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Old 08-06-2012, 08:04 PM
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Rocker Geometry Question

I know much has been made here about the way to properly set up rocker geometry; i.e. smallest pattern vs. centered on the tip. My brain is firmly in the "smallest pattern" camp.

My question is, since Comp Cams continues to suggest the "centered" method as correct (showing right now on their website ), what exactly does that say about their cam patterns? Were they designed and tested using the "centered" geometry? If so, what effect will using the "smallest area" method have? Will "doing it right" actually cost performance? (I would think it would be very small, but if we're pursuing perfect geometry, still significant.)

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Old 08-06-2012, 08:42 PM
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K the center of the valve is where the tip of the rocker needs to ride this is so the rocker does not knock the keeper and retainers plum off the top of the head and destroy the motor the movement of the tip of the rocker will change over the geometric path as far as the path of the lifter on the cam it should ride as close to the center of the lobe for wear purposes the reason you want the tip of the rocker in the center of the valve at full lift is to optimize the lift insuring the valve opens all the way.
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Old 08-06-2012, 09:36 PM
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Ripper, that is not what I asked at all.
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Old 08-06-2012, 10:19 PM
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ok to make it easier for you perfect geometry is very important when it comes to performance and as far as the smallest area this is effected by the lift and duration so it will have a big effect
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Old 08-07-2012, 01:28 AM
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Moving to engine section..

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I have tried most all of it and now do what is known to work..
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Old 08-07-2012, 02:00 AM
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Quote:
Originally Posted by nofearengineer View Post
I know much has been made here about the way to properly set up rocker geometry; i.e. smallest pattern vs. centered on the tip. My brain is firmly in the "smallest pattern" camp.

My question is, since Comp Cams continues to suggest the "centered" method as correct (showing right now on their website ), what exactly does that say about their cam patterns? Were they designed and tested using the "centered" geometry? If so, what effect will using the "smallest area" method have? Will "doing it right" actually cost performance? (I would think it would be very small, but if we're pursuing perfect geometry, still significant.)
My thoughts are there will be very little- if any- actual measurable difference in power whether the geometry is set by Comp's instructons or by the narrowest pattern regardless of position.

In extreme cases there might be valve tip or rocker roller/tip wear issues if the narrowest pattern was located on the very edge of the tip of the valve. And obviously there can be no run-off allowed or there WILL be excessive wear or worse. But in my experience, in the vast majority of cases there's going to be a push rod length/rocker stud length/valve stem length and rocker arm type or design that will allow a fairly (or completely) centered, narrowest pattern.

Here's a short page w/some info and links that you can take a look at if you want. Good luck.
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Old 08-07-2012, 07:36 AM
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The cam lobes are designed to meet some basic requirements, velocity, acceleration, jerk, etc... they aren't designed for a specific rocker or anything else. In fact many lobes are used for sbc, bbc, sbf, olds, pont, etc while only changing the base circle.

You give cam companies too much credit for their off the shelf lobes.

Custom lobes are a different story but almost no one gets a custom lobe for the street.
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Old 08-07-2012, 10:23 AM
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The smallest wipe pattern is the best. It will yield slightly more lift and cause the valve to open and close quicker. Again we are not talking HUGE changes, but why the heck are we on hotrodders.com? It is because we LOVE sweating the details over an engine build.

I think Comp and other companies espouse the center of the valve method because (A) its easier and (B) less chance of the tip walking off the valve stem. Both are aimed at newbies.

A few thousands of lift are gained by switching to roller rockers over stamped rockers. Oil temps are reduced by using roller rockers. Reduced detonation is achieved by selecting the proper head gasket. A little power is made by using flat top pistons, etc, etc. All these little things add up to a great engine.

I suggest you read this: mid lift rockers
and pay attention to the posts made by "millerrockers". He built his own company centered around the mid lift rocker idea: Jim Miller, MID-LIFT Geometry, Rocker Arms, Race Engine Parts
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Old 08-07-2012, 10:50 AM
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Quote:
Originally Posted by Silver Surfer View Post
The smallest wipe pattern is the best. It will yield slightly more lift and cause the valve to open and close quicker. Again we are not talking HUGE changes, but why the heck are we on hotrodders.com? It is because we LOVE sweating the details over an engine build.

I think Comp and other companies espouse the center of the valve method because (A) its easier and (B) less chance of the tip walking off the valve stem. Both are aimed at newbies.

A few thousands of lift are gained by switching to roller rockers over stamped rockers. Oil temps are reduced by using roller rockers. Reduced detonation is achieved by selecting the proper head gasket. A little power is made by using flat top pistons, etc, etc. All these little things add up to a great engine.

I suggest you read this: mid lift rockers
and pay attention to the posts made by "millerrockers". He built his own company centered around the mid lift rocker idea: Jim Miller, MID-LIFT Geometry, Rocker Arms, Race Engine Parts
Lift is not gained by a roller, its by the ratio, which can vary for stamped or roller.
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Old 08-07-2012, 10:54 AM
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Guys, thanks for the responses, but most of you are not understanding my question. More likely, I didn't ask it clearly enough.

I understand the geometry issue. I know how to measure it, set it up, etc.

Ap72 answered it the way I intended.
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Old 08-07-2012, 10:55 AM
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Quote:
Originally Posted by ap72 View Post
Lift is not gained by a roller, its by the ratio, which can vary for stamped or roller.
Although a roller has more accuracy, meaning between a set of sixteen they are all about the same. You cant say that about a stamped rocker. Roller also reduces friction, and theoreticaly should reduce oil temp.
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Old 08-07-2012, 10:58 AM
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Quote:
Originally Posted by ap72 View Post
Lift is not gained by a roller, its by the ratio, which can vary for stamped or roller.
I never said that. Given two identical roller rockers, one with mid lift (aka smaller wipe pattern) vs. center wipe (aka Comp Cams), the mid lift system will yield more lift.

EDIT: OK I see this statement, "A few thousands of lift are gained by switching to roller rockers over stamped rockers." I meant to say that stamped rockers are not 1.5:1, they are less. And when you use a roller rocker you can be sure the ratio is in fact correct.
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Old 08-07-2012, 11:12 AM
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Quote:
Originally Posted by Silver Surfer View Post
I never said that. Given two identical roller rockers, one with mid lift (aka smaller wipe pattern) vs. center wipe (aka Comp Cams), the mid lift system will yield more lift.

EDIT: OK I see this statement, "A few thousands of lift are gained by switching to roller rockers over stamped rockers." I meant to say that stamped rockers are not 1.5:1, they are less. And when you use a roller rocker you can be sure the ratio is in fact correct.
True, but neither gives max lift. There's only a gnat's fart in difference between midlift and max lift but it is there. Max lift requires a hair shorter pushrod than midlift, but the difference is very very small.
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Old 08-07-2012, 11:19 AM
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Quote:
Originally Posted by ap72 View Post
Max lift requires a hair shorter pushrod than midlift, but the difference is very very small.
Whatchoo talkin bout Willis? I don't see how this is possible. You are translating linear motion through an arc. Therefore the maximum lift will be realized when half of the travel bisects the total arc. That is the most efficient means to translate the majority of the linear motion straight down via an arc.
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Old 08-07-2012, 11:28 AM
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You'll see max lift when the line through the fulcrum and rocker tip is exactly perpendicular to the valve stem at peak lift, not midlift. The difference is really really small but its there.
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