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View Full Version : 2.5" or 2.25" cat back ?? thoughts??



02blksi
09-15-2002, 09:04 AM
getting my N1 installed this weekend, and i was wondering what everyone thinks is good for replacing the b pipe. I read the last issue of honda tunning and they did a write up on the k20a engine and they quoted DC sports talking about the issues that they had making an exhaust that benifited the k20 engine. They stated that when the car was fitted with a 2.25 exhaust and a 2 and 3/ 8ths exhaust they BOTH LOST POWER. Then they fitted a 2inch exhaust (cat back) on the car and power levels rose to "almost stock levels" finally they tried 2.5 and peak hrspwr rose 6hp and trq 3ftlbs across the entire rpm range. Now usually i would think that a 2.5 exhaust is rather larger diameter for an N/A application, BUT, i feel that this new engine prolly would benefit it better than the b series due to its larger capacity and seeming better flowing head. But hey what do i know. Any thoughts ??? what would you do ? 2.5 cat back? --thanks - joe

David K.
09-15-2002, 03:30 PM
I currently have a crush bent 2.25" mid on and I will be replacing it with a 2.25" mandrel bent mid w/ resonator. I don't think 2.5" will be too severe considering our displacement; you might lose a little low end but it should support your future performance mods no problem. I say go for it, dyno it and let us know. Good luck.

02blksi
09-15-2002, 04:50 PM
yea the more i think about it and re-read the article written in Honda Tuning referrinf to what DC sports found to be beneficial on our cars, i feel rather confident that 2.5 is the way to go. But im going to be sure and save the old piping just in case i do not like it. -thanks --joe

JSIR
09-15-2002, 05:11 PM
for a complete aftermarket system I think 2.5 inch mandrel bent piping may work, but you may lose a bit of bottom end. If someone is keeping the stock muffler I think you can go as low as 2 inch mandrel bent considering other restrictions in the system. 2.25 inches may be a better overall diameter .

I also read the DC sports opinion, but I think their testing related to the RSX-S engine more than our k20. The RSX-S engine revs much higher than our engine and it has larger cams, thus much higher flows. I know DC uses 2.5" piping for all k20's as does Comptech but I think 2.25" is better for the Civic and base RSX. I lost some decent bottom end torque going from a 2.25" midpipe to a 2.5" midpipe with no gains on the top end or anywhere else.
So I am kind of biased towards 2.25".

Just my opinion.

02blksi
09-15-2002, 06:26 PM
"I lost bottom end torque going from a 2.25" midpipe to a 2.5" midpipe with no gains on the top end or anywhere else. "

How do you know.?? did you dyno the car?? not trying to flame rather im just curious.! thanks for the input, i really dont know what to do 2.25 or 2.5? since my N1 inlet is 2.5..--joe

02SilverSiHB
09-16-2002, 04:27 AM
if you're going to go turbo or supercharger later, get a 2.5

JSIR
09-16-2002, 02:24 PM
Originally posted by 02blksi
"I lost bottom end torque going from a 2.25" midpipe to a 2.5" midpipe with no gains on the top end or anywhere else. "

How do you know.?? did you dyno the car?? not trying to flame rather im just curious.! thanks for the input, i really dont know what to do 2.25 or 2.5? since my N1 inlet is 2.5..--joe

yeah I dyno'd the car at each level. Stock - 2.25" pipe, and 2.5" pipe. I lost torque at all levels after the stock piping.

I guess if you have a muffler that has 2.5" piping you should try to match it up, a 2.25" midpipe probably wont save you any torque if the muffler is opened up like that . So match it up and hope for some good top end to offset the loss on the bottom end. You may want to test it out first with some cheap crush bent aluminized pipe before you committ to a permanent more expensive pipe. That's what I have been doing.

