Originally Posted by Park2670
I believe (just a guess) that the reason for the gains when removing the DPF is from removing the restriction in the exhaust. If you do this on a stock car, obviously the car has been programmed with the knowledge that there would be a DPF restricting flow. If you add a tune that has been programmed for not having the DPF, they can make the engine flow more air, have more boost, rev higher ETC without causing damage to the DPF or the engine.
Just like with the gas cars, if you put on aftermarket downpipes, not much really happens until you tell the computer that it CAN flow more air now. This is the reason that you can put some bolt ons to the N54s and gain 100 HP or more. The factory has to limit air flow with the factory IC, downpipes, intake, etc.
When I worked at the dealer they started a 335d with no mufflers, no resonators, just the DPF. Still as quiet as can be. That DPF really is an exhaust restriction. And the only reason it is there is to meet the restrictions for what comes out of the exhaust.
I do not doubt the dpf might prove to be a tad restrictive when hp is increased, I just don't believe it's as restrictive as some of you would like to think it is.
The reprogram/tune to remove the dpf is just telling the dme thw dpf is no longer there - not to increase airflow. The increase in exhaust gas/heat is a bi-product of increasing the hp and thus reducing back pressure by increasing the diameter of the downpipe ccan prove to be beneficial under certain circumstances eg significant increase in hp via a tune.
Downpipes/cat removal have proven to yield gains in a turbo charged gas car because they are tuned to yield significant hp increases/running much higher boost - more air in, more air out so a larger diameter downpipe is necessary to take full advantage of the tune. Our diesel engines are tuned to make more power in a different fashion - primarily by increasing the amount of fuel being delivered. This in itself does/will increase exhaust gas temps, so allowing that heat to more easily escape has its" benefits - helps to lower egt's, reduces back pressure and yes as a result I suspect a hp bump.
Like I said, I hope removing the dpf does yield 30hp more, but I suspect it will be far less at the expense of an increase in soot/black smoke output.