Is a downpipe really worth it?
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Car Info: 2002 White WRX Wagon (WWW)
Is a downpipe really worth it?
"Since we live in the clean air era, SPD Tuning Service does not recommend removing the large down pipe cat on road car. First, the down pipe cat is actually the same unit used on the STI 280hp cars. This main down pipe cat is known to run a clear 300-320hp before is considered a power restriction. It is just not a significant restriction at the power levels we can achieve on pump gas. Yes, if we eliminate the main cat there is a mild improvement in spool-up of the turbo, but not huge gains that justify turning you car into a pollution machine. Just do not worry about it. My 260hp WRX runs a down pipe cat. It runs just fine. For a mild tuning of the engine please keep your main cat. The power gains will be quite similar with or without, if other details are taken care of."
That is taken from SPD USA's website. They suggest for exhaust work you leave the cat in the downpipe and you'll be set replacing everything but the downpipe (up-pipe, catback and elimination of the 3rd cat) unless you are going north of 300hp. All the information I have found on their site is pretty damn accurate. Things like their STRONG suggestion of leaving the stock bypass valve in, their shifting techniques page, all in all they seem to know their ****... So I was wondering what you all thought? It seems from what I've read on these boards that the dp is one of the main sources of restriction. I wonder what gain one would observe when going from the stock dp to an aftermarket one if all the rest of the exhaust has already been replaced... Anyone got any evidence one way or the other? Oh here's their website, lots of good reading: http://www.spdusa.com/
That is taken from SPD USA's website. They suggest for exhaust work you leave the cat in the downpipe and you'll be set replacing everything but the downpipe (up-pipe, catback and elimination of the 3rd cat) unless you are going north of 300hp. All the information I have found on their site is pretty damn accurate. Things like their STRONG suggestion of leaving the stock bypass valve in, their shifting techniques page, all in all they seem to know their ****... So I was wondering what you all thought? It seems from what I've read on these boards that the dp is one of the main sources of restriction. I wonder what gain one would observe when going from the stock dp to an aftermarket one if all the rest of the exhaust has already been replaced... Anyone got any evidence one way or the other? Oh here's their website, lots of good reading: http://www.spdusa.com/
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You are probably going to get a lot from both sides on this one. Good to take care of the environment, though. Anyway, no one part is really going to make a huge difference. It is when you take it as a whole that those parts make a difference. Each part compliments another. Basically, what you want is to elliminate back pressure after the turbo. Back pressure will 'push' on the turbine making it work less efficiently. As a side note, that is what the bypass valve does for the compressor side. Anyway, freeing up the exhaust after the turbo will help elliminate this backpressure. Ask any of the performance shops about the 'ideal' exhaust on a turbo charged car and they should tell you that this exhaust would be very very short. It would start as big as the turbine and quickly cone out in size to be quite large in diameter. Before the turbo, you want high exhaust velocity and less heat. This high velocity and low heat will enable to turbine to spin faster which turn makes it work more efficiently. Now, with a WRX that has 200-250 HP, then yes, in the grand scheme of things, just changing the down-pipe isn't going to change your world. It is just a matter of personal preference.
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Something to note: it seems that SPD's hp numbers are flywheel hp numbers. "280" for the older JDM STi would not be wheel hp. Using a very appropriate 25% all-wheel drivetrain loss for our cars, 300 to the wheels would be 400 at the flywheel. However, 300 flywheel hp is 225 to the wheels. With that cat you wont be putting 300 to the ground.
jason
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One of the points that the SDP tuning site makes again and again is that most of the components that US drivers change out to make more HP are also stock on the JDM WRX and are even used in the engine build-up of the WRC cars. The commentary about intakes is extremely interesting; turns out the intake on the 300HP WRC car is 1/3 the size of our stock intake! Clearly, burning race fuel helps, but I do think he does a good job debunking some common myths... Certainly his advice for anyone wanting to run a 300HP setup should give everyone pause (hint: rebuild pistons, valves, injectors and cylinder jackets first!)
