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Old 09-26-2010, 09:14 AM
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AIR RAM AIR RAM is offline
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Originally Posted by Race1Mopar View Post
Back pressure is created by exhaust scavenging and how efficient it is. My guess is that's it's scavenging better and more than before, creating less back pressure. Not sure in tis engine, but in the magnum engines if you took too much of the back pressure away you lost your bottom end grunt.

I hope you don't take this the wrong way but it sounds like you have Back Pressure and Scavenging mixed up to where they some how work together...

Back pressure is NEVER good... No engine on earth has ever needed or needs back pressure. Back pressure is a product of restriction, the words Back pressure and performance should never be used in the same sentence... the term back pressure does not belong in the performance minded vocabulary.

Back in the day when old timers did not understand the physics and dynamics of the exhaust system they would relate loss of torque to loss of back pressure simply because they went too large on the pipe they assumed they had lost some back pressure... There for they preach its importance and they have passed it on through the generations. Many people will speak about the importance of back pressure but have very little understanding on why or how it effects the system... they just know somone told them it was important. Understanding the theory of operation will help anyone put myths like that to sleep.

What they had lost was velocity. Velocity through the system is needed to keep the exhaust gasses flowing through the system... with high velocity you create vacuum... other wise know as the "scavenging effect". If you go too large on the ID of the system for the amount of air the engine can pump, you lose the velocity.... NOT BACK PRESSURE.

This velocity is pulling the gasses through the system. This effect is most important at the exhaust valve. The more vacuum your exhaust system makes the more toque you will make... but how? What is it about the vacuum and exhaust that makes toque... Hint... its not magic!

Most all NA cams are designed with an overlap of the exhaust and intake valves. This means that for a very short duration both the intake and the exhaust are open at the same time... Why? This is so your intake charge can benefit from that exhaust vacuum your system SHOULD be creating. Just before your exhaust valve closes the intake side starts to open... the vacuum on the exhaust side pulls the intake side into the cylinder effectively filling the cylinder with more air. This is a form of force induction... Not like a supercharger or turbo... but the same theory apply s. More air = more power! When you install an exhaust system that is too large it does not create the needed vacuum until the higher RPMs when the engine is moving enough exhaust to make create the scavenging effect. This is why you lose bottom end when you go too large. Its not the loss of back pressure, instead its the loss of low RPM velocity.

To get a better idea how the scavenging effect works, think of it like when draining a pool of water.
With a hose in the pool hanging over the edge from the pool onto the ground you simply create an initial vacuum on one end to get the water moving... BUT once the water is moving it continues to flow with no power source.

Think of the pool as the cylinder, the water as the exhaust gasses and the hose as your exhaust system. Go to large on the hose and it will not have the needed velocity to keep the flow moving go too small and it will be restrictive. But with the right amount of velocity in the exhaust system the vacuum created in the system continue to pull from the cylinder end.

Exhaust systems are not a one size fits all... But we are limited to whats available... so most of us have to make a sacrifice going either too large or too small. But never forget...

Back pressure = BAD (Restriction)
High Velocity = GOOD (Scavenging)


Last edited by AIR RAM; 09-26-2010 at 10:29 AM.
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