Increasing Rocker Ratio.....
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Seasoned HDF Member
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From: Poolville
Increasing Rocker Ratio.....
I'm thinking about going to Roller Rockers with an increased ratio to 1.725. The way I understood what others have said is that not only will it increase lift but will also increase duration... I understand that increasing the ratio will increase the lift but how is it possible that it changes the duration? Duration (opening and closing times) are ground into the cam. I don't see how a larger ratio rocker can change that. Has anyone that has installed 1.725" rockers done before and after ccp tests to confirm if there has been a change? We all know that a change in ivc will change ccp... It would be the only way I know to determine whether or not it will change the duration (without the use of a degree wheel and dial indicator).. Any explanation or actual tests would be appreciated.. Thanx...
Last edited by 98hotrodfatboy; Feb 3, 2019 at 06:59 AM.
Road Warrior
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From: Alorton, Illinois
Valve opening is reported when valve opens at .053. Higher ratio rockers accelerate lift. So, the degree the lobe starts to lift the valve off the seat. To time that valve makes .053 lift might change a half of a degree. All in all most people will consider it negligible.
Last edited by hrdtail78; Feb 3, 2019 at 10:01 AM.
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From: Poolville
Thanks but i's not about more lift.. I'm considering the Rollers to ease the valve train a little.. I was just considering more lift as a bonus if I was gonna change to rollers.. Why not...?
That's good to know cause I really don't want to change the duration of the cam I have. The LSA is 103* and I really like the characteristics of the cam..... It's a custom grind I got through Bob Wood and I really like the way the cam comes in at lower rpm's. So my lift @tdc is definitely gonna change. I'm currently at .203"/196" and with the increase in the rockers I believe i would be at .215"/.208", does this sound right? Also I shouldn't have an issue with valve to valve or to piston cause my heads were original set up Stage III for a Woods W9B... so clearances should be ok..
Valve opening is reported when valve opens at .053. Higher ratio rockers accelerate lift. So, the degree the lobe starts to lift the valve off the seat. To time that valve makes .053 lift might change a half of a degree. All in all most people will consider it negligible.
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From: Viet Vet, crossroads
You may do as you wish-
But a cam change out will be more efficient and cheaper
The valve geometry is already a problem and increasing the load on the P Rods may or may not be ok.
I would think the final word would be a outfit that does this for a living, such as Baisley would be the final say as to yes or no.
But a cam change out will be more efficient and cheaper
The valve geometry is already a problem and increasing the load on the P Rods may or may not be ok.
I would think the final word would be a outfit that does this for a living, such as Baisley would be the final say as to yes or no.
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From: river
Some good questions you have and I definitely cant answer them as I am curious too. On the other had I do run high lift ratio roller rockers, but everything was set up to do so. Good luck and Im Anxious to read more replies on here.
Without getting into the roller debate. My roller rockers stopped my guides from leaking and that was on a 575 lift. Im at 600 now and I think rollers are a great part especially if one spends a lot of time in the upper rpm range. Again, this is only my opinion.
One more rambling thought. I would think the company that does the port work on the heads should have an answer one way or another. Or at least an explanation 🤷🏽*♂️
Without getting into the roller debate. My roller rockers stopped my guides from leaking and that was on a 575 lift. Im at 600 now and I think rollers are a great part especially if one spends a lot of time in the upper rpm range. Again, this is only my opinion.
One more rambling thought. I would think the company that does the port work on the heads should have an answer one way or another. Or at least an explanation 🤷🏽*♂️
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degree will pretty much stay the same as it is fix mechanically. to compare apples, it is common practice to compare at some degree of lift and as brought out, .053, this is not set in stone as each mfg can set his own standard, read the card that came with the cam.
the point here is that at the so called .053 lift, the valve opening will be more. the valve opens sooner and closes later so total duration of seat to seat is longer. and i would believe you are ballpark in your assumptions, only a degree wheel and indicator will tell the whole story. if you are going by .053 open to .053 before closing, duration is the same only lift is diff.
a higher lift rocker can be like a higher lift cam, just cheaper. will the engine respond, hummmm, good ?, many factors.
as far as roller tips, they do reduce friction but look at the design of the system. on a harley rocker, they are shaft mounted and being such, hardly and movement compared to a fulcrum or pedestal mount like in a small block chevy. not to say there are not advantages but on a street hd, your money, i have close to 200,000 miles on some, had to re-arch them once, but still kicking.
the point here is that at the so called .053 lift, the valve opening will be more. the valve opens sooner and closes later so total duration of seat to seat is longer. and i would believe you are ballpark in your assumptions, only a degree wheel and indicator will tell the whole story. if you are going by .053 open to .053 before closing, duration is the same only lift is diff.
a higher lift rocker can be like a higher lift cam, just cheaper. will the engine respond, hummmm, good ?, many factors.
as far as roller tips, they do reduce friction but look at the design of the system. on a harley rocker, they are shaft mounted and being such, hardly and movement compared to a fulcrum or pedestal mount like in a small block chevy. not to say there are not advantages but on a street hd, your money, i have close to 200,000 miles on some, had to re-arch them once, but still kicking.
Last edited by bustert; Feb 3, 2019 at 02:14 PM.
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From: san antonio
Hi lift rockers, geometry, roller tips, etc. are not witchcraft or some dark art. Too many get wrapped around the axle on the minutia surrounding making basic changes to a 100 year old tractor motor; it's not a nuclear reactor. Duration increase is negligible and many run 1.7 rockers with no issues and have seen some improvement, all things being equal, over standard lift rockers. But it one is concerned about duration change, bustert points out the way to put that to rest. The math used to calculate the higher TDC lift is not rocket science either but before purchasing and installing hi lift rockers, valve to piston clearance at TDC should be checked and if new TDC lift still allows .060" clearance at TDC lift, all is good. I have run cams, not intentionally, with TDC lift clearance as low as .030" with no issues but, it was an accident and I would not do it again. Check clearance at TDC lift and if the clearance is there, run them.
Might even consider running them on intake only depending on cam profile.JMHO.
Might even consider running them on intake only depending on cam profile.JMHO.
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From: poway
Cams are speced at 0.053 tappet lift as stated so you won't see that spec change but overall duration will.
With a 0,053 lifter spec, the valve is at 0.053 x 1.625 = 0.086 at 0 lash (assume hydraulic lifter). If you add 1.725 rockers, the calve lift is 0.053 x 1.725 = 0.091 so the valve is lifted higher. Working back to what duration you could get compared to a 1.625 rockers you need to take the 0.086 and divide by 1.725 = 0.050. On measurement it works out to about 2-4 degrees.
With a 0,053 lifter spec, the valve is at 0.053 x 1.625 = 0.086 at 0 lash (assume hydraulic lifter). If you add 1.725 rockers, the calve lift is 0.053 x 1.725 = 0.091 so the valve is lifted higher. Working back to what duration you could get compared to a 1.625 rockers you need to take the 0.086 and divide by 1.725 = 0.050. On measurement it works out to about 2-4 degrees.