5th gen gt engine re-build

[fahr_side]

So when Fahr-side mentioned mechanical wastegate pressure being about 11 psi, that is with an intact reference line and actuator is what you are saying? The actuator alone with no reference can cause compressor surge which would damage the engine?

 

Very sorry. I missed the page to this thread.

 

In a lab test scenario if the engine is not running or the turbo is off the car, putting pressure on the actuator reference fitting via a hand pump will see it moving at about 14psi. In practice on a typical stg2 car with a high-flow cat pipe and a stock catback I see about 11psi MRP peak with zero WGDC or the actuator connected directly to boost reference. That's a combination of what the actuator will open at given a boost reference signal and the EGBP pushing on the spring via the flapper. EGBP makes it easier for the boost reference to overcome the spring and so it lowers the reference pressure at which the wastegate opens.

 

On the other question, no, direct mechanical boost control does not cause compressor surge. EGBP rises to a very high level during spool because the turbine is a great restriction to exhaust flow while spinning at low rpm. Drive pressure (EGBP) then can easily be as high as at peak revs, such as over 2x peak boost pressure. That's easily enough to overcome the actuator spring and leak pressure out the wastegate, slowing spool enough to avoid surge. This is at least true in factory turbos. It might not work out the same in hybrids and pick-and-match tuner turbos with (frankly) mismatched hot and cold sides, whatever wastgate flapper is on the hotside available etc.