BMW Turbo- und Kompressor Forum

A turbocharger is suitable or in other words fits an engine if the exhaust back pressure is maximum 1,5 times the intake pressure. For instance, if you read 0,7 bar on the boost gauge (in the manifold) then the exhaust back pressure should be 1 bar. Ideally exhaust back pressure should be equal to the intake manifold pressure.

If you have a very restrictive exhaust turbine housing, dump pipe and exhaust system then back pressure will be 2 or 2,5 times the intake manifold. This is what we experienced.

As back pressure increases, you have less room for the clean air/fuel in the combustion chamber and in the cylinders. Residual gases, meaning burnt fuell/air mixtures will stay in the cylinders and the engine's volumetric efficiency decreases.

Why does a bigger turbocharger add more power? Because with the bigger exhaust turbine housing and dump pipe, scavenging of the cylinders is improved. Less burnt gases stay in and there is more room for clean air/fuel mixtures. Surely, bigger turbochargers flow cooler than smaller turbochargers above certain boost pressures and flow rates. Where the smaller turbos max out, bigger ones keep on flowing efficiently.