Turbo Diesel Exhaust Diameter
Discussion
Hi,
I've got a 102bhp VW T4 2.5 TDi which has been remapped to 148bhp on a 2" dia exhaust.
It currently makes boost / power at 2k but I'd like to get this lower if possible and with 250bhp petrol turbo cars I've seen great benefits using 3" downpipes / systems both in power and getting boost earlier.
I can't seem to find much info on turbo diesel tuning - would running a 2.5" exhaust cause any negative results by reducing exhaust temps etc ?
Thanks
EDITED SIZE (poor memory)
I've got a 102bhp VW T4 2.5 TDi which has been remapped to 148bhp on a 2" dia exhaust.
It currently makes boost / power at 2k but I'd like to get this lower if possible and with 250bhp petrol turbo cars I've seen great benefits using 3" downpipes / systems both in power and getting boost earlier.
I can't seem to find much info on turbo diesel tuning - would running a 2.5" exhaust cause any negative results by reducing exhaust temps etc ?
Thanks
EDITED SIZE (poor memory)
Edited by V8RX7 on Wednesday 28th August 13:57
Thanks for the replies, there is no cat fitted and only straight through type boxes and the exhaust length has been halved.
It's still quiet.
I'm aware there may well be no more power but I was hoping further reducing back pressure might make the turbo a little more responsive.
The gain from std to now was huge but after 2yrs I just want a bit more.
It's still quiet.
I'm aware there may well be no more power but I was hoping further reducing back pressure might make the turbo a little more responsive.
The gain from std to now was huge but after 2yrs I just want a bit more.
Sorry guys, just checked and my exhaust is only 2" as is the std downpipe.
I've been googling and it seems that the 96-01 2.5D Volvos used the same VW 5 cyl making 140bhp and they used a 2.5" downpipe (unsure about system size)
So it would seem to make sense to upgrade the lot to 2.5"
It won't cost much more than £150ish (after I've sold my old one) so it seems worth a try.
I've been googling and it seems that the 96-01 2.5D Volvos used the same VW 5 cyl making 140bhp and they used a 2.5" downpipe (unsure about system size)
So it would seem to make sense to upgrade the lot to 2.5"
It won't cost much more than £150ish (after I've sold my old one) so it seems worth a try.
Have a think about this:
What is the back pressure,post turbine, when the turbo isn't spooled up?
Found the fundamental flaw yet?
when you have no turbine work, you have no compressor work, hence no compressor pressure ratio, hence engine mass flow is low. Exhaust back pressure linearly increases with mass flow. So before the turbo has "spooled" there is no mass flow to cause a restriction to that spooling......
What is the back pressure,post turbine, when the turbo isn't spooled up?
Found the fundamental flaw yet?
when you have no turbine work, you have no compressor work, hence no compressor pressure ratio, hence engine mass flow is low. Exhaust back pressure linearly increases with mass flow. So before the turbo has "spooled" there is no mass flow to cause a restriction to that spooling......
V8RX7 said:
Ok
BUT - why did my 200bhp Greddy turbo MX5 make full boost at 2800rpm with a std exhaust but that dropped to 1800rpm when a full 3" exhaust was fitted ?
Sorry, but, it didn't......... You may have convinced yourself it may that much of a difference, but i'm afraid there is no physical effects that could create that kind of difference in boost threshold. Unless you ran the engine on an engine dyno, under controlled conditions, then it's a meaningless conclusion. For example, try the following:BUT - why did my 200bhp Greddy turbo MX5 make full boost at 2800rpm with a std exhaust but that dropped to 1800rpm when a full 3" exhaust was fitted ?
1) idle drive (i.e. no accelerator) along a flat road in 1st gear. Floor the accelerator pedal quickly. At what engine rpm did the turbo make full boost?
2) repeat same test in 5th gear. At what rpm did the turbo now make full boost
I haven't got the car anymore but I compared it back to back at the time by rolling along at 10mph then flooring it in 2nd.
Similar results are well documented on the forums - people get huge reductions in rpm by using big downpipes / exhausts (bare in mind these are 115bhp NA cars as std).
I'd assume that as the turbo starts to work, pressure builds up which slows the turbine.
I'm not one to say there were benefits if there weren't - on my PD supercharged MX5 it makes no difference (within reason) what exhaust is fitted.
Similar results are well documented on the forums - people get huge reductions in rpm by using big downpipes / exhausts (bare in mind these are 115bhp NA cars as std).
I'd assume that as the turbo starts to work, pressure builds up which slows the turbine.
I'm not one to say there were benefits if there weren't - on my PD supercharged MX5 it makes no difference (within reason) what exhaust is fitted.
V8RX7 said:
I haven't got the car anymore but I compared it back to back at the time by rolling along at 10mph then flooring it in 2nd.
Similar results are well documented on the forums - people get huge reductions in rpm by using big downpipes / exhausts (bare in mind these are 115bhp NA cars as std).
