Here is a writup I did and posted on Supraforums. The friend I mentioned told me to post the same information here and see how this crowd responds.
-snip-
Today, a friend who is looking to do a custom intercooler setup for his MKIII Supra told me he was going to use 2.5" intercooler piping instead of the 3" piping that I have left over that I offered him for cheap. His defense of 2.5" piping was that it has been reported that there is a quite noticable difference in response time with 2.5" piping vs 3" piping. In trying to convince him that 3" piping was better, I mentioned that in my opinion, the flow advantages of larger piping outweigh the potential loss in response of the larger piping.
So, I started doing some math.
Boring Stuff
The area of a 2.5" pipe is (1.25" x 1.25" x 3.14), or 4.90625 square inches.
The area of a 3" pipe is (1.5" x 1.5" x 3.14), or 7.065 square inches.
Lets say that the intercooler piping length we need is 6 feet (72 inches).
A 6 foot length of 2.5" piping holds a volume of 353.25 cubic inches. This is equal to 0.204427083 cubic feet. A 6' length of 3" piping holds 508.68 cubic inches, or 0.294375 cubic feet. The difference in volume between the two pipes is 0.089947917 cubic feet.
A 3L 7M, with assumed 90% VE, at a pressure of 1 (0 PSI, at WOT, waiting for the turbo to spool), at 3000 RPMs, flows 143.109541 cubic feet per minute, or CFM. This is equal to 2.38515902 cubic feet per second. The formula for airflow is (Volume x RPM x Pressure Ratio x VE) / 5660.
This means that on a stock longblock 7M at 3000 RPM with zero boost, the 6 foot length of 2.5" piping is "emptied and refilled" 11.6675295 times per second, or one time in 0.085707947 seconds. The same length of 3" diameter piping is emptied and refilled 8.10245102 times per second, or one time in 0.123419444 seconds.
At 3000 RPMs, at 0 PSI, right before your turbo spools, it takes 0.037711497 seconds longer - thats 38 thousands of a second - to fill the extra volume of a 6 foot long, 3" diameter pipe than it does a 6 foot long, 2.5" diameter pipe.
To continue...
Lets assume that you are running 18 PSI (a pressure ratio of 2.2244898), are at your 6500 RPM redline, and have the same displacement and VE. Your engine is flowing 689.749046 CFM, or 11.4958174 cubic feet per second.
At this flowrate, the air in the 2.5" piping is getting replaced 56.2343171 times per second, or once every 0.0177827357 seconds. With 6' long piping, this comes out to 337.405903 feet per second, or 230.049479 mph. The 3" piping is flowing at 234.309654 feet per second or 159.756582 mph. The generally accepted maximum reasonable airspeed through a pipe is around mach .3, or 200 MPH. This is primarily due to the friction between the air and the pipe wall, which, gets exponentially higher as speed increases. One must also realize that it is not only until mach .3 that the piping that creates this airspeed becomes a "restriction," but it will always pose more of a restriction, regardless of how low the airspeed is, than a larger diameter pipe of the same length.
End Boring Stuff
1: There is virtually no noticable loss in response time between 2.5" intercooler piping and 3" intercooler piping.
2: Without doing more math, I would venture to say that I would not use 2.5" intercooler piping for more than 350RWHP. Still, if most of the time you are doing custom piping anyways, why not go with 3"? I am using full 3" piping on my MR2, and with the A2W I only have almost exactly 2 feet of piping.
Here is a link to the thread of SF for those who are interested in the replies and what I answered with.
http://www.supraforums.com/forum/showthread.php?t=377429
-snip-
Today, a friend who is looking to do a custom intercooler setup for his MKIII Supra told me he was going to use 2.5" intercooler piping instead of the 3" piping that I have left over that I offered him for cheap. His defense of 2.5" piping was that it has been reported that there is a quite noticable difference in response time with 2.5" piping vs 3" piping. In trying to convince him that 3" piping was better, I mentioned that in my opinion, the flow advantages of larger piping outweigh the potential loss in response of the larger piping.
So, I started doing some math.
Boring Stuff
The area of a 2.5" pipe is (1.25" x 1.25" x 3.14), or 4.90625 square inches.
The area of a 3" pipe is (1.5" x 1.5" x 3.14), or 7.065 square inches.
Lets say that the intercooler piping length we need is 6 feet (72 inches).
A 6 foot length of 2.5" piping holds a volume of 353.25 cubic inches. This is equal to 0.204427083 cubic feet. A 6' length of 3" piping holds 508.68 cubic inches, or 0.294375 cubic feet. The difference in volume between the two pipes is 0.089947917 cubic feet.
A 3L 7M, with assumed 90% VE, at a pressure of 1 (0 PSI, at WOT, waiting for the turbo to spool), at 3000 RPMs, flows 143.109541 cubic feet per minute, or CFM. This is equal to 2.38515902 cubic feet per second. The formula for airflow is (Volume x RPM x Pressure Ratio x VE) / 5660.
This means that on a stock longblock 7M at 3000 RPM with zero boost, the 6 foot length of 2.5" piping is "emptied and refilled" 11.6675295 times per second, or one time in 0.085707947 seconds. The same length of 3" diameter piping is emptied and refilled 8.10245102 times per second, or one time in 0.123419444 seconds.
At 3000 RPMs, at 0 PSI, right before your turbo spools, it takes 0.037711497 seconds longer - thats 38 thousands of a second - to fill the extra volume of a 6 foot long, 3" diameter pipe than it does a 6 foot long, 2.5" diameter pipe.
To continue...
Lets assume that you are running 18 PSI (a pressure ratio of 2.2244898), are at your 6500 RPM redline, and have the same displacement and VE. Your engine is flowing 689.749046 CFM, or 11.4958174 cubic feet per second.
At this flowrate, the air in the 2.5" piping is getting replaced 56.2343171 times per second, or once every 0.0177827357 seconds. With 6' long piping, this comes out to 337.405903 feet per second, or 230.049479 mph. The 3" piping is flowing at 234.309654 feet per second or 159.756582 mph. The generally accepted maximum reasonable airspeed through a pipe is around mach .3, or 200 MPH. This is primarily due to the friction between the air and the pipe wall, which, gets exponentially higher as speed increases. One must also realize that it is not only until mach .3 that the piping that creates this airspeed becomes a "restriction," but it will always pose more of a restriction, regardless of how low the airspeed is, than a larger diameter pipe of the same length.
End Boring Stuff
1: There is virtually no noticable loss in response time between 2.5" intercooler piping and 3" intercooler piping.
2: Without doing more math, I would venture to say that I would not use 2.5" intercooler piping for more than 350RWHP. Still, if most of the time you are doing custom piping anyways, why not go with 3"? I am using full 3" piping on my MR2, and with the A2W I only have almost exactly 2 feet of piping.
Here is a link to the thread of SF for those who are interested in the replies and what I answered with.
http://www.supraforums.com/forum/showthread.php?t=377429
