I was just wondering what size exhaust originally came on our V6 cars. I have a 85 Sport Coupe w/2.8L engine if it helps. I want to put on a new muffler and cat but don't know what size inlet and outlet to get.

 

Good news and bad news.
Good a 305 V-8 HIPO set up is perfect for our rides.
2 1/2" in n out CC to same sized I pipe to same sized muffler set up.
Bad news
Open hood
See passenger side straight down?
Deduct 5-10 HP for that junction bend.
Have a shop make a newer junction bend w/smoother flow & you'll be sizzling on the street.
NO street legal smog approved headers.
Meaning ya open up smooth that junction flow & add newer exhaust system you could eaasily gain back a solid 15 HP plus upper RPM strength.
BUT age of engine determines how much fun ya get.

 

I don't really understand what you said about junction bend, and 305 v-8 HIPO, CC. Mabye you cold be a little more detailed please.

 

Ok, I don't know what he was talking about... but the stock exhaust on 2.8's, 3.1's and LG4/LO3 engines is 2 1/4 inch and that is plenty of pipe for a 2.8, although you can get better sound out of a bigger exhaust there is no benefit powerwise until you're making upwards of 250 Hp.

Also, the stock y-pipe is horrible on most thirdgens.... headers would be a great addition, but if you can get a muffler shop to custom make a more subtle y-pipe then you'd gain a few HP... that's what I think he was saying, if you look where the exhaust exits the manifold on the passenger side you will see a 90 degree junction... that's why it sucks so much.

FYI any third or fourth gen exhaust will bolt into any third or fourth gen with only minor adjustments to the hangers depending on the exhaust you get.

 

Ohhh yes there is a power increase in upgrading pipe size! But only to 2.5 in...more than that and you've got torque loss. I've got a 2.5 in mandrel bent catback and I easily dropped 1 second off my 0-60. I ran 10-10.5 before ANY of my mods, and after my K&N filter, catback and high flow cat, and minor ignition mods which don't do much for performance, I pulled 8.28. And yeah, the stock Y-pipe is disgustingly bad. I believe a Y pipe off of a Blazer can be modified to fit ,but don't quote me..I heard it through the grapevine.

 

Yes, I have meant the junction pipe outta passenger side exhaust set up yeah the suck away 5-10 HP "Y" pipe.
I have a "Y" from my latest 3.4 purchase.
So that Item will fit, easy?
THANK YOU!
The factory 3.4 sized pipe I got ith the 3.4 for my Blazer is WAY LARGER than the old 1985 Firebird "Y" pipe stuff
I SO NEED A NEW CC AND MATCHING 2 1/2" I PIPE for my 3.4 Firebird.

My 3.4 Blazer?
I'm not sure what I'll do.
I can get Blazer headers, easy.
I already run the original exhaust manifolds & "Y" pipe to a 2 1/2" in & out CC to a 2 1/2" I Pipe to a Flowmaster 2 1/2" to two 2" out rear exit split pipes. Flowmaster said my exhaust design easily can handle up to 300 HP.
When I installed that set up on the 1985 stock running 2.8 carb set up, ZOOM! MY POWER INCREASED SO STRONG!
I truly want to gain the smoother power of the MPFI or the SFI set up I have still, under Blazer hood.
That MPFI SFI set up HP gain over the current factory carb set up is like 20+HP.
ONLY WISH
GM's new 60* manifold had an EGR set up on them.
They are smooth flow design, and seem to LOOK larger ports.

For a very cheap premade exhaust set up & HP gain, seek to purchase an exhaust set up from/for a 305 powered F Body, max to 2 1/2" pipes.
Recreate your current "Y" pipe to a smoother "Y" pipe set up & GO have fun! Keep factory manifold exit pipes to 2 1/4" MAX, to a larger sized "Y" pipe juction to a 2 1/2" exit of "Y" pipe.
You gain a solid RPM range of up to 5,000+.
Current factory "Y" pipe set up chokes up and peak HP is made by 4,000 RPM range and only wasting gas staying at higher RPMS except for an upshift advantage.

 

If your y-pipe is appropriately sized the there should be no torque loss from any [larger] sized pipe after that point.. it is a common misconception that has not actually been proven that big exhaust will lose you power... people that get big exhausts may not feel the same SOTP after putting the exhaust on are actually experiencing a smoothing out of the pattern... more power everywhere so it seems like off the line power is robbed.

With the v6, it's really the muffler and cat that holds you back power wise... headers probably help immensely too, if possible I'd change the whole exhaust from start to finish.... 2.5" just to optimize everything.

 

For EASE OF PASSING CA SMOG RULES, I keep exhaust manifolds.
I just got exhaust ideas and answers.
Pipe work ain't cheap.
I got some extra pipes and stuff and will one day do the $$$$.
Wish I could bend/weld the stuff myself.
Ain't worth the effort, as I go to a great muffler guy!
I rent his talent & tools for the price of the exhaust system. How often ya do pipes? Once!

 

I can't see how the pipe diameter wouldn't matter past the Y pipe. Ok, so let's say the Y pipe is restrictive and small, but the exhaust system is large past it. The exhaust gas flow will be hindered by the Y pipe, and will pick up velocity and maintain heat from this because it's more dense, correct? BUT, what happens once it gets past the Y pipe? It expands out to the large side of the pipe. With that expansion, it loses velocity because the exhaust constantly exiting to push the older fumes out is too thin so it lacks the same force. This results in loss of exhaust velocity. The larger diameter at the same time causes loss of heat of the exhaust gases. Heated particles move faster than colder particles. Therefore, in theory, larger diameter pipes, even past the Y pipe, should slow down the exhaust in the pipe, thus actually CREATING backpressure on the freshly exiting exhaust. If anyone can make sense of any of that.........

 

I don't think it matters. I think that the only thing that matters is a restriction at some point... so the engine has to push a certain ammount to expell the gasses from the headers... I don't think the velocity of the exhaust has much of anything to do with the whole thing... All the performance exhaust info I've ever read say the same thing. Feel free to look aroud for it, I know that leaving the exhaust small probably doesn't hurt performance for these engines, but I think a better sound quality can be reached with the right size.

Edit: the temperature of the exhuast has nothing to do with the speed of the particles, only the density... Hot particles are more active, but when they are launched at some speed they don't go different speeds because of their temperature.

 

*** I wish I could remember who it was....but there's a guy on the boards that's like some kind of exhaust genius or something...he could straighten this out. I remember there being a similar debate on this kind of topic and he had last words I think.... Ahh...maybe it'll come back to me. I see what you're saying though, but I still think exhaust size matters past the initial restriction.

 

For a 2.8l, 2.5 inch exhaust is plenty of pipe, another wider is just unnessary.


I think the stock ones were 2 1/4"

 

Right on that one, Dev...

 

Too large of primaries cause exhaust gases to cool down & slow thus restricting flow just as much as too small of pipes would. It needs to be married to the flow of cfm flow of the motor. the displacement and cfm flow of these motors will not hold primaries any larger than 1 1/2" and y-pipe collector/primaries 2", then y-pipe collector to catback at 2 1/2". Any larger will hurt performance.

 

A FREAKING GOOD TIME is the genius of the pipes.
Well, a few women in my past, also!

 

I always assume bigger is better, but that is not the case for everything. The power gains and losses is a bit more complicated than I ever knew.

 

Without a degree in physics
It's really simple

Take what the factory created for ya.
Add your real world usage (RPM driving range, racing, street driven, hill climbing, Stock car racing!)
Add your budget
Add recommendations
Ask manufactures/engineers (use their brains, that's what they do for a living)
Keep the project common sense
Keep it street legal
Simple!