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Back pressure

Introduction to Engine Tuning

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Hello guys, I have a question that I have been thinking about recently. Do cars need back pressure or it is another car mit? They are different opinions in the net, but only from amateurs, so I want to find out the truth. Thanks

In short, NO! - but that's qualified...

Back pressure is the basically resistance to the flow of exhaust gas from the cylinder to the exhaust system, this pressure has to be overcome by the piston to push the gas out and it robs the engine of potential torque (part of the parasitic/pumping losses you may have read about). If you have 5 psi (gauge), a piston diameter of 4" and a stroke of 3", that means the force on the piston is 62.8 lbf (piston are times pressure) and a peak torque load of 94 lbs.ft (force times crank throw - half the stroke). As the pressure in the exhaust ports of turbo-charged engines can be twice the boost pressure, or worse for restrictive housings, you can see how that can rob you of a LOT of potential.

That same back pressure can also reduce the amount of incoming fuel-air charge at the top of the stroke, when there is overlap, and can actually pass exhaust gas into the intake ports - with carb's cars with lumpy camshafts this can be seen as fuel stand-off, which is a mist of fuel around the intakes caused by the air in the intake being pushed out throught the carb and it causes a very rich condition (part of the incoming air passing through the venturi three times!). With fuel injection it isn't as critical, but can still cause issues with, especially, MAF based systems.

By this point, you're probably wondering why some people swear engines need back pressure? It comes down to the engine breathing characteristics and, especially, how the high and low pressure pulses, or waves, are moving in the ports. With a NA engine, especially, careful design can 'tune' the exhaust so a low(er) pressure is at the exhaust valve as the engine is around TDC to help draw the charge into the engine - old farts like us sometimes call this coming onto the pipe, coming on song, coming onto the cam, and you can usually hear it clearly. That sounds great but at different rpm you can have a higher pressure pulse there which you DON'T want - so, what to do? Most OEM manufacturers will opt for exhaust designs that either/or dampen these pressure waves or move them outside the operating range so they have less affect on the engine.

So, what happens if you make a change to the exhaust to lower the back pressure? It changes the breathing, especially around overlap, and if this isn't catered for, with fuelling corrections, there will often be a power loss and so the ignorant will say - you need back pressure... The truth is you would make the best torque/power if the exhaust was connected to a vacuum - BUT the engine will need to be designed and tuned for it.

Get the intake and exhaust tuning (where the term originated - you 'tune' the lengths of the pipes, and it dates back to early steam engines...) right and you can get amazing efficiencies - the bike and F1 racing engines are/were approaching 160%, while a normal NA 4 valve road car is around 85-90%.