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Rocker Ratio

What is Rocker Arm Ratio?

 

Rocker Ratio
Rocker Ratio
Rocker Arm Ratio is the length of the valve side of the rocker arm to the center (or pivot point) of the rocker arm divided by the length of the Cam or Pushrod side to the center of the rocker arm.  Typically the valve side is longer than the pushrod or cam side, so the Rocker Arm will "multiply" its motion by its Rocker Ratio.   This ratio is typically in the range of 1.45 to 1.7, meaning that for each .100" of pushrod or cam motion you would get .145" to .170" of valve motion.

Typically, increasing the Rocker Ratio is a simple way effectively a higher lift cam in your motor without having to actually put a different cam into the motor.  For this reason, it is a common "bolt on" modification.  It does increase the amount the valves open and therefore can improve breathing.  However, by just increasing the lift without increasing the Duration (amount of time) the valve is open, the power improvement is limited.  

Now for the down side.  The auto manufacturers put a lot of design time into the valve train geometry which goes into their engines, which includes picking the proper Rocker Arm Ratio.  Increasing Rocker Ratio means more valve lift, which could cause valve spring coil bind, valve retainer hitting the valve guides, or reduced piston-to-valve clearance (valves hitting the pistons).  (This is also true for many camshaft changes.)  

Also, increasing the Rocker Ratio will increase the stress on the valve train of the engine.  To get more valve motion, the cam lobe must push harder on the lifter, increasing the contact stress on the cam lobes and somewhat on the cam Bearings . The Lifters will also have increased stress because they ride on the Cam Shaft and push on the Rocker Arms. 

Typically, larger Rocker Ratios could mean the Rocker Arm could be more flexible, which is bad for high RPM valve train dynamics. This could increase the possibility of valve toss, which is when the valve springs cannot hold the valves in the desired position at high RPM.  

Larger Rocker Ratios could also mean putting more side loading on the valve stem, causing higher wear.  

 

True Rocker Ratio

 

True Rocker Ratio
True Rocker Ratio
Another important note is that the Rocker Ratio is always changing because the Rocker Arm tip is moving on an arc around it's center pivot point, but the valve stem goes straight up and down. This means that as the Rocker Arm moves, the distance from the pivot point to the contact point on the valve stem is always changing.  If this distance is changing, the Rocker Ratio is also changing.  Typically, the amount of change for a nominal 1.5 Rocker Ratio in a pushrod engine is from 1.45 to 1.55 as it goes through it's arc.

For Over Head Cam valve trains, the change in Rocker Ratio is much more dramatic.  It is not unusual for the ratios to change from 3.0 to 1.5 during the entire valve lift event.

Programs that take Rocker Ratio into account

We have 3 programs that work with Rocker Ratio when conducting Engine Simulation:  Engine Analyzer v3.4, Engine Analyzer Plus v3.4, and Engine Analyzer Pro v3.9 .

And we have 5 other programs that take Rocker Ratio into account when making calculations. Cam Analyzer v3.8, Valve Spring Tester v1.1, Port Flow Analyzer v3.5 with Valve Opener and Engine Log Book v1.1.



 

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