Exhaust Backpressure

Good Or Bad ?

Early on in my fixed wing training I was told in a throw away kind of fashion that exhaust back pressure [EBP] is a good thing from an engine performance point of view, like a good student I accepted what my instructor told me and stored it away in the back of my mind in case I was ever bored at work and wanted something to impress everyone with. Now a few years down the line I can find no evidence for this anywhere if anything it would appear that EBP is only ever bad.

It would appear that what we need is to make the pressure at the exhaust port equal to or less than ambient pressure to allow the exhaust gas to flow freely out of the cylinder creating a partial vacuum into which the the fuel air charge can now flow ready for the next cycle.
Tuned exhaust pipes can help the situtation over a narrow band of RPMs by setting up a resonant sound wave which causes a low pressure just as the exhaust port opens, this will cause an increase in EBP outside the tuned RPM band, but is ideal over the narrow normal operating band required for the cruise in an aeroplane or the constant ERPM required for constant Rotor RPM in a helicopter.

Turbochargers increase the inlet manifold pressure , once they are rotating fast enough and allow an easier flow through the engine, albeit with a reduced fuel efficiency for the increased performance. Turbochargers are driven by exhaust gas, so at low RPM provide little if any performance improvement.
Superchargers perform a similar function by increasing the inlet manifold pressure and forcing more air fuel mixture into the engine.

Here I present a list of statements made on various newsgroups in the internet, in time we will prove or disprove each one, this has no yet been done and they are present here for the curious student to research for himself.

• 1 psi increase in EBP is worth a 2% reduction in power
• Tuned exhausts are good, as are capped exhausts, not for EBP increase, but for resonant pressure decrease.
• Performance figures from engine manufacturers are with straight exhausts.

Web References

Lycoming Article On Exhaust Systems
Comparative Flight Efficiency
Detail Of What Goes On Inside engine, ideal and actual Powerflow Exhausts