In a message dated 96-09-04 17:15:48 EDT, you write:
> The Comp/Prep Manual states that the uprated cams will give poor
>performance below 3000 rpm unless the compression is raised to at least 10.
David-
With regards to compression ratio and lack of low end torque due to the cam,
you are talking about two different, although related, things.
This gets a little complicated, but I'll give it a shot...
Higher compression ratios tend to excite the incoming fuel/air charge more
than lower ratios (all else being equal). What this means is that the
incoming mixture is more thouroughly mixed within the chamber more quickly.
This tends to give more power when the mixture is ignited, primarily by
having a larger percentage of the flame front applied to the piston top
sooner. (This is also seen with many modern type engines with a lower
compression ratio, but using much better mixing of the incoming charge eg.
Honda's CVCC system and Jaguars Mays cyl head). Basically, a higher
compression ratio, or a better mixing system, gets the power happening
sooner.
Camshafts, unfortunately try to do just the opposite. The wilder the cam, the
worse the idle quality will be. This is due to the fact that the intake valve
is held open long enough for the piston to start back up the bore and
actually push the incoming charge back OUT of the carb.At higher RPM's, this
is not a problem since the incoming column of air/fuel has a good deal of
inertia, and will continue to fill the cylinder, even though the piston is
trying to force it back out, but at idle the incoming mixture has very little
inertia built up. This leads to the next variable, the carbs!
1.75" SU's will flow about 300cfm in stock form. 45 DCOE Webers choked to
about 32 mm or so, will flow almost as much through each barrel. Since the
SU's are feeding two cylinders, to the Webers one cyl/barrel, they will have
a higher gas flow (column of air/fuel) through them than the Webers, hence a
better idle/low RPM performance.
Now for the big finish...
Most any cam up to about an "F" factory competition grind can be used on the
street. A good combustion chamber modification will help make more power,
sooner. A higher compression ratio will help more. 2 barrels fedding the
engine , instead of 4, will also help the bottom end. Installing the cam 2-3
degrees advanced will also help the bottom end.
Have I confused you totally ? I hope not. If you get to a point that this
just doesn't make any sense, perhaps you would like to give me a call on the
phone. I'm a little hard to catch, but if you want to try, let me know.
The Car Curmudgeon
Nick in Nor Cal
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