Stroke, bore, cam & RPM's

ricematthews at ricematthews at
Sun May 13 10:08:45 EDT 2001

I think what you are asking (at least part of what you are asking) has to do
with the factors which determine the red line of your engine.  One of these
factors is piston speed.  I forget the actual magical number, but 5252 seems
to come to mind (feet per second? - I'm sure you could look this up).  What
this boils down to is that as you increase the stroke of an engine, you tend
to decrease the redline.  This is why a lot of the high torque, big block
American muscle cars of the 60's have low red line numbers (like 5000 rpm).
Another factor affecting the engine's red line is in the valve train.  You
need to avoid a phenomenon called valve float - in which your cam turns too
fast for your valves to keep up with it.  I'm not sure how to determine the
red line based on this limiting factor, but if you were looking for a new
cam, the supplier could probably tell you what effect this would have on
your red line.

A couple of thoughts:  One thing to think about here is that if you are
reaching your maximum horsepower (300 or whatever) at 8000 rpm, what good is
it to you unless you plan to run your car in this range.  Flat and high
horsepower and torque curves are really what you want so that you don't have
to worry about running out of revs before you reach your peak.
A good ticket for horsepower with the I-5 engines is with a chipped 20 valve
turbo motor (preferably not from Hack).  To add a bunch of expensive
internals to your 10 vt motor may cost you more and get you less.


  If you recall I wanted to take the turbo charged engine out of my rear =
ended 1985 5000 S Turbo and place it in the 1986 4000 CS Quattro. I am =
aware of the site with the change over to turbo but my question is this.
  I want to take it as close to 300 horse as possible, planning on a =
complete rebuild I had noticed some comments on the size of the bore =
affecting the RPM or on how high the RPMs will go.
  I guess is can the wrong piston, bore and cam affect the rpms? This is =
a do it my self because I cant afford a mechanic type thing and I want =
to do it right any input would be appreciated.

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