Cam shaft rules-of-thumb: lobe center angles and duration

In the page on TR6 performance cams, lobe center angle was calculated because this specification is rarely "revealed" by the cam grinder. They were calculated assuming each lobe of the cam is symmetric. The assumption that a lobe is symmetric is a bad one if the cam is a modern design. Modern cams have more aggressive opening ramps that closing ramps. But I don't know what else we can do here, since vendors are typically not inclined to share that information with us (part of the 'magic' in making a cam). So the job is the best that we can do.

This page contains an article dicussing the impact on the running of your motor by changing the lobe center angles, duration and cam timing.

Tradeoffs for lobe center angles, duration and cam timing:

Assuming a constant lobe center angle

 Longer duration
more peak power, rougher idle,
 higher fuel consumption,
 higher emissions,
 less torque at low RPMs,
 power peak occurs at higher RPM,
 power rpm band widens and moves up

 Shorter duration
 the opposite of the above results

Assuming constant duration

 Tighter lobe centers
 more peak power,
 higher fuel consumption
 rougher idle
more torque at low RPMS,
peak power occurs at LOWER RPM,
 higher emissions,
 power RPM band gets narrower and moves DOWN

 Wider lobe centers
 opposite of above

Assuming constant lobe centers and cam duration

 Advancing the cam
 slightly improved low rpm torque,
 slightly reduced peak power
imperceptible change in emissions, idle & fuel consumption

 Retarding the cam
 opposite of the above

All of the above scenarios assume that the values change over some reasonable range. Obviously if the lobe center spacing went to 0 (zero), they engine simply wouldn't run. Likewise for duration. Duration can typically vary over a much wider range than can lobe centers. The cam advance/retard typically only works over a fairly narrow range (+/- 6 degrees?).


All the following data were generated using Dyno 2000 on a 2.5 liter TR6 motor.


This graph should be used as the reference point.

This graph is for the stock 1974 USA configuration. The simulator predicts 106HP @ 4500 rpm at the crank. This is very close to the factory rating of 104HP. Click here for a chart of the numbers.

The following figures were generated with a 9.5:1 compression ratio, mild porting, stock valves, stock Strombergs, and a header. In short, a mildly hot-rodded TR6. All plots and data assume a valve lift of 0.390 intake and exhaust, and intake duration = exhaust duration (i.e., symmetrical).

  This graph shows the effects of cam duration on HP and torque for two hypothetical cams, one a 258 degree cam @ 108 lobe center angle, 0 degrees advance, and the other 278 degree cam @ 108 lobe center angle, 0 degrees of advance (S2 cam). Click on the links to see a table of the results.
  This graph shows the effects on HP and torque of advancing the cam timing by +5 and -5 degrees. The cam has a duration of 278 degrees, and 108 lobe center. Click on the links to see a table of the results.
  This graph compares the effects of lobe center angle on HP and torque. The two cams are both S2 with 278 degrees duration and 0 advance, but one has 103 degrees lobe center angle, and the other has 113 degrees lobe center angle. Click on the links to see a table of the results.

Remember, other factors affect cam performance. A red hot cam will produce less power than the factory cam if intake, exhaust, compression, etc are all modified to accommodate the cam. No cam will perform unless the rest of the engine is matched to it. Use these numbers for comparison, not as an absolute indication of power.

Back to my Triumph TR6 home page

Last update: 25 July 2002.