when does the standard,de-catted exhaust get restrictive?
11 July 2007, 08:10 PM
#1
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when does the standard,de-catted exhaust get restrictive?
As title, if i was to decat the whole of my standard exhaust at what point in BHP does it start to get restictive?
Also im guessing a standard WRX 02 plate is 2.5" Dia??
thanks for your help.
steve
Also im guessing a standard WRX 02 plate is 2.5" Dia??
thanks for your help.
steve
12 July 2007, 09:10 AM
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I think the stock WRX 02 is 60mm, when you say stock decatted, I hope that means you will insert tubes and not just smash out the cat, as the expansion/contraction through the empty cat cans creates almost as much flow restriction as a cat!
60mm isn't significantly restrictive to about 240bhp.
Simon
60mm isn't significantly restrictive to about 240bhp.
Simon
12 July 2007, 07:58 PM
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I think the stock WRX 02 is 60mm, when you say stock decatted, I hope that means you will insert tubes and not just smash out the cat, as the expansion/contraction through the empty cat cans creates almost as much flow restriction as a cat!
60mm isn't significantly restrictive to about 240bhp.
Simon
60mm isn't significantly restrictive to about 240bhp.
Simon
Yeah it has de-cat pipes in place lol, so only 240bhp??
bugger.
12 July 2007, 08:30 PM
#5
60mm is 30mm radius. 2.5" is 31.75mm radius.
Scoobynet lore suggests that 2.5" is good for up to 350 bhp, although john banks didn't notice any "pants to the seat"-dyno differences on his 400bhp scooby when changing from 2.5" to 3" (There's some thread floating about on 22b.com about it).
Assuming (and this may be wrong) that the flow is reasonably proportional to the cross sectional area, we get the following:
Cross section area for 60mm: 30*30*3.14 = 2826mm^2
Cross section area for 2.5": 31.75*31.75*3.14 = 3165mm^2
Now, let's calculate bhp per mm^2 for the 2.5" system: 350/3165 = 0.111 bhp/mm^2
The 60mm pipe would then be able to sustain 0.111*2826 bhp = 313bhp.
The above are only musings of a rambling idiot. They may or may not be representative for the real world. Also, it might be that e.g. the back box and centre resonators etc are more restrictive in nature than the "performance" silencers you can buy (e.g. due to different baffle design etc), which may further restrict the point where the stock exhaust becomes restrictive.
However, the standard STI pipes are the same diameter, as far as I know, and the stock 02 STI produces 265bhp AFAIK, and I'm sure it's not engineered to be exactly on the limit where it becomes restrictive.
Having said that, there isn't a discrete point where the exhaust becomes restrictive, it's more of a gradual change, so as you get into higher bhp numbers, you may find that the exhaust holds you back more and more.
If I was doing my exhaust again, I'd see what I could get with the stock exhaust, to be honest (assuming I wasn't aiming for much more than 300 ponies), and use the saved money for some suspension and/or brakes
Scoobynet lore suggests that 2.5" is good for up to 350 bhp, although john banks didn't notice any "pants to the seat"-dyno differences on his 400bhp scooby when changing from 2.5" to 3" (There's some thread floating about on 22b.com about it).
Assuming (and this may be wrong) that the flow is reasonably proportional to the cross sectional area, we get the following:
Cross section area for 60mm: 30*30*3.14 = 2826mm^2
Cross section area for 2.5": 31.75*31.75*3.14 = 3165mm^2
Now, let's calculate bhp per mm^2 for the 2.5" system: 350/3165 = 0.111 bhp/mm^2
The 60mm pipe would then be able to sustain 0.111*2826 bhp = 313bhp.
The above are only musings of a rambling idiot. They may or may not be representative for the real world. Also, it might be that e.g. the back box and centre resonators etc are more restrictive in nature than the "performance" silencers you can buy (e.g. due to different baffle design etc), which may further restrict the point where the stock exhaust becomes restrictive.
However, the standard STI pipes are the same diameter, as far as I know, and the stock 02 STI produces 265bhp AFAIK, and I'm sure it's not engineered to be exactly on the limit where it becomes restrictive.
Having said that, there isn't a discrete point where the exhaust becomes restrictive, it's more of a gradual change, so as you get into higher bhp numbers, you may find that the exhaust holds you back more and more.
If I was doing my exhaust again, I'd see what I could get with the stock exhaust, to be honest (assuming I wasn't aiming for much more than 300 ponies), and use the saved money for some suspension and/or brakes
12 July 2007, 09:22 PM
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60mm is 30mm radius. 2.5" is 31.75mm radius.
Scoobynet lore suggests that 2.5" is good for up to 350 bhp, although john banks didn't notice any "pants to the seat"-dyno differences on his 400bhp scooby when changing from 2.5" to 3" (There's some thread floating about on 22b.com about it).
Assuming (and this may be wrong) that the flow is reasonably proportional to the cross sectional area, we get the following:
Cross section area for 60mm: 30*30*3.14 = 2826mm^2
Cross section area for 2.5": 31.75*31.75*3.14 = 3165mm^2
Now, let's calculate bhp per mm^2 for the 2.5" system: 350/3165 = 0.111 bhp/mm^2
The 60mm pipe would then be able to sustain 0.111*2826 bhp = 313bhp.
