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Camshaft and Valvetrain
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The camshaft and valvetrain directly determine not only an engine’s personality, but how reliably it
will perform throughout its service life. Unfortunately, a lot of folks have more misconceptions about
camshafts than they have facts. Hopefully, in the following pages we can set you straight on the
myths and get you headed in the right direction.
To understand how to pick a camshaft and valvetrain, you must first understand how it all works.
Choosing a camshaft profile must be based on how you want an engine to perform. Are you
building a streetable engine where low- and mid-range torque are important? Or are you building a
high-performance racing engine that makes peak torque in the high revs? Whatever the
application, it is vital for you to choose the right combination of components.
A camshaft manufacturer’s catalog lists dozens of camshaft types for the same type of engine. This
is where it gets mighty confusing for the novice. We see words like lift, duration, lobe separation,
base circle, lobe centerline angle, and valve overlap. What does all of this information mean and
how will it affect your engine’s performance?
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CAMSHAFT SHOP TALK
What makes one camshaft different from another? Call it profile. Profile refers to the lobe’s design,
dimension, and positioning, as well as its functionality. Functionality refers to when the lobe opens
the valve, when it closes the valve, how long it keeps the valve open, and how much it opens the
valve. All of these factors influence an engine’s performance.
Following are a few terms that you will hear in the shop when talk turns to camshafts. Lift is the
maximum amount a lobe will open a valve. Duration refers to how long the lobe will keep the valve
open. Lobe Separation or centerline is the time or duration between intake and exhaust valve
action. Overlap plays into lobe separation because it is the period when the exhaust valve is closing
and the intake valve is opening. The Ramp is the ascending or descending side of the cam lobe
coming off the base circle when lift begins to occur. The Flank is the ascending or descending
portion of the lobe past the base circle nearest maximum lift. The camshaft’s Base Circle is the
portion of the lobe that doesn’t generate lift. The bottom-most portion of the lobe is called the Heel.
Flat-tappet camshafts work differently than roller-tappet camshafts, which means you have to think
differently with each type. Flat-tappet camshafts limit what you can do with lobe profile if you want
streetability. If you want an aggressive profile with flat tappets, you can only go so far with a street
engine or suffer with poor driveability (rough idle, low manifold vacuum). If you want an aggressive
profile in a street engine, we suggest stepping up to a roller camshaft, which can handle the
aggressive profile better using roller tappets.
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STREET CAM FACTS
Based on everything we have seen in nearly 30 years of experience, the best street performance
cams are ground with a lobe separation between 108 to 114 degrees. When you keep lobe
separation around 112 degrees, you improve driveability because the engine idles smoother and
makes better low-end torque. This is what you want from a street engine. Any time lobe separation
is below 108 degrees, idle quality and streetability suffer. However, there is more to it than just lobe
separation.
Compression and cam timing must be considered together because one always affects the other.
Valve timing events directly affect cylinder pressure. Long intake valve duration reduces cylinder
pressure. Shorter duration increases cylinder pressure. Too much cylinder pressure can cause
detonation (pinging). Too little and you lose torque. You can count on cam manufacturers to figure
stock compression ratios into their camshaft selection tables, which makes choosing a camshaft
easier than it has ever been. Plug your application into the equation and you will be pleased with
the results most of the time.
The greatest advice we can offer the layman is to be conservative with your cam specs if you want
reliability and an engine that will live a long time. Stay with a conservative lift profile (under .500-in.
lift). A high-lift camshaft will beat the daylights out of a valvetrain, and will put valve-to-piston
clearances at risk. Watch duration and lobe separation closely, which will help you be more
effective in camshaft selection. Instead of opening the valve more (lift), we want to open it longer
(duration) and in better efficiency with piston timing (overlap or lobe separation).
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Always bear in mind what you are going to have for induction, heads, and exhaust. The savvy
engine builder understands that in order to work effectively, an engine must have matched
components. Cam, valvetrain, heads, intake manifold, and exhaust system must all work as a team.
If you are opting for stock heads, your cam profile doesn’t need to be aggressive. Select a cam
profile that will give you good low- and mid-range torque. Torque doesn’t do you any good on the
street when it happens at 6500 rpm. Choose a cam profile that will make good torque between
2500 and 4500 rpm. Otherwise, you are just wasting engine.
The thing to remember with camshaft selection is how the cam will work with your engine’s cylinder
heads. We need to take a close look at valve lift with a particular head and determine effect. Some
camshafts will actually lose power with a given head because there’s too much lift or duration. This
is why it is important to understand a given cylinder head before choosing a camshaft. You want to
seek optimum with any cylinder head/camshaft combination. This means having to really do your
homework before making a decision. Part of building a successful budget engine is doing a lot of
the homework yourself because you cannot afford a wasteful experience.
