Does a higher ratio rocker change duration?
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: Does a higher ratio rocker change duration? (ddn)
David,
Thanks for the kudos, but apparenrly it IS that hard to understand.
The duration at .050" is measured at .050" LIFTER rise. Not valve lift. The rocker arm does not, cannot, affect the relationship between the lifter and the lobe. Duration at .050" does NOT change with a rocker ratio change.
Because the valve will reach the height at which it begins to flow, sooner, the engine will SEE a very minor increase in effective duration. The more aggressive the lobe, the greater the increase. The valve would reach minimum flow lift earlier with a roller cam than with a cheap flat tappet hydraulic cam, due to the steeper lobes on the roller set up. And that is exactly WHY guys shell out the big bucks for a roller cam.
I don't mean to beat a dead horse, here. And I hope I haven't offended anyone. I'm just trying to restate it so EVERYONE will get it. I truely AM trying to help, not argue.
Thanks for the kudos, but apparenrly it IS that hard to understand.
There is a change in duration at .050 because it reaches the magic lift number sooner.
It doesn't change the cam, but it changes the duration at .050.
It doesn't change the cam, but it changes the duration at .050.
Because the valve will reach the height at which it begins to flow, sooner, the engine will SEE a very minor increase in effective duration. The more aggressive the lobe, the greater the increase. The valve would reach minimum flow lift earlier with a roller cam than with a cheap flat tappet hydraulic cam, due to the steeper lobes on the roller set up. And that is exactly WHY guys shell out the big bucks for a roller cam.
I don't mean to beat a dead horse, here. And I hope I haven't offended anyone. I'm just trying to restate it so EVERYONE will get it. I truely AM trying to help, not argue.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: Does a higher ratio rocker change duration? (CFI-EFI)
Thank you. I've been trying to point out what really goes on.
If I have failed in that I am extremely sorry and I appologize to everybody. I would however ask you to go back an re-read my posts and not see if I've been trying to say the exact same thing. Both sides of this are correct. A motor will ACT like it has an effective increase in duration, but for different causes.
So again, to answer the original question, it depends on how the dyno program works. What does it ask for? If it asks for VALVE LIFT plain and simple you might have to tweak the duration a little. If it on the other hand asks for CAM lift and then asks for a rocker arm ratio, it will do all the calculations for you. If you then go back and change the duration, you have counted the same change twice, you essentially have a different cam.
Everybody cool? I'm tired for arguing.
:cheers: :cheers:
If I have failed in that I am extremely sorry and I appologize to everybody. I would however ask you to go back an re-read my posts and not see if I've been trying to say the exact same thing. Both sides of this are correct. A motor will ACT like it has an effective increase in duration, but for different causes.
So again, to answer the original question, it depends on how the dyno program works. What does it ask for? If it asks for VALVE LIFT plain and simple you might have to tweak the duration a little. If it on the other hand asks for CAM lift and then asks for a rocker arm ratio, it will do all the calculations for you. If you then go back and change the duration, you have counted the same change twice, you essentially have a different cam.
Everybody cool? I'm tired for arguing.
:cheers: :cheers:
Drifting
Joined: Jul 2001
Posts: 1,418
Likes: 1
Re: Does a higher ratio rocker change duration? (BlueDemon)
Pulled from 1st page
(The motor WILL see/run/act like it has effectively 2-4 degrees more duration when 1.6 rockers are added.Call my long time friends Jay Adams @ Comp Cams to verify.
Dave Crower@ Crower cams and George Striegle@ Clay Smith cams.Tell them CH Racing sent ya!)
Man,those Crane Cheeseburgers are good! Eat while waiting on the dry cleaning.Cool concept! Wish I had though of that!
:D
(The motor WILL see/run/act like it has effectively 2-4 degrees more duration when 1.6 rockers are added.Call my long time friends Jay Adams @ Comp Cams to verify.
Dave Crower@ Crower cams and George Striegle@ Clay Smith cams.Tell them CH Racing sent ya!)
Man,those Crane Cheeseburgers are good! Eat while waiting on the dry cleaning.Cool concept! Wish I had though of that!
