Intercooler piping

[sr vegas 240]

So I'm about to fab som intercooler piping for my s13 with s14 sr and bottom mount gtx2867. All the kits i see on the market are 2" or 2.5" on the hot side and 3" on the cold side or somewhat in that range small on hot side bigger on cold side. Now i was going to make all my piping from 2.5" both hot n cold side. Is there any reason to why the pipes are different size or does it really matter.

[nisileighty]

PBM’s website explains why. Just find their intercooler on their website and read the description

[sr vegas 240]

Their kit is designed for stock turbo t25/28 and is not capable of supporting 400whp I already talked to them. I wanna know if i can make 2.5" both sides because i already have 2.5" if not i will get some 2.75" or 3"

[Kingtal0n]

The larger pipe on the outlet enhances pressure drop and slows air down.
Slower moving air supports higher flow rates in larger tubes, and creates less friction (frees up potential additional density improvements)

There is an ideal tube size range where max flow rate at a given pressure is realized.
Because the compressor discharge is high temperature and the manufacturer has set the outlet diameter based on max flow conditions, it is optimal to keep the same size tube until it reaches the intercooler.
Once the air moves through the intercooler and experiences the friction and turbulence and cooling, there is an overall pressure drop and the flow is reset to "cool and turbulent". Thus when it encounters the outlet of the intercooler, using a smaller diameter pipe is no longer favorable since you are not conserving or matching any of the compressor discharge's investment of potential and kinetic energy, unless the intercooler was so lame and basically hollow that it allowed airflow molecules with an inlet velocity vector to carry that energy all the way to the other end, in which case it isn't much of an intercooler.

[nisileighty]

From personal experience, the PBM HMIC does support 400 whp.

[Kingtal0n]

Well. What does that even mean? A straight piece of 2.5" pipe supports like 500 horsepower.

Intercooler is heat exchanger with a given volume/construction. On the inside the air encounters resistance to flow. On the outside the temperature and rate of conductivity through materials exists. What an engineer might do is model both situations as partial differential equations. There are standard formulas used in mathematical methods for these common scenarios. For example fourier's law, diffusion equation for the airflow on the inside of the intercooler. If you had a question about the rate of conduction the mathematical methods exist to create a model used to find a solution.

The thing is, a solution to what? What was the question? What are we trying to compare or discuss here.
1. for the air inside the intercooler, the resistance to flow is an important property
2. for the air outside the intercooler, the rate of heat exchange through the materials and temperature differences are important

So notice right away, because all the intercoolers are made of aluminum, and because aluminum has a pretty standard well known rate of conduction K of material, we can generally cross #2 off the list as comparable features between intercoolers. i.e. one intercooler's aluminum isn't going to be better than another company's aluminum. Note that if the construction is different (maybe one company used LESS or MORE aluminum some places) this would alter the effectiveness or weight. "Bar/plate" vs "tube/fin" I guess is an example. So be aware when comparing intercoolers to be sure you compare similar construction or account for that.

So lets deal with or compare #1
#1 the air inside the intercooler encounters resistance as it moves from one and to the other. The difference in pressure and temperature at the far end suggests that the lowest temperature, highest pressure air is desirable.
The thing with this condition is that it has to be studied empirically, experimentally on a running engine.
The resistance to flow through an intercooler will reduce engine flow rate which has varied consequences on exhaust gas pressure, compressor outlet temperature (efficiency) and turbine behavior, i.e. adding a significantly restrictive intercooler would reduce the flow-rate of the compressor at the same pressure, moving it "to the left" on it's compressor map, suggests a lower impeller rpm in the turbine. The bottom line is: intercoolers will reduce power output when "all else is equal" if all you do is add one and don't compensate.
The way we typically compensate for adding an intercooler is by using the "cold side" (after intercooler) pressure to operate the wastegate. This replaces the pressure lost from the restrictive intercooler. it means the pressure on the compressor outlet will be higher than it was originally also- that is where the additional power is coming from. Additional power comes from the compressor flow rate being turned up.
Also notice if the intercooler isn't doing any additional cooling of the air at low flow rates then there will be no additional power due to flow rate. Only an increase in pressure to compensate for any restriction of the intercooler.

