A common misconception is that ‘you have to have back box pressure to get power’, this could not be more from the truth, back box pressure, although will change the torque curve, will not give more peak power.
The exhaust has 2 main functions:
• To get the exhaust gases & heat away from the engine.
• To reduce the noise.
So in theory an engine without any exhaust would be best for power right? Wrong-
The exhaust has a much more important use to performance engines. If designed right they can utilize the pressure waves present (which are traveling at over the speed of sound, much faster than the actual speed of the gases) to reduce the pressure of the exhaust header & thus ‘scavenge’ the cylinders for burnt gasses. This creates more room for the fresh charge to be drawn in… (on the engine exhaust stroke not all the gasses may be expelled, leaving heat & unwanted gasses in the cylinders) on the exhaust stroke the pressure within the cylinder will eventually reduce to a pressure similar to the exhaust & the speed of the exhaust gasses will slow down.
Once the exhaust valve has closed the pressure on the exhaust side now drops & a properly tuned exhaust will use the pressure wave changes to ‘suck’ exhaust gasses out on the opposite cylinders & visa versa.
The need for utilizing these pressure waves is significantly reduced on turbo engine’s as the flow charge will ‘blow’ out most of the remaining exhaust on the valve overlap.

Silencers are used primarily to well, silence! As well as look good & give a desirable sound. They are not good for peek power at all & in fact a silencer designed to give no power loss would be no silencer at all! All silencers create restriction to power.

Generally increasing back box pressure will create more torque at the given rpm (torque being generated by the engines ability to process more fuel & air charge efficiently per engine cycle without raising the given rpm) thus raising the volumetric efficiency of the engine. This is because e.g.
If a engine is turning at 2000rpm, increasing the back box pressure will create a restriction to exhaust flow, thus creating a ‘load’ on the engine and in turn lowering the engines rpm- If you now slowly open the throttle to increase the revs back to 2000rpm, the engine is now turning at the same speed as before BUT it is using more air & fuel to do this. Burning more fuel & air per engine cycle increases the torque but will lower the engines max RPM possible & thus lower the engines peek power.
But the increase in back pressure may reduce the cylinder scavenging effect & thus reduce the volumetric efficiency, so it’s a vicious circle.

Volumetric efficiency is the engines ability to fill the cylinders with air & fuel. If the engine has a cylinder 'swept' volume of 500cc but it can only process 250cc’s of air per cycle then the engine has a volumetric efficiency (or VE) of 50%.
VE will always be at a maximum at peak torque- as will the compression of the engine be at a max (this is why your engines compression ratio will have to be suited to your cam.

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