:D
Instructor
Joined: Sep 2001
Posts: 159
Likes: 21
From: McKinney Tx
Re: Does a higher ratio rocker change duration? (mountainmotor)
when you up rocker ratio, timing AT a certain valve lift DOES change, period. The seat to seat is fixed but that's at zero lift. Everything else changes. So everyone's right to some degree. :chevy
Most don't quote the whole or advertised duration because they include the clearnce ramp which really isn't real timing. :flag
Most don't quote the whole or advertised duration because they include the clearnce ramp which really isn't real timing. :flag
Advanced
Joined: Jan 2002
Posts: 80
Likes: 0
From: Clarksville Tn
Re: Does a higher ratio rocker change duration? (mountainmotor)
Pulled from 1st page
(The motor WILL see/run/act like it has effectively 2-4 degrees more duration
(The motor WILL see/run/act like it has effectively 2-4 degrees more duration
Safety Car
Joined: Nov 1999
Posts: 3,891
Likes: 2
From: Houston, TX
Re: Does a higher ratio rocker change duration? (mountainmotor)
Duration of a roller camshaft is measured at .006" lift(general rule of thumb followed by comp, lunati, etc.) This leaves duration/timing virtually unchanged. Seat to seat timing is also not changed, which is what Nathan is very correctly pointing out to you.
Flat tappet camshafts duration is measured at .200" lift. At this point, you would see a 2-4 degree duration increase. Does that mean that the duration that the cam opens the valve, measured in degrees of rotation, is increased. NO!
This has turned into a heated discussion for absolutely no reason. We are just trying to hash out the truth. You are both correct in what you are saying. Mountain Motor is correct if you go by industry standard measurement of duration of a FLAT TAPPET cam. Nathan is completely correct referring to ROLLER TAPPET cams and measuring mechanical/seat to seat duration.
Nathan,
Nice drawing! That is exactly correct! I like that. I also like you analogy of the door! :)
That is all for now.
:seeya
Flat tappet camshafts duration is measured at .200" lift. At this point, you would see a 2-4 degree duration increase. Does that mean that the duration that the cam opens the valve, measured in degrees of rotation, is increased. NO!
This has turned into a heated discussion for absolutely no reason. We are just trying to hash out the truth. You are both correct in what you are saying. Mountain Motor is correct if you go by industry standard measurement of duration of a FLAT TAPPET cam. Nathan is completely correct referring to ROLLER TAPPET cams and measuring mechanical/seat to seat duration.
Nathan,
Nice drawing! That is exactly correct! I like that. I also like you analogy of the door! :)
That is all for now.
:seeya
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: Does a higher ratio rocker change duration? (Chris@SpeedDemon)
I think after this thread EVERYBODY should have a better understanding about how the entire camshaft works. Heated at times, yes it certainly has been, but I think in the end everybody should see the total picture and have a much better understanding of what rocker ratios and such do.
I've had fun with this one, I think everybody who reads it will have fun as well.
Next week we're gonna have a heated debate over lobe seperation angles!! :jester
:chevy :chevy :chevy :chevy :chevy :flag :cheers:
I've had fun with this one, I think everybody who reads it will have fun as well.
Next week we're gonna have a heated debate over lobe seperation angles!! :jester
:chevy :chevy :chevy :chevy :chevy :flag :cheers:
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Team Owner
Joined: Apr 2001
Posts: 22,267
Likes: 3
From: Dallas TX
Re: Does a higher ratio rocker change duration? (Nathan Plemons)
Next week we're gonna have a heated debate over lobe seperation angles!! :jester
:chevy :chevy :chevy :chevy :chevy :flag :cheers:
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: Does a higher ratio rocker change duration? (Nathan Plemons)
Next week we're gonna have a heated debate over lobe seperation angles!! :jester
:chevy :chevy :chevy :chevy :chevy :flag :cheers:
:chevy :chevy :chevy :chevy :chevy :flag :cheers:
Burning Brakes
Joined: Jun 2000
Posts: 865
Likes: 6
From: Auburn Michigan
Re: Does a higher ratio rocker change duration? (Nathan Plemons)
Next week we're gonna have a heated debate over lobe seperation angles!! :jester
