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How is the car "hard to shift?" High effort shifter? Bitchy synchros? etc.
I do not think that the addition of a BOV will allow you to execute a shift any faster. If the issue is indeed boost lag between shifts then a BOV will help, as the turbo will not slow as much while shifting. An additional, possibly greater benefit will be extended turbo life. If you think the noise is cool, well, I suppose that's a plus too.
Fitting the valve in the intake piping is the easy part. Find a website/retailer that sells silicone or rubber intake boots, maybe even adapt the Saab piece, and splice a boot with a 1" tee into the intake between the turbo and the carb. Install the valve in the 1" hole you now have in your intake system.
You will want to source the vacuum line from between the carb and the head. The BOV works thusly: When the pressure on both sides of the actuation mechanism (the vacuum line and the vent port) in the valve is equal, force from the internal spring holds it closed. When the pressure on the control side drops below a certain level (the throttle is closed suddenly, the engine is pulling vacuum but the turbo is still making pressure) the pressure on the vent side overcomes the spring and the valve opens. So for the valve to work at all, you need vacuum in your vacuum line and pressure in your intake when you shift.
The next step is to decide whether you want a recirculating valve (diverter valve, DV) or a blow off valve. The stock Saab configuration employs the valve as a DV: it diverts pressure from between the turbo and the engine to the intake tube before the turbo. A blow off valve simply vents the pressure to the atmosphere. Not knowing the intricacies of carbuetion I cannot affirm that the Bosch valve that Saab uses will work as a BOV. It causes "comfort" issues in fuel injected Saabs when used this way because it allows air to enter the engine unmetered when the vacuum signal is strong enough to open it. My car is temporarily configured this way due to an oversized turbo. The car requires a foot on the gas during the first couple minutes after a cold start otherwise it will die. Throttle tip in is also very stumbly and unpredictable. Again, I do not know whether you will face these problems on a carbureted car, but undoubtedly someone here does. If you need a DV then an additional tee boot is necessary upstream of the turbo. Run piping of some kind to it from the discharge port of the BOV.
There are aftermarket turbo kits for factory NA cars that run 7ish psi and do not use BOVs under the premise that the pressure is low enough that the turbo can survive. I do not doubt that this is also a cost cutting measure. I would want a BOV regardless because turbos are expensive and unnecessary lag sucks. However, the true necessity/benefit of said valve may be something to confirm on a Lotus forum.
Someday I will buy an Elise and turbo it.
Best of luck,
Andy
91 SPG
Boulder, CO
posted by 67.173.2...
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