CatBack/ Exhaust size.

[TheStupidFock]

I'm looking at maybe getting a Borla Cat Back in the near future. I just don't know if I should get the 2.5" or 3" piping for it. Some people are telling me that the 3" is too big, and that I should go with the 2.5" others disagree.
I havn't been able to find any dyno charts between a 2.5" and 3", but since I plan on modding the car, should I just go ahead with making a 3" system?
If it helps any, after the Cat Back I plan on getting a high flow cat, ceramic headers, and then a performance chip.

Thanks
-Fil

[99W68Firebird]

yea i had same decision to make, so all i did was get size in between .. stock I-pipe off Z-28.. 2.75" and just had shop hook it up to my flowmaster muffler when i got rest of my exhaust installed.

[KBeezy]

3" stainless steel system here on my 3.4..

I love it. [img]smile.gif[/img]

[1DangerousV6]

3" stainless here on my 3.8 with hollow cat ...it rocks! [img]smile.gif[/img]

[AaronG]

3" here, very nice too

[BobsSilverSix]

3" kicks ***.

[HBHRacing]

3 is too big. you run into a problem with trying to fill more exhaust volme then your motor can push out. when you get that you lose low and untill you hit high enough RPM's to compinsate. Even on the V8's 3 inch is a lot, we still lose low end from it.

[KBeezy]

Originally posted by HBHRacing:
3 is too big. you run into a problem with trying to fill more exhaust volme then your motor can push out. when you get that you lose low and untill you hit high enough RPM's to compinsate. Even on the V8's 3 inch is a lot, we still lose low end from it.

WHAT!? 3" is a lot on a V8? Kiss my *** dude, they make 4" catback systems for 4th gens. I know tons of LS1 and LT1 guys all motor running 4" exhausts. 3" too much? Dude, you dont know wtf you are talking about. You probably think the motor needs a lot of back pressure. BS. I ran open downpipe on my 89 Probe GT Turbo and you know what.. I didn't LOSE any power, it just ADJUSTED the power curve and actually INCREASED my power on the top end of the curve. I didn't LOSE anything. With exhaust you have 2 options. Go narrow and get low end grunt.. Or go big and have top end power. If you want speed, you want a lot of power up top -- if you have it all down low you'll have a good hole shot but once you start to really rev you aren't going to have ****.


And in fact, there was a 3.4 guy who dyno'd after he got his 3.4 kit and he picked up I think 8rwtq and 6rwhp over stock. Sure, its not much, but when you consider he was running stock intake, stock cat converter and forward...thats not bad at all. Lose low end power my ***.

[Brandon83]

3in here and love it. P.S Dont waste your time on a chip their crap. And on the lose power part.... [img]graemlins/bs.gif[/img]

[BLLDOGG]

Originally posted by HBHRacing:
3 is too big. you run into a problem with trying to fill more exhaust volme then your motor can push out. when you get that you lose low and untill you hit high enough RPM's to compinsate. Even on the V8's 3 inch is a lot, we still lose low end from it.

some how my post never got posted so Kbeezy bet me to it. 3" is too big is total [img]graemlins/bs.gif[/img]

i have 3"

[98Silver6]

3" all the way baby, don t even think twice!! lol
just my opinion<<<

[HBHRacing]

maybe you should go and get an education before dableing in the relms of exhaust theory. or better yet. go read a book or 2. If I saw a fairly stock LS1 owner that didn't have a high compression large lift cam or FI of some kind running 4 inch exhaust I would slap him. 4 inch is for disels, not fbodys. Even with boost 3.5 would be fine. do you understand the concept of volume of a cylinder? do you understand what actualy happens in the exhaust system? look at a dyno graph with JUST a 3inch exhaust. you :might" gain some HP but look at the TQ curve and you will see that it shifted far up the RPM range.

I realize that people hate change. I also understand that what I say goes against what most people think. but do you know why? because learned reactions are not always right. They are just preveiced to be correct.


and before you go stabbing guesses at what I belive about back pressure why don;t you ask me. tell you what.. lemme ask you. in PSI tell me what would be considered high back pressure at 2500 RPM and idle. And why don't you tell me what is considered normal. I would be suprised if you even come close to that.

now. if you ever frequent some of the other boreds you will see things like.. "installed true duals and lost power" This is due to what I was saying before. There is not enough exhaust flow to fill the voulme of the larger pipe untill a higher RPM. Obviously a V6 will not make as much air flow as a v8 so the condition is even more severe. This is why disels have such large exhaust and why small motors like a honda have a tiny little exhaust system.

