Originally Posted by COSMO

*.4 Mach is the point at which air becomes turbulent and losses in efficiency start to occur exponentially. The key is to stay under that speed. You want to use the smallest piping possible that still flows enough to meet your needs. Larger than necessary piping increases lag time with no measurable gain

The velocities are in miles per hour and mach, and the flow rates are in cfm. Measurements for the piping are in inches.




I use the rule of thumb that every 1 psi drop is roughly 10-11 h.p. lost at the 450 h.p. level. So if I have a 2.25" lower I.C. pipe that has 2 90* bends with 4 feet of straights, I get a psi loss of about 1.22 psi loss. If I upgrade to a 2.5" pipe with the same configuration I get about .74 psi loss. For 3" I get about 0.32 psi total pressure loss. At best, the switch to the 3" pipe on the lower I.C. pipe is worth about 0.9 psi gained back in the intake manifold, or about +9.0 h.p.

Also keep in mind when you switch to bigger I.C. pipes the transition off the turbo outlet has to be taken into consideration, which the psi loss actually increases as the pipe size increases, so the gains going to a bigger lower I.C. pipe are not as great as the case I just illustrated. Which is why you need to sketch out exactly the pipe configuration you have now and what you will have when you uprgrade for an accurate loss analysis.

You can play with the numbers all day long but you get the idea. There are gains to be had optimizing the piping size IF your turbo compressor is already dropping off boost at high rpms. Otherwise the turbo will simply command itself to spin up a little faster to compensate. When the turbo is pinned wide open at high rpms and your already seeing boost drop off due to the engine outflowing the turbo, THIS is where reducing pressure losses in the intake system pays off.

2.5" piping
4.90625 sq in = 2.453125 x 2
300 cfm = 100 mph = 0.13 mach
400 cfm = 133 mph = 0.17 mach
500 cfm = 166 mph = 0.21 mach
600 cfm = 200 mph = 0.26 mach
700 cfm = 233 mph = 0.30 mach
800 cfm = 266 mph = 0.34 mach
900 cfm = 300 mph = 0.39 mach



2.75" piping
5.9365625 sq in = 2.96828125 x 2
300 cfm = 82 mph = 0.10 mach
400 cfm = 110 mph = 0.14 mach
500 cfm = 137 mph = 0.17 mach
600 cfm = 165 mph = 0.21 mach
700 cfm = 192 mph = 0.25 mach
800 cfm = 220 mph = 0.28 mach
900 cfm = 248 mph = 0.32 mach
1000 cfm = 275 mph = 0.36 mach



3.0" piping
7.065 sq in = 3.5325 x 2
300 cfm = 69 mph = 0.09 mach
400 cfm = 92 mph = 0.12 mach
500 cfm = 115 mph = 0.15 mach
600 cfm = 138 mph = 0.18 mach
700 cfm = 162 mph = 0.21 mach
800 cfm = 185 mph = 0.24 mach
900 cfm = 208 mph = 0.27 mach
1000 cfm = 231 mph = 0.30 mach
1100 cfm = 254 cfm = 0.33 mach
1200 cfm = 277 mph = 0.36 mach