Re: compression/cam

[Scott Williams <sfwilliams@home.com>]

Nathan Malone wrote:

> I have been told that when installing a wild
> racing cam, you need high compression to match.  I
> have no argument against this, but I wonder if it
> is a absolute necessity and why.

A "wild" cam is going to have a lot of lift and duration. The duration
is the time that the valves stay open -permitting more complete
evacuation of exhaust and filling of fuel and air. The downside to this
is that there is an rpm  range where the engine is not efficient and
loses power. Overcamming an engine will usually make the powerband very
"peaky". That is, it will lose bottom end torque in favor of hp up top.

On the other hand, more compression ratio will help augment the torque
-making for a less peaky powerband. There is a practical limit for
compression, though. Cars running on street gas can rarely exceed 11:1
without problems. However, a very wild cam can sometimes permit a higher
ratio because it will keep the valves open for long time, thus blowing
some of that pressure out the exhaust. So, there is your relationship
between high compression and hot cam profile.
--
Scott F. Williams
NJ Scirocco nut
SCCA ProRally driver
Hotrod Rabbit GTi

Check out our rally team's website!
http://www.usrallyteam.com



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