View Single Post
911nut 911nut is offline
Metal Guru
911nut's Avatar
Join Date: May 2001
Location: Michigan
Posts: 2,511
Originally Posted by Ag02M5 View Post
One member plugged the intake and dumped the air to atmosphere.
That is me.
Why did the factory rig the bov to do this? For emissions. When the butterfly snaps shut in an n/a or turbo car, a tiny amount of fuel isn't completely burned, which raises the hc content of the exhaust. On a n/a car a vacuum-activated dash pot with a linkage to the throttle bell crank resists the sudden closure of the butterfly, allowing the fuel to combine with air and burn completely. On our turbo cars, the high pressure air trapped between the compressor and the butterfly is dumped into the intake, accomplishing the same thing.
I switched my recirc valve to a Forge bov after I noticed that my revs were dropping very slowly. Having to wait 3 seconds for the revs to drop isn't much fun. What I noticed after making the change was that, when shifting at 5000 rpm, I had an instant 2 psi with the bov vs. 0 psi with the diverter valve holding up my fun. My assumption is that taking a few tenths of a second to shift keeps the turbine from slowing down as much.
As for concerns about driveability, my car runs great; no issues. As far as unburnt mixture igniting in my turbo, well, I've seen that happen with cars that are running the recirc valve. The turbine wheel and housing are cast from nickel superalloy so it can stand up to raw fuel burning in it and a lot more. In checking my AFR's with an LM-1 I could not detect a rich spike when letting off the throttle to shift.
IMO making this change is a non-issue regarding durability and driveability and improves acceleration performance.
Paul B.
'91 964 Turbo
Port matched, SC cams, K27/K29 turbo, Roush Performance custom headers w/Tial MV-S dual wastegates, Rarlyl8 muffler, LWFW, GT2 clutch & PP, BL wur, factory RS shifter, RS mounts, FVD timing mod, Big Reds, - 210 lb
Old 11-05-2009, 02:24 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #62 (permalink)