<DIV>Dean,</DIV>
<DIV> </DIV>
<DIV>That is my observation. I had an AAC exhaust system on my OS1.20 years ago, and at most I saw an increase of about 150 rpm if I tuned the length. Hardly worth the trouble, IMHO. I had problems with that exhaust system, and ended up removing it and running the OS stubby muffler.</DIV>
<DIV> </DIV>
<DIV>Bob.</DIV>
<DIV><BR><BR><B><I>Dean Pappas <d.pappas@kodeos.com></I></B> wrote:</DIV>
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<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>Hello Dean ... Dean here.</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>Echo ... echo ... echo ...</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>The short answer is that no tuning is necessary. </FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>4-cycle exhausts do indeed tune. This tuning is not at all crirtical, and produces only a marginal horsepower improvement.</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>What happens is that as the exhaust wave front makes its way down the long header it reaches the muffler can,</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>or the expanding cone part of the muffler. In the case of a race car, it reaches the open atmosphere. In all of these cases,</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>the gasses expand rapidly, causing a "reflected" vacuum wave to travel back to the exhaust valve. If the length of the </FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>header is correct, then the reflected wave will return to the engine at the right time to help pull the exhaust out,</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>and fresh intake mixture into the combustion chamber, behind it. This happens because of the valve-overlap. T</FONT></SPAN><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>he intake valve</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004></SPAN><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>is already open </FONT></SPAN><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>during the last 60 or 70 </FONT></SPAN><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>degrees of the exhaust valve opening</FONT></SPAN><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>. This is desireable, even with supercharged engines.</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>As I said, this tuning is very non-critical. A good figure for the length, from exhaust valve to place where gasses may expand,</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>is anywhere from 12inches to 16 inches, for our RPM range. Back when I ran my YS 120-ACs at under 7,500 RPM (16-13 N prop)</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>I did run slightly longer headers.</FONT></SPAN></DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=147295914-29112004><FONT face=Arial color=#0000ff size=2>Regards,</FONT></SPAN></DIV>
<DIV><FONT face=Arial color=#0000ff size=2></FONT> </DIV>
<P><FONT face="Times New Roman">Dean Pappas</FONT> <BR><FONT face="Times New Roman">Sr. Design Engineer</FONT> <BR><FONT face="Times New Roman">Kodeos Communications</FONT> <BR><FONT face="Times New Roman">111 Corporate Blvd.</FONT> <BR><FONT face="Times New Roman">South Plainfield, N.J. 07080</FONT> <BR><FONT face="Times New Roman">(908) 222-7817 phone</FONT> <BR><FONT face="Times New Roman">(908) 222-2392 fax</FONT> <BR><FONT face="Times New Roman">d.pappas@kodeos.com</FONT> </P>
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