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Exhaust Backpressure and Cylinder Compression

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Please note:  the following discussion has been restated from the original thread for improved clarity.  Follow this link to the original thread if you wish to leave a comment:  Compression and Backpressure - is there a direct relationship?

Exhaust backpressure is the effective resistance to normal exhaust flow and limits an engine's ability to breathe.  The greater the backpressure, the lesser the exhaust flow, the lesser the airflow through the engine, and the lesser the engine torque development. 

Compression ratio is a fixed arithmetical relationship expressed as cylinder volume with the piston at bottom dead center (BDC), divided by cylinder volume with the piston at top dead center (TDC).  Compression ratio is directly proportional to (but not the same as) cylinder pressure development and relates directly to torque.  The greater the compression pressure (short of detonation), the greater the engine torque development.

From a practical perspective, typical factory backpressure parameters have a negligible direct impact on selecting "streetable" compression ratios for the majority of mildly built gas engines discussed on this forum.  Nonetheless, decreasing exhaust backpressure by substituting headers, less restrictive mufflers & cats, and mandrel formed tubing is generally beneficial to power production and power adders, such as cams, manifolds, heads, air valves, and boosting compression, because they allow the engine to breathe more efficiently - assuming exhaust energy (temperature and velocity) is maintained by using a modest diameter exhaust tubing.

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Exhaust backpressure originates from engineered (intended), or consequential (unintended), restrictive "effects" that typically impede airflow through an engine.  Restricting airflow at any given RPM, decreases torque, so "excessive" backpressure will diminish power output.  Yet, in some cases engineered backpressure, properly tailored and applied to a specific powerplant and drivetrain, can benefit performance.  However, determining the correct amount of backpressure seems to beg a trial and error method of discovery.  There is no universal formula that applies to all power systems.  Nevertheless, if backpressure is properly tuned to valve timing, a reverted exhaust pulse moving up-pipe will reach the exhaust port of the cylinder head at the optimum moment to reflect back down-pipe and assist exhaust gases exiting the combustion chamber - essentially drafting spent gases from the chamber much as a bicyclist drafts behind a vehicle.  This process is referred to as scavenging and is beneficial to power production because it purges the combustion chamber of spent gases and promotes greater filling of the chamber on the subsequent intake cycle.  Therefore, for any given engine, a unique amount of backpressure is desirable for optimal scavenging across the RPM range of maximum torque.

*** As a sidebar, the newer breed of smaller displacement engines producing heretofore unattainable power and fuel mileage on pump gas are evidence of engineering subtleties and finesse that employ concepts, such as backpressure, to real world mechanical advantage. ***

"Exhaust reversion" occurs when excessive backpressure is so poorly timed that it pressurizes the exhaust port at valve opening and prevents adequate purging of the combustion chamber, or worse, forces spent gases back into the intake port, contaminating subsequent cylinder charging.

There are numerous variables that affect backpressure - e.g., an actual physical restriction in the exhaust; reverting pressure waves reflected off of both, (a) solid surfaces and edges within the exhaust conduit, as well as (b) dynamic pressure boundaries, resulting from changes in conduit cross section and changes in gas density in the exhaust stream; the length of individual header and head pipes before joining together; overall length of the exhaust conduit; volume and placement of cats and mufflers; energy content of exhaust gases; turbulence; etc.  The cognitive tuner is compelled to develop a balance between engine use, build parameters, conduit diameter, conduit length, conduit bends, and the size, shape and placement of acoustic traps, such as mufflers and cats.  See the two subsequent discussions for interesting additional details...

http://www.veryuseful.com/mustang/tech/engine/exhaustScavenging.pdf

Exhaustvideo's How To Calculate Muffler Size and Exhaust Pipe Diameter

Hotrod's X vs Y pipe

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