ELI5: Exhaust System for the Carrera

[ksluna]

Hi all,

Wondering if you all can ELI5 (Explain like I'm 5)

Recently purchased a Carrera have decided my first mod will be a SW chip and modifying the exhaust. I know there are a million threads on that but first if someone can help me explain (or confirm if my knowledge is correct) the exhaust system in a 911.

Looking at this image belowand reading some articles this is what I've concluded

• Heat Exchangers are just headers with metal boxes around them to help heat the cabin
• Headers connect to the cylinders and push the exhaust fumes through the exhaust system
• Cat converter makes bad gasses into good gasses (ELI5 version)
• Muffler/baffles make exhaust noise quieter

Here is an image I made from what I understand is the stock 3.2 exhaust system. I just don't see the catalytic converter in it, I did google searches of what it looks like and can never find it in the pictures of stock exhausts.

Now in my research for SW + exhaust there is a lot of talk about pre muffler and cat-bypass.

Can someone ELI5 what a pre-muffler does and how it is different from a cat-bypass? One can assume that a cat bypass is just a pipe that replaces the cat converter, thus making exhaust fumes bad for environment at the cost of a better engine note.

However, upon my reading it seems that if you were to install a pre-muffler or cat-bypass, it is one or the other.

If one were to modify an exhaust system, am I correct when I say you MUST have:

• headers/heat exhangers (these cannot be replaced by other components the way a pre muffler replaces a cat)
• piping from headers to muffler (your choice of cat, pre-muffler or cat-bypass)
• muffler to tail pipe (e.g., M&K sport muffler)

Lastly, if you get a SW chip, would you have to tune it differently depending on what muffler you have? I'd like the Bisimoto Pulse chamber but as far as I know that only replaces the muffler/tail pipe, to make the most out of a SW chip, I've read that it's best to get a pre muffler or a cat bypass as well.

I intend on keeping the stock heat exchangers on there unless that is a bad idea.

[Bill Verburg]

Originally Posted by ksluna

Hi all,

Wondering if you all can ELI5 (Explain like I'm 5)

Recently purchased a Carrera have decided my first mod will be a SW chip and modifying the exhaust. I know there are a million threads on that but first if someone can help me explain (or confirm if my knowledge is correct) the exhaust system in a 911.

Looking at this image belowand reading some articles this is what I've concluded

• Heat Exchangers are just headers with metal boxes around them to help heat the cabin
• Headers connect to the cylinders and push the exhaust fumes through the exhaust system
• Cat converter makes bad gasses into good gasses (ELI5 version)
• Muffler/baffles make exhaust noise quieter

Here is an image I made from what I understand is the stock 3.2 exhaust system. I just don't see the catalytic converter in it, I did google searches of what it looks like and can never find it in the pictures of stock exhausts.

Now in my research for SW + exhaust there is a lot of talk about pre muffler and cat-bypass.

Can someone ELI5 what a pre-muffler does and how it is different from a cat-bypass? One can assume that a cat bypass is just a pipe that replaces the cat converter, thus making exhaust fumes bad for environment at the cost of a better engine note.

However, upon my reading it seems that if you were to install a pre-muffler or cat-bypass, it is one or the other.

If one were to modify an exhaust system, am I correct when I say you MUST have:

• headers/heat exhangers (these cannot be replaced by other components the way a pre muffler replaces a cat)
• piping from headers to muffler (your choice of cat, pre-muffler or cat-bypass)
• muffler to tail pipe (e.g., M&K sport muffler)

Lastly, if you get a SW chip, would you have to tune it differently depending on what muffler you have? I'd like the Bisimoto Pulse chamber but as far as I know that only replaces the muffler/tail pipe, to make the most out of a SW chip, I've read that it's best to get a pre muffler or a cat bypass as well.

I intend on keeping the stock heat exchangers on there unless that is a bad idea.

Here is a stock 3.2 Carrera exhaust setup, the cat is hidden by the heat shield on the left, This is a very inefficient system from a performance perspective, but is reasonably quiet


Same from the top, to the left is a cat by pass, this only replaces the cat and is only a small improvement


instead of a cat by pass you could use a euro premuffler, also only a small improvement




The best way to improve it is to replace the whole thong from heads to tail pipe w/ something like this

This is a common upgrade used to be SSi but now sold by Dansk, various mufflers could be used w/ different noise and performance levels. The issue here is that these are 1 1/2" systems(1 3/8" ID) and will restrict flow at high rpm, for a 3.2 you really want 1 5/8"(1 1/2" ID) these are expensive. So the question becomes how much do you want to spend for what is going to be a small but meaningful increase in hp?

[darrin]

ksluna --

1) the exhaust system link you posted is not of a stock 911 Carrera setup, which only has one connection to the muffler -- instead, it's of the earlier (and pre-catalytic converter) SSI system.

