It may be a matter of the smaller size increases the velocity of the exhaust?

 

Zach, you are right, there is a point where you do have diminishing returns, and at the HP our trucks make stock to slightly modified(under 600hp) 4 inch is all we need. however if you are pushing over 600-650 the 4 inch exhaust does become the bottle neck since it cannot flow as well as the 5 inch and that is when upgradeing to a larger becomes a factor in increasing HP.

 

Drew, did your exhaust come factory 4", or did you upgrade to the 4".

Why do you think my exhaustis only 3.5" from the factory? Do you think I would have benefitted more by going up to a 4"?

 

The problem with going to a 5" exhaust, like Drew pointed out, is that you lose volumetric efficiency (like that $5 phrase).

What happens is that the motor isn't pushing a large enough volume of air through the exhaust pipe to get a smooth, even flow. Instead the exhaust tumbles and slows in the exhaust and causes the engine and turbo to push more dead air. If it has to push dead air, it's just like having a restriction in the exhaust or using crimped exhaust pipes vs. mandrel bent.

As exhaust exits the engine, it's hot, expanded and traveling fast. As it travels the the exhust stream it cools, condenses and slows. By putting a big 5" sewer pipe on your truck, you ar emaking the motor push MORE of this cooler, denser, dead air out rather than it flowing smoothly out.

Thatis my redneck thermodynamics class for the day....