Post by ThatChinskiGuy on Jan 5, 2015 12:28:18 GMT 10
Forester Exhaust and Up-pipe Info
What does a GT/XT exhaust look like?
The exhaust typically consists of three sections, being the dump-pipe (which starts at the turbo exhaust housing), centre pipe, and diff-back/axle back (you guessed it… it’s the rear-most part of the exhaust, including the rear muffler).
This diagram gives a good visual representation of a MY01+ Forester turbo exhaust system. The main difference with the MY98-00 is that the rear 02 sensor is at the top of the dump pipe, and there is no cat in the up-pipe.
Will a WRX/STI exhaust bolt onto my turbo Forester?
The dump and centre pipes will generally fit fine. Sometimes you may need to fit a new 02 sensor bung and plug the old one depending upon your year model (turbo WRXs and Foresters up to MY00 have the 02 sensor near the turbo; MY01+ have the 02 sensor down underneath the floor near the join between the dump and centre pipes). Be warned - early model dumps may have clearance issues with the steering mechanism on late model XTs.
Any WRX/STI diff-back will always require modification. You need to do a combination of things – adding 100mm length to the diff-back pipework or the exhaust tip, as well as either lengthening the exhaust hangers (or running extended Kartboy rubber hangers) or modifying the tip to clear the rear bumper.
What should you look for in an exhaust?
There's a number of things to consider:
* Design of dump-pipe - the factory item has a flat plate immediately after the turbo. This directs wastegate exhaust gasses directly into the path of the exhaust gasses that have just exited the turbo. This creates a bit of turbulence which is what we are trying to avoid. Aftermarket dump pipes come in bell-mouth (with or without splitter) or divorced wastegate setups that separate the wastegate gasses from the main exhaust gasses to help minimise turbulence and back-pressure. Here are some pics:
Flat plate dump:
bell-mouth dump (open):
bell-mouth dump (splitter):
Note: the splitter lengths vary depending upon which turbo you are running.
Sleeve bearing VF series turbos (VF30/35/39) require a 27mm splitter.
Ball/roller bearing VF series turbos (VF22, 23, 24, 28, 29, 34) require a 33mm splitter.
TF035/TD04/TD05 turbos require a 22mm splitter.
divorced/separated wastegate dump (note the separate tube for wastegate gasses):
Generally-speaking, a bellmouth and splitter dump does is the best combination of overall flow. The functionality of a divorced wastegate setup really depends on the design and diameter of the seperate wastegate pipe. If it is too small it can present excessive backpressure at the wastegate flap, causing boost creep. This becomes worse on high powered examples. Be sure to thoroughly research any divorced wastegate dump to ensure it is hig quality and suits your needs.
Splitter design is also important. The wastegate doesn't open at a 90 degree angle, so this means that the wastegate gasses do not exit in a uniform manner. Accordingly, the splitter should seal against the inside of the turbo housing to prevent wastegate gasses from leaking into the path of the gasses passing through the exhaust turbine:
* Style and location of cat converter - some exhausts use ceramic substrate cats whereas others use high-flow metal substrate cats. Good quality metal cats flow very well and tend to be less fragile than ceramic substrate cats. Some exhausts have the cat in the dump immediately after the turbo, whilst others have the cat under the body. Victoria is particularly strict regarding the location and number of cats required in an aftermarket or replacement exhaust system. Be sure to check what applies for your state/area.
round metal cat:
Ceramic cat left, metal cat right:
Ceramic:
Metal:
* Ground clearance - some aftermarket exhausts hang low and don't hug the underbody of the car. This means that you are more likely to scrape your exhaust on a lowered car. Most exhausts will hug the underbody fairly well, especially custom exhausts that can be made to suit your car perfectly.
* Resonators - some exhausts will have a centre resonator to help reduce noise levels or to refine the sound of the exhaust. Hot-dog resonators come in different styles which can either reduce noise or help reduce a raspy exhaust note. Large flat oval-style resonators are generally used for noise reduction purposes only.
Hotdog-style resonator:
Flat oval-style resonator:
* Noise levels and droning - generally speaking, cannons are loud! However, high quality Japanese cannons can be quite subdued. High quality straight through oval mufflers provide a decent balance of flow vs noise. Baffled rear mufflers can flow well if designed properly, whilst offering virtually no drone and a nice subdued burble. Adjustable mufflers like the X-Force Varex are an option (they can switch from loud to factory-silent) but bear in mind that the authorities are cracking down on these (they will be illegal shortly).