I recomend that you read a book or 2 on exhaust theory and then come back to me. I hear that Scientific Design of Exhaust & Intake Systems - Third Edition
by Philip H Smith and John C. is very good. with that you can understand things like wavelength, pressure pulse, reflection and more.

there is consideration to velocity based on pipe diamiter but only when there is enough volume to fill that pipe. there is a LOT that goes into exhaust tuning. but if you actualy take the time to learn the basics that you will come to see the same things that are tought to me. If bigger indeed was better then every car out there would have 3 inch exhaust and sports cars would have 4 inch. look at what manufactures do and you can see that there is logic behind it.


am I a genius? hell know. But I do pay atention to the people that are automotive gods.

Maybe next week I can tell you why a motor running 250 degrees or more actualy makes more power thena motor running 140 and throw even more traditional ideas out the window [img]smile.gif[/img]

[TheStupidFock]

Ok, thanks for that part of my decision, now for what brnad to go with.

I know that Borla is good, but it is a bit pricey. Others have been telling me that the better buy is a Magnaflow or a FlowMaster. What I really want to know is which system works the best and has the best sound.

Thanks again
-Fil

[BLLDOGG]

Originally posted by HBHRacing:
maybe you should go and get an education before dableing in the relms of exhaust theory. or better yet. go read a book or 2. If I saw a fairly stock LS1 owner that didn't have a high compression large lift cam or FI of some kind running 4 inch exhaust I would slap him. 4 inch is for disels, not fbodys. Even with boost 3.5 would be fine. do you understand the concept of volume of a cylinder? do you understand what actualy happens in the exhaust system? look at a dyno graph with JUST a 3inch exhaust. you :might" gain some HP but look at the TQ curve and you will see that it shifted far up the RPM range.

I realize that people hate change. I also understand that what I say goes against what most people think. but do you know why? because learned reactions are not always right. They are just preveiced to be correct.


and before you go stabbing guesses at what I belive about back pressure why don;t you ask me. tell you what.. lemme ask you. in PSI tell me what would be considered high back pressure at 2500 RPM and idle. And why don't you tell me what is considered normal. I would be suprised if you even come close to that.

now. if you ever frequent some of the other boreds you will see things like.. "installed true duals and lost power" This is due to what I was saying before. There is not enough exhaust flow to fill the voulme of the larger pipe untill a higher RPM. Obviously a V6 will not make as much air flow as a v8 so the condition is even more severe. This is why disels have such large exhaust and why small motors like a honda have a tiny little exhaust system.

I recomend that you read a book or 2 on exhaust theory and then come back to me. I hear that Scientific Design of Exhaust & Intake Systems - Third Edition
by Philip H Smith and John C. is very good. with that you can understand things like wavelength, pressure pulse, reflection and more.

there is consideration to velocity based on pipe diamiter but only when there is enough volume to fill that pipe. there is a LOT that goes into exhaust tuning. but if you actualy take the time to learn the basics that you will come to see the same things that are tought to me. If bigger indeed was better then every car out there would have 3 inch exhaust and sports cars would have 4 inch. look at what manufactures do and you can see that there is logic behind it.


am I a genius? hell know. But I do pay atention to the people that are automotive gods.

Maybe next week I can tell you why a motor running 250 degrees or more actualy makes more power thena motor running 140 and throw even more traditional ideas out the window [img]smile.gif[/img]

your theory about why manufact.s make them with small exhaust pipes is wrong. its to keep them quiet, not to gain HP.

[WickEdSix9838]

well i know this because ive done it on my own personal car....I used to have a 2.5" catback with a flowmaster and a 3" cutout right before the muffler, best MPH i ever ran at my local track was 86.7 on a cold winter day...I put the 3" TSP Rumbler on my car and ran a new best MPH of 88mph in late spring early summer at the same track. This was all on a stock motor w/stock manifolds and cat. I myself used to be the same way as you HBH but I got a small discount for being TSP V6 test car for the Rumbler and said screw it i'll do it. I now know 3" isnt too big and will outrun a 2.5" exhaust. If you're still scared of 3" atleast use the 2.75" I-pipe off of a Z28 or TA.

Maybe next week I can tell you why a motor running 250 degrees or more actualy makes more power thena motor running 140 and throw even more traditional ideas out the window

and regarding this statement, all i had ever been told was that same statement, a hot motor will make more power BUT why would NHRA Pro Stock, the pinnacle of NA racing engines, be running ice water through their motors after ever pass and their engine water temperature only be 103* at the end of a pass? one of my dads customer saw some teams doing this w/some kind of pump between passes at the nationals in Ennis, Texas this past weekend. Also we have some customers that run Competition Eliminator and they run their motors as cold as possible every pass. The motors always make more power on the dyno cold and run down the track quicker cold.

[ September 29, 2004, 11:00 PM: Message edited by: WickEdSix9838 ]