Do any of you know if changing rocker ratio from lets say 1.5 to 1.6 will effect the duration all that much to make any kind of change in the motor?
Thanx.

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crower says 5 degrees

Thanx Brian.The cam you suggested for my motor
00322,thats on a 108 deg yes?

Steve,

I suggest the 00321 custom ground on a 108LSA.

That is the exact cam that i have in my motor, and i'm happy with it.
12.30 @110.5 mph in extremely hot/humid weather, shifting at 5500rpms, and turning the tire on the launch, to have a 1.65 60ft.

Your motor is in the same RPM band as mine, when the motor is dialed in i'll bump the shift point to 6000-6200rpms.

the 00322 is on the large side.

graemlins/thumbsup.gif Thanx again.Big help...

It's the duration effect as much as the intensity effect. Bigger ratio implies a faster opening/closing rate. Duration and rate are changed.

I have a hard time understanding how the duration will change with differend rocker ratios. I can see the lift intensity being greater with bigger rockers, but wouldn't the closed point of the valve be the same degrees as with a smaller rocker? Right before opening the valve, the lift is 0.000. 0 times 1.5 is the same as 0 times 1.6, still zero. Wouldn't the duration stay the same regardless of rocker ratio, because there's no difference between rockers until something actually moves? (which would be the same open/close point, regardless of ratio)

Not trying to start an argument, I'd just like a better understanding of how this works.

chris

edit: Does it have something to do with adv duration being at SOME lift point, like .002 or .006.... if this is the case, I can understand how the adv duration would be a little bigger with a bigger rocker. But the duration at 0.000 lift would be exactly the same regardless of rocker ratio.

Im also not understanding how a rocker ratio change can change the duration of a cam because absolutely nothing happens till you leave the base circle of the cam. In order to do 1.1 more lift in the same amount of time(duration), that would increase the speed at which the vavle opens and closes, so kinda the same as a cam being given steeper ramps it seems.

Seat duration is not changed with higher ratio rockers. .050" dur is increases by about 5º, .200" dur by about 7-8º. This is because the valve is opened and closed quicker. With 1.5 rockers, the valve lift at .050" is about .075". With 1.6 rockers at .050" tappet lift the valve is open .080. If you measure the dur at the valve at .075" (the 1.5 ratio lift) using 1.6 rockers the tappet lift is .0468". As you can see the tappet has not traveled up the ramp as far. This means the valve is open longer given the same valve lift. While the measured duration at the tappet doesn't change the effective dur at the valve increases.

Also, while the overlap in degrees does not change the overlap area increases. This is because the valve comes off the seat quicker with high ratios and is open farther at overlap centerline. With some cams this can give the engine a slightly more "cammy" sound at idle.

what pat said.

Thanx. That makes it alot easier to understand and to deal with.