Madmax199 said:
I wouldn't say that it's not true that there is no long term negative effects from running without some form of compressor bypass. Are these effects something that will kill a turbo in short order like most people think? Not at all, and I am always preaching this on forums.
However, the long term and high pressure stress can't be argued, that's the whole reason why bypass valves were created. If not having a compressor bypass had zero effect on turbo life, Ford or Buick wouldn't have incorporated them in their later cars. Below is something I posted here just a couple hours before this thread, this will show you that I fully understand and agree that at moderate boost levels the stress is minimal and can be lived with. But that doesn't negate the cold fact that there is extra and unnecessary stress on the turbo from running this way.
Although I'll be the first one to say that not running a compressor bypass valve will not kill a turbo short term (many turbocharged race cars don't bother running one). It does however have long term negative effect on the turbo bearings. Some old school turbo cars like the infamous Buick Grand Nationals came without a DV, they lasted several hundred thousand miles with no issue at stock boost. However, if the boost was increased like even a "stage 1", the No-DV approach took a toll on the turbos in a couple of years (I know because my friend has 4 and a half GNs in his driveway and backyard). The device is clever, works, and help reduce long term stress on the turbo. At the boost levels that I run on my TT for example, no DV would mean a turbo rebuild every year. Not worth it at all on a street car... unless you're sponsored by BorgWarner.
THIS.
I've worked on several turbo designs for newer vehicles over the past 5 years including electrically actuated ones and you are spot on.
Something that isnt often known is the fact that older turbo's were usually able to take higher loads than the modern day ones, especially at the lower end of the spectrum. Shafts, bearings and in some cases even the casings are all setup for about 10-15% more than they are able to take according to their specific charts. This wasn't the case in the 80's or 90's.
Introducing flutter creates large spikes in the loading cycle of all of these components and will degrade them faster, though instant failure would only occur on a car creating levels of boost above what the turbo was designed to handle. On older turbos that had less optimisation than modern day units they may have failed slower, but given the modern life time is about 120,000 miles MBT its still going to last probably longer than you own the vehicle.
I wouldnt try it on a unit thats already done high mileage or hard duty cycles though.