2) What are you trying to accomplish through an exhaust modification? important that you think of the engine (intake>engine>exhaust) as a system (or, more specifically, one big pump) -- improving the system can only take place if you're fixing a part that's holding other parts back. Put another way, putting a freer-flowing exhaust on your car isn't going to improve power if the intake system is the source of restriction for the motor system/pump. Conventional wisdom suggests that there's little to be gained power-wise by simply swapping out a functional stock muffler and since the SSI headers referenced above have smaller header tubing diameter than a stock Carrera's headers, they actually are MORE restrictive than what you already have. New mufflers can make your car SOUND faster but will generally have minimal effect on actual performance

3) pre-muffler vs. Cat vs. cat bypass -- if your car is subject to smog testing you need to keep the catalytic converter in place. Premuffler/cat bypass tubes essentially perform the same function (replace the space taken up by the cat with something else -- a hollow tube for the cat bypass or a baffled pre-muffler for the pre-muffler), they just each create a slightly different sound when doing so. NOTE -- many people have found out the hard way that the catalytic converter burns the "stink" out of modern gas formulations and that replacing the catalytic converter with a bypass/premuffler results in an unexpected exhaust aroma.

4) Yes, you need headers/heat exchangers to route the exhaust from the engine and provide a place for heat exchangers for in-car heating and defrost. Some heat exchangers bolt directly to the muffler (i.e. the SSI exchangers in the image you linked to) -- others, like the stock Carrera, are designed to accommodate a catalytic converter and require something between the headers and the muffler, and finally, if you want/need to attenuate engine noise you need some sort of muffler.

5) think this is key, you need to decide what you want/can best tolerate sound-wise. The SW chip (or Carrera tuning in general) can be tweaked to optimize the reduced back pressure caused by a freer flowing exhaust system, but you need to live with what you build. See this post Dual exhaust for 2.7 1976 (which also contains a picture of the header configuration that should have come stock with your Carrera) where the poster is looking to replace his bisimoto pulse chamber because it has made him deaf in his left ear . . . similarly, many dual out muffler solutions have been reported to create unacceptable drone while driving at highway speeds.

I've chosen to retain my stock muffler and headers (still going strong at over 180k miles) but have added the M&K active exhaust catalytic converter to the mix (since my Carrera needs to pass annual smog tests) -- am hoping this will give me the best of both worlds -- free flowing exhaust at wide open throttle along with a relatively quiet car when cruising along. Haven't had a chance to drive it yet, but bodywork is nearing completion.

Hope this helps

[ksluna]

Thank you Bill and Darrin fo the explanations! The pictures certainly helped and also thinking of the engine as a conglomerate system rather than components.

I hope this thread can serve others like me who are new to the mechanics of a car and are wanting to learn.

The car is completely stock, I actually haven't even received it yet. Will be getting it next week, but really mainly looking or some quick and subtle HP gains and a better exhaust note. I read good things about bisimoto with the baffle but I will look into it more because I just read another thread on how it was too loud.

[Bill Verburg]

Originally Posted by ksluna

Thank you Bill and Darrin fo the explanations! The pictures certainly helped and also thinking of the engine as a conglomerate system rather than components.

I hope this thread can serve others like me who are new to the mechanics of a car and are wanting to learn.

The car is completely stock, I actually haven't even received it yet. Will be getting it next week, but really mainly looking or some quick and subtle HP gains and a better exhaust note. I read good things about bisimoto with the baffle but I will look into it more because I just read another thread on how it was too loud.

JMO but I'd spend some time listening to different mufflers, You will never get the feel from reading someone else's opinion. What's fine for one set of ears may be less so for another pair.

[darrin]

Well said Bill -- the other point I'd make is to start by getting your car properly set up and running 100% on spec before starting down any "upgrade" path -- while not as "sexy" as new shiny bits, you'll be very well served by first getting the car running as it's supposed to and then assessing what, if any areas of its performance require further improvement. Otherwise, you'll be dumping even more money into making a car running at 75% of capacity with a loud exhaust approach 80% of its capacity, etc.

This can either be done on a (experienced) diy basis through forum research into air leaks, AFR settings, etc. or by enlisting a reputable local mechanic with deep knowledge / experience of/with these air-cooled cars.

Welcome!

[Peter M]

Originally Posted by Bill Verburg

Here is a stock 3.2 Carrera exhaust setup, the cat is hidden by the heat shield on the left, This is a very inefficient system from a performance perspective, but is reasonably quiet

Bill,
I think you are being a bit harsh.

I think from a "bang for the buck" point of view, keeping the standard headers and changing the mufflers is probably unbeatable. Their effective heat exchangers are just the icing on the cake.

I can't see how anyone could justify putting SSI's on a 3.2 except that they look nice.