Originally Posted by supershaft
Cam specs are all at the crank. A cam that is advanced three degrees is advanced three degrees at the crank (and valves). Six degrees at the crank would adversely effect the exhaust timing too much by most tuners' opinions. Most tuners consider five degrees the absolute max to advance or retard a single cam. Most don't go past three degrees (at the crank). That's 1.5 degrees at the cam. Advanced or retarded at the cam? That all depends if it is gear or chain drive but most all tuners call what ever it is doing at the crank how they are modifying the setup at the cam.
Advancing the sport cam had nothing to do with emissions since the cam was never in a regulated bike. Advancing the 308 had to do with emissions but . . . . I don't know anyone that has changed a cam timing for idle. The timing was changed to lower peak torque around 500 rpm. Most single cams can be retarded and advanced three degrees for a total of six degrees to play with in order to move peak torque around about 1000 rpm.
As I understand it, they advanced the 336's timing before they changed the oil pump drive so this one could be either straight up or advanced.
I have a 336 in my '92 R100 which includes the later, lighter clutch and single row timing chain. 9.5:1 pistons. .025 off the heads. Titanium valve spring retainers. Custom made 4130 pushrods. 44mm intakes with modified valve head/seat ID ratio. Heavily re-shaped ports. Dual plugged with stock bean can and re-mapped ignition curve. Completely re-jetted 38mm Dellorto carbs. Heavily modified airbox. Staintune Sport mufflers. 33/11 FD. The thing makes WAY more power EVERYWHERE in the rev range including at least 1000rpm on top that it never had to start with! I love it!
I ran mine for over ten years without raising my stock 8.5:1 CR or much port work at all with great improvements from around 2500 rpm on up but now I wish I had raised the CR and ported my bike like I did just recently a long time ago. The thing is now running a LOT better than it was and it was already running a lot better than stock!
The strange thing is that valve timing is usually retarded for emissions. BMW advanced the 308 and the 336 the same amount. Some say it's only 3 degrees some say 6. Anybody ever take a degree wheel and figure it out for sure? There's a lot of ambiguity....when and why? Either style of 336 will give you more punch than a stock 308. Since you've already got one, I'd consider using it, especially on a 90S.