buddy
Active Member
It's only 4 cylinders worth of exhaust going through it.
If you think 4" pipe is good for 8 cylinders, consider 4" pipe has a cross sectional area of 12.6 square inches
2.25" pipe has cross sectional area of 4 square inches.
3" pipe has 7 square inches.
If looking for half the flow capabilty, just realize that you dont just drop the diameter of pipe in half.
2.25" would be appropriate for carrying 4 cylinders into a combined 8 cylinders into 3" pipe, but it is less than a third the flow capablility of 4" pipe.
So although 2.25" might make sense in this instance its not just because it only carries 4 cylinders of exhaust.
Now, in fluid dynamics, the restrictiing of the pipe doesnt mean that it negates all of the flow benefit of the previous pipe. The more area/volume behind a restriction, the more force it will exert on the restriction and the faster it can push out of the restriction. So if back pressure is 10 pounds per square inch, then 4 sqaure inches (of 2.25" pipe) of 10psi is 40 pounds, whereas 5 sqaure inches (of 2.5" pipe) is 50 pound. Once pressurized, it could speed up exhaust through the passenger manifold when it brings it back into a smaller pipe. Although, still a detriment to spool time because it will take longer to pressurize.