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Just to add to the debate - I've worked on Subaru engines for a few years, and they use a relatively simple reed valve to pull fresh air into the emission stream - no pump involved. Air is drawn from the air cleaner assembly and distributed to each pair of horizontally opposed cylinder heads near the exhaust ports. In addition, the exhaust system on these cars does not appear to be anything special. A single exhaust pipe services each pair of cylinders, and then merges before the catalytic converter. Erik Pullman 73 Squareback 71 Campmobile (and 87 Subaru) -----Original Message----- From: Jim Adney [SMTP:jadney@vwtype3.org] Sent: Tuesday, July 21, 1998 6:43 PM To: type3@vwtype3.org Subject: Re: Backfire > >JimA=> the exhaust is under positive pressure and will almost never > >draw air INTO itself. > > I disagree. the exhaust is not always under positive pressure but > under a series of pressure pulses. The are some basic air injection > systems for emissions that are nothing but a simple valve that > allows fresh air to enter the exhaust during the low pressure period > between the pulses. The systems I'm familiar with all have an air pump. Who gets by without one? > (And to push this thread into a different > direction) This is also how scavenging works on headers... The > pulse from one cylinder will help draw the exhaust from another > cylinder at the collector improving the overall flow... I don't think this is quite correct. I believe equal length headers allow the pulses from the different cylinders to enter the muffler with even timing, which allows each cylinder to operate under the same conditions, thus giving more even running. There is a resonance effect with a tuned exhaust, but I really don't know if the pressure ever goes below atmospheric. All it has to do to have a beneficial effect is to go below average (at the right time, of course.) Jim - ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA *******************************