Quote:
Originally Posted by tasdrouille
Overall they surely are, but not just because of back pressure. Every now and then the DPF goes into a regeneration cycle to burn the trapped soot. This uses extra fuel.
Regarding back pressure, on previous generations TDIs, people gutting out everything back of the turbo and replacing it with a straight pipe have not reported increased FE. Probably because the turbo is the biggest restriction in the exhaust system to start with.
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The Californian Emissions model of my car ( which I unfortunately have
) emits about 1 ton more of CO2 a year than the federal emissions model. The only real difference is the californian emissions model has a DPF and gets much worse mileage. This is a year that did not have the equipment to do regen cycle. Also its filtration requirements were much lower than todays filters.
The TDI is not a good example for showing what removing a restriction in the exhaust can do. Its not that the TDI isn't a great car, its that the stock intercoolers on these cars suck. I think it was you that mentioned you measured a 3 psi drop across the intercooler in another thread. I questioned your method of measuring this because it was so unusually high. After doing some research and talking with a TDI guy I know I found out that this is actually true. On a TDI a reduction in exhaust back pressure after the turbo pales in comparison to the restriction caused by the intercooler in the intake. Note the guy I know replaced his stock intercooler with an aftermarket one and his pressure drop is now about 0.7 psi and he gets about 30% better cooling.
I replaced my muffler on my mercedes with a cherry bomb muffler ( essentially a straight pipe)and got about a 0.5mpg improvement ( around 2.5%). This isn't all due to the drop in back pressure its also due to better combustion caused by more fresh air in the cylinder. The better combustion not only means I'm burning more fuel that wasn't but it also doesn't plug my DPF as much.