Greetings . . .

The idea of rocker arms and "Increased Duration" can be counterintuitive if not kept in context. Higher Ratio rocker arms cannot change the basic valve events of a given cam. The lobe doesn't push on the lifter any sooner or raise any longer.

Many cam grinders talk about 'advertised duration' as measured at .020 valve lift. In the old old days 'advertised duration' generally described the valve events directly from open to close. Some consider only the duration at .050 valve lift as useful for comparing one cam to another.

Consider a Chevy 216 cam with an advtersied 250 degrees of intake duration (@ .020 valve lift).

On this particular cam the valve doesn't reach .020 lift until the valve has been off the seat for 16 degrees of crank rotation with a 1.5 ratio rocker. With a 1.7 ratio rocker this event occurs 3 degrees sooner, and the valve stays above .020 for 3 degrees longer on the down side as well. So the effect is an increase in the 'advertised duration' from 250 to 256.

And the effect at higher lobe lifts is even more prounounced because of the multiplicative effect of the higher ratio. Again from the 216 Chevy cam described above. With 1.7 ratio rocker arms the valve reaches .050 lift some 6 degrees sooner then with 1.5 ratio. This has the effect of increasing the 'usable duration' by 12 degrees of crank rotation.

But let's not forget that there is downside to this increased duration - because as Roger Huntington always points out "everything with a cam is a compromise, you don't gain something without giving up something in return".

In this case the higher valve lift results in an additional 4 degrees of 'significant' overlap with both intake and exhaust above .020 valve lift. As you probably know any amount overlap effects low RPM response, lugging ability etc. Low end response is further impaired by reaching .020 exhaust valve lift sooner duing power stroke and later intake valve closure below .020 lift during the compression stroke.

regards,
stock49

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