When searching for automotive-related debates or suspect products to test and expose for Fact or Fiction, it's easy to make the mistake of looking a little too hard, and forgetting about what's right in front of our faces. Like this month's subject: bypass valves (or blow-off valves) on turbocharged engines, and whether venting their charge to atmosphere can decrease performance as opposed to recirculating it.
When installed on a turbocharged engine, bypass valves use an engine's intake manifold vacuum at throttle let-off to open a diaphragm that vents excess boost pressure from its turbocharger's compressor. Without releasing that pressure, the turbocharger would otherwise force or "surge" boost pressure against a closed throttle plate, bringing it to a series of rapid stops that eliminate spool and damage the turbo. In most stock applications, the bypass valve recirculates boost pressure back into the turbo compressor's inlet, but in many modified instances, the bypass valve is made to vent directly to the atmosphere; some would argue, a move that decreases spool and slows throttle response.
The Test:
We knew testing this theory would be a little trickier than strapping the car to the dyno and testing peak power, so we enlisted the help of Mavrik Motorsports and their DynoLog chassis dynamometer, where lead tuner Teddy simultaneously logged boost pressure, throttle position, and time delay throughout a series of runs with our '08 Mitsubishi Lancer Ralliart test car, first with its factory bypass vlave recirculating boost, then again with it venting to atmosphere. Since this was a throttle response test and not one of peak power, we loaded the dyno to keep engine speed at a consistent 4,000 rpm in Fourth gear throughout testing, and tracked the amount of boost pressure our 4B11 engine produced at a series of wide-open-throttle jabs at specific intervals after throttle let-off from full boost, for each orientation of the bypass valve. And for kicks, we also tested peak power. Here's what we found:
The Verdict: Fact
After sifting through mounds of numbers, results were averaged for a few clean throttle tests for each variation of the bypass valve taken at different times, and the amount of boost our 4B11 produced after throttle let-off with its bypass valve recirculating boost was consistently higher than with it venting to atmosphere. But not by much-the benefits of recirculating will be more apparent with larger turbos, and larger, aftermarket replacement bypass or blow-off valves will likely offer improved results over original equipment, provided they offer the option to recirculate boost. Sure, recirculating boost may not sound as cool as blowing it off... but neither do First Place trophies, and we like those, too.