Cam Timing: Lobe Centers Explained

When you hear engine tuners bandying about numbers like "100-95" or "98-98," they are not talking about last night's game but rather camshaft timing and what is commonly referred to as the "lobe centers" of the intake and exhaust camshafts. These numbers are important because they go a long way toward determining your bike's power characteristics, and with a set of adjustable camshaft sprockets (which come standard on Kawasaki's new ZX-6R and ZX-6RR, in fact) you can tailor the cam timing to your own needs.

We all (hopefully) know the basic events in a four-stroke engine: the intake, compression, power and exhaust strokes. And as you were probably taught in high school auto shop, the exhaust valve is open during the exhaust stroke, and the intake valve is open during the intake stroke. But in reality, the exhaust valve opens before, and closes after, the exhaust stroke, and similarly the intake valve is open for longer than just the duration of the intake stroke. This means that, at the finish of one complete cycle (an exhaust stroke) and the beginning of the next (an intake stroke), there is a period of time where both the intake and exhaust valves are slightly open at the same time--commonly referred to as valve overlap. This is the critical time, because keeping the exhaust valve open as the intake opens can actually suck fresh charge into the cylinder, depending on the design of the intake and exhaust systems. However, with too little valve overlap, not all the burnt gases will exit out the exhaust valve, hurting power (not to mention emissions and fuel mileage).

The lobe center refers to the center (generally, but not always, the peak) of the cam's profile, and the cam timing numbers represent the crankshaft angle before top dead center (BTDC) for the exhaust cam's center, and the angle after top dead center (ATDC) of the intake cam's center. Figures 1 and 2 show two different representations of the important camshaft events in relation to the crankshaft's rotation. Note the overlap in figure 1, where both intake and exhaust valves are slightly open. Retarding the exhaust camshaft (moving the profile to the right) will increase overlap, as will advancing the intake camshaft (moving the profile to the left).

While it may be easy to look at a lobe's profile and point to the center, actually finding that point when the camshaft is in the engine is a different matter--especially when a couple of degrees can result in a big change. The lobe center is defined as the midway point between the opening and closing of the valve, but because a camshaft's profile is so shallow at small lifts, it's difficult to find that exact angle. For that reason, in the motorcycle world, the center of the profile is found by referring to the angles when the valve is open by 1mm, and calculated as follows:

For intake valves:
Lobe Center = [I_open+I_close+180]/2 - I_open

I_open = angle the valve opens BTDC

I_close = angle the valve closes ABDC (after bottom dead center)

And for exhaust valves:
Lobe Center = [E_open+E_close+180]/2 - E_close

E_open = angle the valve opens BBDC (before bottom dead center)
E_close = angle the valve closes ATDC

For example, using numbers from Suzuki's SV650, the intake cam's lobe center is:
I_open = 28

I_close = 62

Lobe Center = [28+62+180]/2 -28 = 107 ATDC

The exhaust cam's lobe center is:
E_open = 43

E_close = 24

Lobe Center=[43+24+180]/2 -24 = 99.5 BTDC

For applications of changing cam timing and valve overlap, we spoke with J. Andrew Milton, former Willow Springs champion, proprietor of SuperSport Engine Development, and husband to SR's art director, the divine Mrs. Milton. According to Milton, there are few generalizations that can be made as far as actual cam timing numbers go. Numbers that work for one particular engine will not necessarily work for a different setup, and experimentation is the key. Typically, however, small lobe center numbers (more overlap) equates to more midrange power at the expense of top-end power, and big numbers (less overlap) will give more top end, sacrificing midrange. Milton also notes that the airbox, ram air setup, and especially the exhaust system on newer bikes, all interact with and affect optimum cam timing settings and that again, experimentation is required when changes are made elsewhere.

One of the dangers of playing with cam timing is the potential for a collision between internal engine parts. As the piston approaches and passes TDC (top dead center) on overlap, the exhaust valve is closing. If the exhaust cam is too retarded, contact between the piston and valve will result. Similarly, the intake valve is opening as the piston approaches TDC on overlap. After TDC, the intake valve chases the piston as it opens--another potential danger zone. And with too much overlap, there is a chance the intake and exhaust valves could tangle. While not usually a source for concern with stock parts, this could present problems with aftermarket cams or oversize valves. After adjusting cam timing, Milton checks valve-to-piston clearance at 15, 10 and five degrees before and after TDC for both intake and exhaust valves.

In the practical world, engine builders will experiment with two- or three-degree changes in cam timing, and will be familiar with the clearance limits for each cam. For quick and easy changes, the sprockets can be marked for different values, meaning the tedious job of adjusting the cam timing by calculating the lobe centers need only be completed once for each setting. As with any modification, changes that show an improvement on the dyno are not necessarily reflected on the racetrack, and Milton recommends testing at both venues.

In the practical world, engine builders will experiment with two- or three-degree changes in cam timing, and will be familiar with the clearance limits for each cam. For quick and easy changes, the sprockets can be marked for different values, meaning the tedious job of adjusting the cam timing by calculating the lobe centers need only be completed once for each setting. As with any modification, changes that show an improvement on the dyno are not necessarily reflected on the racetrack, and Milton recommends testing at both venues.

Contact SuperSport Engine Development at
(310) 993-4065 or racewithssed@attbi.com.

Want to find out how to change your cam timing?
Check out Hands On: Cam Timing

This article originally appeared in the June, 2003, issue of Sport Rider