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Originally Posted by Coopdog01us  I read in one of the diesel magazines that unless you upgrade your turbo you shouldn't go any bigger than 4" exhaust. I don't remeber the reasons, if i come accross the artical i'll let you know. |
This is correct. You need some back pressure to keep the velocity of the exhaust gases flowing. There is a simple equation for cross sections on intake manifolds that can be applied to exhaust systems. Volume versus Velocity. If you have a greater than needed volume, obviously you can flow more air but it will take longer for that air to actually fill its given volume. Velocity is with respect to how fast the air is flowing through an intake or exhaust. The smaller the exhaust, the greater the velocity but the smaller the volume.
The same example can be said for traditional (not Variable Geometry Turbos ) smaller turbochargers versus bigger turbochargers. The smaller the turbo, the quicker it will typically spool up but the less it will produce in terms of power at a higher RPM. This is do to the fact that the air flows faster because it has a smaller space to fill. With a bigger turbo, at a low rpm, it will spin slower but it can hold a great volume of air at a higher RPM.
A 5" pipe would be best suited for a high horsepower truck or one that is used at wide open throttle often (WOT). Drag racing usually requires a greater volume of air to be moved through the engine since the upper RPM band is mainly used. Some back pressure is always needed in street driven applications. In order for you to keep healthy torque numbers in check, maintaining equal pressure in the cylinders is a must.
Anyone that can ether explain it better or possibly challenge what I have to say then I'm all ears. In a nutshell, I think I got my thoughts across clearly but I'm not sure everyone can understand it.
Erik
2005 6.6 Duramx
Linear Mode
