While doing my "homework", I came across some interesting information. But I do have some questions too! I'll first share what I've found to be interesting.

Some may know better, but if your intercooler has 2.5" in and out, you'll just use 2.5" piping and use reducers to mate those with the turbo and tb? Hmmm... Actually, there is a little more to it.

I'll just go straight to the numbers. Listed below is intercooler piping size in inches, the corresponding flow of air at 0.35 mach and a max horsepower estimate at 0.35 mach.
2.00" 514.3 CFM Everything below 360 whp
2.25" 660 CFM up to 460 whp
2.50" 825 CFM up to 580 whp
2.75" 975 CFM up to 680 whp
3.00" 1166.7 CFM up to 820 whp

Dude, wtf is this 0.35 mach you speak of? Very simple, around this 0.35 mach is the efficient range in which air moves through your intercooler piping . Which means the numbers shown are in the "efficient range". Now if your intercooler piping is too tiny and you reach 0.4 mach, air will become turbulent. Beyond that point your effective air flow will drop exponentially. Take note that the calculation's "perfect conditions" do not apply to your intercooler piping. Your piping will not be a perfect straight tube. So if it's a close call, just step up one size. It's not too big of a deal. Now the downside of bigger intercooler piping is the same as an overkill intercooler. The bigger the piping, the bigger the volume of air between the turbo and the intake. This effectively means a slower spool because it just takes longer to build pressure in a bigger volume. Now I should add that oversized intercooler piping will often increase the actual horsepower number by a little, however this little gain does not outweigh the huge advantage of a quicker spool by any means.

To give you a perspective (with metric numbers ) when air moves at 0.40 mach, at 20°C this would be about 490 kilometers per hour. If an object exceeds mach 1, it will break through the sound barrier.

Calculations for the CFM numbers are not exact as there are a lot of variables, but they are very close. The horsepower number on the other hand, is a very rough estimate. Now wtf is this CFM number? This is just a number that in this case is the flow of air. Your turbo may have a flow rating in LB per minute listed in the specs. With this number you can calculate the CFM value your turbo "could" put out by multiplying the LB per minute with 14.27. An easier approach might be to just use your horsepower goal to find the most efficient size.. since not everyone will be maxing out their turbo. I do not advise to stretch those horsepower numbers, because ~10% over the stated whp numbers will "theoretically" hit 0.40 mach, which, if you've been paying attention, is not good! Again, if it's a close call I would step up one size to be safe.


Now I've read about people using the apropriate piping for the hot side of the intercooler, and use an often larger size for the cold side. This side will then be a (closer) match to the throttle body. Apparently this helps with air starving on initial throttle sinnce there is just more volume after the intercooler to suck air from. Is this also a thing with miata's? Let me know if you have info or experience in using different size piping! I would like to know.


Useful source: https://www.dsmtuners.com/threads/ho...post-152057611

 

We have 2" hotside piping on 500hp setups, seems to work awesome.

Dann

 

If you go back and search 10-13 years ago we also have some posts floating around with the math involved and such. It was a large reason why I had no issue dropping to 1.75" IC piping on the hotside for my turbo setups. It will flow more air then a stock motor will take no problem. So trying to make a larger pipe fit just because it is bigger did not make sense to me.