Rocker Ratio affect on Duration

[enkeivette]

Learned something, thought I'd share.

Camshaft duration is a measurement of how long the valve stays open in crankshaft degrees. And the measurement is taken from one specific lift point to that same closing point on the closing end of the ramp.

http://images.circletrack.com/enginetech/ctrp_0805_06_s+cam+degree_wheel_3.jpg

But, since duration is measured at the cam itself from the manufacturer you would think that rocker ratio is totally independent of this. Well yes, and no, and yes again.

What really matters is the lift at the valve (obviously) and if we measure the lift at the valve at a specific point (say .05 again) with a higher ratio rocker, the valve will reach that point slightly sooner and slightly later, creating a slightly longer degree arc.

Although, if we were to measure valve lift at .00000000000000000000001 (you get the point) there would be no difference in duration, as the cam begins to lift the valve when it begins to lift the valve, period. Rocker ratio cannot change this. However, we do not end the measurement at the very closing point of the ramp. Why not? Probably because there is a negligible amount of air flow at .00000000000000000000001 lift. :huh:

Is your head spinning? Mine is. :sm_up_there:

So in conclusion, yes rocker ratio can affect dynamic duration. The rule of thumb seems to be 2 degrees for every .1 ratio on the rocker arm.

In otherwords, if you had 220 degrees of duration with a 1.6 rocker on an LS1, you'd have 222 degrees with a 1.7 rocker. My cam is a 236 in, 242 ex. split duration with a 1.5 rocker and with my new 1.65 rockers, my duration will jump to 239 in. and 245 ex. :D

[BRUTAL64]

Learned something, thought I'd share.

Camshaft duration is a measurement of how long the valve stays open in crankshaft degrees. And the measurement is taken from one specific lift point to that same closing point on the closing end of the ramp.

http://images.circletrack.com/enginetech/ctrp_0805_06_s+cam+degree_wheel_3.jpg

But, since duration is measured at the cam itself from the manufacturer you would think that rocker ratio is totally independent of this. Well yes, and no, and yes again.

What really matters is the lift at the valve (obviously) and if we measure the lift at the valve at a specific point (say .05 again) with a higher ratio rocker, the valve will reach that point slightly sooner and slightly later, creating a slightly longer degree arc.

Although, if we were to measure valve lift at .00000000000000000000001 (you get the point) there would be no difference in duration, as the cam begins to lift the valve when it begins to lift the valve, period. Rocker ratio cannot change this. However, we do not end the measurement at the very closing point of the ramp. Why not? Probably because there is a negligible amount of air flow at .00000000000000000000001 lift. :huh:

Is your head spinning? Mine is. :sm_up_there:

So in conclusion, yes rocker ratio can affect dynamic duration. The rule of thumb seems to be 2 degrees for every .1 ratio on the rocker arm.

In otherwords, if you had 220 degrees of duration with a 1.6 rocker on an LS1, you'd have 222 degrees with a 1.7 rocker. My cam is a 236 in, 242 ex. split duration with a 1.5 rocker and with my new 1.65 rockers, my duration will jump to 239 in. and 245 ex. :D




Good, job on this. I already knew this but reading it again was good for me. ;)

The only time you feel any difference is when you add 6* or more on intake and 10* more exhaust. The open and closing points on the intake are more important than a couple of degrees of duration. Just my .3 cents.

[C5Natie]

Wont a higher ratio also increase the lift? That might be something to look at on a setup with an already tight ptv. The reason I didnt go with 1.8 vs the stock 1.7.

[Shaolin Crane]

Yes

[94cobra69ss396]

I'll use the E303 cam in the Cobra as an example to explain that. The advertised valve lift is .498 with a 1.6 rocker. The lift on the cam lobe is .311. So with my 1.7 rockers the valve lift is .528. I measured the lift at the cam on mine though and every lobe was .312 making the lift at the valve .530 with my 1.7 rockers.

[Shaolin Crane]

Advertised vs. Actual. That's its very very important to check ptv EVERY time