increased- It’s a known fact that in over 90 when we let go, but it didn’t, did it? overlap, high-performance cams. per cent of engines, a faster burn In fact, if we try to stop it spinning The result is an engine that is slow and a more turbulent mixture are now it resists us. This is inertia. to get on the cam as engine speed obtained when the compression picks up some velocity in the height of the piston is set to take EXHAUSTEXHAUST GASGAS exhaust port, and very poor low- the maximum advantage of the fl at VELOCVELOCITY & INERTIA speed engine operation and bad, quench face of the cylinder head. InIn thethe exhaustexhaust port,port, manymany ofof thethe sometimes non-existent idle. In other words, if the area samesame considerationsconsiderations applyapply asas Ultimately, both the intake between the fl at face of the head withwith thethe inletinlet ports.ports. InIn thethe exhaustexhaust and exhaust ports must balance and the piston top is suffi ciently ports, instead of drawing in air fl ow velocity and fl ow capacity to close, as the piston reaches awareaware thatthat thethe samesame headhead wwillill onlyonly drainpipe-sized inlet and exhaust velocity means that the air column and fuel to fi ll the cylinder we are achieve fl exible performance over TDC on the compression stroke, supply 30 lb of air per minute to the tracts and fi t massive combustion is moving faster and this means actually pushing it out with a piston.