Cam Timing Notes – Part II

Checkout Cam Timing Notes Part I here and The Brain of Your Triumph B Range Engine- the Camshaft.

By Mike Grange, Customer and Technical Support at The Bonneville Shop

This cam timing blog is intended to be informative and educational only.  The Bonneville Shop is not responsible for any damage that may occur when performing any of the tasks outlined in this tech article.  Please be careful so that you do not bend your valves and pushrods.

Every gear head has had those garage talks about camshafts and cam timing. One common misconception is that installing higher lift and longer duration cams is a simple bolt in performance modification. This is not true; camshaft selection depends on the application. The more lift/duration you have, the more compression is needed to create enough negative pressure to fill the cylinder. Too much lift/duration will slow down the intake charge and underfill the cylinder.

Then there is the valve-to-valve clearance (at least .060”), valve to piston clearance (at least .080”), retainer to guide clearance (at least .060”) and valve spring coil to coil clearance (at least .060” total) that need to be considered. All of which affects the most important thing to consider, rideability.

There are a few terms and measurements that should be understood when discussing cam timing and selection. After all this is one of those subjects where bigger numbers are not always better. And major engine damage can occur if cam timing is not done right.

Lobe Centers

The main specification used in cam timing is lobe centers.  There are many ways changes to lobe center timing change performance characteristics, there will be more information further down.  The following illustration is similar to a cam profile map, for mine I prefer to use graph paper and go from left to right.

The changes and resulting characteristics in the illustration are accurate. The missing detail is that the range of changes to cam timing is approximately +/- 5 degrees of crankshaft rotation. That is less than one tooth in a Triumph 500cc or 650cc engine (1 tooth = 14.4 degrees of crankshaft rotation).

The best starting point for cam timing (or degreeing) is with symmetrical lobe centers. The 100.5 degree symmetrical lobe centers in Triumph 500cc and 650cc engines is one of the main reasons why they were popular racing bikes in the 1950s and 1960s. Race mechanics/tuners used different cam gears, keyways, and marks to advance or retard in their cam timing to fine tune their bikes in for different tracks during the British bike glory days of racing.

Lobe center timing has to be done with the more radical cams because valve to piston contact will occur if the factory cam timing marks and keyways are used. Needless to say, it’s very important to know the parts of a camshaft and understand how to take the necessary measurements in order avoid catastrophic failures before fettling with cam timing.

The Parts of a Camshaft and Their Functions

(Note: Illustration does not represent a Triumph camshaft)

• Journal: These are the parts that fit inside the bearings.
• Base circle: The center of the cam, excluding the lobe.
• Clearance ramp: Not exactly the same as Triumph quieting ramps.  These are necessary with solid lifters to prevent seat bounce and to gradually take up the valve clearance.
• Flanks: Control the acceleration/deceleration of the valve opening/closing.
• Nose: Where valve max lift occurs. The shape is critical for preventing valve float.
• Dwell: The time the valve is fully open, measured in degrees of crankshaft rotation.
• Lift: Difference between base circle and max lift.
• Duration: Total time the valve is effectively open, measured in degrees of crankshaft rotation.

Engine Timing Locations/Quadrant and Related Events

The illustration below shows the locations on the degree wheel.  The lobe centers (LC) are After Top Dead Center (ATDC) degrees for intake lobe centers and Before Top Dead Center (BTDC) degrees for exhaust lobe centers.  This illustration is for a clockwise engine rotation, this is how it would read on the timing side of a Triumph pushrod engine.  The primary/drive side would be a mirrored image.

(Clockwise engine rotation)

• BTDC: Before top dead center, intake valve opens.
• ATDC: After top dead center, exhaust valve closes.
• BBDC: Before bottom dead center, exhaust valve opens.
• ABDC: After bottom dead center, intake valve closes.
• TDCO: Top dead center overlap, details further down.
• TDCC: Top dead center compression, the big bang happens here.

The exhaust valve opens BBDC to let the expanding gasses start the exiting the cylinder. When the intake valve opens BTDC the exiting exhaust gasses start to draw in the intake gasses, which also cools the combustion chamber, this happens until the exhaust valve closes ATDC. The intake gasses are moving fast enough for the laws of momentum to take effect and keep the intake gases going into the cylinder until the piston is ABDC. Then we have the big bang, and it starts all over again.

Rocker Arm Ratio

(Note: Triumph B range twins have a 1:1.1 ratio)

How Cam Lift is Measured

“The first place where this geometric problem can show itself in the difference between the standard follower (.750” radius pre-DU24875) and the R follower (1.125 radius) on a high lift long duration camshaft.”

While engine and part manufacturers give specifications for wear measurements of a camshaft that are to be checked with a micrometer, these do not indicate lift or duration. The cam lift and duration measurements are always taken at the valve with no valve lash/clearance. The reason for this is to eliminate all variables caused by the geometry of the valve train.

There are two places where this geometric problem shows itself in a Triumph B range engine (or any pushrod engine). The first place where this geometric problem can show itself in the difference between the standard follower (.750” radius pre-DU24875) and the R follower (1.125 radius) on a high lift long duration camshaft. The second place is in the rocker arm ratio. The best way to measure cam lift is with a dial indicator on the valve spring retainer. To do this on a British bike, I always used a magnetic base dial indicator on a fabricated 3/8″ steel plate mounted on the torque stay cylinder head bolts.

