What is the Stock Ratio on a 3800

Apr 25, 2005

What is the stock rocker ratio on a 3800?

The stock rocker arm does not have a roller tip the section of the rocker tip that touches the valve changes with valve lift. This means that the higher you lift the rocker (with pushrod/cam) the higher the rocker ratio becomes. On a car with a stock cam the “rocker ratio” will not be as high as on a car with say a .550 lift. Looking at the picture below, you can see what I mean.


When measuring the above rocker using the shiny part as a contact point the rocker ratio varies from 1.53 to 1.81. Pretty crazy huh? This is fine though, for now.

Next comes in to play rocker deflection. This is how much lift you lose due to the rocker arm flexing under load. With a stock rocker arm this amount can be quite significant depending on the springs you are using. In fact with 130 pound springs you could have over 40 thousands of deflection while using stock rockers!



Stock 3800 cam lift of .257


HEAVY SPRING: Comp 130LBS SEAT 280LBS @ .500 

ROCKER TYPE             LIFT WITH            LIFT WITH           DEFLECTION

                                      TEST SPRING      130#  SPRING

                                      (Effective ratio)   (Effective ratio)

Standard steel               .431                    .423                             .008

                                        (1.67)                     (1.64)

1.9 mod rocker             .518                    .481                             .037

                                        (2.01)                    (1.87)

SLP 1.8 rocker              .471                    .464                             .007

                                      (1.83)                     (1.80)