[Prev][Next][Index][Thread]

Re:<02> rocker ratio

Troy D. Brewster asks about rocker ratios in the 02:
>I am attempting to determine what cam I have in my 2002.  The machinists that
>working on my engine suggested that I find out the rocker ratio he could
>determine the degree of the cam.  (There isn't a casting on the cam indicating
>its duration).  This car is running 40 DCOEs and high CR pistons so I'm
>thinking that the cam may not be stock (264 degree duration) but I'm not sure.
> Is there a rocker ratio associated with the different cam durations?
    BMW used exactly the same rocker arm geometry for the original 4 cyl.
engines from '63 through '83.  Unlike some American car tuners, where the
effective lift of a cam can be increased through changing the rocker arm
geometry, this was never done with BMW's to the best of my knowledge.  So
all cams, from stock to Schrick 336 used the same rocker arm geometry.
    The precise ratio is a subject of debate, though.  Schrick says its
1.31 (valve lift/cam lift); everyone else has measured a smaller number:
Metric Mechanic, Lee Johnson, and Webcam get 1.25; Pete McHenry measures
1.23; I've measured 1.24.  In any case, the best way to determine what type
of cam you have is to measure the cam itself, which is easy to do in the
head.  Measuring cam lift is much more accurate than trying to measuring
duration.  Measure the base circle (stock is 26.8 mm); then measure the
"heel to toe," which is the top of the cam lobe to bottom of the circle.
Stock is 6.88 mm; the BMW/Alpina 300 is 7.66; a Schrick 304 is 8.15.  I
have measurements for a number of other cams if you're interested.

Terry Donohue
'71 2002Ti
'90 Audi 200TQ
'95 M3