02blksi
09-16-2002, 03:45 PM
jsir so what were your numbers? on the dyno when you tested them all? (rough guess is all im looking for). stock, 2.25, 2.5? thanks oh btw what setup do you have now a crush bent 2.25 cat back with no resonator? thanks again -joe

CleanBlackSi02
09-16-2002, 06:12 PM
My Thermal catback is 2.5" and it feels great. It's just begging for some forced induction though. :D

JSIR
09-16-2002, 06:46 PM
Originally posted by 02blksi
jsir so what were your numbers? on the dyno when you tested them all? (rough guess is all im looking for). stock, 2.25, 2.5? thanks oh btw what setup do you have now a crush bent 2.25 cat back with no resonator? thanks again -joe

My torque started right around 126 ft lbs at 2500 rpms, then you get the mid range dip and then it climbs back up across the upper rpm band on the stock setup. Well after the 2.25" mid pipe it dropped to around 124 ft lbs and there was a consisted 2-3 ft lbs lost torque across the low to mid range rpm band. Upper rpm band was the same as stock, maybe a 1 hp gain. Then I tried the 2.5" mid-pipe and got a further loss of about 2-3 ft lbs torque from 2500 rpms to mid range, torque was down to 121 or so, upper mid range may have picked up 2 hp max. , if not less . I was very frustrated to say the least, as my earlier posts have indicated. The car felt good on the street still, but I did notice a bit less bottom end just going to the 2.25", but on the dyno it seemed to have lost a good deal of torque.

One thing I have yet to test is an intake, it may very well be the intake will make the other parts work better, I have yet to test it though. Also the stock mid-pipe is so damn small it could only be for maximized bottom end torque, there is no exhaust that could increase bottom end torque more than the stock midpipe that thing was made for torque.

02blksi
09-16-2002, 07:11 PM
so after all your findings what would you recomend me to do? im still undecided.
my choices:
1. leave the stock b pipe and just add a rear section to it with my N1 on it -looks, sound, same power output
2. change the entire catback to 2.25 or 2.5 with a resonator
3. same as above but without a resonator



what would you do???????

JSIR
09-17-2002, 05:14 AM
I don't like talking people into mods especially since Im still trying to work out the bugs in my setup.

But it all depends on your future setup, if you plan on getting a header and intake or if you already have such parts then I would say ditch the stock midpipe and go with 2.25 or 2.5 midpipe with a resonator (unless you dont mind very loud exhausts in which case you can ditch the resonator, but it will be loud). I will probably settle for a 2.25" mandrel bent midpipe for myself once I decide to get a high quality midpipe, but I am keeping the stock muffler, I may even consider a 2 inch midpipe for my setup, but it is quite a bit different from yours.

If you aren't in a hurry to get the exhaust finished and you want to hold on to your money for a little while , you can try the N1 muffler on the stock midpipe and see how you like it first. You may end up hating the muffler and want to get rid of it. I would probably try that first and see how you like things. Then work on a midpipe in the near future. Its hard to say, as everyone has different expectations. hth

CleanBlackSi02
09-17-2002, 06:20 AM
Originally posted by 02blksi
so after all your findings what would you recomend me to do? im still undecided.
my choices:
1. leave the stock b pipe and just add a rear section to it with my N1 on it -looks, sound, same power output
2. change the entire catback to 2.25 or 2.5 with a resonator
3. same as above but without a resonator



what would you do???????

If I was planning on minor mods I'd get the 2.25 without the resonator....with more serious stuff in the future I'd get the 2.5 w/o. Just me.... Then again, that DC sports article said that we lose HP with 2.25, so I duno.

