Hi folks ) I've been absent for several months missing some money to get that car tuned. I still don't know what will be my future plans for that car, however I still like to get in touch with that Cobalt community. Those who knows me will agree that I doesn't like to make some statement without testing or at least without some valid or proven theories. I also think that not enough people (of someone) dynoed test a 3" catback along with a long Header (not a extruded manifold or a shorty "header"). IMO the stock manifold is way too restrictive to establish a valid dyno test of the true optimised gains of a 3" catback vs a 2,5" catback on a modded LSJ. Therefore, here is a simple text I found that is a good starting point for an exhaust calculation.

"As far as ability of an exhaust system to flow enough air for a given amount of horsepower without causing significant back pressure, this is from one of David Vizard's books : For avoiding significant restriction from back pressure, the pipe should flow at least 2.2CFM per horsepower produced. Also, a straight pipe will flow about 115CFM per square inch of area (using inside diameter of the pipe). Mufflers usually won't flow as much as a straight pipe of the same inlet/outlet diameter, so you have to factor that in, also."

According to that text, let's make some calculation for a midly modded LSJ : I+E+H+2.6+HPTuned = 265WHP (Dynojet) = 315 BHP.

315BHP x 2.2 CFM (MIN) per BHP = 693 CFM.
693 CFM divided by 115 CFM per square inch = 6.02 square inches.
Square root of (6.02 square inches divided by 3.1416) = 1.385 inches (Radius of pipe) = 2.77 (diameter of pipe - unfactored). So because you have a muffler + resonator + CAT anything less than a 3" exhaust won't be optimised for the car stated in example. And it should be mandrel bent.

Also, from my own experience, I know Vibrant Performance to be a damn good company making some exhaust systems. I already bought one Catback for my RSX-S and it was 2,5". That's why Vibrant use 3" for the Cobalts SS SC Catback ...

Best regards,



Jean-Marc

 

good post, most of what i've seen about the 3" exhaust is that its too loud and thats the only complaint i've seen.. but i haven't looked long or hard about it

 

but still your saying that only over about 300hp should the 3in have any real effect right?

if im understanding right?

 

You said that all so simply and accurately. I hope a bunch of idiots don't come in and try to argue with it. Bigger exhausts FTW.

 

NO. Let's make the reverse calculation for a 2,5" and 3" ID pipe.

Calculation for a 2,5" straight pipe

(1,25 inch (radius) x 1,25 inch (radius) x 3.1416 x 115 CFM per square inch) divided by 2.2 CFM per BHP = 256 BHP = 215 WHP. Plus the fact that this is unfactored with no bent and no CAT and no muffler.

Calculation for a 3" straight pipe

(1,5 inch (radius) x 1,5 inch (radius) x 3.1416 x 115 CFM per square inch) divided by 2.2 CFM per BHP = 369 BHP = 310 WHP. Plus the fact that this is unfactored with no bent and no CAT and no muffler.

Yes my friends a 3" straight pipe flows 44 % more (310 WHP divided by 215 WHP) than a 2,5" pipe.

This doesn't mean anything more that was is written here. We don't want to start over a war between both Catback sizes. If wou want an OPTIMISED setup, IMO go 3".

Excuse me I don't understand your post. English is not my primary language ...

 

 

I never said that...

 

I've been saying it and saying it and saying it for weeks now and i was flamed for it everyday by noobs with there opinions that it was too big. Ahh...

 

Sorry. If you're being serious then you write very well for a non-english speaking person.

Basically, I agreed with you and made fun of the idiots on this site at the same time.

 

Are the LSJ exhaust ports large enough to get full advantage of the 3" catback ? I don't know ...

Therefore, JBPerformance got 31.2 WHP out of a 3" JBP catback + JBP Intake + JBP 4-1 3" Header. And Weapon-R states 20.3 WHP out of a Long Header with 2.5" collector. This is not a comparison of exhaust sizes, just random dyno data here.

I knew you would have been proud of me ahah )

Talk with me over the phone and you'll see I'm a Quebecer LOL.

 

Don't we know better than to quote or trust JBP by now?

I was looking through the GM Build Book today for an unrelated reason, and GM used a custom Corsa 3" system, not 2.5 or 2.25 or 2.75. They were building a race system for max power, on a basically stock engine with a slightly smaller pulley, still at stock redline I think. The 77.9mm pulley is only 3.06, most of us are smaller than that. I don't really see any reason to go with less than a 3", I know that i'm gonna be rebuilding my system to a full 3" split to dual 2.25 exit, to handle the power I want to make. I shot temps at my DP-Cat-back flange, and there was a 60 degree increase where the pipe stepped down, from 410 to 470F. That alone tells me that there is a lot of restriction there.

 

Good info! I have been waiting to get an exhuast and now when I do I know what size to go with.

 

Also the problem is the head, doesnt flow for ****.

 

ANyone have flow numbers for the stock head?

 

I found this on a Fiero site that swaps to the LSJ. I don't know how accurate it is but here it is:

"The stock four-valve aluminum cylinder head's generously sized intake and exhaust ports provide excellent airflow (250 cfm at .400-inch intake valve lift)"

This is from: http://www.fastfieros.com/turnkey_so...ne_install.htm

 

Hmm.. I wonder if a race port could get close to 300cfm numbers from our head. Wonder what the exhaust flow's.. 220?

 

This is also on the site that I posted: "The cylinder heads were ported and outfitted with dual-coil valve springs and titanium retainers; reground production camshafts actuate the valves through stock finger followers. The original camshaft drive gears were slotted to allow adjustments in camshaft phasing. The camshaft drive uses a stock timing chain, guides and hydraulic tensioner."

And then he goes on to say that it was supposed to make 650HP in a powerband that lasts from 5500-9200 rpm.

Edit: Sorry for the editing the article skips around quite a bit.

 

The GM race head (basically a heavily ported LSJ head), flowed something like 289 cfm. I believe the exhaust flowed around 226 or there abouts.

 

Eww.. 226 on the exhaust? Sad

 

http://performanceunlimited.com/illu...s/headers.html

 

So Jmc007 does this mean that we could possibly run a straight pipe, say possibly a 2.5" inch pipe and a glasspack which has no resistance and not screw our engine up? Ive got an intake, stage 2, about to be a 2.9 pulley with it, and imma be getting a shorty header and a higher flowing down pipe. I will also be getting a Front Mount heat exchanger and dual pass kit. I know my bhp will be close to 300 after the exhaust.

 

https://www.cobaltss.net/forums/show...ghlight=header

 

Also take into account the HP loss from spinning the blower with that 2.6 pulley. I would guess it would be 60+ hp just for that.

 

i don't think i will say this often. but i do agree.

 

Interesting ! Increase of the temp at catback flange says all ...

If you mess with some exhaust tubing, why not use 3" ... At least that is what I would do now ...


LSJ head (exhaust ports) is very restrictive. GMPP build book says the stock head will be efficient (no loss) up to ONLY 275 BHP (230WHP). You now see the problem when trying to make big HPs out of this head ...