feature head games - eliminator€¦ · the engine's power curve and personality. the...

Acylinder head is much morethan a casting that tops offthe block, holds the valves

and forms the combustionchambers. The head works incombination with the camshaft,induction and exhaust systems todetermine how the engine breathes,the engine's power curve andpersonality. The "right" cylinderhead on an engine will deliver peakpower in the RPM range where youwant it. The engine will have goodthrottle response and produce thekind of torque and horsepowernumbers you want. Head selection,therefore, is a key ingredient inbuilding a winning performanceengine.

Let's start with the basics.Assuming you are going to choosean aftermarket cylinder head, youhave to find out what's availablefor the engine you want to build.For popular engines like smallblock and big block Chevy, Fordand Chrysler engines, there aredozens of head configurations,brands and product lines fromwhich you can choose. Theselection can be so overwhelming

that sometimes it comes down toeenie, meenie, minee, moe to pick ahead.

Some people pick a set ofcylinder heads based on namebrand, previous experience orword-of-mouth recommendations.Some will shop around for the leastexpensive set of heads that promise

to meet their expectations. Otherswill make their selection basedstrictly on which set of headsclaims the highest air flow

EngineBuilderMag.com 45

Bolt-on horsepower for street performance engines (such asthis Edelbrock head) take theguesswork out of choosing a performance head.

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Head Games:Don’t Gamble When ChoosingCylinder Heads

BY LARRY CARLEY, TECHNICAL EDITOR

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numbers. But there's a lot more thatshould be considered whenchoosing a set of cylinder heads.

Determine The VenuePrice, performance and availabilityare all important considerations inthe head selection process. Butequally important is choosing a setof cylinder heads that are right forthe engine and the application.Building a street performanceengine is different than building acircle track engine, a drag engine, amarine engine, or a truck pullengine.

Each type of application has itsown unique requirements, so theheads have to have the right flowcharacteristics for that application.

Basic considerations include suchvariables as engine displacement,compression ratio, camshaft lift andduration specifications, RPM range(where the engine should make themost horsepower and torque), andtarget horsepower (be realistic!).

46 April 2014 | EngineBuilder

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Heat reflecting thermal coatings applied to combustion chambers in aluminumheads improve thermal efficiency and horsepower.


You also need to consider vehicle weight, type of trans-mission (manual or automatic), torque converter stallspeed (if automatic), gearing (transmission and differen-tial), and most importantly the application itself (street,street/strip, drag, circle track, road race, off-road, etc.).

Street engines spend most of their time between idleand part throttle so they must have good low and mid-range torque and throttle response to be drivable —especially in heavier cars with automatictransmissions. For this type of application, you want aset of heads with stock to moderately larger intakerunner volumes to keep air velocity high. Peak valvelift is probably going to be no more than about half aninch with a typical street cam, so a set of heads thatclaims huge airflow numbers at extreme valve liftswould not be your best choice. Too much head can becounter productive in this type of application.

A drag engine runs at full throttle for a quarter mile.For this kind of application, you want lots of valve lift,duration and airflow at high RPM. Bigger is better interms of intake runner volumes, valve size and peakairflow numbers provided the engine has the cam,induction system and cubic inches (or boost pressure)to handle it.

Circle track engines are usually rule constrained.Some tracks only allow cast iron heads depending on

the class. If we're talking small block Chevys, somerules only allow heads with stock port locations andstock 23 degree valve angles. Others may allow anyhead configuration with raised ports and shallowervalve angles. A shallower valve angle helps unshroudthe valve for more airflow and power. For circle trackengines, heads that deliver good mid to high RPMthrottle response and torque out of the corners willusually win more races than heads that deliver morepeak RPM horsepower.

Every cylinder head manufacturer offers a varietyof different cylinder heads for this reason. They offerheads with various intake runner volumes, stock andraised port locations, various intake and exhaust portconfigurations, valve sizes, valve angles, combustionchamber volumes and spring pad sizes toaccommodate a wide variety of possible applications.Some manufacturers concentrate on a narrow segmentof the market (high end race only, street/strip, circletrack, etc.) while others offer a broader range ofproducts.

The people who make aftermarket performancecylinder heads know their product lines and canprovide the kind of guidance that's often needed tochoose the right head combination. “The mostcommon mistake people make is wanting the biggesthead that will fit their engine,” said one headmanufacturer we interviewed for this article. Othermanufacturers agreed. “Bigger isn't always better. Justbecause a head makes a lot of power on Bubba'sengine doesn't mean it's the right head for yourengine.”

