Honda_Lover

hey guys,
i am running a rotrex c38-92 supercharger.
according to rotrex its capable of 1090 cfm.
my intercooler supports 1050 cfm.
throttle body is 2,75" (70mm) inner diameter
cold side piping is 3" (76mm) outer diameter
hot side piping is 2,5" (63,5mm) outer diameter

on a lot of internet forums (here too) you can find this information:

0.4 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.
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
<FONT COLOR="blue">913 cfm max = 304 mph = 0.40 mach</FONT>


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
<FONT COLOR="blue">1100 cfm max = 303 mph = 0.40 mach</FONT>


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

should i upgrade my hot side to 2,75" on a supercharged setup producing around 550 hp or is it a waste of time?

thanks for your opinions.

hatrickstu

FWIW there are a ton of turbo manufacturers with 2.5" discharges on the compressor housing that are capable of 1,000+hp. Mine is, although it opens up to 2.75" shortly thereafter. Been fine at 40+psi of boost and 950+whp.

Spoolin

I've always thought that 3" or even 2.75" pipe (hot and cold side) was a little too large for anything under 1200whp and figured 2.5" probably would work best at up to 1200whp, but that's just my opinion. I could be totally wrong here, but plan on staying with 2.5" unless I'm told otherwise.

06S2k07Si

Hello, I know that there are other factors in what I did with mine regarding the overall piping routing but mine is currently 2.75" hot side and 3" cold. I have a C38-92. You can read more about my testing here... https://www.s2ki.com/forums/s2000-fo.../#post24378393

scottdh20

i run 2.5" hot side and 2.75 cold side. only reason for the 2.75 cold size was its the same size as the throttle body for smooth transition into the intake manifold, but im also v-mounted so i only have like 2.5 feet of total piping on my car. 2.5" will support alot of hp

Spoolin

Very interesting! So the 3" piping all the way through is much better than the 2.5" and even showing so at only 450 whp. So even better results should be had at over 1000 whp with 3" piping over 2.5". Who knew? Looks like I will now be going with 3".

scottdh20

its not always so cut and dry. just with anything do deal with fluid dynamics if you increase the piping size it will increase the flow but your also losing velocity. its a trade off. the air will slow and you will have a larger internal volume to fill with compressed air causing more lag. how much im not really sure, would depend greatly on the setup (cfm, length of piping, number of bends,etc). just to throw out some b.s. numbers to provide a theoretical example of moving to 3" piping over 2.5" is you may gain 10whp at peak but spool time may take an extra 400rpms. it would be upto the end user to figure out what setup would be more beneficial to the setup. if the additional lag causes you to drop out of your peak power band between shifts that 10 extra hp is actually going to be slower.

edit: as to the other user who said he saw a increase in power that very well could be true but as far as his lag times are concerned he did mention a complete cold side reroute with far less bends. if you made a more direct path with shorter piping then he likely masked the slower spool time by making less internal volume to fill than the 2.5" causing spool times not to change much or maybe even improve. i have seen phenomenal gains from shortest routing possible in terms of lag time.

Spoolin

Very interesting. I'll have to do some research on this then before making my final decision.

BuggyofMildDiscomfort

Part of the reason I went with small intercooler piping on my build was because I was more concerned about loosing throttle response than I was gaining 10-15bhp right up by the rev limiter - my intake pipework is already pretty long, probably 8ft of pipework plus intercooler - so bigger pipework adds considerable volume to the system.