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View Full Version : larger exhaust diameter on a forced induction car?

Sonic
03-10-2010, 10:32 PM
I will soon be ordering my new header & down pipe but I'm looking for some info before doing so. I currently have 2.5" exhaust from the cat back but most Cobalt SS owners have gone with a full 3" exhaust. I've heard mixed opinion both pros & cons about doing so (not enough back pressure, etc). So I'm asking the forced induction guys here on CSS for your suggestions. Is it worth upgrading everything to full 3" or just keep it 2.5" I already have?
1Panel2NV
03-10-2010, 10:41 PM
What header and dp did you have in mind?
Sonic
03-10-2010, 10:44 PM
What header and dp did you have in mind?

Clear Image Automotive Mid-length 4 into 1 header with 1 3/4" primary tubes and their down pipe with cat delete

Link to Clear Image (http://www.clearimageautomotive.com/Chevy%20Cobalt.htm) :checkit:
1Panel2NV
03-10-2010, 10:56 PM
What cat back do you have now? That's probably going to be loud lol. Are there any videos of a similar setup? I'd be worried about a raspy sound if you don't have a resonator with a catless dp.

I would go with 2.5''. What do you plan on doing as far as tuning?
Sonic
03-10-2010, 10:59 PM
What cat back do you have now? That's probably going to be loud lol. Are there any videos of a similar setup? I'd be worried about a raspy sound if you don't have a resonator with a catless dp.

I would go with 2.5''. What do you plan on doing as far as tuning?

I'm keeping a long resonator with a Dynomax Super Turbo muffler. The car will also be getting a 2.8" supercharger pulley, 60# fuel injectors & a tune from OTT Performance (www.ottperformance.com)
Mr. Incredible
03-10-2010, 11:09 PM
I wouldnt waste the money on upgrading from 2.5 to 3 if you already have it.

I'm sure there is some perf gains to be had but couldnt you make more gains somewhere else w less money
sr20ser
03-10-2010, 11:17 PM
Okay, here is what I have learned from various tuners, and some practical exercise. Back pressure is bad. Back pressure is not good... Ever. The thing that you should care about is exhaust gas velocity.

When there is back pressure, it will slow the gas. When a tube diameter is too large, once again velocity is lost.

As an example, we lost about 30 horse power on a TT Viper when we went with an all 4" exhaust. We regained velocity by adding a 3.5" reducer after the downpipes, and then another reversed to go back to the 4" and subsequently we regained the 30 and then some.

We used a velocity gauge and a magnehelic gauge to test the back pressure, and the HP was on a mustang dyno (without unstrapping).
1Panel2NV
03-10-2010, 11:21 PM
Whatever you decide, it should be fun to drive. You should get your car dynoed this summer...I know where you could go.

Okay, here is what I have learned from various tuners, and some practical exercise. Back pressure is bad. Back pressure is not good... Ever. The thing that you should care about is exhaust gas velocity.

When there is back pressure, it will slow the gas. When a tube diameter is too large, once again velocity is lost.

As an example, we lost about 30 horse power on a TT Viper when we went with an all 4" exhaust. We regained velocity by adding a 3.5" reducer after the downpipes, and then another reversed to go back to the 4" and subsequently we regained the 30 and then some.

We used a velocity gauge and a magnehelic gauge to test the back pressure, and the HP was on a mustang dyno (without unstrapping).

Turbo cars are a little different than supercharged as far as back pressure, but I don't want to get into that.
sr20ser
03-11-2010, 12:50 AM
Turbo cars are a little different than supercharged as far as back pressure, but I don't want to get into that.

I disagree. An engine is an engine, is an engine, regardless of what is bolted to it. Back pressure is bad. I am not saying that I am an expert in the matter, but I have been around enough tuners, engine builders, head porters, and as well as exhaust fabricators. The consensus among all of them, is that back pressure kills.

