[200q20v] Re: [audi20v] Re: Camshafts

Bernie Benz b.m.benz at prodigy.net
Fri May 25 07:15:29 EDT 2001

Thanks for the response, Scott.
Further comment interlaced below.

From: QSHIPQ at aol.com
Date: Fri, 25 May 2001 02:04:32 EDT
To: b.m.benz at prodigy.net, audi20v at rennlist.org, urq at audifans.com,
200q20v at audifans.com
Subject: Re: [audi20v] Re: Camshafts

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
> 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.
I still can't buy this concept, Scott.  First, I think that you are
talking cam lobe centers relative to engine TDC in this statement.
But further, how does one "effectively" change a hard machined lobe
profile, except by introducing another component in the valve train,
such as a spongy lifter, float, rubber valve stem, ect?

  Call it optimizing, tweeking, VVT or specific
to what you are doing, cam phasing..  I don't disagree that DOHC overlap is
valve timing characteristic, but if you aren't physically changing one cam
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
cams fully? 
Not yet, Scott.  Soon, when I take the car out of service for the
I will fully characterize the 7A cam lobe profiles and cam timing, and
the plot for those interested.  At this point, although I'd love to have the
the same info on the 3B, I do not feel it necessary to do the same with the
3B, inasmuch as I have the comparitive Bentley cam timing specs for both and
speculate that the lobe profiles are identical.  (Divorce would result if I
had both cars down simutainously for my fun and games.)

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
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
function of valve timing.  I always understood that valve timing and phasing
affects volumetric efficiency only.

To further clarify, effective CR of the engine only is the geometeric CR
reduced by that portion of the compression stroke during which the intake
valve is still open.  System, or overall CR would include any FI
Please correct me if my terms do not relate to normal convention.

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