Cam timming change question?

[itsafatboy]

OK talked with mackie he said it would work fine but does not know from experience, so i would like some info on changing timming on cams .

the cams are mackie 598 numbers are
int. 24/50
ex. 56/20
int. dur. 254
ex. dur. 256
overlap 44
lobe center intake 103
lobe center ex. 108
LSA 105.5
cam lift int. .598
ex. cam lift .598
int tdc lift .213
ex tdc lift .179

i have a 11.5 comp 116"
run 1.67 rocker ratio so that would five me about .015 more lift and tdc lift.

so i was thinking on either the 2 or 4 degree advance what would the results be? i like how they ran straite up but they were a little soft down low compared to the tw8g(but the valve train ran quite)

so when you advance a cam does that change the lob centerline , would you just take all the specs and subtract 2 0r 4 from those numbers.

just wondering if it would just be a waste of time.

[Mike]

Currently your corrected cylinder pressure is 10.1. With four deg adv. it would be 10.3 and with 4 deg ret. it would be 9.9. All of these are very high numbers and you should be experiancing a fair amount of detonation (unless this was tuned out, in which case a fair amount of power was also lost). Advancing cam timing would make matters worse, I would recommend retarding by 4 degrees. I forgot which exhaust you are running, but your current setup should be VERY strong down low, but with the low duration, will not run hard in the upper ranges. The soft running may be a function of the exhaust and/or the tune required to prevent detonation. My favourite corrected pressure is 9.4-9.6 for street aplications to be run on 91 pump fuel. Better combustion chambers can stand higher numbers but these are typ. four valve setups that allow for a much flater/shallower chamber/piston setup with a much more aggressive quench (I.E., V-Rod, not a big twin design).

FYI; changing the timing changes when the valves open and close, changes the lobe center timing (relative to open and close), changes TDC lifts (when advanced, the intake TDC increases and the exh. TDC decreases; retarding timing reverses this) and changes cylinder pressure (advancing increases pressure, retarding lowers it).
Adv or Ret timing does NOT change duration, overlap, Lobe Seperation Angle (LSA) or max. lift at valve.

With the 1.67 rockers, you will have to be very careful about advancing the timing. TDC's will have to be checked since increasing this ratio also increases TDC lifts in addition to the increase with advanced cam timing.

In summary, it really looks like a more optimized cam and exhaust for your application (what you are asking the bike to do) may need consideration.
Thanks, Mike