Down/Straight Pipe

[HuGo]

i have a question. I was looking under my car and seeing how the pipes run down. I drew a picture of how it looked like...

Heres my questions, The piping goes in like zig zag order with 2 cats right

So, How much more hp or gains would i get if i get a pipe that goes straight without curves threw the bottom without the zig zags.

Lets say i did a pipe going straight with the 2 cats, how much HP gain is it?
Also, Same question except with the 2 cats.

I would like it going straight back with the 2 cats so i can still pass smog.

Well here are my 2 pics, they are tacky but its example, First one is now, and second is what id like.

So anyway, yea how much HP gain would i get with 2 cats, and how much hp would i get without the cats. Or would it make no difference once o ever. Thx.

Also, If i were to get atleast 20 HP for this, i would consider doing this as my next mod.

Edit- I always here people say to make it 3', so id make a 3' pipe. Since stock is 2' i think?

[Viper3812]

First of all there is only 1 Cat, the other thing is a resonator. I highly doubt you would get 20hp for changing your exhaust around cause it doesn't do much and you would be paying more money to get a custom setup. BMW designed the exhaust in the zig zag for a reason to fit it in properly. If you take off your Cat you are really getting more torque than hp. You are better off getting an after market exhaust like Supersprint or Stromung, they are 2-1/4" pipes I think. The 3" setup will just be big, bulky, and take you to Honda world. Sorry if this bums you out, but it's the truth.

[GDB]

If you are looking a legal hp increase then stick a straight pipe where the resonator (what you referred to as the second cat) is. Maybe swap to a dynomax or similar straight through muffler. Either way these mods are not going to get you 20hp, probably not even 10, but it should be the best bang for your buck while still allowing you to pass emmissions testing.

[HuGo]

ive seen in magazines before where they mod and they do dyno test after each mod, and i remember seeing on a car that was down piped 3' it increased 30 HP.

[SlickmisterN]

Down Pipes are for turbo cars It was probobly a turbo car that the increase you saw was on, which will be completely different from our N/A cars.

[HuGo]

Originally Posted by SlickmisterN

Down Pipes are for turbo cars It was probobly a turbo car that the increase you saw was on, which will be completely different from our N/A cars.

n/a?? Not Available? Would increase that much if it was done to a S/C car?

[Variance]

Your gains from your idea will be slim to none. However, if it were to have any effect, you would gain a little bit of top end, but lose a little bottom end. Exhaust backpressure is one of the things that gives you torque, so reducing backpressure will cause you to possibly gain high-end HP, but lose low-end torque.

In short, even if it were to work, it wouldn't be worth it in the least.

Edit: I guess I was kinda slow in getting my post in. N/A stands for normally or Naturally Aspirated. I.E. no forced induction. You would not see the same gains on an SCed car. Having the larger downpipe allowed for better flow to the turbo, which caused the power gain.

[HuGo]

hhhmm, i guess ill leave it as is.

[pdxmotorhead]

Actually just like VTEC,, Vanos should be tuneable to take
advantage of a big pipe. BUT!!! On all the race cars I work
on there is definately a sweet spot / compromise without
variable valve timing. Just like on Porsches BMW did not skimp
on exhaust, it takes a LOT of engine mods to take advantage of
a bigger pipe on a BMW.

I had a friend in search of the ultimate Porsche exhaust, and
he had the $$ to try them out on his chassis dyno. Guese
which one perormed the best across the board?
There were 2. The stock unit and the one DESIGNED by the porsche
race team engineers. Then there was one compromise system, he
gutted the stock 911 pickle muffler and welded it back.
Got him exactly 12 hp over stock on a 3.2 litre turbo.
That was already clear of the 500HP mark. The factory porsche design
gave 23.

I guess what I'm saying it is a pretty good driving machine

If you look across the board at 2.0 engines the 318 motor
produces respectable power and life.
Your gaining little for a lot of $$ with a lot of the mods folks
burn money on.

Now if someone has systematic dyno numbers that prove me wrong show em!