The Numbers GameAll too often, the only thing people look at are flownumbers. Yes, flow numbers make horsepower, butyou have to keep in mind how the flow numbers weredetermined on a flow bench. If you hog out the intakeports and shove the valve open far enough, manyheads can deliver impressive flow numbers. But ifthose numbers are rated at .700 inches of valve lift andthe engine you are building only has half an inch ofvalve lift, you are better off choosing a head that flowsbest at .400 to .500 inches of valve lift.

According to one head manufacturer, good airflownumbers measured at .200, .400 and .500 inches ofvalve lift are the most important for a streetperformance engine.

As for intake runner volume, generally speaking,smaller intake port volumes produce more low-endtorque and throttle response while larger intake portvolumes allow more flow at higher RPMs for peakhorsepower numbers.

But airflow also depends on runner height (raisedports typically flow better), the contour of the port(especially the profile of the short side radius wherethe runner flows into the bowl area above the valve),the cross-sectional area of the intake runner and itsrelationship to the size of the valve opening, the angleof the valve stem (shallower usually flows better), and


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the angles on the valve seat andvalve face. The shape of thecombustion chamber can alsoinfluence airflow.

Every head manufacturer andhead porter has their own recipefor combining these factors tosqueeze the best performance outof a given cylinder head. You cantake two cylinder heads fromdifferent suppliers that haveidentical intake runner portvolumes and valves and end upwith very different airflow andhorsepower numbers. Why?Because the profile of the intakerunners in one head flow betterthan the other. Consequently, oneset of heads may deliver 15 to 20more horsepower on the sameengine than a competitor's heads.

When choosing a cylinder head

for a particular engine application,one of the first variables that has tobe considered is enginedisplacement. Are you building a350 with stock bore and stroke, or a383 stroker or something bigger?The more cubic inches the enginehas, the more airflow the headshave to deliver.

A head with 180 or 185cc intakerunners will work well on a typical350 Chevy street engine that makes400 to 450 horsepower. If it's a 383stroker, you can probably go with200cc intake runners.

Putting a higher flow head with220cc runners on a relatively stock350 would be going in the wrongdirection.

On the other hand, if you'rebuilding a high revving raceengine, or a big displacement



Last year, Mike Androwick’s big-block head designs won the Northeast Dirt Modi-fied championship. According to Androwick, one of the two chief advantages ofusing a low, 10-degree valve angle is that it yields a shallow combustion chamber.Source: www.mooregoodink.com

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stroker (over 400 inches), you could probably go withlarger runners for increased airflow. Some aftermarketSB Chevy racing heads have intake runner volumesfrom 240 to 270cc or more!

If you're building a big block Chevy streetperformance engine, heads with 300 to 320cc intakerunners will probably work best. Again, the larger thedisplacement, the more intake volume and airflow theengine needs.

On the other hand, if you're building a 565 cubicinch big block stroker motor, you'll probably wantheads with 330 to 340cc intake runner volumes. If it's adrag motor, you can go even bigger, maybe 350 to360cc with a CNC-ported head. It all depends on thecam, RPM range and where you want the engine tomake the most power and torque.

Aluminum vs. Cast IronAluminum heads are lighter than cast iron, savingmaybe 25 to 30 lbs. per head depending on theapplication. In a 3500 to 4500 lb. street car, that's not alot of weight savings, but in a 2,500 lbs. race car it is.Even so, lighter is usually considered better forperformance.

Aluminum heads are easier to machine than castiron because the metal is softer, and the heads areeasier to repair because aluminum can be TIG welded

to fix cracks and other damage. Cracks in cast iron can be drilled and pinned, or

even furnace welded, but the latter is more difficultand requires a high level of skill and experience toprevent repeat cracking.

Aluminum may be your only choice if you want acustom billet head. Billet aluminum heads can bemade for almost anything, but are very expensivebecause of all the machine work that is required todesign and fabricate the head.

This is what the big boys use on many top fueldragsters and some Pro Stock mountain motorsbecause nothing else will work. We've also seen somevery slick billet aluminum heads for diesel enginesused in Super Stock Pulling Tractors.

