IC Pipe and TB Sizing

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So, I was doin some reading, and started to wonder about how to size intercooler pipe and the throttle body. One point that was made was, to keep good throttle response through out the throttle angle, you need to keep the throttle body size close to the charge pipe size (a lil bit bigger is best). This makes sence, because it would keep the intake velocity up, but is it true? Another thing I was thinkin about is, with my mid 400 horse power goal, would coupling a 2.5" charge pipe to the 2" outlet on my turbo be reasonable? I see plenty of cars makin more than 400 horse using 2.5" piping, so I know that a 2.5" pipe can flow enough volume to support my goal. But, will that extra half inch of diameter right off the snail of the turbo cause a pressure drop that will kill power output? Should I maybe run 2" from the turbo to the IC, then 2.5" from the IC to a 65mm (2.559") throttle body? Im just lookin for tips on gettin the best flow I can, and questioning rather or not the "typical" setup is good enough in my case. Thanks for any info.
 
Last edited:
This is something Ive thought a lot about as well. And for all the same reasons :)

I usually do 2" piping to the Intercooler and then 2.5 up to the TB. Im building my sube just as described right now with a 350awhp goal in mind.
 
I see some ppl sayin run 2.25" from the turbo to the IC and 2.5" from the IC to the TB, but they dont say what size the outlet on the turbo is, or why they go with 2.25". I dont see that being anymore beneficial to me than runnin 2.5" to the IC, but maybe Im wrong. One reason Im not sure about runnin a different size pipe on each side of the IC is because theres already a pressure drop through the IC, and runnin a bigger pipe on the cold side will only make it worse. Maybe Im wrong tho. Blanco usually clears things up for me in short order....

<----Patiently awaiting Blanco's input.
 
Once to the top, lookin for tips and ideas.
 
i questioned this too but more so when i was at a local shop and saw a skyline with like a 2in all around my buddy i was with that worked there didnt know why and no one else was around to answer. ive also see a few "built" sr20dets running smaller pipes than ive seen some civic guys running on smaller motors.

the only asnwer ive ever gotten about pipe size is that the hot side should be smaller than the cold side because its like breathingn threw a paper towle roll as opposed to a straw. dunno just what i was told (kind of a pointless responce by me o well )
 
Ya, I was hopin a few more ppl might have ideas about it, or be able to give reasoning for some of the more common setups, but I guess its not that big of deal to the others on this forum. My thought would be that having smaller pipe on the cold side would give you more velocity into the TB and give you better throttle response. I dunno, Ill just hafta wait till my cars done and buy a few different sizes of pipe and try a few different combos.
 
This is something I will need to know for my build, hence; I'm marking this thread down. :D
 
Luis998 said:
This is something I will need to know for my build, hence; I'm marking this thread down. :D
Click to expand...

Lol, ya, you can see that this thread hasnt had very much action, but it looks like things might pickup now. Everyone can see my thoughts on this, waiting to be corrected and taught something new. :)
 
It's a shame it hasn't had much action because it really is a good thread. I would have never thought that the pipes had to be different sizes at all.
 
Remember that the higher the volume of the charge pipes, the more lag your system will have. The perfect size is found by calculating the system's CFM, then figuring the volume of the pipes, and choosing the pipe size by how much lag is tolerable.


I'll whip up the calculations sometime.
 
That guy is the BOMB! Since you were trying to figure out all of this stuff I was going to ask you, how does one figure out the size of the down pipe?
 
heres my thought...cooler air is denser..there for bigger piping after the IC..and smaller before..
so 2.5 after and 2 or 2.25 before?!?!
maybe even 2 if its a smaller turbo..
 
INJEN78 said:
heres my thought...cooler air is denser..there for bigger piping after the IC..and smaller before..
so 2.5 after and 2 or 2.25 before?!?!
maybe even 2 if its a smaller turbo..
Click to expand...

Makes sence. My turbo isnt exactly small, but its no monster either, and the outlet is only 2 inches in diameter. Would it be best to match the outlet size or go slightly bigger, like 2.25"?
 
Luis998 said:
That guy is the BOMB! Since you were trying to figure out all of this stuff I was going to ask you, how does one figure out the size of the down pipe?
Click to expand...

You want the lowest back-pressure you can get for a downpipe. It's all dependent on how much the turbo and engine flow. Ideally you want an exhaust manifold pressure (remember, not absolute pressure) of 0 PSI, however this will rarely be the case. To reduce spool time, you want lots of exhaust at a high velocity. You want the exhaust flowing away from the turbine as fast as possible.


Now to the charge pipe fill equation. Ok this is simpliflied, and a lot of the math has been reduced to easy-to-use intergers, here we go:

So, let's say you flow 300 CFM (not a bad estimate for a 1.6l), into about 14' 8" of 2.5" pipe (1.5mm sidewall, but we'll simplifly this), and an intercooler (24 x 3 x 12" with tanks), all at a pressure of 15PSI.

2.5" pipe has an area of ~4.9", which leads to volume
Code: 
(2.5/2) * (2.5/2) * pi = 4.90873852

7 feet 4 inches is ~88, we need volume, so:
Code: 
(88" * 4.9") =  432ci

24" x 3" x 12" is our intercooler, we need volume, so:
Code: 
24 * 3 * 12 = 864ci

864 cubic inches in cubic feet, since our flow is rated in CFM
Code: 
1 (cubic inch) = 0.000578703704 cubic feet
864 * 0.000578703704 = 0.5 cubic feet
432 * 0.000578703704 = 0.25 cubic feet

Convert everything to seconds:
Code: 
(300 CFM) / (60 seconds in a minute) = 5 cubic feet per second

Now the number you want:
Code: 
((0.5 Cubic feet of pipe) * (0.25 cubic feet of intercooler) ) / (5 feet per second of fill)
= .15

.15 seconds just to fill all the pipe with an extra atmosphere. That's a lot of lag due to volume. One way to fix this is to keep the turbo spooled up by reducing exhaust back-pressure (simply put). Another way is to down-size the charge piping. Try 2" and see what happens.

The bigger your downpipe is, the less backpressure you'll get, the faster it spins. Just don't be a tool, and figure the downpipe for the *nominal* flow of the turbo + engine. Figure it for the turbo at the lowest effiecency, and the highest boost, with all temperature expansion taken into account. For most people on a d16, it'll be about 3". Don't do any smaller.


I got a little side-tracked, but here it is.
 
newb said:
Makes sence. My turbo isnt exactly small, but its no monster either, and the outlet is only 2 inches in diameter. Would it be best to match the outlet size or go slightly bigger, like 2.25"?
Click to expand...

Protip: Match the turbo's outlet. The turbo compresses, so you'll want to get it as much air as possible at first, while maintaining a sane velocity. Most inlets are larger to help this.
 
:worthyp: Rep'ed

Maybe someone could put that info in a sticky?
 
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