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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > How do after market downpipes increase power?



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      01-16-2008, 11:32 PM   #1
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How do after market downpipes increase power?

Simple question...I understand that increasing pipeing diameter, all other things being equal with the exhaust comming from the manifold, would increase flow and thus reduce turbo lag. I'm just drawing a blank on how horsepower would be increased, does boost turn up at a lower rpm?

I would really appreciate any insight.
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      01-16-2008, 11:36 PM   #2
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Think of a turbo like a water wheel in a river. What happens when you remove that beaver dam just downstream of the water wheel? The water flows faster and the wheel spins more quickly. More flow = more boost + more fuel = more power. The primary catalytic converters are major restrictions in the exhaust flow.
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      01-16-2008, 11:37 PM   #3
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Turbo spools up faster as well as you'll be removing restriction of the stock catalyzers.
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      01-16-2008, 11:42 PM   #4
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Quote:
Originally Posted by bullmrkt View Post
Think of a turbo like a water wheel in a river. What happens when you remove that beaver dam just downstream of the water wheel? The water flows faster and the wheel spins more quickly. More flow = more boost + more fuel = more power. The primary catalytic converters are major restrictions in the exhaust flow.

dude i started typing about "seeing the engine as a bigass air pump" you hit the nail on the button. short but too the point, my description was atleast 15 lines long
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      01-16-2008, 11:45 PM   #5
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And the catted DPs work slightly better than the stock ones because the bends are smoother or something? Otherwise why bother with catted DP's?
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      01-16-2008, 11:50 PM   #6
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Less cells (less dense) so they flow better. What's easier to see through, canvas or a tennis racket? Might as well get rid of them, it stinks a bit but I like it. Reminds me of the racetrack.
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      01-17-2008, 12:04 AM   #7
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perfect

Quote:
Originally Posted by bullmrkt View Post
Think of a turbo like a water wheel in a river. What happens when you remove that beaver dam just downstream of the water wheel? The water flows faster and the wheel spins more quickly. More flow = more boost + more fuel = more power. The primary catalytic converters are major restrictions in the exhaust flow.
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      01-17-2008, 02:01 AM   #8
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Quote:
Originally Posted by E90076 View Post
Simple question...I understand that increasing pipeing diameter, all other things being equal with the exhaust comming from the manifold, would increase flow and thus reduce turbo lag. I'm just drawing a blank on how horsepower would be increased, does boost turn up at a lower rpm?

I would really appreciate any insight.
Our cars are currently blowing through a straw with a sponge in the bottom (catalytic converter). Downpipes allow you to blow through a waterhose with no sponge. No restrictions. More flow. More horsepower. More fun.
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      01-17-2008, 06:47 AM   #9
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Points of view on leaving the stock catback on after the catless DPs ?

Driver72 indicated that short of about 10hp and the obvious weight advantage the stock exhaust after the cats wasn't THAT restrictive, what do you think ?

SJ
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      01-17-2008, 08:41 AM   #10
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With all things being equal, including the final boost number, the primary power gain is by reducing exhaust port back pressure.

In order to generate X amount of boost at Y Engine RPM, the compressor wheel needs to spin at Z RPM. To get the turbine to spin the compressor wheel at Z RPM, you need enough flow through the turbine housing which is accomplished by having a pressure differential on either side of the turbine housing. By decreasing pressure on the outlet of the turbine housing, the pressure on the inlet will also be lower to achieve the same compressor RPM and boost. Less back pressure on the inlet of the turbine housing means there is less power robbing back pressure at the exhaust ports; the piston can push the burnt gases out easier on the exhaust stroke.

So, the immediate gains are by way of reduced exhaust port back pressure. Another benefit to this is that you can generally run a little more boost safely as the EGT's have dropped some which just adds to the gains.
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      01-17-2008, 08:43 AM   #11
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ya but the 800 bucks doesnt suit what they r... R&D couldnt be that much... someday ill get them
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      01-17-2008, 12:12 PM   #12
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Quote:
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ya but the 800 bucks doesnt suit what they r... R&D couldnt be that much... someday ill get them
Keep in mind there are 2 of them. For my turbo volvo we were looking at $1k for a single downpipe. They might be expensive, but I dont find the pricing too bad.
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      01-17-2008, 08:18 PM   #13
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Originally Posted by 335i4u2nv View Post
ya but the 800 bucks doesnt suit what they r... R&D couldnt be that much... someday ill get them
Well, there really aren't many products that can produce 30hp and 50lb. ft. of torque. to the wheels for $800.00 other than a tune. Its definitely worth it and optimizes full power out of a turbo car.
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      01-17-2008, 08:28 PM   #14
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Quote:
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Keep in mind there are 2 of them. For my turbo volvo we were looking at $1k for a single downpipe. They might be expensive, but I dont find the pricing too bad.
Downpipes on an Audi RS6 are $2,500.00. Pricing really isn't too bad.
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      01-17-2008, 08:29 PM   #15
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I honestly didn't notice any more power increase after I dyno'd my DPs.
So, I believe that if you already have a good tuner on your car then you won't have an increase.

