[200q20v] Re: [audi20v] Re: Camshafts - demimed - sorry

QSHIPQ at aol.com QSHIPQ at aol.com
Fri May 25 03:07:36 EDT 2001

In a message dated 5/24/01 12:24:58 PM Central Daylight Time, 
b.m.benz at prodigy.net writes:

Scott Justusson writes:
> Depends on what system is employed.  The simpler VVT changes overlap without
> really changing cam profile per sae.  Cam timing changes in DOHC motors
> effectively changes the cam profile, since you are affecting the overlap
> characteristics of them.
Still talking only DOHC systems, IMO, no way is it possible for "Cam timing
changes in DOHC motors effectively(?) changes the cam (lobe) profile, - -."
You're implying that a cam timing adjustment can change a hard machined lobe
profile, no way!  DOHC overlap is exclusively a valve timing charisteristic!

You change from a set of cams TDC to a different TDC you have effectively 
changed the cam lobe profile.  Call it optimizing, tweeking, VVT or specific 
to what you are doing, cam phasing..  I don't disagree that DOHC overlap is a 
valve timing characteristic, but if you aren't physically changing one cam or 
the other,  by changing the relationship of centerline, separation and TDC 
you are effectively changing the cam profile. Cam phasing doesn't change the 
duration or the lift as you say, only making a cam open a valve sooner or 
later, which also closes it sooner or later  You can physicaly change the 
lobe profiles (leaving centerline, separation and TDC alone) OR change the 
relationship of the cams, both do the same thing.  The latter obviously the 
cheaper and sometimes more effective method.  Have you compared the 2 sets of 
cams fully?

Scott concludes with:
> All that said Bernie, I'm still not convinced that changing cam timing to
> yield compression ratio numbers isn't taking a step back, not forward in
> forced induction application.  Why not just change out the pistons and dip
> into the audi 20vt parts bin, then your volumetric efficiency loss is at
> least no worse than the way audi did it.
Wrong perspective, Scott.  I plan to change valve timing only to reduce the
7A's NA overlap to a value more suitable to street FI use.  The fact that
this action will also reduce effective CR by some desireable, but as yet
undetermined amount, is a freebe.  IMO, this action will increase volumetric
efficiency re: to just adding FI to the 7A's NA valve timing.  Show me

Now I understand what it is you are trying to do.  Thanks for the 
clarification.  No problem here.  I'm still skeptical that you will be really 
reducing effective CR, since I remember CR to be figured as cylinder + 
chamber volume/chamber volume, not a function of valve timing, and effective 
compression ratio is a functon of geometric CR and boost pressure, also not a 
function of valve timing.  I always understood that valve timing and phasing 
affects volumetric efficiency only.


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