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Wastegate spring pressure vs. Manifold pressure

Off Topic Discussion

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Hello everyone,

I have a 5.3 LS engine with a single turbo on a log manifold. We have a 12 PSI spring in the waste gate and the manifold is only seeing 7.5 PSI.

Is it normal to see this much of a difference in pressure from waste gate spring to manifold pressure? If not what should I be looking for? Boost leak? Intercooler having a large pressure drop? I had built the intercooler piping all from 2.5" but the intercooler is a 3" core. I replaced from the intercooler to the throttle body with 3" piping but it doesn't seem like this made any difference.

thanks for any input!

The wastegate spring is only a guide on the actual boost you'll see in the inlet manifold. The reason for this is that while you have boost pressure trying to open the wastegate, you also have exhaust pressure trying to force the wastegate open too. Ultimately the boost pressure you'll see for a given spring will depend on the level of exhaust pressure which in turn comes down to turbo sizing.

With your situation this could happen if your turbo is quite a small match for the 5.3 as the exhaust pressure will be very high. There may be other reasons for this though such as boost leaks or restriction in the system somewhere. A common mistake I see people make is forgetting to fit the wastegate seat into the wastegate when they assemble it onto the manifold. This causes a large exhaust leak around the turbo and artificially drops your boost as well as making the turbo very lazy to come on boost.

It might be worth having your plumbing smoke tested for leaks to eliminate this as a possibility. Beyond that I'd suggest checking the boost at the compressor outlet to see if it's showing a large boost drop from there to the manifold. Finally if you can check the exhaust pressure this will tell you a lot about the turbo sizing. For a responsive street setup I normally look for the EMAP to be 1-1.5 x the pressure in the inlet manifold.