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Forge question

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iainalpine:
When you lift off for changing gear the car runs lean as the injectors are shut off (if your TPS) is set right. When we fitted the dump valves and were running the cars up on the rollers the lean condition seemed to be worse which was backed up with very high exhaust gas temp's. Cant explain why this happened.

Iain

Mark_GTIV:
Hi Ian,

That sort of doesn't make sense to me. The condition you describe would surely be made *worse* when a recirculating or standard diverter valve is fitted, because there would be *more* air present.

Cars with Air Mass Meters run rich on gear changes with an atmospheric valve because they're dumping *pre-metered* air. The car has already calculated the correct mixture, has injected the fuel and is waiting for the correct amount of air to show up in the combustion chamber that never arrives for that moment, because on the gear change / lift off, that air gets dumped out the system - therefore there is more fuel than air at that moment, the air fuel calculation is temporarily incorrect with more fuel, which gets burned off or just hits the exhaust - hence the usual pop / brown smoke puff on shift - so I'm not sure what was happening with the cars you mention running lean.

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