Valve clearance adjustments - The Unofficial BMW M5 Messageboard (m5board.com)

m5james · 27th February 2001, 01:25

I was thinking about the tolrances allowed in the valve tolerances, and I had a couple of questions that some others might be able to help me answer. First, doesn't an aftermarket cam just increase the size of lobes and/or duration. And second, if they increase the size of the lobe meaning that the valve stays open longer, couldn't the same thing be done by increasing the shim size to the bigger of the tolerances allowed, or maybe even go a couple (thousandths, mm, or whatever it is) higher? Would this increase in duration cause the vavle to stay open longer, which would increase the amount of airflow in and out of the engine, in order to make maybe even a few extra horses. I don't know the HP increase in the new cam, but would the bigger shims be able to do something similar?

BillK · 28th February 2001, 02:00

I'm no expert in these matters, but it seems to me that increasing shim size is equivalent to adjusting them too tight on my low-tech non-shimmed Volvos. The result then is burned valves because they don't close soon enough (or not at all if way tight). Cam design allows for higher lift AND closing all the way.
Don't do it....
Regards,
Bill

m5james · 1st March 2001, 22:29

Now that I thought it through a little more, it makes more sense to me now. At least I killed the curiosity by asking instead of doing!