JeffTsai

iTrader: (1)

Well, backpressure is when you have too small of an exhaust diameter or there is some kind of restriciton. The restriction could either be caused by too small diameter piping, bad design such as a log manifold, or clogged cats. Having no backpressure at all in the exhaust is good if you're boosted because the turbo itself is a restrction in the exhaust flow already. On a NA engine you want the exhaust to be as free flowing as possible and have piping that is sized to allow for the exhaust velocity to be fast but not so small that if causes restriction. It's really down to fine tuning at that point to get maximum power. Then there are long or short tube headers that can affect power properties as well. There's a lot that goes into designing a high performance exhaust system.

jcat_350

no, you've got the concept backwards. backpressure is not the amount of pressure pushing the exhaust out, its the pressure of the restriction on the exhaust.

a bigger pipe means LESS backpressure, resulting in loss of exhaust velocity (if you have the same piping size all the way back; going from this to stock piping would yield a negligible increase in backpressure). Hence, less low end. However by the same token, too much backpressure will hurt your performance as well. It has to be mated just right to the other performance mods on the car, hence 3" would be appropriate for a forced induction application due to the increased amount of air being forced in and then pushed out, but it would be overkill for an n/a engine (not talking about big cubes btw, more relating to the gs3 in question) because there isn't as much need for huge flow like that.

GSteg

iTrader: (15)

Yes correct. It's not the pressure that the engine is pushing out through the exhaust side of the valve. It's any pressure that opposes in the opposite direction. In this case, back pressure is due to the atmosphere. When you have a larger pipe, you're enlarging the area for the atmospheric pressure to push on the exhaust. It's physically impossible to have less exhaust velocity by decreasing back pressure (everything else constant).This can be shown by Bernoulli's Principle, unless someone has defied the laws of physics .



Ignore the equation. Look at the relationship between cross sectional area and pressure.

Less back pressure is always good. Ideally you want no back pressure so your exhaust velocity can be at its greatest. However to get less back pressure, you'll need to decrease the exhaust piping, but you run to the point where the smaller piping becomes the bottleneck simply because it does not have the capacity to transfer gas at a high rate, not because of back pressure.

I think the problem is the word back pressure itself. People tend to mix it up with a different kind of restriction (piping size). They see that adding on a 4" piping to a stock GS300 decreases power because of loss in exhaust velocity. So somehow they relate the loss of power to back pressure.

jcat_350

that's actually the perfect way of putting what i tried to get across, but i'd like to add one thing.

super high flow (i.e. 3" or 4" exhaust) is only useful when there is a turbo or something else pushing a ton of air INTO the motor that is requiring more pipe volume to exit.

if you have the same force applied, one to a smaller opening, one to a larger opening, the velocity in the smaller opening will be higher. thus why you need something to create the push on one end in order for less resistance on the other end to work in your favor.

its just a really complicated principle, thats why the engineers get paid so much to do the thinking for us

GSteg

iTrader: (15)

Ideally, we want some kind of pressure that cancels out the atmospheric pressure. But it's kind of hard to drive around in a vacuum camber

Cliffnote: Bigger the pipe --> More back pressure --> less velocity --> possibly power loss

Forced induction utilize the bigger piping better just because of the the sheer volume you can transfer.

Any engine, big or small, will benefit from less back pressure as long as there is no restriction on the piping size. We dont need back pressure. We dont want it at all!

pingu

I would also be all over this but the liberal democrat green loving hippies run CA. I think that the b-pipe does have a cat. Magnaflow sells a catalytic converter that replaces the entire b-pipe.

http://cgi.ebay.com/MAGNAFLOW-CATALY...1%7C240%3A1318

SorrGwa

iTrader: (1)

here's a pic of the megan racing y pipe next to the stock one.



here's the picture of it mounted on.



the pipe itself sits a smug lower than the stock pipe. there are no hangers.

it make the car sound a bit more throaty w/ the tanabe touring medallion exhaust
i found the piping size almost identical to the stock one, except for 1 thing, the stock y pipe was made bye 2 halves sandwiched and welded together, the megan racing one was a round pipe that was bent and flanges welded at the ends. it still has the bung hole for the o2 sensor.

pingu

What year is your GS?
Have you thrown a CEL?
Can you look at the end of your b-pipe?
Does it look like a honey comb?

SorrGwa

iTrader: (1)

mines a 99 GS300
the only CEL code i have is for the stupid charcoal canister p0440. ive eliminated every possibility, gas cap, vacuum lines, evap switch under the hood. my friends a toyota tech, so we used the toyota scanner to turn on and off everything, checked every line, swapped the gas cap, and narrowed it down to the charcoal canister. i hook up an obdII reader to my car every week or so to clear the CEL

ill dig up my ypipe/pipe after the weekend meet. its in storage with the rest of my stock parts. ill check to see if its got the cat converter in it (the honeycomb) ur probably wondering if its true that we got 3 cats. i know we have 2 cats for sure. 2x(3 into 1 header w/ cat in the piping)

here is the thing, it should not produce and CEL codes for this reason. look where the bung hole is on the stock and in the megan racing pipe. its pre 3rd cat. even if it wasnt a 3rd cat, it wouldnt matter. if the o2 sensor (where the bung hole is on the pipe) is located in front of the 3rd cat, then the air going thru it would not really affect the readings at all. if it was after the 3rd cat, then it would run the chance of throwing a CEL.

pingu

Yea, I also thought that. Was the Megan down pipe lighter than the stock b-pipe?

SorrGwa

iTrader: (1)

if i remember, it was a tad lighter, not much difference, the only thing i really remember was it sat lower. the stock pipe hugged the under carrage a lot closer than the megan racing one. i had to remove one of the metal braces that goes perpendicular across the exhaust piping cuz the piping was right up against it causing the whole car to shake.

which was fine, cuz i stupidly without thinking gunned on the 10mm little screw/bolt that held one side of the flat bar and so snapped the screw/bolt it right off.

i had access to a full car shop and so was a lot braver at attempting my own installs.

the install itself took 30mins, 10mins was sitting around waiting for the wd40 to soak into the bolts.

rule of thumb, first remove the o2 sensor. its much easier when the exhaust is still bolted in place.

pingu

Thanks for answering my questions. I am curious how the megan down pipe performs.

VEGAS DRE

MR makes very good products at great prices !and no i dont work for megan racing ! just speaking from experience from selling and installing their products over the past 3 years. the main purpose of an aftermarket exhaust is to have more free flowing burnt gas to exit out as fast as possible ! rule of thumb the faster you can get air in and out of an engine the more hp you'll make ! id say buy it !

SweetS160

On my 99 gs3 I have a resonator on the b pipe from the factory. Maybe kali cars come with a extra cat.

wowitztai

iTrader: (1)

i like..i wanna buy one now.