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[quote][i]Originally posted by vibrio[/i] [quote][i]Originally posted by tiga77[/i] This is an extract from a guy called Chris Wilson who is a race car engine preparation and developer. ----------------------------------------------- "For those sick of reading about my blow off valve saga, stop right now. For those who would learn from my mistakes read on To those that proffered advice, sincere thanks, it's been an interesting learning curve. The following is based on my Skyline experience, with air usage measured by air flow meters before the turbos. [b]It is not applicable to cars that soley use a MAP sensor and throttle angle for air measurement .[/b] To recap I put in a bigger intercooler and the standard Skyline duct that runs across the lower front of the car taking dumped air from the 2 blow off valves to the intake between the air flow meters and the twin turbo inlets was masking air exiting the lower 4 rows of the new IC. Having spent a good deal of money on this item it pained me to see some of effectively blanked off. [b]So I decided to blank the inlet off altogether where the cross pipe entered the turbo inlet ducting, remove the cross pipe and vent to atmosphere. The dump valves were being held open at idle by the idle vacuum level, so after ages of pondering i made a set up of solenoids to control when vacuum was applied to the dump valves.[/b] This set up worked perfectly. *HOWEVER* a much more serious problem occurred, which any similarly inducted turbo car will potentially suffer if the boost air is dumped to atmosphere rather than as standard into the air intakes after the AFM (s). On the overrun, after a period of boost running, the standard set up will recirculate the excess boost back into the turbos, through the IC, and back through the dump valves to the turbo inlet again until the turbos inertia has slowed them to the point of little or no boost. The AFM's do not see any more air entering the engine, as it is being recirculated. However, when we dump to atmosphere, that air is lost from the system and the turbos draw in fresh air via the AFM (s). This causes the engine to go mad rich, as the throttles are closed and no fuel is really needed. [b]Hence the black smoke seen on the overrun after my mods.[/b] Worse still was a noticeable but very short period of detonation when coming hard back on the throttle. This puzzled me totally, then it dawned. As the fuel system started working normally again the Lambda sensor "caught up" with what was happening, saw a very overly rich mixture, and shut down the injector pulse width, creating a very lean mixture, causing a brief, but very dangerous period of detonation! I spent the afternoon and evening making new bracketry for the IC, new hoses and adaptors and shifting the IC forward that critical 40 mm or so, enabling me to fit the original moulded duct from the BOV's behind the IC without blocking it. Quite a lot of work and fabrication... [b]However, the mod I was planning could well have caused damage, and is certainly something to be very aware of if one has a similar AFM pre turbo set up changed to dump boost to atmosphere. [/b]The type of BOV matters not, it's the fact that the air no longer re circulates but is lost from the system, confusing the AFM (s) into thinking the engine is wanting more air/fuel mixture. An oscilloscope on the O2 sensors confirmed what was happening. We live and learn, often the hard way, but in this case not as hard a lesson as it might have been. I hope this helps. I can now finally make some proper seals to make sure all air entering the radiator ducting passes through the IC, something HKS totally fail to address, at the moment most air will go around rather than through the IC core. Best Regards, Chris Wilson http://www.formula3.demon.co.uk (Race car and engine preparation and development)" :wave::thumbs: [Edited on 08-05-2004 by tiga77] [/quote] note the highlighted areas. [Edited on 08-05-2004 by vibrio] [/quote]
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