2.5" High Flow Cat with 2.25" Pipe??

[Pro_Mod_Accord]

Hey guys just need some feedback please......

Getting my CAT-Back finished this weekend.

I'm using a Pacesetter Header:

I have purchased a 2.5" Magnaflow High flow CAT.

Should I go down to 2.25" out the back or should I stay with 2.5" out the back?

The Pacesetter has a 2" down pipe, should I also get this replaced and up it to 2.25"?


The reason I went with a 2.5" Cat was to have less restriction then a 2" or 2.25" Cat.


Anyfeed back would be appreciated...

[SteveDX89]

2.5" is a little too big on a stock or slightly modified motor. 2.25" is ideal. You can use your 2.5" cat since you already bought it. You'll just need to use reducers to get it in there.

[A20A1]

My thoughts are that the 2" is good cause it helps velocity... it is a restriction but not in the same way the cat is. Going from 2" to 2.5" will slow down the gasses when it expands but if you want more flow from a cat, thats one way of doing it. You can probably get the exhaust shop to weld on a tapered adapter pipe to go from 2" to 2.5" close to the cat, that way you have the velocity high right up untill you get to the cat.

From the 2.50" cat down to 2.25" wont be too bad. Further down the exhaust pipe the gasses are cooiling and slowing down anyways so the smaller pipe will keep up velocity.

Your muffler will be the only restriction you have to worry about other then the cat. The muffler should be the same diameter as your piping though.


BTW: I always thought the pacesetter was 2.25" after the secondaries. I guess I'll need to edit the header FAQ.

[racerx]

yeah, i thought it was 2.25 as well... hrm...

anyway, don't worry about your cat too much. as soon as the gases go into it, they spread to about 4-4.5 inches anyway, so having a 2.5 inlet is only gonna give it a smoother transition into the honey comb thing.

going back into the cat-back, definately put a reducer on it, even if it's a direct bolt-on. having a 1/4 inch wall at the end of the cat is gonna create some serious turbulence.

what kind of muffler do you have? if you're really worried about restriction, just get a straight-through muffler and an extra resonator. piece of cake. sounds about the same (if not better, in my opinion) and it's totally 100% restriction free. ;)

[Pro_Mod_Accord]

PACE SETTER DIM.

Accord
86-89
2.0L (F.I & Carb)
1-1/2" < TUBING DIA
2" < COLLECTOR DIA
1, 3, 6, 11 < design
G7250
70-1250
72C1250

Link http://www.pacesetterexhaust.com/headers_detail.htm

Yah it's only 2" Collector Dia

[Pro_Mod_Accord]

Thanks guys for the info.

I'm going to be using a straight-through muffler with a resonator right after the cat.

Trubulance????hhmmmm a tappered joint from 2.5 to 2.25 should it limit that?

Gonna go 2.25" out the back, so the 2" collector should be sufficient? :dunno:

[bobafett]

my opinion, even with the few mods i have, is to go with 2.5"

my motor isnt all that modded, but i am definetly wishing i had gone with 2.5 for topend... my fiances old accord had 2.5 hfcat with 2.5 catback, and was a great system. she only had header and intake (and full exhaust) and the car had a really really nice powerband (not quite as high rpm as i liked, but whatever)

my car has 2.25 catback with 2.5 high flow cat, and its "ok" but belive me, i have every intention of upgrading to a 2.5" fulllllll system. just have to find someone to build me a header with 2.5" collector and 2.5 all the way back. of course this is with my new motor in mind, and im pretty sure nobody else except maybe rjudgey has mods that would merit that type of exhaust system...

2.5 also sounds a hell of a lot nicer than ricey 2.25// ;-)

[bobafett]

i know there isnt any dyno proof of this, maybe i can manage to do a dyno run with my 2.25 and then another after my 2.5... we willl see...

[Pro_Mod_Accord]

I would go 2.5" out back, but I don't want to lose any driveable power...Don't like red-lining the shit outta my baby all the time...Like a smooth bottom end with good mid-to-top end hit.

[Pro_Mod_Accord]

Definetly run the dyno, I'm sure we would all love to know the reality of the exhaust myth......