What type of fuel do you intend to run in your engine? This also affects camshaft selection. We can
actually raise compression if we’re running a mild camshaft profile or using a higher octane fuel. It
all has to work together. Camshaft timing events must be directly tied to actually raise compression
if we’re running a mild camshaft profile or using a higher octane fuel. It all has to work together.
Camshaft timing events must be directly tied to compression ratio. The longer our duration, the
lower the cylinder pressure and resulting compression. The shorter the duration, the less air we’re
going to bring into the cylinder, which also affects compression. Our objective needs to be the
highest compression without detonation, which will harm the engine. With this in mind, we want the
most duration possible without compression extremes. Duration is what gives us torque as long as
compression is sufficient.
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Valve overlap, as we have stated earlier, is the period between exhaust stroke and intake stroke
when both valves are slightly open. This occurs to improve exhaust scavenging by allowing the
incoming intake charge to push remaining exhaust gasses out via the closing exhaust valve. Were
the exhaust valve completely closed, we wouldn’t get scavenging. The greater the overlap in a
street engine, the less torque the engine will make down low where we need it most. This is why we
want less valve overlap in a street engine and more in a racing engine, which will make its torque at
high RPM. Increased valve overlap works best at high RPM.
Street engines need 10 to 55 degrees of valve overlap to be effective torque powerhouses. When
valve overlap starts wandering above 55 degrees, torque on the low end begins to go away. A
really hot street engine will need greater than 55 degrees of valve overlap, but not much greater.
To give you an idea of what we’re talking about, racing engines need 70 to 115 degrees of valve
overlap.
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Camshaft design can be
confusing. Think of the cam
lobe in geographical
regions as it travels against
the lifter: opening ramp,
opening flank, nose, closing
flank, closing ramp, then
the heel. The base circle is
the part of the lobe that
doesn’t generate lift.
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For a street engine, we want valve overlap to maximize torque, which means a conservative
approach in the first place. Push overlap as far as you can without compromising torque. We also
have to figure in lift and duration with valve overlap to see the complete power picture.
Lobe separation angle is another area of consideration in street cam selection. This camshaft
dynamic is chosen based on displacement and how the engine will be used. Rule of thumb is this.
Consider lobe separation based on how much displacement and valving you’re going to be using.
The smaller the valves, the tighter (fewer degrees) lobe separation should be. However, tighter
lobe separation does adversely affect idle quality. This is why most camshaft manufacturers spec
their cams with wider lobe separations than the custom grinders.
Duration in a street engine is likely the most important dynamic to consider in the selection process.
We increase duration whenever less lift is desired. Why? Because we get air flow into the cylinder
bore two ways: lift and duration. We can open the valve more and for less time to get air flow. Or,
we can open the valve less and keep it open longer via duration to get air flow. Each way will have
a different effect on performance. Duration is determined by how much cylinder head and
displacement you have, and how the engine will be used. Excessive duration hurts low-end torque,
which is what we need on the street. So we have to achieve a balance by maximizing duration
without a loss in low-end torque. We do this by using the right heads with proper valve sizing. Large
valves and ports don’t work well at all for street use. Mix in too much duration and you have a real
slug at the traffic light.
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This is a flat-tappet
camshaft. Notice the cam
lobe profile (shape). It is
more aggressive by nature
even with a stock grind.
Streetability suffers when lift
and duration are increased,
making the idle rough and
eroding manifold vacuum.
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So what does this tell us about duration? Plenty. We want greater duration whenever displacement
and valve sizing go up. Increasing duration falls directly in line with torque peak and RPM range.
This does not mean we necessarily gain any torque as RPM increases. It means our peak torque
simply comes in at a higher RPM range. An example of this is if our engine is making 350 ft./lbs. of
torque at 4500 rpm and we increase duration. We may well be making that same amount of torque
at 5200 rpm. In short, increased duration does not always mean increased torque.
Compression has a direct effect on what our duration should be. When we’re running greater
compression, we have to watch duration closely because it can drive cylinder pressures too high.
Sometimes we curb compression and run greater duration depending on how we want to make
power. When we have greater duration, our engine is going to make more power on the high end
and less on the low end. This is why you must carefully consider duration when ordering a
camshaft. Higher compression with a shorter duration helps the engine make torque down low
where we need it most in a street engine. The thing to watch for with compression is detonation and
overheating. Maximum street compression should be around 10.0:1.
Valve lift is an issue we must think about as it pertains to an engine’s needs. Small blocks generally
need more valve lift than big blocks. As we increase lift, generally we increase torque. This is
especially important at low- and mid-RPM ranges where it counts on the street. Low-end torque is
harder to achieve with a small block because these engines generally sport short strokes and large
bores. Your objective needs to be more torque with less RPM if you want your engine to live longer.