02blksi
09-17-2002, 11:23 AM
thank guys -- Im off to the muffler shop right now, i think im going to go with what jsir said, since i want to make sure i even like the N1 im just going to have the muffler shop do a rear section replacement, this way i can see if i like the muffler enough to start hacking up my oem b pipe, plus im going to wait and see what combo b pipe/ exhaust/ intake makes the best performance.. THANKS FOR THE INPUT!!!!! --(JSIR especially:) ) --joe

BTW :
I will post some pics soon after my exhaust is on. --

02blksi
09-17-2002, 06:21 PM
WELP, i just got back from getting my exhaust on. I have spent the last 4 hrs. drving my car and loving every second of it ! This is what i went with. I had the shop here make a "CUSTOM" rear BOLT ON section for my N1. MAN they did a great job, and believe it or not i noticed the car is a bit peppier, I REALLY believe the true bottleneck of our exhaust system is not the b pipe diameter rather it is the last resonator and the stock exhaust itself. Even though the stock muffler is 2 inch it is really just a big cork, to many baffles in it (to silence the exhaust) which inhibits flow. Also i did not know that inside the resonator was another pipe (smaller diameter 1.75 inches) I chose to leave the stock resonator in place since i wanted to be able to swap exhausts if i get annoyed with my new one (which wont be happening anytime soon!! LOVE IT!!! GET THE NA N1).. The sound is GREAT, i compare it to my Av6 which had the same distinctive Vrroooom Vrroommm and not that tiny raspy screw bouncing in a coffe can sound.. I LOVE THIS EXHAUST!!!!! --I wil post some pics tomorrow when i get back from work! I was out later than expected tonight enjoying my new exhaust!


cliffnotes:
N1 and custom Flanged rear section (kept the stock flange so i can swap rear sections when needed :) ) feels peppier, crisper throttle response, and killer sound!

Notes:
I believe that the stock piping is just fine for now, i dont have enough mods to justify bigger piping at the moment (only other mod is a SRI), plus i dont want to hack my car already -to new-
my observations on the oem exhaust - not all that bad stainless, decent size, if i were to go for performance i would just elminate the resonator and the stock muffler. just my observations. --joe

02blksi
09-18-2002, 03:26 PM
PICS as promised. --:) joe
http://boostedhx.20megsfree.com/images/mvc-008f.jpg
---back view
http://boostedhx.20megsfree.com/images/mvc-007f.jpg
---custom rear section with OEM flange so i can take the exhaust off and replace with my OEM one with ease!
http://boostedhx.20megsfree.com/images/mvc-002f.jpg
---side view MMmmmm
http://boostedhx.20megsfree.com/images/mvc-006f.jpg
--- CLOSEUP APEXI :)


ENJOY, --

CleanBlackSi02
09-18-2002, 03:32 PM
Looks great! I wish Apex made a full catback N1 for our cars.....I have a feeling they might one of these days. :rolleyes:

Ritteri
09-18-2002, 04:27 PM
I myself have decided to wait until next summer before I make ANY mods to my car. There will be alot more data out by this time next year, and my car will be fully broken in by then. I dont see the point in wasting hundreds or thousands of dollars on mods for MINIMAL if any hp or torque gains.


Instead, Im doing the IMPORTANT modification to the car. The sound system! Stripping out the garbage factory setup and going a nice high end system is the best investment that can be made presently on the car.

02blksi
09-18-2002, 04:59 PM
GREAT ! now what the hell exactly was the point of your post? :confused: :rolleyes:

LazyAzzAzian
09-18-2002, 08:26 PM
hey it looks like the exhaust tip is touching the rear bumper, is it?

fishboy
09-19-2002, 11:18 AM
looks good man! damn i neeeeeed exhaust bad!

CleanBlackSi02
09-19-2002, 12:44 PM
Originally posted by Ritteri
I myself have decided to wait until next summer before I make ANY mods to my car. There will be alot more data out by this time next year, and my car will be fully broken in by then. I dont see the point in wasting hundreds or thousands of dollars on mods for MINIMAL if any hp or torque gains.


Instead, Im doing the IMPORTANT modification to the car. The sound system! Stripping out the garbage factory setup and going a nice high end system is the best investment that can be made presently on the car.

Preach on brotha!

2sloSi
09-19-2002, 02:03 PM
02blk, sweet muffler dude, looks nice, hey when am i gettin a ride in that badboy car of your's i feel deprived! Call me!

P.S. When are the rims goin on, u gonna wait until next year or what!