Re: way to test wastegate?
 

ghettoturbo is right, its gonna change somewhat when you have 35psi of hot ass exhaust gas pushing on the wastegate flapper. Remember, exhaust manifold pressures can normally be 2.5 x the psi of boost you are running...

Re: way to test wastegate?
 

No ---- Sherlock. Nobody is arguing against that. The point isn't what the wastegate is doing while it's on the car. The most accurate way to see how/when your wastegate is opening is to remove it from the car and used pressurized air to visually see when it moves. This number is obviously going to be absolute when done this way. 8 psi is going to be 8 psi whether you use a bike pump or fish pump with a pressure gauge. You're right that 8 psi might be 6 psi when the wastegate is installed on the turbo and the car is moving. This can be from sheer force of the engine combined with boost pressure combined with heat combined with age of a tired wastegate spring. The easiest way to check when it's opening while installed on the car is to use two boost gauges, better yet two pressure sensors and install one at the compressor outlet to wastegate vacuum line, before the intercooler so pressure drop doesn't come into play, and to be absolutely sure, one at the manifold and watch the difference between all three gauges.

Figure out your manifold pressure and go from there. I had a car once that my manifoold showed somewhere around 35 psi, but the pressure ratio stayed the same acrossed the turbine with the stock exhaust or aftermarket. I added atmospheric pressure of around 15 psi (14.7 if you're a stickler) which gave me in the neghborhood of 50 psi of backpressure in the manifold.

Stock, I tested around my cat and got roughly 22 psi (which would be 7 plus 15) and a pressure ratio of about 2.25:1

Where it gets interesting is that in doing a turbo back, I took out 4.5 psi from the exhaust. Add atmospheric pressure back in 5 + 15 + 20 psi X 2.25 and I got 45 psi. Subtract atmospheric pressure and I now had 30.5 psi a drop of arounf 4.5 in the manifold....where it matters.

Do yourself a favor and make yourself a backpressure tester.

Re: way to test wastegate?
 

that is exactly the reason im open dp :P