When does a 2.5" exhaust become a restriction?

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martin33100

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In general at what power levels can a 2.5" exhaust become a restriction?.
I can measure manifold EMP but I want to add a test point into my current downpipe as well to see actually how much back pressure I have in the exhaust.
Vehicle is a tuned VW T5 2.5PDvan, the exhaust on a van is obviously longer than say a golf.

I usually use a 4x4 dyno so I haven't been stood at the back while it's being dyno'd, yesterday we use a local 2WD dyno so I was at the rear.
All I can say is the exhaust made a similar rushing of air kind of sound like I have heard before on tuned petrol 1.8T's/2.0TFSI's when there exhausts get restrictive.
 
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martin33100

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I will have a look later, I have read some of the David Vizard stuff before as I was building an A series.
Most of his stuff is petrol though.
I still want to measure back pressure in my current downpipe so I am not waisting money on a bigger 2.75 or 3" system.
 
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martin33100

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red16vdub said:
This is actually a great suggestion, just disconnect it after the down pipe which is probably easiest.


Bajan
Click to expand...
Before I remove the downpipe to add the test point I was going to disconnect the downpipe from the system and log EGT's.
I am only thinking of going bigger on the exhaust to see if EGT's go down slightly.
 
CNGVW

CNGVW

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At no time on this forum has anyone shown A stock or super build TDI to make more power with a 3-inch ex-system then a 2.5-inch system!!
There are dynos everywhere so those that build this big system should step up and show us some real back to back dyno tests.
I did do my own test on the Road Race Beetle with a controlled dump just after the turbo and felt no power gain.
Bob Mann
 
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[486]

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I just recently went to 3", mainly because that's what the outlet V-band was off the turbine housing.

Dunno man.
running a muffler, cat, dpf?
 
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martin33100

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CNGVW said:
At no time on this forum has anyone shown A stock or super build TDI to make more power with a 3-inch ex-system then a 2.5-inch system!!
There are dynos everywhere so those that build this big system should step up and show us some real back to back dyno tests.
I did do my own test on the Road Race Beetle with a controlled dump just after the turbo and felt no power gain.
Bob Mann
Click to expand...
I am guessing you are running big power on your Race Beetle?.
 
Festa

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I can say that when I was with Martin on Saturday the exhaust was screaming like a girl to the point you had to walk away from the back of the van as it hurt your ears. Mine running the same power with a 3" pipe didn't make the same noise! Mines a straight through with no box's and a long wheel base but not the same turbo..
 
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Yourbuddysatin

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I know on trucks bigger the exhaust lowers egts. I had the factory 3.5 on my Cummins and went to 5” and saw a drop. Not sure if there was ever a power gain.
 
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Yourbuddysatin

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I have a golf I’m in the process of building and should be over 300hp at the wheels. I have the whole gtb2260 kit from dark side on it with their 3” down pipe. The goal of that power is I’m sure why they use 3” instead of 2.5. My Jetta on the other hand only making about 180hp has 2.5 which I’m sure is plenty for that set up.
 
turbobrick240

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I'd guess that a 3" system might add 1 hp @ 250 hp, and maybe 5 hp @ 300 hp vs. a 2.5" setup.
 
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martin33100

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Fix_Until_Broke said:
Generally speaking, if you're using bolt on parts (no welders involved :)) my opinion is that 2.5" is "sufficient". 3" will be louder, heavier and more expensive for marginal/minimal (if any) gains.

Here's some data behind that opinion

http://forums.tdiclub.com/showthread.php?t=205022
Click to expand...
I have just looked at that page and there is some very useful info in there, really I am trying to lower EGT's, currently peaking around 900-950 if you do a WOT in 4th and then 5th, 6th etc.
The company that built my exhaust do offer a 3" and said one customer found lower EGT's when they changed from 2.5-3"
 
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Yourbuddysatin

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Like I stated earlier on my Cummins going larger lowered my EGTs. 900-590 isn’t too bad for WOT in my opinion. A friend has a tubular manifold and 3” on his alh. Going from a vnt17/22 to that set up we saw a couple hundred degree drop. I don’t think there is much of a hp gain as there is lowering EGTs.
 
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martin33100

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Yourbuddysatin said:
Like I stated earlier on my Cummins going larger lowered my EGTs. 900-590 isn’t too bad for WOT in my opinion. A friend has a tubular manifold and 3” on his alh. Going from a vnt17/22 to that set up we saw a couple hundred degree drop. I don’t think there is much of a hp gain as there is lowering EGTs.
Click to expand...
My 900-950 is degs C.
 
TDIJetta99

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I'm at 255whp and I didn't notice much of anything as far as high rpm power goes from stepping up to 3", but I did see slightly lower EGT (~100F@5500) so I feel it was worth it.. It sounds pretty neat too..
 
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martin33100

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Thanks for the reply's guys, it's the same old story, I originally wasn't going to run this power so I bought PPT's 2.5" downpipe and system, they do offer a 3".
I started off with tubular manifold, Wagner intercooler and a bigger compressor wheel 44/60 in my gt2056v hybrid and got 245hp.
Then I had the turbo machined to take a bigger turbine and power went up to 280hp.
And then recently I had the turbo machined again to take a slightly bigger again turbine again and a 46.5/60 compressor wheel and Firad plus 50's, power went up to 300hp on diesel and 335hp with water meths.
Currently can't really run any extra diesel due to EGT's so it would be good if a 3" exhaust helps.
I have also thought of going in the middle at 2.75" like the twin turbo 3L BMW's, they can run big power on that size.
 
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