John B

iTrader: (10)

I need to knock the duration down a few degrees on my cam to sneak it through tailpipe emissions testing, thanks!

Nasty N8

iTrader: (33)

We could probably make a set of custom Harland Sharps but I am not sure how much that would effect the rest of the geometry.

Nate

J-Rod

Check with Patriot. I think Terry can do smaller ratios on his shaft rocker setup.

94form2000z

iTrader: (10)

Umm doesn't ratio effect lift not duration? Maybe that's what you meant.

J-Rod

Nope it will affect duration slightly. Think about it. Your duration @ .050, on the cam with a 1.7 ratio is your lobe lift @ 0.02941

When you go to a 1.4 ratio your cam now has to have a lobe lift of 0.03571 to get to .050 at the valve. That addition lift means less duration @ .050

With 1.4 your duration that you had with a 1.7 is now really your duration now @ .041. For instance, on a stock Z06 cam going to a 1.85 nets you about + 2 degrees of duration. I would expect about the same drop going down in ratio.

Another_User

Not really. You are just reducing your lift. The valves will all open and close at the exact same time as before...just less lift.

Nickn20

seat to seat, yes they will open and close at the same time, I agree with J-rod with higher ratio RRs you achieve .050 early in the cam lobe and it ends later, thus increasing duration @ .050 they will not increase overall duration though, so high lift rockers increase the ramp of a cam lobe.

94form2000z

iTrader: (10)

Makes no sense how changing rockers changes the amount of time a lifter is on a cam lobe Which is what determines when the valve opens how long and what lift. All you changed from what I'm understanding is the amount of lift after fulcrum on rocker

WAHUSKER

iTrader: (4)

True, but since all that is spec'd is the cam lift & duration, that's what everyone calculates the rocker arm ratios back to. You don't see people stating how far the valves will open or for how long or how quickly they transition.....

J-Rod

There is actual lift and duration. Then there is effective lift and duration.

By changing a rocker arm ratio you don't change the lobe, but for instance if you go with less rocker arm ratio it take more actual "time" to open the valve to .050 so the observed duration is less than it was with the larger rocker.

Conversely if you put a bigger ratio rocker arm, the cam lobe doesn't get any bigger physically. But, you get to .050 lift "quicker". Thus the effective duration is bigger. If you want an actual number I have two campros comparing a Z06 cam with a 1.7 vs a 1.85. Effective duration went up around 2 degrees.

DaleMX

iTrader: (4)

Rhodes lifters would help, but the heads would have to come off. Go the easy route and change the cam.

Dale

Another_User

Yeah, I can see that. Duration @.050 would be down. The duration overall would be the same, it would just appear to have a teensy bit less duration @.050 because the ramp would be lower.