Re: head gasket, any mods while I am in there?

[Kevin Ford <cheryl_ford@bc.sympatico.ca>]

Graydon D. Stuckey wrote:
> 
> Bruce,
> 
>         Some interesting points you make here.  I havealways understood
> that there are two distinct sets of tunings going on in the exhaust.
> The exhaust gas itself has a pulse which can be reflected, as you discuss
> below, but there is also the noise pulse which travels at the speed of
> sound (in the exh gas, whatever that new speed is).  My understanding is
> that this noise pulse travels much faster than the exhaust gas itself and
> can have a significant impact on exhaust tuning.   Am I confusing things?
> 

Yes.  

The resulting low pressure area that is formed as the pulse passes by 
will help to scavenge the cylinder.

A reflected pulse (echo) will not scavenge, it will impede the fresh 
charge's progress into the cylinder.  The antireversionary step will 
help lessen the reflected pulse, thus lessen the impedence on the fresh 
charge's entry into the cylinder.  

In 2 stroke engines, it is quite common to use a tuned exhaust pipe.  
The pipe is sized to have the reflected sound wave (resulting from 
combustion) reach the exhaust ports at a time when the fresh charge has 
entered the cylinder & is starting to flow out the exhaust ports.  The 
sound wave tends to impede the flow of the fresh charge.  This helps 
avoid losing the fresh charge, and thus increases the charge % captured 
when the ports are sealed by the piston. As the speed of the charge 
sound wave is fairly consant (thus the time in which the echo reaches 
the exhaust ports), but engine rpm's vary, the tuned pipe has a limited 
rpm range in which it is effective.

 The valves in a 4 stroke allow for intrinsically much greater control 
over the flow of the air charge, and do not open in a manner in which 
we can exploit the sound wave for impedeing the fresh charge's flow.  
These phenomenum seem to lessen the effect of a tuned pipe on a 4 
stroke.

I have not encounted much gain by tuning a 4 stroke exhaust pipe for 
sound wave reflection, while equal length headers show modest power 
gains (maybe by equalizing the scavenging effect on all cylinders?) in 
a NA engine.

 Kevin Ford
 Chase, B.C., Canada
 email: cheryl_ford@bc.sympatico.ca