ziggs

hey guys just wondering about rocker arm ratios. wheres the ratio meausured from and wats the ratio of? any help out there fellas?
zig

Rhett

Picture in your mind a see-saw at a playground. The same length each side, giving a ratio of 1 to 1. Hold that thought.

Look at the rocker and imagine a line going exactly thru the centreline of the main axis (where the rocker is bolted down). In your mind break the rocker arm into 2 sections, either side of that imaginary line, (remember the see-saw). One part stretches over to the push rod, while the other part stretches over to the valve stem.

Imagine that the part that stretches to the pushrod is exactly 2 cm long and the part that stretches over to the valve is 4 cm long. If you get the 4 and divide it by the 2 you have a rocker ratio of 2.0.

If the valve side was 3 cm long instead of 4 then the ratio would be 1.5, (i.e. 3 divided by 2). The higher the ratio the further the rocker will push down on the valve, (i.e. giving you a higher lift without having to change cams).

This is a common trick for Chevys which have a standard ratio of somewhere between 1.5 and 1.6, (sorry I don’t know exactly, because I really don’t care), but Cleveland’s come from the factory with a ratio of 1.73.

I’ve heard of some people bolting on BB Chevy 1.8 ratio rockers onto their Clevelands. It may not sound much, but you will feel the difference. Be careful that the valve doesn’t go down so far that it hits the piston.

The other thing to remember is what the law of physics (levers) refers to - that the higher the ratio, the more effort the pushrod has to exert against the rocker to open the valve.

I hope this helps.

ziggs

thanks rhett exactly wat i was after. I thought that but needed it confirmed. obviously by increasing the lift the valve will stay open longer and u'd achieve better scavenging howeva i could see it playing funni buggers with the power and compresion stroke if the duration in the cam wasnt suited to the rocker ratio, would u agree?