I would like to expose my opinion about the exhaust on Cobalt SS / SC. By some theorical calculations I've made and personal experience ... here is also a good web site which helps me a lot.

http://popularhotrodding.com/enginem...exh/index.html

Firstly, most stock Cobalt SS / SC have been dynoed (on a Dynojet) to an average of 208 WHP (on 4th gear), which equals to roughly 245 BHP (brake horsepower; power of the engine) (due to the drivetrain loss).

According to the web site, a section of 2,5" pipe will flows 560 CFM. To be sure there isn't any HP loss due to unwanted backpressure of a too small pipe, your engine needs 2.2 CFM per brake horsepower. So 560 CFM divided by 2.2 is good for 254 BHP. It simply means that a 2,5" catback on a stock Cobalt won't cause any HP loss (versus a 3" one), unless other modifications arrives.

If you plan to go up to 300 BHP (roughly 255 WHP) but not more, the 2,5" catback (flowing a max of 560 CFM) will allow the engine to breath a maximum of 1,9 CFM per BHP. At 1,9 CFM per HP, the HP loss due to backpressure is roughly 2 % only, which won't affect the engine too much (5 WHP or less). Personnaly, since I don't plan to go higher than 300 BHP (or roughly 255 WHP), I won't go larger than a 2,5" catback.

About the stock catback, there is a rear section which is 2,25" in diameter. This section is already maxed for the stock 245 BHP. About the stock rear muffler, it doesn't flow enough too. Those two parts together cause a HP loss of 8 to 10 WHP on a stock SS / SC. A very simple thing to do is to buy the 2,5" Magnaflow catback (available from Modern Performance). But for those like me who want to make a custom one, here is a how-to link which will give roughly the same results than replacing the entire catback.

https://www.cobaltss.net/forums/2-0l-lsj-performance-tech-47/how-custom-magnaflow-vibrant-2-5-ss-catback-intake-mod-parts-needed-5478/

About the stock header on the Cobalt SS / SC: It has roughly the same inside dimensions than the 2,2 header, so GM has voluntarily got the car detuned the. It is the most restrictive part on this very car (2.0 SS / SC's and Ion Redlines). For those who are replacing the stock catback, but don't plan to replace the stock restrictive header : The stock header inside diameter (where it meet the downpipe) is less than 2 inch; it totally kills the car and cause several HP loss. You will be able thus to get some HP gains with good kits (like the PSI-FI one), but won't be able to achieve maximum results. Also, the stock header stresses the engine a lot.

Have a nice day ! More to come ...

 

great write up, thanks for the info.

 

gm didnt build and desing everything for max power

they wanted to keep the tq up in the range you drive the car the most , that is how they set things up


nice info though

 

Have you dyno'd that beast of a car you have? You have seem to upgraded all of the major weeknesses. Thanks.

-Rich

 

I will get the header and the CAI installed this very friday, stay tuned for dyno results and pics.

 

tagg

 

This is a good write up. My question is how can you base everything on the diameter of the exhaust? Stock SS/SC exhaust is 2.5" but the higher flowing GMPP exhaust is only 2.25" so what happens to the calculations there?

 

surface finish plays a big role in this..

and i imagine if you have a certain amount of back pressure it will increase torque values.. GMPP exhaust is probably 2.25 alll the way throuhg the muffler and everything, and the gmpp extrude honed manifold is probably roughly the same, but just honed out a bit, and honing creates an amazing surface finish, as if the inside was polished..

lots of little things to think about.

 

The stock exhaust is 2.5" until before the axle when it goes to 2.25". Also, the stock exhaust is crush bent versus an aftermarket one which is typically mandrel bent. The piping makes a difference, but some of the biggest restrictions are in the muffler.

 

Decent write up

Just listening in on this one

 

Yeah this is exactly my point. We can't base everything on diameter.

 

I have the GMPP manifold sitting on my lap right now and I can tell anyone who wants to know that it sure aint that great.I consider it a starting point.The only place inside that is anything close to being polished is at the very exit where it opens up to meet the Cat.I'm guessing that is the end that they force the honing grit through.Its still real rough inside from being cast.Me,my die grinder,a couple of dozen grits and buffs,then I'll have something ready to send for Jet-Hot coating.

 

Reading back at the article from Popular Hot Rodding (link in my first post), the maximum CFM (maximum CFM level not causing restriction) for a 2.5" straigth pipe should have been posted at 509 CFM (not 560) using their own formula of 115 CFM per square inch of pipe area. Now this is without any bents or muffler.

I also found this on the Corsa web site and their 2.5" (O.D.) catback flows "only" (maximum CFM level not causing restriction) 411 CFM, which is still an improvement over the 303 CFM stock catback.

http://www.corsaperf.com/media/pdf/CHEVY%20COBALT.pdf

http://www.corsaperf.com/product_dis...px?product=222