B204 head on B2x2 cam timing

You have read my forum posts about putting a B204i head (sometimes referred to as a T5 head)* on a B2x2 block. I’ve written about the cam timing change due to the head being thinner, and how it comes to 10 crankshaft degrees.

I was apparently wrong.

A well-respected person on a forum noted that the thinner NG head retards cam timing by 10 crankshaft degrees. I decided to do the math myself, and found him to be mistaken. Here’s the equation, from a book on engine performance:

D = 720h/NP

where
D = amount of retard (crankshaft degrees)
h = change in cyl head height (1mm or 0.0394") [source: Saab manuals. Basically, the NG and OG heads differ by a millimeter]
N = number of cam sprocket teeth (mine's 38 teeth) [source: me – I counted ]
P = chain pitch (mine's 9.525mm or 3/8") [source: Saab OEM chain company in Germany]

So running the numbers : 720*1mm/38*9.525mm = 1.989 degrees of retard at the crank, not 10.

I posted this information at an engine-building web forum for professional builders and got confirmation that it’s correct from two people using two different methods:

Quote:

--Since the slack side tensioner will take up the slop, the cam will retard 1 mm at the pitch circle circumference. --The circumference is 0.375" or 9.525 mm x 38 teeth = 361.95 mm. --Thus, 1 mm cam rotation = 1/361.95 of 360° or 0.9946 cam degrees = 1.99 crank degrees.

Quote:

I laid it out in SolidWorks(Hey, it's pretty close to the end of the day ! ), and got the same .9934 cam degrees, or 1.987 crank°.

Anyway, the moral of the story is this: DON’T BELIEVE WHAT YOU READ. EVER. Not even from people whom you believe to be knowledgeable, experienced, whatever. They can be mistaken, flat out wrong, etc.



*Calling it a “T5 head” isn’t exactly accurate, because these heads were fitted to many non-turbo engines that were controlled by Motronic!