rocker ratio vs valve lift

[thevic24]

how dose rocker ratio affect valve lift? i know (ford related) 1.6 is standard for most fords, how would going to 1.7 rockers affect valve lift and motor preformance. its kinda funny, i now how to assemble things but just tring to learn how certian things affect others and how things actually work thanks
-vic
if this helps- b303 cam, gt-40 heads, fords hi-rise duel plane(eddle. preformer rpm)10-10:5.1 comp, 1.95 intake valves, heads shaved .040

[BlackoutSteve]

Maximum lobe height of 0.300" + rocker ratio of 1.6:1 = 0.480" lift at the valve.
1.7:1 ratio would = 0.510" at the valve..
Easy!

You'll get a very slight increase in duration also.

[thevic24]

cool, iam learning somthing new every day 8). so increasing rocker ratio is allmost like using a diffrent cam with more lift and duration? would the power curve change much?
thanks
-vic

[PackardV8]

FWIW, increasing the rocker arm ratio does not increase duration. It increases the area under the existing curve. The valve opens .1 times quicker and higher, within the same duration. It makes the cam "bigger", not longer. You may need to stiffen up the springs a bit to handle the quicker, higher lift, as well as check the coil bind.

thnx, jack vines

[TC]

Like Steve said. Basically you're multiplying the Lobe Lift by the rocker ratio to get the valve lift.

And in the case of not knowing the lobe lift of the cam, just figure that changing from 1.6's to 1.7's will give you about another .030+/- in lift.

[TC]

FWIW, increasing the rocker arm ratio does not increase duration. It increases the area under the existing curve. The valve opens .1 times quicker and higher, within the same duration. It makes the cam "bigger", not longer. You may need to stiffen up the springs a bit to handle the quicker, higher lift, as well as check the coil bind.

thnx, jack vines

Actually changing the rocker ratio does increase you duration by about 2-3 degrees.

[mopower58]

Actually I was going to ask the same question. I am going to a 1.6 ratio from a 1.5 with the High Torque .440 roller lobe from Comp. I will go from .660 at the valve to .704 at the valve. As a baseline do I need to jet up a couple of sizes?

[TC]

It's hard to say, I'd leave the carb alone, and do some WOT passes and see what the sparkplugs look like. Or if your a rich guy go have it dyno'd.

And one more thing, I'm sure you've thought of this but have you checked the retainer to guide clearance. Going .040 higher on your lift can cause interference problems if the heads aren't set up for the added lift.

[std]

FWIW, increasing the rocker arm ratio does not increase duration. It increases the area under the existing curve. The valve opens .1 times quicker and higher, within the same duration. It makes the cam "bigger", not longer. You may need to stiffen up the springs a bit to handle the quicker, higher lift, as well as check the coil bind.

thnx, jack vines

Actually changing the rocker ratio does increase you duration by about 2-3 degrees.


How does it increase duration? Sounds impossible to me.

[BlackoutSteve]

Think of degreeing your cam in at the 0.050" opening and closing points.

Now imagine a rocker with a higher ratio. The valve will have slightly more lift at those points. In other words, it's like the valves have opened earlier.

When degreeing in a cam, we use lobe lifts, but as far as the valve is concerned, it has reach it's (0.050" x 1.7:1) 0.85" sooner than before with the 1.6:1 rockers..

Technically, the actual duration hasn't changed, but airflow past the valve has begun sooner, and that's the same effect as adding a few degrees on both sides of the lobe.

It's only a slight increase, but you'll notice a slightly lower idle vacuum on an already large cam.

[mopower58]

It's hard to say, I'd leave the carb alone, and do some WOT passes and see what the sparkplugs look like. Or if your a rich guy go have it dyno'd.

And one more thing, I'm sure you've thought of this but have you checked the retainer to guide clearance. Going .040 higher on your lift can cause interference problems if the heads aren't set up for the added lift.


I have a F.A.S.T. air fuel ratio monitor on it with o2 sensors in each header. I just like tuning on the safe side when making a change. The heads have plenty of guide clearance, and I am by no means Rich! ;D