How to Find True TDC with a Degree Wheel

Duration is measured in degrees of crankshaft rotation as they relate to top dead center (TDC) and bottom dead center (BDC). True TDC must be located before any readings can be taken. To do this a crankshaft degree wheel (not a D605/8 cam timing disc) must be fitted to the crankshaft (preferably the primary side on vintage British bikes) and a bendable fixed position pointer must be fitted to a bolt/fixed point on the engine. The pointer can be made out of a coat hanger or tig welding rod.

True TDC can be determined with a dial indicator before the head is installed. I prefer to find true TDC with the head installed, it saves time since clearances can be checked during the process. To do this rotate the engine ATDC and screw a positive stop in the spark plug hole, TBS-0112W can be used for this after a slight modification to stop the inside piece from sliding. Then rotate the engine back until the piston contacts the positive stop, record the ATDC degree. Then rotate the engine forward until the piston makes contact with the positive stop, record the BTDC degree. After that adjust/bend the pointer and repeat until contact degrees with the positive stop match.

How Clearances are Measured

The valve-to-valve clearance is the main factor limiting valve lift and duration in engines with hemispherical combustion chambers like in Triumph B range engines. Measuring this clearance should be done while creating a profile map of the camshaft. The best way is to place a small amount of modeling clay in the valve pockets machined into the piston, and balls on the crown of the pistons.

Then remove the cylinder head after the profile map is created, use the depth gauge on a dial caliper to make sure the valves didn’t smash the clay thinner than .080” (.060” for steel conrods), and the OD jaws to make sure the intake and exhaust valve smashes are at least .060” apart. These clearances are the main reason why Megacycle and other manufacturers give TDCO lift specifications for some of their high lift race cams.

How to Profile a Camshaft (Duration)

This is necessary to find lobe centers, opening and closing degrees. You are ready to go once you have found true TDC and have your dial indicator set up to take readings off the valve spring retainer. The first thing you do is rotate the engine until the valve is lifted .040 (Triumph’s spec is to set valve clearance to .020) and write down the reading on the degree wheel, this is your opening degree.

Next write down the degree wheel reading every .020 of lift. Cam dwell is measured from the degree .0005 before max lift to .0005 after max lift. The cam closing degree is .040 before the valve is fully seated. Personally, I like to map it out on graph paper similar to the first illustration in this blog.

How to Determine Lobe Center

Use the following to determine LC after creating a profile map: (opening degree + 180 + closing degree) divide that sum by 2. Then subtract the BTDC degree for intake LC and the ATDC degree for exhaust LC. For example: the intake E3134 opens 34 BTDC, closes 55 ABDC. So, 34+180+55=269, 269 div by 2=134.5, and now we subtract 34 so the time will relate to ATDC and the result is 100.5 ATDC/LC. This is the specification used for changes to cam timing. Advancing both will lower the effective RPM range, retarding both will raise the effective RPM range, and increasing overlap will allow the engine to rev up more freely.

Valve Overlap

The amount of valve overlap is determined by adding your intake opening degree to your exhaust closing degree. The importance of valve overlap is often neglected in camshaft discussions. The amount of valve overlap is important because exhaust gasses leaving the cylinder help start the intake charge and cool the combustion chamber.

The right amount of valve overlap can result in volumetric efficiency over 100% (more than 325cc air/fuel mix per cylinder in a 650cc twin). More overlap will make the engine run more efficient at high RPM by reducing low end cylinder pressure. Less will increase low end cylinder pressure resulting in more low RPM torque. The only applications where less valve overlap helps is with forced induction intake systems (turbo and superchargers).

Additional Information

The theoretical optimum intake gas speed is 300ft./sec., so it is possible to decrease performance when increasing lift and duration. Going above the optimal gas speed will cause turbulence in the intake tract and underfill the cylinder, going below is self-explanatory. The engine builders I learned from recommend a maximum lift of 32% of the valve diameter (in most cases) when designing a camshaft, this number is based on SuperFlow flow-bench and other related testing. However, .509” lift for a 1.592 intake valve is nearly impossible in a Triumph B range engine due to the ~45 degree valve angles needed for the hemispherical combustion chamber.

While there is a general rule of thumb for lift, duration depends on the application. Most stock engines have between 200-220 degrees of duration. For most sporting applications where a broad powerband is needed 230-250 degrees works well, that will provide a broad powerband with a decent amount of horsepower. Road racing, where the mid to high RPM power delivery is needed will require 250-300 degrees of duration. When the engine is WFO for short periods of time 300+ degrees is the way to get that fast ¼ mile run.

Needless to say, there are a lot of camshaft options for each specific application. If you understand what I’ve explained in this blog, our cam timing notes blog will be an application chart. I could go into details on all the camshaft specifications listed on that page, but it would end up being longer than any edition of the Machinery’s Handbook.  Hope you enjoyed this and be sure to read Dave Porter’s blog on the history of the Triumph B range engine camshaft.