Limited Prep - Alternate Rocker Arms

T

tschreyer

Well-known member
I must have missed the subtle verbiage change for '24.

Just curious on rationale? Being unrestricted, especially rocker arms, could lead to a whole host of changes. Assuming since valve lift is the major outcome of a geometry change, and lift is as measured at the valve, we don't really care what you do to achieve the total lift... What is the word "type" defining in the lifter language?

HP
1. #33984 (Eric Vickerman) Prep L2 Rocker Ratios

In Production Category Specifications, GCR Section 9.1.5.E.2.f.1., change as follows:
"Camshafts are unrestricted. Any lifters, tappets/cam followers of the same type and diameter as stock are permitted. The interchange of hydraulic and solid lifters is permitted. (Effective 1/1/2024)

"In Production Category Specifications, GCR Section 9.1.5.E.2.f.5., change as follows:
"Pushrods are unrestricted. Rocker shafts and/or shaft pedestals, when utilized in the stock type system, can be replaced by alternate shafts and/or pedestals and are unrestricted. Valve rocker arms and/or rocker type cam followers are unrestricted, but the rocker ratios and/or follower ratios must be stock. (Effective 1/1/2024)"
 
Type for the cam, means you can’t go from solid flat tappet to a roller tappet. Rules do state you can go from a hydraulic to a solid of the same type (ie: hydraulic flat tapper > solid flat tappet, hydraulic roller > solid roller). But you cannot change from a flat tappet to a roller. Remember, OHC bucket valvetrains are just big flat tappets.

As for diameter, on a solid flat tappet, or bucket, the diameter constrains the maximum velocity of the lobe. Too much velocity for a given diameter, and will run over the edge of the lifter destroying it and the lobe. The better your lifter bores are machined / blue printed relative to the cam lobe, the closer to the edge you can be, but you can never go over.

This ultimately dictates area under the valve lift curve. Going larger than stock and having a cam lobe designed to take advantage of this allows for more peak velocity and thus adding valve lift area under the curve. This is the advantage of a roller as it is acceleration limited not velocity limited.

Type on the rocker part means going from a non roller tip to roller tip is allowed.

As for more rocker, there are a few advantages. 1) you can run less lobe lift and make up for it with rocker ratio. For most people this means the base circle doesn’t have to be cut down as much making for a more stable and reliable valvetrain 2) On a flat tappet, you can make up some of the velocity limitations vs a roller. This is why NASCAR had 2-2.1:1 rocker ratios when they were flat tappet and why they didn’t gain anything going to a roller… they were already at the limit of the valvespring (source: Hendricks R&D guy, JJ’s old engine builder).
 
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Yes, but lift is regulated at the valve, so they don't care how you geometrically get there as long as the total lift is compliant. Makes sense to me as it allows a bunch of aftermarket options that would never work for LBCs valvetrain with the old language. Can't comment on the word "type" being added because the last sentence about hydraulic and mechanical swapping being permitted seems a little contradictory now.
 
Jason Stine said:
Yes, but lift is regulated at the valve, so they don't care how you geometrically get there as long as the total lift is compliant. Makes sense to me as it allows a bunch of aftermarket options that would never work for LBCs valvetrain with the old language. Can't comment on the word "type" being added because the last sentence about hydraulic and mechanical swapping being permitted seems a little contradictory now.
Click to expand...

That’s for the rocker, not the lifter. So lifter has to be the same type, but rocker can change from a non roller to roller tip, thus allowing aftermarket rockers.

You’ve always been able to convert a hydraulic lifter to solid though.

There are two seperate paragraphs there pointing to seperate things.
 
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Making the rocker unrestricted is nice, and it makes policing it a whole lot easier. Trying to find a rocker that matches stock can be a challenge. Finding documentation of what stock is can also be a challenge.
 
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