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Old 12-04-2008, 11:51 AM   #121
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Originally Posted by 03kbcobra View Post
Does anyone have an idea about this?
I'm just going to hazard a speculation about this, but less lift on the exhaust side is probably made up for (and made better by) the extended duration. The whole point of the longer duration/shorter lift is that the valve train lives an easier life.
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Old 12-04-2008, 11:57 AM   #122
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The Helix 2s I've profiled have insignificantly lower exhaust lift, which I imagine helps avoid valve-piston contact with the lobes' much longer exhaust duration.

IMO, high-ratio rockers are pointless w/o extensive internal engine modifications... the window of potential valve action determined by degrees of crank rotation is limited by the space available between the piston & valve faces.
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Old 12-04-2008, 03:20 PM   #123
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Old 12-04-2008, 07:21 PM   #124
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Personally, I'm a big fan of ZDDP... it's absence is often accompanied by wiped lobes, mushroomed crank gears and galled rocker shafts.
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Old 12-04-2008, 10:40 PM   #125
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Quote:
Originally Posted by partsguy662 View Post
I'm just going to hazard a speculation about this, but less lift on the exhaust side is probably made up for (and made better by) the extended duration. The whole point of the longer duration/shorter lift is that the valve train lives an easier life.

I thought that was the trade off, but the other part of question still remains. With head porting and more flow at higher lift, would higher ratio rockers on exhaust side be beneficial with increased RPM?
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Old 12-04-2008, 11:01 PM   #126
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Quote:
Originally Posted by 03kbcobra View Post
I thought that was the trade off, but the other part of question still remains. With head porting and more flow at higher lift, would higher ratio rockers on exhaust side be beneficial with increased RPM?
Re-read post 143, sums it up good
 
Old 12-04-2008, 11:26 PM   #127
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Re-read post 143, sums it up good

Read it. Just did not know if the statement was reality or speculation.
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Old 12-05-2008, 12:21 AM   #128
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Nothing speculative about the mechanical valve/piston relationship... I can't read Don's mind, but if I designed a cam profile it couldn't be much different from a Helix 2 based on it's intended usage.



An efficient path targeted to a particular power level using a majority of OEM parts is defined by the laws of physics, and as such is readily described by the relevant mathematical equations.

Helix cams work well more as a result of common sense than rocket science.
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Old 12-05-2008, 10:04 AM   #129
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Max lift has no impact on Piston-To-Valve issues, only duration & LSA. At max lift the piston is near the BDC. Cam duration and LSA (Lobe Separation Angle) are what you need to look at. For a given intake/exhaust duration as the LSA decreases two things happen - 1. Overlap increases (or becomes less negative) and 2. PTV clearance decreases. For a fixed LSA, increasing durations has the same effect. LSA is a number that basically "falls out" of a properly designed camshaft when selecting your Valve Closing & Opening Points (IVC, IVO, etc.) based on where you want your power band, etc.

For example:
180/180 .550/.550 cam would have no PTV issues
220/220 .350/.250 cam would
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Last edited by banshee; 12-05-2008 at 10:11 AM.
 
Old 12-05-2008, 11:24 PM   #130
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Max lift has no impact on Piston-To-Valve issues, only duration & LSA. At max lift the piston is near the BDC. Cam duration and LSA (Lobe Separation Angle) are what you need to look at. For a given intake/exhaust duration as the LSA decreases two things happen - 1. Overlap increases (or becomes less negative) and 2. PTV clearance decreases. For a fixed LSA, increasing durations has the same effect. LSA is a number that basically "falls out" of a properly designed camshaft when selecting your Valve Closing & Opening Points (IVC, IVO, etc.) based on where you want your power band, etc.

For example:
180/180 .550/.550 cam would have no PTV issues
220/220 .350/.250 cam would

Well put. Thanks for the good write up. The question of more duration decreasing clearance is appropriate to the conversation. It may take the cam designer to determine any clearance issues with higher ratio rockers and its benefits with the use of higher than stock flowing head at higher lift rates. Higher ratio rockers would add lift at peak of cam but add very little to lift on the outer edge of duration. Thanks again.
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Old 12-06-2008, 02:52 AM   #131
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Originally Posted by 03kbcobra View Post
Well put. Thanks for the good write up. The question of more duration decreasing clearance is appropriate to the conversation. It may take the cam designer to determine any clearance issues with higher ratio rockers and its benefits with the use of higher than stock flowing head at higher lift rates. Higher ratio rockers would add lift at peak of cam but add very little to lift on the outer edge of duration. Thanks again.
If the stock rockers are like 1.5:1 or so, if you add high ratio rockers that are 1.7:1 (persay) it would improve lift at all amounts of lift throughout the entire duration of the camshaft lobe not just at full lift. It would be "chasing" the piston down faster and also be "slower" coming back up. Atleast that is what I think but I have had SEVERAL drinks.
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Old 12-06-2008, 04:58 PM   #132
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You're right Muddin. With a higher ratio rocker every lift would be 1.7/1.5 X original lift (if you went to a 1.7 ratio from a 1.5 ratio). They basically add a percentage of lift everywhere but add no duration. Higher ratio rockers can be an okay way to add flow, but there are more downsides IMO. Because the ratio is higher they accelerate the valve faster and allow it to come back to the seat faster which adds valvetrain instability and valve bounce if the spring can't control your new ramp rate. Coil bind of the spring along with potential accelerated valve guide wear due to side loading the valve (different wipe pattern) are other big concerns. Like mentioned earlier, for the cost of a set of rockers it really makes as much sense to install a cam that has the lift ground in.
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