Thanks, Jim. Why do cams with a wider lobe seperation idle better than those with a narrow lobe seperation? I'm still trying to understand this area of cam selection a little better. In your opinion would a cam with a 116 degree seperation with 230/240 duration (TA288-96H) idle poorly when set straight up (intake on 116)?
A wider LSA means that there is less overlap. Overlap is great for making high RPM power (sucks in a lot of air during the intake stroke), but at idle it can cause an inefficient combustion since part of the intake charge can go directly out the exhaust valve.
This is an interesting topic...I just learned something. If I may ask a question, if retarded valve timing makes a rougher idle, what is to be gained by retarding valve timing?
Using those numbers that cam has 3 deg of overlap @0.050" which means not very much overlap. That will have a good (not smooth, but not choppy either) idle. The explanation that I've heard as to how it effects idle is that it's the exhaust charge contaminating the intake charge; you get a good explosion sometimes and not others, making for an erratic idle.
In the past I heard someone say something about a "scavenging effect".......is that somehow related to the valve overlap?
retarding your initial advance will help you start, to much advance at start up causes the moter to turn over slow, its real bad when the moter is hot. i think that why some guys have a hard time stating there cars when they run alot of advance on there big cams to get them to idle.
scavenging yes, it is what makes alot of top end power, but will kill your low end due to the lose of commpression at low rpms, that is why you need more commpression as your overlap increases, there is alot more to it, that just my 2 cents worth
Ahhhhhh, so that's why race motors run so crappy at low speeds. I never really understood what caused that. :bglasses:
