Rocker Arms

[Tom keedle]

Originally Posted by Travis Miller

If you are talking about an engine that has a spec of 1.5 rocker arm ratio, the first senerio would be okay. The second one would mean you have used a rocker arm that is sold as a 1.6 ratio and that would be illegal. While the total lift at the valve retainer would check correct, the ratio would be incorrect if it were to be checked.

The difference is you have used a rocker that has altered the rate of lift as the valve is opening and closing from the original rocker arm ratio spec but hid it by meeting the total valve lift spec once the cam reaches max lift.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

how would that be any different from a cam being ground different?

[Alan Roehrich]

With a given tappet diameter, especially a flat tappet, there is an absolute limit as to how fast you can accelerate that tappet. So, given the tappet diameter, and a factory rocker ratio, there is a design limit on how fast you can accelerate the valve. Increasing the rocker ratio allows you to accelerate the valve faster. So, if you reduced your maximum lobe lift in order to use a higher ratio rocker, you'd be disguising a method of exceeding the design limit of the original valvetrain with regards to valve acceleration.

When you have a limited amount of lift, the secret to power, within reason, is how fast you can get to that lift limit from the valve being on the seat, how long you can hold it there, and how quickly you can get it closed. It's called, in general terms, "area under the curve". Again, this is a generalization, not an exact rule. For a given combination, there is an ideal, that combination may or may not be able to achieve that ideal. And that ideal may not always be the maximum amount of "area under the curve".

It is really hard to explain this, especially using generalizations as opposed to actual examples and the math involved. That's about as clear as I can make it here.