Rocker arm ratio

[Spoonbill]

When you are looking rocker arms what is the difference between 1.5 and 1.6 ratio? Also, some are self aligning and some aren't. What's the difference?

[Spoonbill]

What is the difference between a 1.5 ratio and a 1.6 or 1.7??

[Rattler12]

The amount the valve opens. Say a cam has a lift of .400 with 1.5 rocker arms. Put in 1.6 rockers and it would go to .427 and with 1.7 rockers to .453.  But again you have to consider the whole combination or BANG pistons hitting valves.  Not a good outcome

[bd]

A valve rocker arm is actually two levers and a fulcrum.  The fulcrum is simply a hinge point or pivot.  The rocker arm ratio is the relative length of each lever across the fulcrum (pivot). 

The eccentric lobe of a camshaft pushes against one end of the rocker arm through a lifter and pushrod; the opposite end of the rocker arm pushes directly against the valve tip.  If the lever lengths on either side of the pivot are the same, the rocker has a ratio of 1:1 - for any given amount of movement on one end of the rocker, the opposite end of the rocker moves the same amount.  If the lever on the "valve side" of the pivot is 1.5 times longer than the lever on the "camshaft side" of the pivot, the rocker has a ratio of 1.5:1.



For example, if a camshaft has a "lobe lift" of 0.332", a rocker arm having a ratio of 1.5:1 will open the valve...

1.5 x 0.332" = 0.498"

The same camshaft lobe operating through a rocker arm ratio of 1.6:1 will open the valve...

1.6 x 0.332" = 0.531"

So, the rocker arm ratio provides a lever or multiplication advantage to the camshaft for opening the valve.  Small block Chevy engines are factory equipped with 1.5:1 ratio rockers for both the intake and exhaust valves.


Edit: added illustration.

[Spoonbill]

Thank you. That made perfect sense. Is there an advantage to having s bigger than stock ratio?

[VileZambonie]

I will say that unless you have validity, do not change the rocker ratio. There are adverse effects of the increased ratio.

As bd pointed out, the rocker arm ratio allows for a mechanical advantage [leverage] between the cam and valve. By designing the rocker with a greater distance between the axis of rotation and valve than between the axis and pushrod you can increase the lift of the valve. The distance between valve tip and axis - divided by the distance between pushrod tip and axis establishes the rocker ratio.

A simpler numerical number but less realistic number to depict this is a cam lobe with a lift of .300" inch to open a valve = .450" inch with a 1.50:1 ratio. .300 x 1.5 = .450

By utilizing rocker ratios to increase valve lift we can reduce the eccentricity of the cam lobes - thereby reducing the surface speed between the cam and lifter.

This provides a low cost mechanical advantage, saves space and allows for an easily serviced assembly. Of course you can go to far which increases stress on the cam and valvetrain ultimately reducing efficiency and longevity. More frequent valve adjustments, more prone to bent pushrods, worn cam lobes etc. Also a higher reciprocating mass reduces top end performance and where you will see more suitable gains by utilizing the appropriate cam grind.

[SkinnyG]

Stock heads may need the pushrod guide holes elongated for this to work.

Stock heads cannot handle much more than about 0.480" lift without machining.

[Spoonbill]

Is there an advantage to a bigger ratio?

[bd]

You would not feel, notice or measure any improvement.