The above are only musings of a rambling idiot. They may or may not be representative for the real world. Also, it might be that e.g. the back box and centre resonators etc are more restrictive in nature than the "performance" silencers you can buy (e.g. due to different baffle design etc), which may further restrict the point where the stock exhaust becomes restrictive.
However, the standard STI pipes are the same diameter, as far as I know, and the stock 02 STI produces 265bhp AFAIK, and I'm sure it's not engineered to be exactly on the limit where it becomes restrictive.
Having said that, there isn't a discrete point where the exhaust becomes restrictive, it's more of a gradual change, so as you get into higher bhp numbers, you may find that the exhaust holds you back more and more.
If I was doing my exhaust again, I'd see what I could get with the stock exhaust, to be honest (assuming I wasn't aiming for much more than 300 ponies), and use the saved money for some suspension and/or brakes
Scoobynet lore suggests that 2.5" is good for up to 350 bhp, although john banks didn't notice any "pants to the seat"-dyno differences on his 400bhp scooby when changing from 2.5" to 3" (There's some thread floating about on 22b.com about it).
Assuming (and this may be wrong) that the flow is reasonably proportional to the cross sectional area, we get the following:
Cross section area for 60mm: 30*30*3.14 = 2826mm^2
Cross section area for 2.5": 31.75*31.75*3.14 = 3165mm^2
Now, let's calculate bhp per mm^2 for the 2.5" system: 350/3165 = 0.111 bhp/mm^2
The 60mm pipe would then be able to sustain 0.111*2826 bhp = 313bhp.
The above are only musings of a rambling idiot. They may or may not be representative for the real world. Also, it might be that e.g. the back box and centre resonators etc are more restrictive in nature than the "performance" silencers you can buy (e.g. due to different baffle design etc), which may further restrict the point where the stock exhaust becomes restrictive.
However, the standard STI pipes are the same diameter, as far as I know, and the stock 02 STI produces 265bhp AFAIK, and I'm sure it's not engineered to be exactly on the limit where it becomes restrictive.
Having said that, there isn't a discrete point where the exhaust becomes restrictive, it's more of a gradual change, so as you get into higher bhp numbers, you may find that the exhaust holds you back more and more.
If I was doing my exhaust again, I'd see what I could get with the stock exhaust, to be honest (assuming I wasn't aiming for much more than 300 ponies), and use the saved money for some suspension and/or brakes
thanks also for a very informative post
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13 July 2007, 11:02 AM
#8
60mm is 30mm radius. 2.5" is 31.75mm radius.
Assuming (and this may be wrong) that the flow is reasonably proportional to the cross sectional area, we get the following:
Cross section area for 60mm: 30*30*3.14 = 2826mm^2
Cross section area for 2.5": 31.75*31.75*3.14 = 3165mm^2
Now, let's calculate bhp per mm^2 for the 2.5" system: 350/3165 = 0.111 bhp/mm^2
The 60mm pipe would then be able to sustain 0.111*2826 bhp = 313bhp.
The above are only musings of a rambling idiot. They may or may not be representative for the real world. Also, it might be that e.g. the back box and centre resonators etc are more restrictive in nature than the "performance" silencers you can buy (e.g. due to different baffle design etc), which may further restrict the point where the stock exhaust becomes restrictive.
However, the standard STI pipes are the same diameter, as far as I know, and the stock 02 STI produces 265bhp AFAIK, and I'm sure it's not engineered to be exactly on the limit where it becomes restrictive.
Having said that, there isn't a discrete point where the exhaust becomes restrictive, it's more of a gradual change, so as you get into higher bhp numbers, you may find that the exhaust holds you back more and more.
If I was doing my exhaust again, I'd see what I could get with the stock exhaust, to be honest (assuming I wasn't aiming for much more than 300 ponies), and use the saved money for some suspension and/or brakes
Assuming (and this may be wrong) that the flow is reasonably proportional to the cross sectional area, we get the following:
Cross section area for 60mm: 30*30*3.14 = 2826mm^2
Cross section area for 2.5": 31.75*31.75*3.14 = 3165mm^2
Now, let's calculate bhp per mm^2 for the 2.5" system: 350/3165 = 0.111 bhp/mm^2
The 60mm pipe would then be able to sustain 0.111*2826 bhp = 313bhp.
The above are only musings of a rambling idiot. They may or may not be representative for the real world. Also, it might be that e.g. the back box and centre resonators etc are more restrictive in nature than the "performance" silencers you can buy (e.g. due to different baffle design etc), which may further restrict the point where the stock exhaust becomes restrictive.
However, the standard STI pipes are the same diameter, as far as I know, and the stock 02 STI produces 265bhp AFAIK, and I'm sure it's not engineered to be exactly on the limit where it becomes restrictive.
Having said that, there isn't a discrete point where the exhaust becomes restrictive, it's more of a gradual change, so as you get into higher bhp numbers, you may find that the exhaust holds you back more and more.
If I was doing my exhaust again, I'd see what I could get with the stock exhaust, to be honest (assuming I wasn't aiming for much more than 300 ponies), and use the saved money for some suspension and/or brakes
You're a bit like me - enjoy a bit of practical maths. ( What I wouldn't do to Carol Vordeman
)JohnD
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