Originally Posted By: Klutch9
Originally Posted By: horse123
I've heard compressor surge, BOV, "flutter dump" (supposedly a combination of the the first 2), among other things. What actually is this sound:
https://youtu.be/HP4IJlLaBn8?t=6m21s
Start it at about 6:20 if it doesn't load there. It's the fluttering noise between shifts.
What you are hearing is what people commonly mistake as "compressor surge".
Let's review: compressor surge is actually when the compressor wheel is providing more pressure than the motor can take in, under load. You hear a similar fluttering noise, but it happens under acceleration. The fluttering you are hearing there between shifts, or under deceleration, is a similar sound, but is produced for a different reason...
Many blow-off/diverter valves completely vent all intake pressure when the throttle plate closes. The downside of this is that all pressure is lost in the intake. In a race situation, this means that between every shift, that boost will have to rebuild once you get back on the throttle. It doesn't take long, since the compressor wheel is still spinning fast. But there is still a small amount of time required to rebuild that boost pressure.
Enter the scenario in the video now. That fluttering is a blow-off valve with a pressure spring inside. This allows the valve to vent most of the air when the throttle closes, but still keep some pressure in the intake. Say an engine is at 20 psi when shifting, and the blow-off valve had a 5 psi spring. All pressure will vent upon shifting, except 5 psi. That's where the fluttering comes from... The valve maintaining the correct pressure level, and SOME air making its way back through the compressor wheel. The downside is that it does slow the compressor wheel a bit, but some boost is maintained. Usually these pressure spring blow-off valves are installed to match the turbo size and boost level. Listen to the group B rally cars, which weren't allowed to have bypass valves. They flutter like mad on every shift!
Of course, any pressure that backs its way through the compressor wheel is harder on the turbo, so a turbo motor without a bypass valve, or with too stiff a spring, will likely lead to premature turbo failure. But the trade off is usually quicker boost response, and one heck of a sound.
THANK YOU!
Finally an accurate and educated supply instead of a bunch of 16 year old civic drivers arguing about some vague noise they've heard on car videos on youtube.
