Quote Originally Posted by Brian P @ speedtalk.com/forum;2064798 said:
The flow velocity in the pipes will govern how much pressure drop you get.
Intercooler is on the high pressure side of the turbo compressor, so the volume flow rate is governed by the displacement and RPM and VE of the engine. For the moment let's disregard the temperature effects (i.e. let's work out the post-intercooler side first).
I don't know what you are turning for RPM. Plug in your own numbers below, if you know them better than I do. Also I prefer staying all metric.
6000 rpm = 100 revs per second = 50 complete 4-stroke cycles per second.
50 (complete 4 stroke cycles per second) x 3.0 (litres pumped per complete 4-stroke cycle) x 1.1 (assume 110% VE - I presume this is a 4 valve DOHC) = 165 litres per second = 0.165 m3/second
2.5" (inside or outside? wall thickness if outside?) let's assume 2.5" OD 1/16" wall = 2.375" ID = 60.325mm call it 60mm = 0.060 m
Cross-sectional area = 0.060 ^2 x 3.14159 / 4 = 0.00283 m2
Flow speed is then 0.165 / 0.00283 = 58 m/s (for the imperial folks, this is 191 ft/s)
Flow losses are in proportion to the "velocity pressure". I don't know what you are running for boost pressure because that affects the density. For the moment let's pretend that it is a normally aspirated engine. If it's running under boost then all the pressures scale by the density factor. Density of air at standard temperature and pressure is 1.2 kg/m3
VP = 0.5 x rho x V^2 = 0.5 x 1.2 x 58^2 = 2018 Pa (for comparison, standard atmospheric pressure is 101,325 Pa).
Now comes something that we really don't know. Every 90 degree elbow is a loss of about 0.25 times the velocity pressure. Every pipe-diameter length is a loss of about 0.02 times the velocity pressure. An entrance from a plenum into the pipe without any particular effort paid to smoothing the entrance will lose around 1.5 velocity pressures. The smoothest possible entrance must lose at least 1.0 velocity pressures because of Mr Bernoulli. Theoretically you can gain some of that 1.0 velocity pressure back at the outlet of the pipe IF it is a tapered expansion but it's normal to assume that it will be thrown away when the pipe opens up into a chamber. Any other sources of flow losses will add something.
OK now for some guesswork (you plug in your own numbers here)
Pipe entrance 1.5 velocity pressures
4 ft of duct = 1.2 metres = 20 pipe diameters x 0.02 = 0.4 velocity pressures
Assume 4 90 degree bends = 1.0 velocity pressures
Total 2.9 velocity pressures
So your pipe from intercooler to engine will lose 2.9 x 2018 Pa = 5852 Pa (about 5% of total atmospheric pressure).
If everything is under boost - say, 2 bar total pressure = 1 bar "boost" pressure above the approximately 1 bar of barometric pressure - then the pressure loss doubles but also the density that you started with doubles. It's still about a 5% loss of the pressure at the inlet of the pipe under those particular conditions.
Now, the thing is, on a turbo engine, you can crank up the boost pressure to make up for that, particularly at higher revs. Still, not having that pressure loss is boost pressure that your turbo wouldn't have to generate.
If you re-crunch the numbers using 3" (call it 75mm ID) then it changes as follows. It makes a huge difference.
Volume flow rate remains the same because it's governed by the fixed-displacement engine.
Cross-sectional area is now 0.00442 m2
Flow speed is now 37.3 m/s
Velocity pressure is now 837 Pa
The number of elbows and the entrance are the same, but now your 4 ft (1.2 m) length is now only 16 diameters so you are now only losing 2.82 velocity pressures, i.e. 2360 Pa.
So going from 2.5" to 3" pipe will cut the pressure losses through the pipe by a little better than half.
The pipe from the intercooler to the throttle body might have some effect on the resonance tuning because it acts sorta like a bit of plenum volume. Whether this is good or bad is anybody's guess at this point.
As for the effect on throttle response ... The total volume inside 1.2 metres of 75mm dia pipe is about 5 litres. Your engine draws in that entire volume in about 3 revolutions. With the 60mm pipe it draws in that entire volume in about 2 revolutions. Think you would really notice the difference? I don't.
I'd go 3" but any more than that would be subject to the law of diminishing returns.
