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?
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:
24" x 3" x 12" is our intercooler, we need volume, so:
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.