Silvia Club of NSW

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PostPosted: Sat May 28, 2011 11:58 pm 
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T28 Hybrid
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Joined: Wed Sep 09, 2009 6:15 pm
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Location: South West Sydney
Car: Holden VT Calais Supercharged
Real Name: Sebastian
Hey guys, now as the post states, correct me if i'm wrong, buuuuuuuut if you are running a N/A engine, and you put a bigger exhaust on it (say from standard 1.5 - 2" to 4 - 5") doesn't it lessen back pressure, therefore decrease engine performance?

Reason it popped into my head was that these fucking wankers and their shit box charades and fucking 5 in cannon exhausts keep driving by my house, and i'm gonna start bagging on them about their shit box cars. But before i do, i wanna get my fax straight.

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PostPosted: Sun May 29, 2011 8:58 am 
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Ahh,the old 'your exhaust needs back pressure'. Is that myth STILL getting around?

Back pressure is NOT desirable,you want an exhaust system to be free-flowing,hence why performance systems are larger than std.
Back pressure is basically a restriction in flow = gas entering the system faster than it can get out of it.

An engine is really only an air pump,you want to flow as much gas through it as possible,as quickly as possible. More air in = more fuel in = bigger burn = more power = more exhaust gas.

Factory exhaust systems,like pretty much every component on a car,are a compromise. Passing legal noise standards,passing legal emission standards,coming in at a certain cost to the manufacturer,being easy and fast to install to speed up assembly times and so on.
If you are willing to alter any of those conditions,you can make gains in power.

Most systems trade increased noise for improved flow. Replacing a multi-chambered design muffler with one that is straight through will improve the flow speed of the gas considerably,which will help power - at the expense of sound muffling.
Careful system design and paying attention to the areas that cause the most restriction can net good gains,without sacrificing legality or increasing volume too severely.
A lot of cars you'll hear on the road haven't had much thought put into their aftermarket exhausts. The main goal is actually noise - to get attention. Which is most likely what the wankers you're referring to have done to their charades.

Film the car as it drives past and send the file to the EPA. Tell the owner you're doing it too using some colourful language. It's unlikely that the EPA will do anything,but the threat may get the car's owner to think twice about how noisy his car is.


Justin...[/i]

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PostPosted: Sun May 29, 2011 8:18 pm 
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T28 Hybrid
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Location: South West Sydney
Car: Holden VT Calais Supercharged
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Cheers Justin, it makes more sense now. So in actuality it does benefit the actual system to have a bigger exhaust, but in order to reach it's full potential, it takes thought and more then just a massive cannon.

So in a N/A system, increasing not only the exhaust size at end (sports or performance muffler), pipe size thought-out, hi-flow cat, and increasing the outlet manifold size should increase the overall flow of the engine, and increase power? What could be done to keep noise down?
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PostPosted: Sun May 29, 2011 8:37 pm 
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you need really large (and good quality) mufflers.

They need to be straight through design for good flow, so generally will be pretty loud.

You can lose some low end torque if the exhaust is too large, because the pulses lose energy and slow down, essentially causing increased back pressure in the exhaust.

In my opinion, anything after the cat doesn't really matter, apart from overall flow - because the gases have already had to slow down, go through the cat, and speed up again into the rest of the piping.

Before the cat you want to keep the pipe diameter the same as the outlet of the extractors to keep the gas velocity up, but after that you just need minimal restriction.

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PostPosted: Sun May 29, 2011 10:06 pm 
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Car: 1998 200sx S14a
i have those similar kiddies as well on my street, i use to have a 4.5 inch custom cannon muffler welded on and it was loud, noisy and annoying then bought a kakimoto racing exhaust, apparently they hit 92db, though i never checked.

i did notice it had a better note to it and wasnt as loud and cranky on idle and accelerating compared to the custom one, plus its stainless steel and more shiny.


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PostPosted: Mon May 30, 2011 2:16 pm 
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As Dumhed has said,the back half of the system can be fairly random,as long as it flows,up front is where you make the most difference to power.

Muffler size,design,and placement make the most difference to the noise level and pitch of the output.

You'll see a lot of cars with a small resonator (or nothing at all) in the middle after the cat and then often a simple cannon or straight-through slight offset muffler at the rear.
This setup is great for overall flow,but lousy at reducing the output volume,and will likely have a drone to it on cruise.
Adding a decently-sized slight offset path muffler in the middle of the system can remove a LOT of the drone,and harsh raspiness from the note,without a drastic decrease in overall flow.
If you check out the design of most aftermarket systems for silvias,you'll see that the quieter types will have a mid muffler as part of the cat-back section.

On an NA car,you need to more careful with system design than on something turbocharged,as the flow of the system depends on gas velocity to promote scavenging,and in extreme engines,a full charge of the cylinder on intake.
You'll need proper extractors of a design and diameter to flow best in your ideal powerband,and to maintain that diameter until at least the outlet of the cat. After the cat,diameter is less critical,but you must maintain a consistant flow rate through the system as well as the layout of the car can allow. The gas will be cooling and expanding less and less as it moves along the lngth of the system,so the further back along the car you go,the less critical piping diameter is.
As an example,an NA 1.6l 4AGE will make it's power in the top end only,so it will run large diameter extractors of a 4-1 design. These will be 40mm+ at each cylinder,before merging to a 60mm or so collector just before the cat. Catback aftermarket systems generaly run 50mm or 60mm piping post-cat,depending on application.

On a turbo car,you want to get the gas out of the engine and into the turbine as smoothly and as fast as possible,so short but equal length runners that merge at the turbine inlet are most desirable. After the turbine outlet,everything is a restriction to flow,so you want to minimise that as much as possible.
A smooth dump pipe,that does not allow the wastegate gas to merge immediately with the outlet of the turbine is most desirable. Everything you do after that is to minimise noise and backpressure. Noise for legal reasons,and backpressure to allow best response of the turbine.
Split dump pipes with a separate wastegate pipe to the main turbine outlet are popular because they work best. Blended designs were popular in the 90's,before we knew better.

Changing a rear muffler alone on a car will give a small gain in overall flow,but is primarily for added noise. The real gains are made in the front half of the system.


Justin...

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