Originally Posted by mattcfish
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.
A lot of people talk about cams and know even less than I do about them and that is saying something!! People talk about check points like they really don't know what they are. Timing cams in general. This question about specs, for instance. I know you have read what you are reporting numerous times but that doesn't make it right. I do have a friend that is good with cams for working with flat track bikes. Cam specs are at the crank. Period. Look at any photo or read any book and look at the photos. NO ONE has a degree wheel on the cam(S). What the cam is doing AT THE CRANK is all that matters. Some people think that since they know that the cam spins at half crank speed they can take some specs and then do some 'figurin'' Wrong! The figuring has already been done with a degree wheel on the crank. Ambiguity? Not from BMW specs. There is all the BS and there is the specs. That isn't ambiguity.
[EDIT] Oops mattcfish! There is ambiguity in the numbers from BMW too! I knew there was in the wording but . . . .
The same amount? Three degrees is THE classic amount to change a single cam. You can't do it much more without running into troubles and if you do it much less it won't make much difference. The room for adjustment is SMALL. If you think about it, hopefully the need for a mid stroke arrived at TDC and the biggest degree wheel that you have room for will start making sense. The tiniest difference is a BIG difference! IMO rocker arm ratios and swing radii are best left out of the picture if possible! Spec duplication is key and super touchy! I have worked with people trying to time cams with the TDC determined at TDC. WAY too much crank to piston movement at TDC to even get close as far as duplicating cam specs! It's simple geometry!
Usually retarded for emissions? I don't know but it makes since to me to advance them. Why? Advancing them makes them more efficient at lower rpm. Emission testing is done at idle and cruise ie low rpm. Remember that it is easy to get all this stuff mixed up in your head when you are writing about it. Just one for instance: A BMW service bulletin that says that 336's had there timing retarded. Wrong. It's advanced.