Downpipe design for max flow
22 September 1999, 12:34 AM
#1
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Mongoose I think have a problem in that they still have the blanking plate over half of downpipe at the back of the turbo.
As the gases are still mixed further down from the BPM system I dont think the gains will be that great but we are all still waiting to see but it seems it may be some time before we get a back to back test of exhaust systems on the same car and that is the very important point comparing exhausts on different cars is pointelss.
Darren
As the gases are still mixed further down from the BPM system I dont think the gains will be that great but we are all still waiting to see but it seems it may be some time before we get a back to back test of exhaust systems on the same car and that is the very important point comparing exhausts on different cars is pointelss.
Darren
22 September 1999, 04:14 AM
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I've been doing some research downpipes for Wrx. There are mainly 2 designs, one which splits the wastgate from the turbo and the other which is a straight though big bore.
I am rather confused which one is better, especially when Japan rally tuners like HKS and June use the single big bore.
What does prodrive and scoobysport use?
Also, I've been told the the STI's ver 5 has only 1 Cat on the downpipe. The mid-section box is just a see-through silencer and need not be replaced on std car.
Can someone shed some technical light on this issue. Thanks.
PS: I really hope the decision to change to HKS downpipe (3" to 2.5") on my STI 5 was the right one! :-o
I am rather confused which one is better, especially when Japan rally tuners like HKS and June use the single big bore.
What does prodrive and scoobysport use?
Also, I've been told the the STI's ver 5 has only 1 Cat on the downpipe. The mid-section box is just a see-through silencer and need not be replaced on std car.
Can someone shed some technical light on this issue. Thanks.
PS: I really hope the decision to change to HKS downpipe (3" to 2.5") on my STI 5 was the right one! :-o
22 September 1999, 09:00 AM
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The Scoobysport one is designed as a single system without a seperate pipe. The BPM system has a seperate pipe.
Pros cons differences etc no one as yet has done any back to back testing on the same car to provide a comparison.
One important thing to look at with the Downpipes is the entry into the downpipe. If you look at a standard Subaru system then wastegate side of the turbo exit has effectively a metal blanking plate of it and some of the after market systems are like this. The Scoobysport system has a large hole at the top which is oval shaped to cover both the wastegate exit and the turbo exit.
If your car is running standard boost levels then a 3 inch system may well lead to lower torque levels.
The center box on the Type R doesnt have a cat in it but I am not so sure about the other models but this box can seem to create some problems and I am looking to replace mine with a straight through pipe with just a possible small silencer in it rather than the 2 that currently exist in it.
Darren
Pros cons differences etc no one as yet has done any back to back testing on the same car to provide a comparison.
One important thing to look at with the Downpipes is the entry into the downpipe. If you look at a standard Subaru system then wastegate side of the turbo exit has effectively a metal blanking plate of it and some of the after market systems are like this. The Scoobysport system has a large hole at the top which is oval shaped to cover both the wastegate exit and the turbo exit.
If your car is running standard boost levels then a 3 inch system may well lead to lower torque levels.
The center box on the Type R doesnt have a cat in it but I am not so sure about the other models but this box can seem to create some problems and I am looking to replace mine with a straight through pipe with just a possible small silencer in it rather than the 2 that currently exist in it.
Darren
22 September 1999, 10:03 AM
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BPM has informed me that the reason for the Spilt of Wastgate and turbo gases is to minimise turbulance caused by mixing too near the turbo housing.
I am no air-flow expert and would like to know if this theory checks out. Most after market performance turbo exhausts like HKS, June, Scoobysport have a single bore design downpipe. Any idea about mongoose and prodrive?
I am no air-flow expert and would like to know if this theory checks out. Most after market performance turbo exhausts like HKS, June, Scoobysport have a single bore design downpipe. Any idea about mongoose and prodrive?
22 September 1999, 03:32 PM
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Seraph
Can't help with specifics of the Impreza as I'm afraid I just drive around in a standard MY99 so havn't taken it to bits, but hopefully I can explain some ideas behind exhaust tweaking as they follow the same lines for any engine:
For a turbocharged engine with a waste gate, the system will reach a steady-state at a given RPM where the wastegate is chattering and the turbo is spinning.
The gases leaving the turbo turbine will be low pressure and have a high velocity, whereas the bypassed gases through the wastegate will be high pressure and low velocity (qualitative rule of thumb only !).
The problem is that these two gas streams now meet up, and to avoid the wastegate gases (low velocity) effectively blocking the way of the turbine exhaust (which creates a back-pressure and slows the turbo down), the wastegate gas speed needs to be reduced.
You can do this by having a bigger bore for the wastegate or a smaller bore for the turbine or a devious combination of eddy-generation with a single pipe.
One is not necessarily better than the other and there are other effects at play as well, but you can see that the exhaust will effectively be tuned to work best at a given gas speed (i.e. engine load and / or revs).