Aluminum conducts heat faster than cast iron. Thishelps cool the engine and allows a higher compressionratio with less risk of preignition or detonation — butit also sucks heat out of the combustion chamber andactually reduces combustion efficiency somewhat. Athigh RPM, there's less time per combustion event forheat to escape through the cylinder head so the loss inthermal efficiency is not as great.

Applying a metallic-ceramic thermal coating to thecombustion chambers can improve heat retention andthermal efficiency. Some who use these type ofthermal barrier coatings say they have gained 30 to 40horsepower on the dyno over uncoated heads on thesame engine.

On a street engine, cast iron heads help an enginereach operating temperature more quickly after a coldstart (good for emissions and cold drivability), whileretaining more heat for improved fuel economy andthermal efficiency. Most cast iron heads are also lessexpensive than aluminum heads because the metal isless expensive.

Go with the FlowA common question that's often asked is whether CNCported heads outperform as-cast heads? It depends onthe head.If the intake runner profiles of an as-cast head are

identical to those of a fully CNC-ported head, bothheads should flow exactly the same.

There may be subtle differences in airflow based onthe surface finish in the ports, but basically the numbersshould not differ significantly.

That said, most CNC-ported heads are marketed asoffering a significant advantage over the typical as-cast head. CNC heads can start out as rough castingswhich are then machined to final dimensions, or ascast heads (stock or performance) which are thenreworked by CNC machining to increase intake andexhaust runner volumes and/or reshape the runnerprofile for more airflow.

You usually pay more for CNC heads because of theextra machine work that's done to them. But if you canget the same performance from an as-cast head withsimilar port configurations, the latter can save youmoney.

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SpeakingVolumesHeads are availablewith variouscombustion chambervolumes. Largerchambers canaccommodate domedpistons and are oftencompatible with awider range ofaftermarket pistons,while smallercombustion chambersallow you to achieve ahigher compressionratio using flat toppistons or smallerdomed pistons.

Many headsuppliers can also milla set of heads toreduce the combustion chambervolumes to your specifications ifthat's what you want.

An important point to rememberhere is to always check valve-to-

piston clearance with the headinstalled on the block to make surethere are no interference problemsat peak valve lift. Milled heads,larger diameter valves, reduced


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CNC porting can open up the intakeand exhaust ports for better airflow.The same port profile can be replicated with a new casting.


deck height, a higher lift camand/or rocker arm ratio can all addup to create interference problems ifyou fail to take everything intoaccount.

Port PointsRaised ports or relocated portsgenerally provide better airflow andmore power. But changing the portsalso means you have to change theintake manifold and/or exhaustmanifold/headers. On big blockChevy, you also have the option ofgoing with rectangular ports or ovalports. Some say oval ports flowbetter, but again compatibility withexisting manifolds may be thedeciding factor as to which port configuration youchoose.

What’s Your Angle?The angle of the spark plug doesn't matter as much asits location with respect to the valves. On many heads,relocating the spark plug closer to the exhaust valveadds some horsepower. But relocated plugs ordifferent plug angles may interfere with some exhaust

manifolds or headers. Consequently, you may have tochoose a set of heads that will work with an existingset of manifolds or headers over a set of heads thatwon't.

Final ThoughtsOther items to consider when choosing a set of headsincludes valve sizes (intake and exhaust), the qualityof the valve seat material, the type of valve guides(integral, bronze, powder metal or cast iron), and thediameter and location of the spring pads (larger padscan accommodate larger, stiffer springs).

Do you want assembled heads ready to install, fullymachined and unassembled heads you can assembleyourself, semi-finished heads so you can do the finalvalve work and assembly, or raw castings that you canCNC machine yourself in-house?

Some head suppliers have mass-produced fullyassembled heads that are typically sold through onlineretailers and performance parts distributors. These area good value for the DIY engine builder or somebodywith a limited budget, but are probably not the bestchoice for most professional engine builders. Manyengine builders want full control over the final valvework and assembly to make sure the heads arefinished to their specifications.

Get AdviceIf you're confused by the bewildering array of cylinderhead configurations that are available in theaftermarket today and need help choosing the righthead for your engine, use the expertise of the peoplewho make the heads to help you in the selectionprocess. As we said earlier, they know their productlines and can steer you towards the best head for whatyou want to accomplish. ■


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The "right" cylinder head on an engine will deliver peakpower in the RPM range where you want it.

Looking for suppliers of stock, diesel or performance heads? Check out the 2014Engine Builder Buyers Guides online at www.EngineBuilderMag.com.


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