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It's easy to measure backpressure...with enough money. Chemical and refining plants use dP (differential pressure) cells by the hundreds. You can measure one pressure versus atmosphere, which is the requirement for this discussion, or you can measure between two points in a system (what the plants use them for). The dP cells we use are the size of a small loaf of bread and made with lots of heavy stainless steel.
Backpressure in the exhaust is relative to atmosphere. There are two competing issues here: Higher pressure means higher motive force to push the exhaust against atmospheric pressure found at the end of the tail pipe, but higher pressure means more work for the engine to evacuate the cylinder, thus less work to the wheel. Lower dP implies higher exhaust velocity, which is a good thing. Zero backpressure implies zero flow, which means all pistons are holed.
Food for thought: back in the Eighties, one muscle car magazine found that they couldn't fit too large an exhaust on a car. Conventional wisdom said 2-1/2" pipes on a dual-exhaust small block. Their testing stopped at 3-1/2" diameter and there was continual improvement in both torque and horsepower without any degradation in power curve shape.
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Bruce in Houston
'94 Nissan Sentra SE-R w/ $tuff, converting to ITA
'98 Suzuki Bandit 1200S w/ $tuff
'02 Dodge Dakota SLT 4-dr tow beast, stock!
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