Revs are what drain the life out of an engine more quickly.
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To make good low-end torque with a small block, we need a camshaft that will offer a combination
of effective lift and duration. As a rule, we want to run a longer intake duration to make the most of
valve lift. We get valve lift via the camshaft to be sure. However, rocker arm ratio is the other half of
the equation. The most common rocker arm ratio is 1.6:1, which means the rocker arm will give the
valve 1.6 times the lift we have at the camlobe. When we step up to a 1.7:1 ratio rocker arm, valve
lift becomes 1.7 times that which we find at the lobe.
When we’re spec’ing a valvetrain, it is best to achieve balance all around. If you run a high-lift
camshaft with a 1.7:1 rocker arm ratio, you may be getting too much lift, which means excessive
wear and tear. It is best to spec on the side of conservatism especially if you’re building an engine
for daily use. Whenever you opt for an aggressive camshaft with a lot of lift, you’re putting more
stress on the valve stem, guide, and spring. The constant hammering of daily use with excessive lift
is what kills engines without warning.
We will take this excessive wear logic a step further. It is vital that you ascertain proper centering of
the rocker arm tip on the valve stem tip when you’re setting up the valve train. We do this by using
the correct length pushrod for the application. Buy a pushrod checker at your favorite speed shop if
ever you’re in doubt. A pushrod checker is little more than an adjustable pushrod that you can use
to determine rocker arm geometry. If the pushrod is too long, the tip will be under-centered on the
valve stem, causing excessive side loads toward the outside of the cylinder head. If the pushrod is
too short, the rocker arm tip will be over-centered, causing excessive side loading toward the inside
of the head. In either case, side loads on the valve stem and guide cause excessive wear and early
failure. This is why we want the rocker arm tip to be properly centered on the valve stem for smooth
operation.
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One accessory that will reduce valve stem tip wear and side loading is the roller-tip rocker arm.
Roller-tip rocker arms roll smoothly across the valve stem tip virtually eliminating wear.
Stamped-steel, roller-tip rocker arms are available at budget prices without the high cost of
extruded or forged pieces.
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Previous | Next
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This has been a sample page from
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How to Build Max Performance
Ford V-8s on a Budget
By George Reid
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Low-cost formulas for building serious horsepower!
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This book addresses high-performance V-8
engines such as the 289, 302, 351ci small-blocks
found in Mustangs, as well as the FE series of
big-blocks. Emphasis throughout is a budget
approach to building high performance powerplants
through the use of over-the-counter factory
components and selected aftermarket pieces.
Includes realistic, low-cost formulas for building
serious horsepower in Ford V-8 engines. Read the
sample pages to learn more!
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Click below to view sample
pages from several chapters
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1 - Engine Building Basics
2 - Making Power
3 - Engine Block
4 - Crankshaft, Rods & Pistons
5 - Cylinder Heads
6 - Camshaft & Valvetrain
7 - Headers and Exhaust
8 - Ford Ignition Systems
9 - Engine Build Ups
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Softbound
8-3/8 x 10-7/8
128 pages
300+ b/w photos
Item #SA69
Price: $18.95
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Click here to buy now!
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If you're serious about building a powerful
Ford V-8 you need this book!
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How to Rebuild the Small-Block Ford
This 144 page book guides you step by step through a rebuild,
including: planning, disassembly and inspection, choosing the
right parts, machine work, assembling your engine, first firing and
break-in. It also gives you helpful hints and tips on performance
upgrades, including cams, heads, ignition, induction, and more. It
also points out problem areas to watch for, professional builder
tips, jobs that need special care or special tools, and more.
Includes 495 color photos and covers the Ford 289, 302, 351W,
351C, 351M and 400.
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Price:
$22.95
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The Step by Step Guide to Engine Blueprinting
Practical methods for racing and rebuilding, selecting and
preparing parts, and how to buy machine shop work. This
completely revised and updated version containing an
additional 32 pages is simply the best book you can buy on
engine preparation for street or racing! Engine Blueprinting
shows the reader how to use precision measuring tools,
calculate compression ratios, degree a camshaft, and much
more!
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Price:
$
18.95
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How to Build Big-Inch Ford Small Blocks
By increasing the bore and stroke of your current engine, you can
add those cubic inches without the hassle of switching to a big
block. George Reid thoroughly explains the building of a small
block Ford stroker, paying special attention to the effect that
increasing the bore and stroke have on the engine as a whole.
Also included is a complete guide to factory head and block
castings, as well as aftermarket block and head guides, so you
can choose exactly the right parts for your project.
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Price:
$18.95
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Payment, Shipping & Sales
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residents must pay 7% sales tax. Items usually ship
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