[bobafett]

well i cant speak for sure, since amber's car DIDNT have a cam or CAI, or ignition mods (which shouldnt effect it) but her car has significantly better low end than mine, which is probably more due to the cam than anything else, but her drivable power was a lot better than mine.

yeah depending on the cost to get a nice 2.5 system on there, and ditch the stock 1.75 cat flange so i am running "true" 2.5 (other than lame DC header), ill see about getting a new exhaust put on in the next couple months... :)

what mods do you have on your car, and what mods do you plan on getting down the line? that will probably have an impact on your exhaust choice.

if you just have bolt ons, and plan on stopping there, 2.25 is probably just right. but if you plan on making as much power as possible, and doing internal mods, and cams/carbs, etc then you might start to think larger than 2.25.

from the sounds of it, since you want to keep "drivable" power, and dont like to take your accord up to 6000+, 2.25 would probably be a better choice...

still sounds worse though... heheh (not that sounds should be a factor in exhaust performance, imo its more about power.)

[racerx]

Like a smooth bottom end with good mid-to-top end hit.

oh, like this?
http://i5.photobucket.com/albums/y175/academyracer/3g/pb/phun.jpg

lol, j/k

in that case, go with straight through 2.25.

[bobafett]

lol damn you i brought that pic up at work and had to close that shit real fast... :( but thanks

[Pro_Mod_Accord]

LOL love the PIC...but yah not good for work...LOL

Here's a list of my Mods:

Accel Coil 10mm wires
Complete VAC removal
EGR by-passed
no emissions
Dialed in modded Stock Carb with 1" spacer, removed choke
Manual Secondary Barrel
Flipped lid
Gapped Pluggs .050"
Pacesetter Header
cam regrind
17"x7.5" Enkei tripple Chrome plated Rims w/ 205/40-17

CAR is waiting to be painted, was to be painted last weekend but my painter bailed out because he had to rebuild a blown motor in his work truck. Yah, it makes me sick, my car is sitting under tarps outside all primed up and torn right down to the bare bones....Getting paint on it in the next 7-10 days...The whole exterior of the car has many custom fabricated parts.

List of Fabbed body work: took over 4 months to complete

> Hand built side skirts molded up into fender wells
> complete seamless rear trunk (shaved no key whole or back up lightd etc.)
> Custom fabricated rear bumper with custom tear drop molds on the back rear corners
>>>it's sick an after market 98 CIVIC BUMPER cut/fitted/molded on
> fabricated seam around lower truck area to match up with the bumper and truck
> Custom front bumper with mold going up into hood (90-93 TEG Spyder)
> all seamless body work
> Candy tri-coat paint, Colour is a secret for now


Not posting any pics until it's ready to be reveal!
Should be SICK!

[Pro_Mod_Accord]

Straight through? do you mean from the Collector or from the CAT?

[Pro_Mod_Accord]

Plan on doing a JDM B20a Swap this winter, gotta save up for the purchase,,,all my body work and Candy paint has put a huge whole in my wallet.

Once the B20A is in then I might consider a t4 turbo w/intercooler with under 10psi boost.

[racerx]

well... mine's straight through all the way (not cat. SHH!!)

but, after the cat in your case i guess.


lol, and sorry bout the pic.

[Pro_Mod_Accord]

so you are running a header with 2.25" all the way back...so how is your cars power delivery?

[racerx]

actually, i have a stock header, with 2.25 mandrel all the way. bolt-on straight-through resonator replacing the cat, so if i ever need to put one there for emissions, i can just bolt one in and out.
then i have an 18" resonator and a Borla Pro XS muffler, all stainless steel, including the pipes and bolts.

power was definately increased, especially with the cat replacement. I can go over mountain passes MUCH easier with MUCH improved gas mileage. Restriction when you already have little air is bad.

[FyreDaug]

I would go 2.5" out back, but I don't want to lose any driveable power...Don't like red-lining the shit outta my baby all the time...Like a smooth bottom end with good mid-to-top end hit.

I donno about all these theories about having a large diameter exhaust. Theoretically it makes sense, but I have the header straight into a glass pack (no restrictions or BACK PRESSURE [arg, that word!]) and it made a big difference all throughout, low end was helped too by a measurable amount. Its too loud though. Straight through headerback, damn.

[racerx]

mine's straight through, no cat, but i have a resonator in place of the cat, another resonator, and a muffler (straight through), and mine isn't loud at all.

it's barely louder than stock. the only thing that really changed was the type of sound.

[Pro_Mod_Accord]

Yah I was going to go with 2 resonator setup just like that, gonna put in a CAT just for emissions Bull-shit issues.....Doing random inspections around here right now....