So did we answer the question? Not yet, really. The rate and temperatures have to be studied on a running engine combination to determine which intercooler is ideal or 'better' because we need to measure and study those variables. In my opinion a good intercooler uses "smooth flow" pathways that also effectively create small pockets of "turbulent flow" where air and aluminum break their laminar layers and transfer heat. If you look around there has been alot of success using both "Cheap" and "expensive" designs which suggests that the more expensive intercoolers may not always be needed and that it isn't difficult to achieve satisfactory results with off the shelf, generic units. In my experience the cheap units work well in daily applications because you can trash them without a care and overall aluminum rate of heat transfer is going to be the same as expensive units so the only real variable is the resistance to flow. And even the cheap units have general "1.5" Hg pressure drop at XXPsi" sort of descriptions which suggests that the design has been tested/copied from a well known unit and is basically 'famous' for it's effectiveness to receive such attention.

[nisileighty]

Originally Posted by Kingtal0n

Well. What does that even mean? A straight piece of 2.5" pipe supports like 500 horsepower.

First of all, I’m not gonna read all that.

[Kingtal0n]

hah it wasn't meant to be directed at you,
it was never intended for you to read

just a general statement about intercooling

[sr vegas 240]

i think i got it all figured out. Thats some great info also found some other stuff here is a great link from DSM guys. https://www.youtube.com/watch?v=nhYlXr-tHpY

My goal 400whp but ill take more
fully built bottom end 8.37 compression rebuilt head with hks 256 cams.
turbo gen 2 gtx2867 flows 30-40 lb/min
@400whp i need pipe capable to flow 570.80 cfm which comes up to a 2.25" pipe if my math is correct.
But i will sacrifice some lag and go with 2.5" hot side and 2.75 cold side just to give my self some room for larger turbo down the road.

[Kingtal0n]

turbo is way too small, compression way to low

thats a turbo for 1998

Its 2019. The turbo should flow at least 63lb/min if the engine is built
Minimum

If a turbo flows 40lb/min
that is about 350rwhp

compression should be at least 9.5:1 and if possible the system should be flex fuel to at least 50% Ethanol IMO

And finally you don't need to do math because the manufacturer of the turbo already did that. Just go slightly larger than the outlet tube to be on the safe size. Bends and turbulent flow reduce effective tube diameter so the tube always needs to be larger than the compressor outlet once it starts to bend or encounters the intercooler.

[sr vegas 240]

So 2.5 hot side and 2.75 cold should be fine

[Kingtal0n]

why do you seem fixated on that

3" would work. 4" would work. At some point though, what would happen? Slow moving fluids in a 'bubble' of space where high velocity input matter enters a control volume, and there is an exit flow as well with the same rate.

Due to conservation of mass, whatever flows in has to flow out. Otherwise we are adding mass to the system. Which is what happens when the pressure starts to rise and boost pressure differential is created. The air will gradually become denser as air molecules squeeze closer together inside the bubble of space that was created by using a 5" diameter pipe with a 2.5" pipe entrance diameter. The more volume inside this space, the slower the outlet pressure will become equal to or near the inlet pressure over time, given the same flow rate into the volume for comparisons.

SO what is really changing as we adjust pipe diameter is the rate at which the outlet of the given volume of pipe we are inspecting will experience (or 'catch up to') the same pressure as in the inlet side of that pipe.

I think. Does that sound right

[Tempo]

Originally Posted by Kingtal0n

Well. What does that even mean? A straight piece of 2.5" pipe supports like 500 horsepower.

Respects to you sir. I haven't read this much in depth non sarcastic, current information on this forum in a long time. It's nice to see legit questions and answers being laid out [emoji106].


Sent from my iPhone using Tapatalk

[SupaDoopa]

Why are you trying to limit yourself on pipe sizing? Bigger is usually better not because we're Americans and everything should be over engineered but because all the bends, twists and turns will make a bent piece of 2.5" far less effective than a straight piece of 2.5". This is why you have kits with the half inch larger piping on one side. They take this all into account. They've done the R&D so people don't have to sit here and try to reinvent the wheel. You're running a tiny tuboski on a SR - you don't have to worry about supporting 500+ HP.