Back Pressure VS Boost

[Josewick]

Can someone explain a little.. on why less back pressure will casue higher in boost??

e,.g. decatted run higher boost than usual... Larger bore runs higher boost.. etc...

Thanks

[IanH]

Less back pressure = Exhaust gas leaving quicker = More gas going through the Turbo per second = Spins the turbo harder = More Boost

*Technical Disclaimer* - I'm a technical Numpty so I may be wrong

[Josewick]

that makes sense Ian, I was just thinking.. higher back pressure, gas leaves slower, higher pressure builds up in turbo...... but thats obviously not the case

[SimonM]

Originally Posted by Josewick

that makes sense Ian, I was just thinking.. higher back pressure, gas leaves slower, higher pressure builds up in turbo...... but thats obviously not the case

You're right in saying the gas leaves slower, but remember the slower the gas through the turbine, the slower the turbo spins, the lower the boost.

[Chris Wilson]

You need a pressure differential before and after the turbine, and good flow. hence de catting and none retrictive exhaust raises boost. HOWEVER, a lot of modern engines rely on exhaust restriction to help control boost, as the wastegate port area and flow becomes inadequate with a very none restrictive post turbine system. Jap spec Supra twin turbos are like that, for example.