Re: DOHC or not?

["Christopher Morgan" <cmor@home.com>]

Regardless of what some of the engine covers say, the 12v VR6 is really a
SOHC motor.  The front cam has the intake and exhaust lobes for the front
cylinders as does the rear cam for the rear cylinders.  In other words, if
you look at any one cylinder, the intake and exhaust valves for that
cylinder are actuated by a single cam.

...however...

The 24v motor is DOHC.  It still has only two cams, but the they are
separate intake and exhaust cams (thus allowing the use of variable valve
timing).  For any one cylinder, the intake valves are actuated by one cam
and the exhausts by another.  It's a pretty clever set-up.  Somewhere I saw
pics of the valvetrain, which showed how each cam uses regular-length valve
stems for its 'local' valves and extra-long valve stems that reach across
the head at an angle for the valves in the opposite bank.

The same write-up also mentioned that the other design goal for the 24v
motor (besides the 30 extra hp) was an increase in fuel efficiency.  Did
anybody else notice that the 12v VR6 is not a gas mileage champ?  :-)  For
comparison, the 174 hp GTI/Jetta GLX 5 spds are rated at 20/28, while the
190 hp Audi A4/Passat V6 5 spds get 20/29.  The 24v VR6 is 204 hp and is
supposed to get the same, if not better, mileage as the 12v.

FWIW, the number of cams does not necessarily relate to the number of
valves.  Using BMW motorcycles as an example, the K75/100 (but not
K1100/1200) motors are 2 valve per cyl DOHC, while the newer R-bike motors
are single cam 4 valve units.

-Chris


From: <VW155mph@aol.com>
> In a message dated 02/28/2000 4:43:56 PM Eastern Standard Time,
> sundie@lunaticfringe.org writes:
>
> > So my question is this!  Does the VR6 have 2 cams or 4?
> >
> >
> >   Breathlessly awaiting answers....
> >
> >
> >  Sundie...
> Just 2..