Durations explained

camshafts 240pxThis week we have an informative article from Cat Cams explaining the concept of camshaft duration and how it's measured. This is a little different to the traditional way that American camshafts are advertised so it may be worth while to make sure you understand the distinctions.

 

Cam Durations

What is the duration at 1mm?

Whenever a duration is quoted it needs to have the checking height (how far the valve is open) listed for the numbers to be of any use.

The more commonly quoted 'Seat to Seat' durations also include the opening and closing ramps of the camshaft.
Below 1mm of valve lift there is very little effectively happening. The valve is still moving relatively slowly and the valves are flowing very little. Because of these factors its much more useful to look at the duration of the camshaft once the opening/closing ramps have been taken out of the equation.Using the metric system these are quoted at 0.1mm for seat to seat duration and 1mm.

This best way to see what is happening to look at a couple of Cam profiles:

  1321806 1321816
This email address is being protected from spambots. You need JavaScript enabled to view it. + Valve clearance 300° 299°
Valve lift (clearance=0) 12.95mm 13.00mm

 

Using this checking height and valve lift, the profiles look to be very similar and you would expect very similar performance from them.

If we now add the durations at 1mm then we get the full picture:

  1321806 1321816
This email address is being protected from spambots. You need JavaScript enabled to view it. + Valve clearance 300° 299°
This email address is being protected from spambots. You need JavaScript enabled to view it. + Valve clearance 263° 274°
Valve lift (clearance=0) 12.95mm 13.00mm

 

Now it becomes clear that the 1321816 profile has 11 degrees more duration at 1mm checking height. This is what the engine sees in terms of airflow and makes this cam much wilder than the 1321806.
When you extend the cam duration on an engine there is always a trade off between low/midrange torque and top end power (and its often not a fair trade off, you tend to lose a big chunk of low rpm tractability for a smaller gain at the top end).
Whats needed is to keep the duration as short as possible but get the valve opening very quickly to a higher lift where the head flows better. This is the basic trend of Catcams Short duration/high lift profiles and the nett result is higher torque figures and wider powerbands. 

If you are trying to compare our profiles to other manufacturers, please not that our Seat (0.1mm) durations are often a lot smaller, however the agressive nature of Catcams profiles means the Duration @1.0mm is often significantly higher. 
The only real way to start to compare profiles is to ask for the duration @ 1.0mm