That's one reason why some exhausts produce more power increases at different revs.
Hope that helps
Russ
Can't help with specifics of the Impreza as I'm afraid I just drive around in a standard MY99 so havn't taken it to bits, but hopefully I can explain some ideas behind exhaust tweaking as they follow the same lines for any engine:
For a turbocharged engine with a waste gate, the system will reach a steady-state at a given RPM where the wastegate is chattering and the turbo is spinning.
The gases leaving the turbo turbine will be low pressure and have a high velocity, whereas the bypassed gases through the wastegate will be high pressure and low velocity (qualitative rule of thumb only !).
The problem is that these two gas streams now meet up, and to avoid the wastegate gases (low velocity) effectively blocking the way of the turbine exhaust (which creates a back-pressure and slows the turbo down), the wastegate gas speed needs to be reduced.
You can do this by having a bigger bore for the wastegate or a smaller bore for the turbine or a devious combination of eddy-generation with a single pipe.
One is not necessarily better than the other and there are other effects at play as well, but you can see that the exhaust will effectively be tuned to work best at a given gas speed (i.e. engine load and / or revs).
That's one reason why some exhausts produce more power increases at different revs.
Hope that helps
Russ
22 September 1999, 03:59 PM
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I got my p's and q's muddled up in my cut-and-paste frenzy on my reply above !
As I mentioned the wastegate gases are high pressure, low velocity so need to be sped up (relative to the turbine gas speed), so the wastegate pipe needs to be a smaller diameter than the turbo exit pipe (or have a suitably-shaped single bore mixing chamber).
Sorry for the confusion !
Russ
As I mentioned the wastegate gases are high pressure, low velocity so need to be sped up (relative to the turbine gas speed), so the wastegate pipe needs to be a smaller diameter than the turbo exit pipe (or have a suitably-shaped single bore mixing chamber).
Sorry for the confusion !
Russ
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22 September 1999, 06:43 PM
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Hi...
This dump system (oooer) for the scoob was used by an Oz guy *I believe*.... he found that on his large turboed/high boost car if he vernted the wastegate gasses seperately from the turbo gases he gained bhp.... I believe he found that he gained upto 30bhp!! But he did vent to atmosphere down by the drivers feet (it didnt join back into the exhaust).
I agree with the thoery... but I personally dont think you will notice any gain if you already have a decent free-flowing exhaust. (that doesnt block of the wastegate gasses - like the standard down pipe or the HKS one)
Most high powered cars dont use the dump pipe (although some do have seperate wastegates).
For most people I dont think they will notice any gain, has anyone dyno'ed an exhaust with a dump pipe for the wastegate and then dyno'ed the same exhaust with the down pipe that hasnt got the seperate pipe ?
Cheers,
J.
ps - I suppose it cant hurt having one designed with it
This dump system (oooer) for the scoob was used by an Oz guy *I believe*.... he found that on his large turboed/high boost car if he vernted the wastegate gasses seperately from the turbo gases he gained bhp.... I believe he found that he gained upto 30bhp!! But he did vent to atmosphere down by the drivers feet (it didnt join back into the exhaust).
I agree with the thoery... but I personally dont think you will notice any gain if you already have a decent free-flowing exhaust. (that doesnt block of the wastegate gasses - like the standard down pipe or the HKS one)
Most high powered cars dont use the dump pipe (although some do have seperate wastegates).
For most people I dont think they will notice any gain, has anyone dyno'ed an exhaust with a dump pipe for the wastegate and then dyno'ed the same exhaust with the down pipe that hasnt got the seperate pipe ?
Cheers,
J.
ps - I suppose it cant hurt having one designed with it
22 September 1999, 11:33 PM
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Seraph,
The Prodrive exhaust is split with a seperate pipe for the wastegate. Some companies tend to use a seperator or a very short pipe from the turbo and wastegate exit. It has been shown the longer you can keep the two streams of gas seperate the better the flow. It makes sense and works.
PS Firefox, the Aussie guy you are reffering to is Ross Wilson who is in our parent company , AVO.
If you have any further queries , please do not hesitate to email me or we can chat online with
ICQ (45940764)
Greg Nikolettos
BPM/AVO Marketing
Phone: (617) 3272-8885
Mobile: 0410 678 989
Fax: (617) 3218 8880
The AVO website will be moving to
The Prodrive exhaust is split with a seperate pipe for the wastegate. Some companies tend to use a seperator or a very short pipe from the turbo and wastegate exit. It has been shown the longer you can keep the two streams of gas seperate the better the flow. It makes sense and works.
PS Firefox, the Aussie guy you are reffering to is Ross Wilson who is in our parent company , AVO.
If you have any further queries , please do not hesitate to email me or we can chat online with
ICQ (45940764)
Greg Nikolettos
BPM/AVO Marketing
Phone: (617) 3272-8885
Mobile: 0410 678 989
Fax: (617) 3218 8880
The AVO website will be moving to
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