Boxerworks.com

I was looking at MAX BMW's parts list and I noticed that there is a difference in cam shafts for the
R100rs-r100C. What is the difference??? I would like my RS to go a little faster. as it would make a good
winter project.

Bob D

All 750 800 900 and 1000 engines used the same 308 spec cam. What changed was the front seal size on points cams and then the bean can drive in 79. In 81 they change the cam timming by 6 deg (advanced I think but could be wrong), which added another part number, but the cam specs remained the same.

What year is your RS?

To the best of my knowledge all stock R100 from the factory were fitted with a 308 cam.
If there is a difference in part #s it will differentiate a "points cam" vs a "bean-can cam",
the post-78 advanced timing, and / or the square end pump drive vs early keyed drive –
rather than any (performance) difference in lift or duration.

The 336 sport cam was never stock issue.

vanzen@rockerboxer.com wrote:To the best of my knowledge all stock R100 from the factory were fitted with a 308 cam.
If there is a difference in part #s it will differentiate a "points cam" vs a "bean-can cam",
the post-78 advanced timing, and / or the square end pump drive vs early keyed drive –
rather than any (performance) difference in lift or duration.

The 336 sport cam was never stock issue.

Is the 6 degree advanced 336 (and 308 for that matter) considered a better street cam? Is there much of a difference from the 0 advance cam in performance?
From what I've read, rodders will often advance the cam timing (by as much as 15 degrees or more on some vehicles) to improve low end power.

Bob Distelcamp wrote:I was looking at MAX BMW's parts list and I noticed that there is a difference in cam shafts for the
R100rs-r100C. What is the difference??? I would like my RS to go a little faster. as it would make a good
winter project.

Bob D

You'll want a 336 for a power boost. But it's not as easy as just dropping it in. Valve pocket clearance needs to be checked, spring preload adjusted, and you need at least 9.5:1 compression. You can easily get sucked in to a bigger project than you bargained for, as I have found out.
http://moragafalconers.org/BMW_336_Cam/ ... 0Draft.pdf

mattcfish wrote: From what I've read, rodders will often advance the cam timing (by as much as 15 degrees or more on some vehicles) to improve low end power.

That very much depends upon the cam. The milder the cam the more it can be moved, but that is not a hard and fast rule. The point of changing the cam timing is to alter the intake valve closeing time, which alters effective compresion at a certain RPM range. If you move too far from design specs you mess with the exhuast timing and intake opening so much that the gains on intake are lost. 10 degrees is probaly the most on our engines.

Garnet wrote:

mattcfish wrote: From what I've read, rodders will often advance the cam timing (by as much as 15 degrees or more on some vehicles) to improve low end power.

That very much depends upon the cam. The milder the cam the more it can be moved, but that is not a hard and fast rule. The point of changing the cam timing is to alter the intake valve closeing time, which alters effective compresion at a certain RPM range. If you move too far from design specs you mess with the exhuast timing and intake opening so much that the gains on intake are lost. 10 degrees is probaly the most on our engines.

Yeah, I think the 15 degrees+ I was refering to was on Double overhead cams. You can adjust intake or exhaust without changing the other.
Still, the questions is...what does the 6 degree change do on an airhead? Was it simply for emissions?

mattcfish wrote: Still, the questions is...what does the 6 degree change do on an airhead? Was it simply for emissions?

What year did they change, 78 or 81?

I thought that perhaps it was to lower the power band to coinside with the removale of the flywheel in 81. Or maybe it was just for emissions, to match the lower compresion engines we where getting.

Did the Euro spec engines get the new cam timing when we did? That would tell if it was for emissions or performance.

Garnet wrote:

mattcfish wrote: Still, the questions is...what does the 6 degree change do on an airhead? Was it simply for emissions?

What year did they change, 78 or 81?

I thought that perhaps it was to lower the power band to coinside with the removale of the flywheel in 81. Or maybe it was just for emissions, to match the lower compresion engines we where getting.

Did the Euro spec engines get the new cam timing when we did? That would tell if it was for emissions or performance.

According to 336 PDF file, all cams after 1978 have the 6 degrees advanced valve timeing (retarded slot). It says nothing about European vs US.

I think the 336 cam was advanced to make it more "streetable". An engine with a 336 is not really very happy below 3500 rpm or so if installed straight up. Other posters have implied that the stock 308 cam were advanced as well, is this true?