* Pipework diameter - the average aftermarket Subaru exhaust is 3" diameter and this generally works quite well. Research and development undertaken by Michael South (MSR) has suggested that reducing the pipework to 2.5" for the diff-back section helps to maintain exhaust gas velocity (many aftermarket exhausts are now replicating this). A 2.5" diff-back also helps to reduce noise levels a bit. www.msengineering.com.au/exhaustsystems.html
* Tip style - whatever tickles your fancy!
What is an up-pipe and when do I need to replace it?
The up-pipe connects the headers to the turbo (refer to the above pic).
MY01-02 GT Foresters and 04-05 XT Foresters have a catalytic converter in the up-pipe for cold-start emissions. The catalytic converter on these models is known to break down on high kilometer or modified examples, which results in pieces of catalytic converter making their way through the exhaust turbine of the turbo. This will cause turbo damage or failure. Accordingly, it is best to replace the catted up-pipe whenever you mod one of these models. A factory catless up-pipe from an 98-00 GT or 06+ XT or 97-00 or 03+ WRX or any STI is the best option as the pipe diameter promotes the best turbo spool. The factory up-pipe diameter is approx. 41-43mm at each end. If you are forced to fit an aftermarket up-pipe, try to find one which is a similar diameter to the above. Avoid the large diameter ebay or aftermarket up-pipes which are 50mm ID and above as these slow down the exhaust gas velocity which is something you want to avoid at all costs!
Catless up-pipes also have an exhaust gas temperature sensor prior to the cat. If the exhaust gas temp gets high enough to potentially cause issues with the up-pipe cat converter, the ECU will enrichen the mixtures to try and bring temps under control. If you are replacing a catted up-pipe with a catless version, the EGT sensor becomes redundant and is best removed. If your car is being reflashed you can make some changes to ensure that a check engine light does not appear. Alternatively, you can insert a 2.2kohm 0.5W resistor in the wiring plug and the ECU will remain happy.
Here's a pic of a factory catted up-pipe on the left, and a factory catless up-pipe on the right:
You can see the cat substrate and the temperature sensor in this pic:
A few measurements of the up-pipe internal diameter (it is about 41-43mm):
MY09 XT exhausts
The MY09 has a different exhaust setup to the earlier models, so the exhausts are not interchangeable.
Hitech have recently released a MY09 XT specific exhaust:
www.hitechmufflers.com.au/product/index.php?m=Forrester+GT+%2F+XT&b=subaru
What does a GT/XT exhaust look like?
The exhaust typically consists of three sections, being the dump-pipe (which starts at the turbo exhaust housing), centre pipe, and diff-back/axle back (you guessed it… it’s the rear-most part of the exhaust, including the rear muffler).
This diagram gives a good visual representation of a MY01+ Forester turbo exhaust system. The main difference with the MY98-00 is that the rear 02 sensor is at the top of the dump pipe, and there is no cat in the up-pipe.
Will a WRX/STI exhaust bolt onto my turbo Forester?
The dump and centre pipes will generally fit fine. Sometimes you may need to fit a new 02 sensor bung and plug the old one depending upon your year model (turbo WRXs and Foresters up to MY00 have the 02 sensor near the turbo; MY01+ have the 02 sensor down underneath the floor near the join between the dump and centre pipes). Be warned - early model dumps may have clearance issues with the steering mechanism on late model XTs.
Any WRX/STI diff-back will always require modification. You need to do a combination of things – adding 100mm length to the diff-back pipework or the exhaust tip, as well as either lengthening the exhaust hangers (or running extended Kartboy rubber hangers) or modifying the tip to clear the rear bumper.
What should you look for in an exhaust?
There's a number of things to consider:
* Design of dump-pipe - the factory item has a flat plate immediately after the turbo. This directs wastegate exhaust gasses directly into the path of the exhaust gasses that have just exited the turbo. This creates a bit of turbulence which is what we are trying to avoid. Aftermarket dump pipes come in bell-mouth (with or without splitter) or divorced wastegate setups that separate the wastegate gasses from the main exhaust gasses to help minimise turbulence and back-pressure. Here are some pics:
Flat plate dump:
bell-mouth dump (open):
bell-mouth dump (splitter):
Note: the splitter lengths vary depending upon which turbo you are running.
Sleeve bearing VF series turbos (VF30/35/39) require a 27mm splitter.
Ball/roller bearing VF series turbos (VF22, 23, 24, 28, 29, 34) require a 33mm splitter.
