Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Wed, 09 Apr 2003 23:58:38 -0400
Message-ID: <redirect-2112156@logan.com>
From: Jeffrey Peterson <jbp@fire.phys.cmu.edu>
Subject: Re: porting or headers.
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII

Rob,

>I'm confused... to get our air pumps, (oops engines) to run as
>efficiently as possible, we strive to make the air path into and out of
>the engine as free as possible.. the freer the airflow the more air we
>can pump (more RPM) but if we are RPM limited by prop tip speed, why
>spend money on headers and tuned intakes? heck, turbos are very
>restrictive, yet they DO compress more air through the engine at a fixed 
>RPM.
> 
>why would someone spend $$ on porting or Chuck's beautiful headers to
>have the same max RPM? 

With a constant speed prop the blade angle changes, biting more air as you
push in the throttle. The prop governor adjusts the blade angle as needed
to keep the rpm at the set point. At higher throttle settings the engine
puts out more torque, doing more work with every turn. You go faster, even
though the prop is still turning at the same rate.

So why bother with tuned exhaust?

Tuned exhaust is a close to a free lunch as you are likely to come across.

Simplifying a bit, it works like this:  When the exhaust valve opens a
pulse of exhaust gas enters the exhaust pipe.  It's a sound wave, a POP,
which travels down the pipe.  When the pulse hits the tail end of the pipe
there is a reflection of the sound.  As it turns out, the reflected POP is
inverted. That is, the reflected pulse is a POP of LOW pressure gas.  
This pulse travels back up the pipe to the exhaust valve and gets there
(assuming a 65 inch pipe and 2600 rpm) while the valve is open for the
next exhaust stroke. The returning pulse actaully sucks the exhaust gas
out of the cylinder.

good for maybe...10-20 HP.

cold air induction is another cheap power booster...more oxygen
molecules in every gulp.

another 10(?)...

and all with no moving parts.

cheers,
 
Jeff Peterson