Even though I prefer the shape of Pat’s original pipes, I installed the Benham pipes at overhaul back in 2006 because of the STC vs field approval issue. I do however prefer hanging them from the engine mount rather than the firewall. (The fewer holes in the firewall, the better, IMHO).
Before ‘98C was rebuilt a few years ago, I used fabric wing inspection covers to close out the cowl holes. Worked well, and didn’t look all that bad.
I doubt any instrumentation exists that is less than the value of the airplane that could show any speed gain over the original pipes. They do however have a throatier note that might make it
sound faster.
As far as CO (carbon MONoxide
), I can see where the exit point behind the firewall would make exhaust less likely to enter through faulty gear leg seals. I have no data on my airplane with the original exhaust, but with the new CO detector I received last week, I see only 1ppm after start up and during taxi with a cold engine. All other phases of flight show zero.(This is with the heater full on and wing vents closed).