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OK, I'm no expert on on this, i pretty much gathered a bunch of info up off the net and am interpreting it the best i can.

INTERCOOLERS

Alot of people think the biggest IC they can fit behind the bumper, the better, but this is far from true. If you have a small 20lb (~280cfm) t15 turbo, and a HUGE intercooler, your system is gonna have alot more lag than a t15 turbo should have and not a very good powerband.

My suggestions when putting together a turbo kit is to get an intercooler is not rater more than 15%+ what your turbo is rated at. For example, if you had a 35lb turbo (~500cfm) I would not use an intercooler rated more than 575cfm. This will give you room to upgrade your turbo a little, but wont be too big to lose out on efficiency.

not all intercoolers will have flow information, nor will you be able to get such information from the manufacturer/distributor.

The intercoolers I use in most of my turbo kits (21"x6"x2.5" core) will usually suffice for most aftermarket turbo setups. They are rated at around 550cfm which is good for 320+hp. A simple upgrade to a (22"x8"x3.5" core) will put you in an efficient range to 400+hp.

For most people either one of these sizes should fit the needs of your turbo setup.

INTERCOOLER PIPING

First things first, before I post all this technical garble. The most common IC piping that I have seen is 2.5" and that is FINE for almost any turbo setup. The benefits of smaller, matched piping is a bit quicker spool up and the piping being more efficient overall. Reason being, the air will have higher velocity at any given CFM.

Even though most setups use 2.5" (including mine) it doesn't mean its the BEST size to use. In fact, its NOT in most cases (including my kits). The reason being, 2.5" is cheaper in almost all cases and more readily available since its good for almost any setup, including HUGE turbos. 2.5" can flow efficiently up to 740CFM which is around a 52lb/min turbo (t3/t4e 60trim) and about 600hp

According to Corky Bell (MAX BOOST) 304mph/.40 mach is the point at which airflow meets increased resistance (drag) and flow losses are experienced. So thats where you would upgrade.

Below is a list of calculations for the most common pipe sizes:

2" piping
1.57 x 2 = 3.14 sq in
300 cfm = 156 mph = 0.20 mach
400 cfm = 208 mph = 0.27 mach
500 cfm = 261 mph = 0.34 mach
585 cfm max = 304 mph = 0.40 mach


2.25" piping
3.9740625 sq in = 1.98703125 x 2
300 cfm = 123 mph = 0.16 mach
400 cfm = 164 mph = 0.21 mach
500 cfm = 205 mph = 0.26 mach
600 cfm = 247 mph = 0.32 mach
700 cfm = 288 mph = 0.37 mach
740 cfm max = 304 mph = 0.40 mach


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
913 cfm max = 304 mph = 0.40 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
1100 cfm max = 303 mph = 0.40 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
1300 cfm max= 301 mph = 0.39 mach




Just for reference again:

Some turbos will be listed in CFM (cubic feet per min) to convert lb/min to CFM, use the following calculation as a base.

lb/min / .070318 = CFM
35 / .070318 = ~497cfm, the turbo would probably be listed as a 500cfm turbo
 

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Finally someone who care of the CFM while selling FMIC KIT... lot of people don't care of the effiency, they just put a big intercooler, while this big intercooler could even make the car lag more and make the car lose about 1 or 2 psi...
 
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