Personally, no offense intended to anyone, but I think running a totally catless car is irresponsible and ultimately quite destructive to the car. If you have unburnt fuel ending up in your exhaust pipe, that indicates incomplete combustion in the actual cylinder -- something that the car's entire engineering (ECU, cams, valve train) was designed to prevent from the ground up. Subaru engineers worked for almost a decade on an engine design that would provide both power and efficiency, and stripping out the entire exhaust side of the engine defeats all of their design efforts and throws a very tightly-engineered setup out of balance. You've just got to ask yourself: who knows the car's core design and potential better, Joe Weldit at the exhaust shop or the Subaru engineers who built it?
Personally, no offense intended to anyone, but I think running a totally catless car is irresponsible and ultimately quite destructive to the car. If you have unburnt fuel ending up in your exhaust pipe, that indicates incomplete combustion in the actual cylinder -- something that the car's entire engineering (ECU, cams, valve train) was designed to prevent from the ground up. Subaru engineers worked for almost a decade on an engine design that would provide both power and efficiency, and stripping out the entire exhaust side of the engine defeats all of their design efforts and throws a very tightly-engineered setup out of balance. You've just got to ask yourself: who knows the car's core design and potential better, Joe Weldit at the exhaust shop or the Subaru engineers who built it?
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I don't think running cat-less is destructive to the car (except from the "More power = More wear"). Sure, it's not great for the environment, but it probably won't hurt the car. Okay, you'll foul the O2 sensor, but that should be about it.
I fail to see your point on how running catless leads to incomplete combustion. If the engine is releasing unburnt fuel, it would be doing it regard-less of cats or not. Is that really not the point of the cat; to remove those very same unburnt fuel molecules from the exhaust, not prevent them?
As for throwing it out of balance, they tune the car to be conservative. Changing the exhaust will allow the car to breath better. "Run more efficiently" is a better phrase then "out of balance." However, I do believe that a Mish-Mash of parts without any tuning/thought could allow the car to run worse then it was before.
Then again, what do I know. I'm a foolish NOOB!!!
I fail to see your point on how running catless leads to incomplete combustion. If the engine is releasing unburnt fuel, it would be doing it regard-less of cats or not. Is that really not the point of the cat; to remove those very same unburnt fuel molecules from the exhaust, not prevent them?
As for throwing it out of balance, they tune the car to be conservative. Changing the exhaust will allow the car to breath better. "Run more efficiently" is a better phrase then "out of balance." However, I do believe that a Mish-Mash of parts without any tuning/thought could allow the car to run worse then it was before.
Then again, what do I know. I'm a foolish NOOB!!!
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Car Info: Subaru Impreza WRX 2002
52 posts is far from noob!
I appreciate what you are saying, and yes, the factory stock setup is conservative. However, it was also engineered as a complete package -- in other words, the backpressure generated by the stock exhaust setup was engineered into the cam timing, valve stroke and ther parts of the combustion cycle. Strip out all the backpressure and something will have to be out of factory spec somewhere.
I've seen a lot of N/A cars with 3" or 4" exhaust on them, and without backpressure those overtuned engines lose ALL of their power... many people turn a Honda Prelude from a 15-second car to a 20-second car this way.
I think of it this way: my father rebuilt 3 Porsche 911s back in the 80s, buying them "blown up" and rebuilding the engines. Every non-stock part he put on those engines came back to haunt him, and even with stock parts, he could never get back to the German factory spec. If Japanese engines are as well-engineered, we should make changes to the drivetrain with caution, not abandon.
I appreciate what you are saying, and yes, the factory stock setup is conservative. However, it was also engineered as a complete package -- in other words, the backpressure generated by the stock exhaust setup was engineered into the cam timing, valve stroke and ther parts of the combustion cycle. Strip out all the backpressure and something will have to be out of factory spec somewhere.