Car just went into a professional customizing shop to get the final prep done and paint. Owner came over and checked out my custom work and wants to sponsor my 3gee...SWEEETTT! Gonna look HOT!!

Working on it latter in the week, can't wait....Getting a Huge Discount...Gonna take them 4 days to prep and paint....He wants his company name tide with the car...LOL....but it's my creation...

[racerx]

umm.... PICS?!?! lol

[Pro_Mod_Accord]

Keep that thought Racerx,,,,was that LOL a call on Bull Shit? Cause it's no Bull my friend...I have over 600hrs work on this Car since Mid June...My labour of love...Don't have a digi cam or even a PC at home,,,but I promise there is gonna be some SICK pics of this car when it comes outta the booth...LOL

I'm as hard-core as anyother hard-core 3g'R in here.....

This is my promise!

[FyreDaug]

:|

You talk like a wigger

[A20A1]

A whater?

[Pro_Mod_Accord]

OK a what? OK...LOL

[Pro_Mod_Accord]

Damm-it////can't figure how to get that link to activate...

[racerx]

no man, i wasn't raising the BS flag, I was trying to say:
PUh-LEEEZ put some pics of that thing up! We'd all love to see some of that action.
:)

do you need help posting pics, btw?

[Pro_Mod_Accord]

Yah A20A1,,,I see you might wanna see you modded intake....why would that be? If it would work I might be interested...

[Pro_Mod_Accord]

There we go....Yah!!! right on!

[Pro_Mod_Accord]

no man, i wasn't raising the BS flag, I was trying to say:
PUh-LEEEZ put some pics of that thing up! We'd all love to see some of that action.
:)

do you need help posting pics, btw?


Don't know how to do it yet///Don't have a Cam...Wish I could post some though...but I promise it'll be worth the wait...
Got some pics before and during my body work,,

Got some really bad pics of my car after it was hit...My friend step-dad was drunk one night and backed into my car after I spent 60 hrs on my custom rear end...it made me sick..took more then 18 hrs to fix...definetly gonna have to find a scanner and post those ones...it was bad.. :uh:

[racerx]

sorry to hear. can't wait to see the pics.

...nice sig, btw. lol where do you guys get this shit?

[Pro_Mod_Accord]

found it on Nexopia.com LOL....it's sweet!!!LOL

[87_HaTcH]

I have 2.5 from the header. 2.5 piping is good if your going to run it through a high flow cat and then a muffler. I think that resonaters are pointless but, i mean no disrespect to anyone one here who has there opinions too.:)

[speedpenguin]

boba,
post back here with the dyno results cuz i'm getting my exhaust soon, and i was going 2.25 but now i'm going to wait.
no rush though, hehe, i'm broke right now, just interested.

[bobafett]

yeah i will definetly post when i change it up... im probably going to wait until closer to the time when i get my motor though, cause i dont want to risk blowing the motor and not have my built one ready. :) but im definetly gonna hook up the 2.5 before i get the new motor in, so i might as well dyno before and after just to settle the damn myth once and for all for a lightly modded 3g. :) im confident i will gain topend and not lose lowend too badly, but only time and numbers will tell. i should setup a dyno funds donation account since i will have to pay $65 per time that i go! :( unless somebody in the nw has a dyno hookup.

[speedpenguin]

can't help w/the dyno hookup, but i do know the optimum backpressure to engine horsepower formula, and an easy backpressure test, i'll post tomorrow after i remember it, it's fairly useful to know how to do. and then you know the optimum size muffler/piping for your engine. doesn't even need a dyno, but having one can help

[speedpenguin]

here we go:
drill a small hole in your exhaust pipe right before the first obstruction of airflow (usually the cat unless you don't have a cat. then it'd be the muffler) braze a piece of copper pipe into it about 2 feet long (it needs to be long enough to dispalce some of the heat) attach a rubber pipe to that and a pressure gauge on the end (something to measure water pressure works best, but you can use a boost gauge, too, it's not quite as accurate, but it works) once you have all this set up, with a way to see the gauge find a long straight where you can floor it (or use a dyno) and let 'er rip. if you're reading over 1 psi you're losing horsepower. 1 to 2 percent (for every pound of backpressure) of your engine's power is being lost if you have short duration cams. if you have longer duration cams, the loss goes up exponentially. what you want is 2.2 cfm of backpressure (measured at 1.5 inches of mercury) for each hp at the crank. example: if you have 120hp, multiply by 2.2 for optimum backpressure.