About the current thread ,
This is based on my experience with the Merkur turbos. Some of you know that
stock they have no IC and therefore no bypass, pop-off or whatever.
Many have added an IC and with it a bypass valve. There is only one current off
the shelf IC kit for the cars so most run with that bypass valve. The valve
from Spearco is very much like a 944 turbo, S6 or Saab bypass. Its problem is
the points from where it gets the boost it recirculates. It connects from the
outlet elbow at the turbo back to the inlet. Not really good enough. Better to
pick up farther downstream. You get more to return and you don't leave the
intercooler and all the plumbing full'O air. Friends have added a pick up port
as close to the throttle plate as possible and re-routed back to the turbo
inlet. Much better.
I use an HKS bypass mounted close to the throttle plate and back to the inlet.
The HKS can flow a greater volume than the Spearco and through a larger hose
back to the inlet.
The big problem with the Xr4s with an IC is because of packaging problems there
are about 5-6 feet of IC plumbing from outlet to throttle plate. WOW.. Without
a bypass, off throttle bucking is HUGE. We call it trailer hitching. So the
bypasses can nearly cure that.
We don't vent to the atmo. Our cars have a vane type air meter with a moving
door in it. If you vent the bypass or a pop-off to the atmo the door in the vane
meter suddenly opens too much and you get an overrich condition during upshifts.
This means you have to drive through all the fuel. It does slow progress. BTTT.
I've tried both the bypass and a pop-off vented to the atmo and I've seen the
richness both in my air/fuel meter and the seat of my pants. And why do it
anyway. Well for the sound but........
So I wonder, does the Audi need a bypass. Compared to the Merkur the turbo
routing is very short so you don't need one for the hitching problem. It may
reduce spool time during upshifting though.
And I wonder with the CIS would venting to the atmo cause the sensor plate to
lift and richen as well. Maybe.
The length of the turbo plumbing on the S6 makes the Merkur look good. I was
amazed the long tubes don't add to the lag more than they do. But the S6s have a
bypass. I've found during both my weeks with S6s that they are very responsive.
That little turbo is working well. The Saabs are also nearly lagless, the 150hp
low boost version especially.
I imagine a 10/20 valve 5000/200 with very short plumbing and a K24 would feel
as near to NA as possible. Maybe.