| |
In a message dated 98-02-26 04:02:19 EST, you write:
<< David E Shreve wrote:
>
> On Wed, 25 Feb 1998 aqn@dev.tivoli<img src=/i/dc.gif border=0 width=35 height=15> wrote:
> > Adjustable sprockets allows adjusting the timing of the intake valves
> > with respect to the timing of the exhaust valves. The power
> > characteristics of an engine is affected by the duration between
> > the "lobe center" of the intake valves & the exhaust valves.
> > That duration can be changed using "adjustable cam sprockets".
>
> If this is true, [...]
Oh, trust me, it's true. (Famous last words. |-)
> [...] then why do so many 8V VW owners have adjustable cam
> sprockets?
Beats the shit out of me. Why do so many people have the P-Flo
filter when a K&N stock replacement is just as good, probably
even better? I am inclined to think that it's the herd mentality
and/or the "internal combustion mod" mentality (if I have to pay
big bux for this mod, it must be good). It is also possible that
I missed something major vis-a-vis adjustable cam sprockets,
in which case I would love to know what's I'm missing.
Oh, I just remember another thing: whether or not adjustable cam
sprockets would do any good do not depend on the number of valves.
It depends on the number of cams, or more specifically, it depends
on whether or not the intake & exhaust valves are operated by
separate cams. I _think_ the 8V VW motors are single cam? But
if for some reason they are DOHC (with only 2 valves per cylinder,
one intake one exhaust), then it will be meaningful to have
adjustable cam sprockets.
>>
Hey Andy,
Adjustable sprockets are used to alter cam timing in relation to the crank
timing in VWs. There is an adjustable sprocket offered for the E30 M3 that
alters the timing between the two cams (in/ex), but since the VWs only have
one cam driven at a time (technically even in the VR6) the idea is to alter
cam to crank index. Opening the valves earlier or later in relation to piston
position can alter the powerband enough to compensate for drivability
problems, or too maximize various areas of the powerband for specific use (ie
road course racing). Since, as you mentioned, the VR6 has both in/ex lobes on
each of the two cams, you cannot alter each cam's timing in relation to the
other. However, you *can* (theoretically) alter both cams' timing in relation
to the crank. This is why there are adj sprockets for both the 8V and 16V
VWs, even though both have a single cam sprocket design. The 8V sprocket
directly drives the cam (containing both in/ex lobes), and the 16V sprocket
drives the exhaust cam which in turn drives the intake cam via a chain.
Another reason that just occurred to me, which is a condition on the 16Vs, is
that ignition timing will be altered on the VR6 if the sprockets are adjusted.
The ignition is controlled by the cams in both dist and non-dist VR6s. The
distributor is run off the front cam via a keywayed drive, and the coil packs
are fired via a cam position sensor on the rear cam sprocket. Altering the
cam timing will alter ignition timing, too, in this case and ignition timing
is not adjustable on either VR6 version. Yet *another* reason to forego the
adjustable VR6 sprockets (if they do in fact exist).
Todd
Air & Water
VW Tuning
Philadelphia
|
>
>
Thu Feb 26 08:14 CST 1998