As Max says, this simply isn't going to happen. Unless the downpipe is grossly undersized than it's not going to make a blind bit of difference to the boost threshold. What people on forums think, or say is happening is most often unrelated to reality.Similar results are well documented on the forums - people get huge reductions in rpm by using big downpipes / exhausts (bare in mind these are 115bhp NA cars as std).
Mr2Mike said:
Unless the downpipe is grossly undersized
But I think that is the case.In the case of the MX5 Turbos it was designed for a 1.6 115bhp NA not a 200-280 bhp turbo.
Similarly my van - it was designed for 102bhp it's currently running 148bhp. The same engine in the Volvo 2.5D rated at 140bhp has a 2.5" downpipe.
V8RX7 said:
But I think that is the case.
In the case of the MX5 Turbos it was designed for a 1.6 115bhp NA not a 200-280 bhp turbo.
Similarly my van - it was designed for 102bhp it's currently running 148bhp. The same engine in the Volvo 2.5D rated at 140bhp has a 2.5" downpipe.
An orifice only becomes a significant restriction above a certain flow rate and with the engine running off boost the exhaust flow is much lower than when it's on boost. If the pipe was so grossly undersized to the point that it was affecting the boost threshold then the engine would be absolutely strangled and making a small fraction of the power it should be. My 200bhp car only has downpipe just over 2 inches wide for example.In the case of the MX5 Turbos it was designed for a 1.6 115bhp NA not a 200-280 bhp turbo.
Similarly my van - it was designed for 102bhp it's currently running 148bhp. The same engine in the Volvo 2.5D rated at 140bhp has a 2.5" downpipe.
Personally I would not change the exhaust pipe. The pipe will only influence things when you run maximum mass flow rate which on a turbo diesel is going to be when the turbo is spinning the fastest (note, subtly different to maximum engine rpm).
What you are trying to do is not going to be easy or cheap - a minimum of a new turbo and mapping at the bottom end of the rev range will be required.
What you are trying to do is not going to be easy or cheap - a minimum of a new turbo and mapping at the bottom end of the rev range will be required.
Your exhaust behind your turbo won't change the spool unless there was a serious issue with the smaller one... A 2inch exhaust will be more than adequate for 200bhp.
By all means upgrade to the larger pipe work as post turbo you can have pretty much what you want without effecting engine performance As long as its NOT restrictive in some way.
Why your old engine had a massive change to spool after the exhaust change will be a mystery, but unless there is something that's not known or being mentioned your highly unlikely to see the same results.
Do you have the same turbo as these people you have found on google?
By all means upgrade to the larger pipe work as post turbo you can have pretty much what you want without effecting engine performance As long as its NOT restrictive in some way.
Why your old engine had a massive change to spool after the exhaust change will be a mystery, but unless there is something that's not known or being mentioned your highly unlikely to see the same results.
Do you have the same turbo as these people you have found on google?
If you want mega power from a diesel then fit LPG, this has the same results as NOZ does in petrol engine's.
Too much & your engine will be melted, i quickly went through two sport's clutch's, two planet gears in Diff & snapped D.shafts. A Quaife diff will not help just gentle throttle application when its switched on or else this happens
... not me but I should not have told wifey about the button.
[/URL]
Was a fast car!
Too much & your engine will be melted, i quickly went through two sport's clutch's, two planet gears in Diff & snapped D.shafts. A Quaife diff will not help just gentle throttle application when its switched on or else this happens
... not me but I should not have told wifey about the button.[/URL]Was a fast car!
530d,e60...ive done a DPF removel and its been fited with a stage 1 remap,really pleased of the car and the way it performed after the remap....after that i decidet to instal a 3" downpipe witch i got from ebay and after that it seems that the car has actually lose torque and it doesnt seem as responsive as before ,removed the back exhaust box as well but the car doesnt performe as before the downpipe was fitted....Any ideas of why this might be hapenning?
From my limited experience of diesels, reducing the restrictions after the turbo allows it to build boost earlier, our 1.9pd 130 used to hit 2k then take off, cat gutted boost builds from 1750-1800 and most of the time allows a higher gear to be used.2.4 Toyota Hilux surf also liked a larger exhaust to standard
Personally a 3 inch exhaust from the turbo back would make quite a bit of difference
Personally a 3 inch exhaust from the turbo back would make quite a bit of difference
ssray said:
From my limited experience of diesels, reducing the restrictions after the turbo allows it to build boost earlier, our 1.9pd 130 used to hit 2k then take off, cat gutted boost builds from 1750-1800 and most of the time allows a higher gear to be used.2.4 Toyota Hilux surf also liked a larger exhaust to standard
Personally a 3 inch exhaust from the turbo back would make quite a bit of difference
Personally a 3 inch exhaust from the turbo back would make quite a bit of difference
Yes there is a big difference between a diesel and a petrol due to the lack of a throttle butterfly a diesel will pump more air so it stands to reason a small increese in exhaust dia should help airflow and also reduse EGT which will help reliability I was always told an exhaust should be about 1 inch per litre on small automotive diesels
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