08-01-2015, 01:28 PM
It's absolutely critical on these motors, I can tell you that from experience!
Remember these engines consume a significant quantity more air, you're running petrol motors at an AFR of about 11:1, these engines you're having to run 18:1 or leaner to keep smoke down, then consider that they're heavily turbocharged... It makes a massive difference to how they respond to exhaust sizing! An XUD9 runs a 2.25" downpipe from the factory and the 16v HDi runs a 2.5" downpipe... These engines need a large amount of airflow and are affected negatively by small exhausts.
2.25 all the way back will support 200hp OK, but I would put a fiver on some horses being released from an exhaust upgrade! Remember with these engines that when you start restricting airflow, smoke levels go up and instantly exhaust gas temperatures rise... It's detrimental to the engines longevity to be running with restrictive exhausts, all sorts of problems start happening.
Put it this way, on my XUD9, there was a full 2.25" system from the downpipe back, if you took it off after the 2.5" downpipe, you'd gain an extra 5-10hp or so, simply from the restriction of the 2.25" pipe back... That's only making somewhere in the region of 175hp!
Remember these engines consume a significant quantity more air, you're running petrol motors at an AFR of about 11:1, these engines you're having to run 18:1 or leaner to keep smoke down, then consider that they're heavily turbocharged... It makes a massive difference to how they respond to exhaust sizing! An XUD9 runs a 2.25" downpipe from the factory and the 16v HDi runs a 2.5" downpipe... These engines need a large amount of airflow and are affected negatively by small exhausts.
2.25 all the way back will support 200hp OK, but I would put a fiver on some horses being released from an exhaust upgrade! Remember with these engines that when you start restricting airflow, smoke levels go up and instantly exhaust gas temperatures rise... It's detrimental to the engines longevity to be running with restrictive exhausts, all sorts of problems start happening.
Put it this way, on my XUD9, there was a full 2.25" system from the downpipe back, if you took it off after the 2.5" downpipe, you'd gain an extra 5-10hp or so, simply from the restriction of the 2.25" pipe back... That's only making somewhere in the region of 175hp!