TF035/TD04/TD05 turbos require a 22mm splitter.
divorced/separated wastegate dump (note the separate tube for wastegate gasses):
Generally-speaking, a bellmouth and splitter dump does is the best combination of overall flow. The functionality of a divorced wastegate setup really depends on the design and diameter of the seperate wastegate pipe. If it is too small it can present excessive backpressure at the wastegate flap, causing boost creep. This becomes worse on high powered examples. Be sure to thoroughly research any divorced wastegate dump to ensure it is hig quality and suits your needs.
Splitter design is also important. The wastegate doesn't open at a 90 degree angle, so this means that the wastegate gasses do not exit in a uniform manner. Accordingly, the splitter should seal against the inside of the turbo housing to prevent wastegate gasses from leaking into the path of the gasses passing through the exhaust turbine:
* Style and location of cat converter - some exhausts use ceramic substrate cats whereas others use high-flow metal substrate cats. Good quality metal cats flow very well and tend to be less fragile than ceramic substrate cats. Some exhausts have the cat in the dump immediately after the turbo, whilst others have the cat under the body. Victoria is particularly strict regarding the location and number of cats required in an aftermarket or replacement exhaust system. Be sure to check what applies for your state/area.
round metal cat:
Ceramic cat left, metal cat right:
Ceramic:
Metal:
* Ground clearance - some aftermarket exhausts hang low and don't hug the underbody of the car. This means that you are more likely to scrape your exhaust on a lowered car. Most exhausts will hug the underbody fairly well, especially custom exhausts that can be made to suit your car perfectly.
* Resonators - some exhausts will have a centre resonator to help reduce noise levels or to refine the sound of the exhaust. Hot-dog resonators come in different styles which can either reduce noise or help reduce a raspy exhaust note. Large flat oval-style resonators are generally used for noise reduction purposes only.
Hotdog-style resonator:
Flat oval-style resonator:
* Noise levels and droning - generally speaking, cannons are loud! However, high quality Japanese cannons can be quite subdued. High quality straight through oval mufflers provide a decent balance of flow vs noise. Baffled rear mufflers can flow well if designed properly, whilst offering virtually no drone and a nice subdued burble. Adjustable mufflers like the X-Force Varex are an option (they can switch from loud to factory-silent) but bear in mind that the authorities are cracking down on these (they will be illegal shortly).
* Pipework diameter - the average aftermarket Subaru exhaust is 3" diameter and this generally works quite well. Research and development undertaken by Michael South (MSR) has suggested that reducing the pipework to 2.5" for the diff-back section helps to maintain exhaust gas velocity (many aftermarket exhausts are now replicating this). A 2.5" diff-back also helps to reduce noise levels a bit. www.msengineering.com.au/exhaustsystems.html
* Tip style - whatever tickles your fancy!
What is an up-pipe and when do I need to replace it?
The up-pipe connects the headers to the turbo (refer to the above pic).
MY01-02 GT Foresters and 04-05 XT Foresters have a catalytic converter in the up-pipe for cold-start emissions. The catalytic converter on these models is known to break down on high kilometer or modified examples, which results in pieces of catalytic converter making their way through the exhaust turbine of the turbo. This will cause turbo damage or failure. Accordingly, it is best to replace the catted up-pipe whenever you mod one of these models. A factory catless up-pipe from an 98-00 GT or 06+ XT or 97-00 or 03+ WRX or any STI is the best option as the pipe diameter promotes the best turbo spool. The factory up-pipe diameter is approx. 41-43mm at each end. If you are forced to fit an aftermarket up-pipe, try to find one which is a similar diameter to the above. Avoid the large diameter ebay or aftermarket up-pipes which are 50mm ID and above as these slow down the exhaust gas velocity which is something you want to avoid at all costs!
Catless up-pipes also have an exhaust gas temperature sensor prior to the cat. If the exhaust gas temp gets high enough to potentially cause issues with the up-pipe cat converter, the ECU will enrichen the mixtures to try and bring temps under control. If you are replacing a catted up-pipe with a catless version, the EGT sensor becomes redundant and is best removed. If your car is being reflashed you can make some changes to ensure that a check engine light does not appear. Alternatively, you can insert a 2.2kohm 0.5W resistor in the wiring plug and the ECU will remain happy.
Here's a pic of a factory catted up-pipe on the left, and a factory catless up-pipe on the right:
You can see the cat substrate and the temperature sensor in this pic:
A few measurements of the up-pipe internal diameter (it is about 41-43mm):
MY09 XT exhausts
The MY09 has a different exhaust setup to the earlier models, so the exhausts are not interchangeable.
Hitech have recently released a MY09 XT specific exhaust:
www.hitechmufflers.com.au/product/index.php?m=Forrester+GT+%2F+XT&b=subaru