Here is a pretty good pdf that explains the whole back pressure/scavenging issue.


http://www.veryuseful.com/mustang/tech/engine/exhaustScavenging.pdf

Here is another link with some more data that shows what I am talking about.

http://www.thrashercharged.com/tech_htm/exhaust.shtm

And of course, here an expert in the exhaust world.

http://www.magnaflow.com/07techtips/faq/question10.asp
http://www.flowmastermufflers.com/index.php?page=54
standard235
03-11-2010, 05:47 AM
Backpressure bad. You want that exhaust scavenging effect. However, on a turbo car, that is null and void. You want it as short as possible and big, but not so big to greatly reduce exhaust velocity.

On N/A and SC cars, its way more important...

Then you get into cam overlap and all that...
IlliniJeeper
03-11-2010, 09:47 AM
What pulley are you running, Sonic? I've heard that 3" isn't effective until you get into really small pulleys. If you're running a 2.9" or 3.0" pulley, I'd think a 2.5" exhaust would be best for you.
blackedout06
03-11-2010, 10:37 AM
Clear Image Automotive Mid-length 4 into 1 header with 1 3/4" primary tubes and their down pipe with cat delete

Link to Clear Image (http://www.clearimageautomotive.com/Chevy%20Cobalt.htm) :checkit:

I'm keeping a long resonator with a Dynomax Super Turbo muffler. The car will also be getting a 2.8" supercharger pulley, 60# fuel injectors & a tune from OTT Performance (www.ottperformance.com)

thats crazy, I wouldnt go with that setup.....:paranoid:

yeah im staying with 2.5", i dont want the hassel of switching it to 3" later. i'll just be happy when my header and downpipe show up so i can order my kit
Sonic
03-11-2010, 04:35 PM
What pulley are you running, Sonic? I've heard that 3" isn't effective until you get into really small pulleys. If you're running a 2.9" or 3.0" pulley, I'd think a 2.5" exhaust would be best for you.

Soon I will have a 2.8" pulley
IlliniJeeper
03-11-2010, 04:41 PM
Soon I will have a 2.8" pulley

You just like to make things difficult, don't you? :lol:
Sonic
03-11-2010, 04:45 PM
You just like to make things difficult, don't you? :lol:

Not trying to. :lol: I'm doing the exhaust now & trying to set it up for what will be coming in a few months. :shut: That's why I'm asking the question 2.5" or 3"
IlliniJeeper
03-11-2010, 04:52 PM
I think most people running 2.8" pulleys use a 3" exhaust.
ryans88gt
03-11-2010, 05:10 PM
Best exhaust on a turbo car is an open turbo.

There is absolutely no benefit to having an exhaust attached to the turbo other than to help dissipate heat and noise away from the turbo location.
Sonic
03-11-2010, 05:17 PM
Best exhaust on a turbo car is an open turbo.

There is absolutely no benefit to having an exhaust attached to the turbo other than to help dissipate heat and noise away from the turbo location.

My car is supercharged though. The turbo Cobalts came out in '08 & mine is an '07
standard235
03-11-2010, 08:34 PM
Pacesetter LSJ Armor Coated header (1.875" primaries) and a 3" headerback with the Super Turbo would be good..
Robin
03-11-2010, 09:06 PM
It's been my experience with any power adder combination Turbo, Supercharged and Nitrous they all gain from an increase in the primary header size as well as exhaust. It's the Naturally Asperated engines that need to have the exhaust in a certain range to properly evacuate the cylinder and aid in filling the cylinder.
I have a TVS 1900 CTS-V engine in my Camaro with a 2.55 upper and a 19% overdrive lower. I run the Kooks 1 7/8" headers and a 3" exhaust.
It is LOUD. Almost too loud for the slight gains from the larger primary and 3" exhaust.

The car makes about 700 at the crank given it is 594 at the wheels through a 4L80E and a 4000 Stall.

I am going to install cats just to quiet it down a little:whacko:

Robin