Someone explain (my head hurts)
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Joined: Feb 2003
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From: Longview, TX
Someone explain (my head hurts) Plz help me out, I am wondering about the effects rocker arms will have on duration. I know that having a 1.7 over a 1.5 will change the total lift, but will it have any effect on duration, considering that most cams are measured at .50 for the actuall vlv opening, would a larger rocker arm change that? Allow the vlv to open sooner? I just can't figure it out in my head...
No. When a valve is closed it's closed.
0 lobe lift * any ratio still = 0 total lift.
The higher rockers will increase the time spent off the seat for higher lifts, but will not change total duration.
0 lobe lift * any ratio still = 0 total lift.
The higher rockers will increase the time spent off the seat for higher lifts, but will not change total duration.
does this help?
it doesn't change actual duration, the cam lobe determines that and that [cam lobe] is fixed. it doesn't allow the valve to open sooner in regards to duration, but at any point within the given duration the valve will be opened more... because you're rocker ratio is a multiplier. higher ratio = greater multiplier = more lift for a given amount of lobe lift.
the downside of higher rocker ratios if you want to get technical is that you're increasing the acceleration rate of the rocker tip, the valve, valve spring retainers and keepers. The more mass and higher the rpms, the greater the forces and the likelihood the rockers and rocker studs start deflecting and eventually break, on top of using correct springs that can handle that acceleration and extra amount of lift.
it doesn't change actual duration, the cam lobe determines that and that [cam lobe] is fixed. it doesn't allow the valve to open sooner in regards to duration, but at any point within the given duration the valve will be opened more... because you're rocker ratio is a multiplier. higher ratio = greater multiplier = more lift for a given amount of lobe lift.
the downside of higher rocker ratios if you want to get technical is that you're increasing the acceleration rate of the rocker tip, the valve, valve spring retainers and keepers. The more mass and higher the rpms, the greater the forces and the likelihood the rockers and rocker studs start deflecting and eventually break, on top of using correct springs that can handle that acceleration and extra amount of lift.
i can currently see it and i'm at work which usually blocks it,
the location of the pic is http://img2.freeimagehosting.net/ima...8fba0fe893.jpg
it's an excel graph showing lift differences for 1.5, 1.6, 1.7 1.8 1.9 and 2.0 rocker ratios.
it ought to answer all your questions
the location of the pic is http://img2.freeimagehosting.net/ima...8fba0fe893.jpg
it's an excel graph showing lift differences for 1.5, 1.6, 1.7 1.8 1.9 and 2.0 rocker ratios.
it ought to answer all your questions
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On The Tree
Joined: Feb 2005
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From: Odessa, TX
Yes it changes lift and duration. Example .352 lobe lift 1.7 (.600 total) with 250 at .050 has your minium requirement for piston to valve clearance you change to a 1.8 it makes .651 total and does not have enough clearance now. But if you change to a .383 lobe and 1.7 with 250 at .050 you would have the exact clearance you started with (.352/1.7/.600) but know you have .651 lift. You typically lose about .015 of piston to valve clearance with a single step in ratio (1.7 to 1.8) because it gives a slight increase in duration and alot of increase in lift.
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Verdad Gallardo Yes it changes lift and duration. Example .352 lobe lift 1.7 (.600 total) with 250 at .050 has your minium requirement for piston to valve clearance you change to a 1.8 it makes .651 total and does not have enough clearance now. But if you change to a .383 lobe and 1.7 with 250 at .050 you would have the exact clearance you started with (.352/1.7/.600) but know you have .651 lift. You typically lose about .015 of piston to valve clearance with a single step in ratio (1.7 to 1.8) because it gives a slight increase in duration and alot of increase in lift.
A bigger rocker does not change TOTAL duration but yes a bigger rocker will change .050 duration.
You kinda it sound like max lift has something to do with PTV clearance, which it does NOT. I understand you're basing it off 2 different lobe lifts, but the way it read to me, I just wanted to state that, not trying to say you're wrong or anything. Maybe it just didn't sound right in my head.
Where did you get the .015 PTV clearance loss from? Do you have a formula I could steal?
A bigger rocker does not change TOTAL duration but yes a bigger rocker will change .050 duration.
You kinda it sound like max lift has something to do with PTV clearance, which it does NOT. I understand you're basing it off 2 different lobe lifts, but the way it read to me, I just wanted to state that, not trying to say you're wrong or anything. Maybe it just didn't sound right in my head.
A bigger rocker does not change TOTAL duration but yes a bigger rocker will change .050 duration.
You kinda it sound like max lift has something to do with PTV clearance, which it does NOT. I understand you're basing it off 2 different lobe lifts, but the way it read to me, I just wanted to state that, not trying to say you're wrong or anything. Maybe it just didn't sound right in my head.