a little confused on Lobe seperation.

What would be the difference in 2 cams that have the same specs. except for lobe sep.

ex. 210/220 @.50 .500/.500 lift (1) Lobe sep. 112
" " (2) Lobe sep. 114

Thanks-

 

Lobe sep is the number of degrees of cam rotation between peak intake valve opening and peak exhaust valve opening.

The exhaust opens before the intake.

Cams are always spec'd by the intake centerline. So in the case of those 2 cams, the one with the higher lobe sep number opens the exhaust valve slightly sooner, and closes slightly sooner, compared to the other.

The exhaust valve begins to open at the end of the power stroke. Therefore, if you open it earlier, you allow exhaust to begin escaping earlier, before the power stroke is complete. This reduces low RPM torque, because some of the pressure in the cylinder is allowed to escape, instead of being forced to continue to do work to the bitter end by pushing the piston down.

On the other hand, since the exhaust doesn't begin moving out instantly as the valve opens, but rather its speed build up slowly, it has less and less of this effect as RPM increases. At high RPMs, it actually can improve engine performance, by improving flow.

It also typically improves idle quality. Since rough idle is usually caused by exhaust reversion back into the intake tract, which in turn is caused by residual pressure in the cyl, the earlier exh opening gives the cyl more chance to emty itself out (lower pressure), which reduces reversion, which improves idle. This is why cams for computer FI usually have wide lobe sep; the wider it is, the better the idle quality, and the easier it is for the computer to measure everything and give the right fuel and spark curves.

So, the higher lobe sep will typically have slightly better idle quality, slightly less low-RPM torque, and the torque curve will be shifted upward and flattened somewhat (less peaky) than the otherwise identical one with lower lobe sep. 2° won't be a radical difference, but it might make the difference between, say, passing California emissions inspection, or not.

 

awesome thanks for the reply. That clears it up a lot.

 

Actually, the intake valve opens first (intake stroke, compression stroke, power stroke, exhaust stroke), but whatever. Same principle and effects apply.

 

Either one could be said to open "first", depending on where you declare the "start" of the cycle to be. E.g. Power, exhaust, intake, compression.

In terms of their SEQUENCE of operation in the cycle though, at the point at which they are adjacent (intake after exhaust), the exhaust is open first; and as it closes, the intake opens. Then after the intake is done, neither does anything for a while. The measurement of lobe sep angle assumes that the exhaust opens first, and that the intake opens x degrees later. The number given is the number of degrees of CAM rotation (most other cam specs are given in crank degrees) that the exhaust lobe centerline occurs BEFORE the intake lobe centerline. The ICL is the "line up" point for timing the cam to the crank (timing chain alignment), and is specified as some number of CRANK degrees that the ICL occurs AFTER TDC.