Reply With QuoteTrouble is Rick that I think half the posters thought Jock was fitting an exhaust to the OME garrett,thats where the 2.5'' size has come from but the VNT has a 3'' outlet so we weren't all on the same page. Pat
Moderator
I wasn't going to comment on this thread, most everything that needed saying had been said, oh well.......Originally Posted by PAT303
Hi Pat, all that Ben, John etc. are saying is that you can't oversize a turbo diesel exhaust.
A 3" system has no negative impact on the torque curve of a Tdi engine but it does allow the turbo to spool more quickly.
Building my 3" system enhanced the bottom end, off idle pull away was much improved.
The major downside IMO is the noise factor. You do need a good muffler and resonator, anyone that says a Tdi doesn't need a muffler on a 3" pipe has never heard one uncorked or is stone motherless deaf
I doubt a smaller system would have this issue, if my experience dyno testing naturally aspirated merge collector/dumps is anything to go by.
Would a 2.75" system work as well on a Tdi ?
Sure, but the tubing is much more expensive and harder to find.
How about a 2.5" ?
Probably little difference, except maybe in a longer 130 system and at high revs.
The 3" pipe allowed about 15km/h higher speeds in third and fourth gear over the OE (2,3/8", 60mm ) system, so you can conclude that the OE 130 system, even with an OE straight through muffler was restricted.
Someone earlier on in this thread alluded to a bigger pipe potentially overheating the exhaust manifold and turbine housing.
For the record, my EGT dropped well over 20*C, probably closer to a consistent 25*C at the same ambient, same pump and timing settings (all stock) on my 2.5km long, 200m gain test hill.
IIRC I used a 15* wall/30* included angle diffuser from the dump to the 3" exhaust, and throated the dump to suit the ID of the 2.5" pipe I swaged up to 3".
The 15* angle is an arbitrary one I use as one of my reference books from years ago mentioned that anything over about 15* taper induces flow separation in a duct and therefore turbulence.
OldBushie
Trouble is Rick that I think half the posters thought Jock was fitting an exhaust to the OME garrett,thats where the 2.5'' size has come from but the VNT has a 3'' outlet so we weren't all on the same page. Pat
Swaggie
Exhaust flow (and intake flow) is almost always completely turbulent. A very rough calculation gives an Re of ~90000 for a Tdi at 2000 rpm with a 3" exhaust and 500oC egts. That is above the limit for laminar flow even with the smoothest possible transitions and a laminarisation screen at the inlet (which a turbo is not).
Not at all. On a Turbo diesel engine, an exhaust serves the purpose of controlling noise and directing the exhaust gases behind the vehicle. The manufacturer sizes the exhaust so it will give acceptable flow restriction within cost (and mass) constraints.
Moderator
So I haven't gained, but haven't lost anything
It looks nice though, and the blokes in the workshop I used were intrigued when I was turning up the flaring mandrel and flared the 2.5" tube.
Swaggie
Smooth transitions are always good.So I haven't gained, but haven't lost anything
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