I've seen a lot of N/A cars with 3" or 4" exhaust on them, and without backpressure those overtuned engines lose ALL of their power... many people turn a Honda Prelude from a 15-second car to a 20-second car this way.
I think of it this way: my father rebuilt 3 Porsche 911s back in the 80s, buying them "blown up" and rebuilding the engines. Every non-stock part he put on those engines came back to haunt him, and even with stock parts, he could never get back to the German factory spec. If Japanese engines are as well-engineered, we should make changes to the drivetrain with caution, not abandon.
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That SPD guy is full of it. Having gone from a stock wrx to one with just an UP/DP, there are definitely HUGE torque gains to be had on the low end. It might not help with peak HP but who wants to brag about peak numbers when the drivability goes way up.
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Exhaust diameter really depends on the output of the vehicles' exhaust. I'm willing to bet that the WRX puts out more exhaust volume then a 2.2 VTEC; therfore the 2.2 doesn't need the larger diameter piping.
You want to make sure there is sufficient velocity in the tube to ensure the extracting effect in the exhaust. Going bigger on the WRX may not necessarily hurt that effect. If it can keep up the same velocity while moving more, it only helps. However, it probably will need some sort of tunning (be it ECU or whatever) to take full advantage of it.
You are definately right though on the reckless abandon. You need to think modifications thru before you go for it.
Wait, wait, wait!!!!!! The Horse power will be out of factory spec (Ah Cha Cha Cha!!!!!!)
You want to make sure there is sufficient velocity in the tube to ensure the extracting effect in the exhaust. Going bigger on the WRX may not necessarily hurt that effect. If it can keep up the same velocity while moving more, it only helps. However, it probably will need some sort of tunning (be it ECU or whatever) to take full advantage of it.
You are definately right though on the reckless abandon. You need to think modifications thru before you go for it.
Wait, wait, wait!!!!!! The Horse power will be out of factory spec (Ah Cha Cha Cha!!!!!!)
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Originally posted by meilers
52 posts is far from noob!
I appreciate what you are saying, and yes, the factory stock setup is conservative. However, it was also engineered as a complete package -- in other words, the backpressure generated by the stock exhaust setup was engineered into the cam timing, valve stroke and ther parts of the combustion cycle. Strip out all the backpressure and something will have to be out of factory spec somewhere.
I've seen a lot of N/A cars with 3" or 4" exhaust on them, and without backpressure those overtuned engines lose ALL of their power... many people turn a Honda Prelude from a 15-second car to a 20-second car this way.
I think of it this way: my father rebuilt 3 Porsche 911s back in the 80s, buying them "blown up" and rebuilding the engines. Every non-stock part he put on those engines came back to haunt him, and even with stock parts, he could never get back to the German factory spec. If Japanese engines are as well-engineered, we should make changes to the drivetrain with caution, not abandon.
52 posts is far from noob!
I appreciate what you are saying, and yes, the factory stock setup is conservative. However, it was also engineered as a complete package -- in other words, the backpressure generated by the stock exhaust setup was engineered into the cam timing, valve stroke and ther parts of the combustion cycle. Strip out all the backpressure and something will have to be out of factory spec somewhere.
I've seen a lot of N/A cars with 3" or 4" exhaust on them, and without backpressure those overtuned engines lose ALL of their power... many people turn a Honda Prelude from a 15-second car to a 20-second car this way.
I think of it this way: my father rebuilt 3 Porsche 911s back in the 80s, buying them "blown up" and rebuilding the engines. Every non-stock part he put on those engines came back to haunt him, and even with stock parts, he could never get back to the German factory spec. If Japanese engines are as well-engineered, we should make changes to the drivetrain with caution, not abandon.
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From: Atlanta/Cayman Islands
Car Info: 2002 White WRX Wagon (WWW)
Originally posted by mexicanpizza
That SPD guy is full of it. Having gone from a stock wrx to one with just an UP/DP, there are definitely HUGE torque gains to be had on the low end. It might not help with peak HP but who wants to brag about peak numbers when the drivability goes way up.