The one thing that I did notice is the better throttle response. The turbo does spool a lot quicker, and I did see better times at the strip.

DPs are a good investment for drivability, but as far as making a whole crap load of power is false.

This is the thread that I started when testing the DPs.
http://www.e90post.com/forums/showth...wnpipe+procede
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      01-17-2008, 08:39 PM   #16
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Quote:
Originally Posted by Mr. 5 View Post
I honestly didn't notice any more power increase after I dyno'd my DPs.
So, I believe that if you already have a good tuner on your car then you won't have an increase.

The one thing that I did notice is the better throttle response. The turbo does spool a lot quicker, and I did see better times at the strip.

DPs are a good investment for drivability, but as far as making a whole crap load of power is false.

This is the thread that I started when testing the DPs.
http://www.e90post.com/forums/showth...wnpipe+procede

You know I think a lot of this is marketing and bull as well. But there are a lot more to be gained then the nothing you got from HFCed downpipes from catless. Even though they are high flow they are still a HUGE restriction being right after the turbo like that.
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      01-17-2008, 09:41 PM   #17
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In my experience no Hi-flow cat will flow as good as straight pipe.

I know there will be people that will speak of some uber-exotic mega expensive Japanese or European cats that do and others who will mention how bad it is for the environment but, I just don't believe it (the flow part not the environment part, although a well tuned car should run quite clean).

There was supposed to be a 5hp difference between the 200 cell HFC I put on my Evo and a straight pipe. After building my own test pipe I will tell you it was more like 20hp and a huge difference in throttle response with the added bonus of fireballs out the exhaust.

IMO if you are going to do DP's you need to go catless. Otherwise I don't think they are worth the cost vs performance. If you are going to spend the money on catted DP's you might as just do a catback that does away with the secondaries and not run into any potential emissions BS and shed some weight instead.
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      01-17-2008, 09:50 PM   #18
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Get catless downpipes, and get a tune for them.. you WILL see big increases in power. Have you ever had a turbo car before?

Quote:
Originally Posted by Mr. 5 View Post
I honestly didn't notice any more power increase after I dyno'd my DPs.
So, I believe that if you already have a good tuner on your car then you won't have an increase.

The one thing that I did notice is the better throttle response. The turbo does spool a lot quicker, and I did see better times at the strip.

DPs are a good investment for drivability, but as far as making a whole crap load of power is false.

This is the thread that I started when testing the DPs.
http://www.e90post.com/forums/showth...wnpipe+procede
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      01-17-2008, 11:20 PM   #19
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Get catless downpipes, and get a tune for them.. you WILL see big increases in power. Have you ever had a turbo car before?
I've owned a turbo car before.

I seriously doubt that the catless DPs make that much more power than the high flow cats. I know that it's already been said where people think that high flow cats are just as bad as the regular cats, but I don't think so.

I agree that if I get a tune specificaly for the pipes then I will see an increase, but that's not the point of this thread. DPs alone do not necessarily increase power. How many tuners have DP programs out there yet?

Right now, people think that DPs will increase power. I was one of them, but it's not the case.
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      01-17-2008, 11:38 PM   #20
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Quote:
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I've owned a turbo car before.

I seriously doubt that the catless DPs make that much more power than the high flow cats. I know that it's already been said where people think that high flow cats are just as bad as the regular cats, but I don't think so.
They do make so much more power, it's HFC in the exhaust that doesn't make a difference. They still make a huge difference in th DP.
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      01-18-2008, 12:07 AM   #21
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They do make so much more power, it's HFC in the exhaust that doesn't make a difference. They still make a huge difference in th DP.
Either you're contradicting yourself or I'm confused.
What are you trying to say? You say the HFC DPs don't make a difference and then you say they do.
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      01-18-2008, 12:10 AM   #22
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Either you're contradicting yourself or I'm confused.
What are you trying to say? You say the HFC DPs don't make a difference and then you say they do.
What part of they make a difference in the downpipes but not in the exhaust didn't make sense to you? And don't say "bla bla downpipes are part of the exhaust" because you know what I meant.
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