That SPD guy is full of it. Having gone from a stock wrx to one with just an UP/DP, there are definitely HUGE torque gains to be had on the low end. It might not help with peak HP but who wants to brag about peak numbers when the drivability goes way up.
Which let's face it, looks like it has a cat. Then getting a borla hush catback. The only stock part of the exhaust in place would be the dp but the system would look relatively stock and still contain one cat. This would work out great if the stock dp is not that restrictive as SPD Tuning suggests...
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Originally posted by meilers
Personally, no offense intended to anyone, but I think running a totally catless car is irresponsible and ultimately quite destructive to the car. If you have unburnt fuel ending up in your exhaust pipe, that indicates incomplete combustion in the actual cylinder -- something that the car's entire engineering (ECU, cams, valve train) was designed to prevent from the ground up. Subaru engineers worked for almost a decade on an engine design that would provide both power and efficiency, and stripping out the entire exhaust side of the engine defeats all of their design efforts and throws a very tightly-engineered setup out of balance. You've just got to ask yourself: who knows the car's core design and potential better, Joe Weldit at the exhaust shop or the Subaru engineers who built it?
Personally, no offense intended to anyone, but I think running a totally catless car is irresponsible and ultimately quite destructive to the car. If you have unburnt fuel ending up in your exhaust pipe, that indicates incomplete combustion in the actual cylinder -- something that the car's entire engineering (ECU, cams, valve train) was designed to prevent from the ground up. Subaru engineers worked for almost a decade on an engine design that would provide both power and efficiency, and stripping out the entire exhaust side of the engine defeats all of their design efforts and throws a very tightly-engineered setup out of balance. You've just got to ask yourself: who knows the car's core design and potential better, Joe Weldit at the exhaust shop or the Subaru engineers who built it?
Nothing personal, DooD...but go to school and get an education on how these things work(like I did) and then see what you think you know.
Last edited by Uncle Scotty; Feb 16, 2004 at 05:38 PM.
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I've been thinking about it and he is definately right if you JUST yank the cat (do nothing else). That's why they got the MIL eliminator out there though, to take care of this problem.
Someone tell me if this is what happens in our cars: YOu yank the cats but put in no MIL. The O2 detects the crap in the exhaust and tells the car to trim the injector pulses (for less fuel which equals less yucka in computer lingo). This causes the car to run lean and generally like my old Metro.
Does our O2 control that or no? (I'm learning too)
Someone tell me if this is what happens in our cars: YOu yank the cats but put in no MIL. The O2 detects the crap in the exhaust and tells the car to trim the injector pulses (for less fuel which equals less yucka in computer lingo). This causes the car to run lean and generally like my old Metro.
Does our O2 control that or no? (I'm learning too)
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Originally posted by blingbling
I've been thinking about it and he is definately right if you JUST yank the cat (do nothing else). That's why they got the MIL eliminator out there though, to take care of this problem.
Someone tell me if this is what happens in our cars: YOu yank the cats but put in no MIL. The O2 detects the crap in the exhaust and tells the car to trim the injector pulses (for less fuel which equals less yucka in computer lingo). This causes the car to run lean and generally like my old Metro.
Does our O2 control that or no? (I'm learning too)
I've been thinking about it and he is definately right if you JUST yank the cat (do nothing else). That's why they got the MIL eliminator out there though, to take care of this problem.
Someone tell me if this is what happens in our cars: YOu yank the cats but put in no MIL. The O2 detects the crap in the exhaust and tells the car to trim the injector pulses (for less fuel which equals less yucka in computer lingo). This causes the car to run lean and generally like my old Metro.
Does our O2 control that or no? (I'm learning too)
straight up-pipe, stock downpipe, straight 3rd cat eliminator, straight catback
Will pretty much be just as good as this setup (as long as you are staying below 300-320hp):
straight up-pipe, straight downpipe, straight 3rd cat eliminator, straight catback
Where I put "straight" I mean aftermarket high flow pipes with no cats...