rexizbest

I just put on a 2.5 inch catback exhaust system and hollowed my cat. Top end feels good but my bottom end power is horrible. My original setup was cat and original midpipe coming off my 4-2-1 header (which is still on there) bolted at the rear axle to a 2.5 inch pipe-to-straight through "muffler". Now i figured because of my hollowed cat and 2.5 inch piping that i have too little backpressure, causing the slow acceleration. It feels like it did when i used to pack my muffler with steel wool to pass my 5-days ..but that gave me MORE backpressure. Am i feeling too much or too little backpressure? I just assumed it was too little because of the larger piping and hollowed cat. I understand how backpressure works..quicker velocity off the exhaust the better, im just wondering how i can fix this. Thanks in advance

uncle

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by rexizbest &raquo;</TD></TR><TR><TD CLASS="quote">I just put on a 2.5 inch catback exhaust system and hollowed my cat. Top end feels good but my bottom end power is horrible. My original setup was cat and original midpipe coming off my 4-2-1 header (which is still on there) bolted at the rear axle to a 2.5 inch pipe-to-straight through "muffler". Now i figured because of my hollowed cat and 2.5 inch piping that i have too little backpressure, causing the slow acceleration. It feels like it did when i used to pack my muffler with steel wool to pass my 5-days ..but that gave me MORE backpressure. Am i feeling too much or too little backpressure? I just assumed it was too little because of the larger piping and hollowed cat. I understand how backpressure works..quicker velocity off the exhaust the better, im just wondering how i can fix this. Thanks in advance </TD></TR></TABLE>

Backpressure is bad. You do not want ANY of it. What you want is exhaust velocity. It might help if you posted what engine you have.

rexizbest

JDM D15B vtec engine in a dx crx..still automatic at the moment
The car had decent accelertion and good top end (for an automatic) before the bigger exhaust. What should i do? Go back to the stock piping or throw on an oem cat? Thanks for the help

daver

you put a 2.5" exhaust on a 1.5l engine?!! Thats like trying to water your garden with a sewer pipe. A 2" exhaust should be plenty for that engine.

Its not the hollow cat's fault; you want the least amount of obstruction in the pipes as possible. You need the most flow velocity possible for your desired rpm range. Thats governed primarily by pipe diameter.

d

Terrible One

2.25" sounds about right for d-series

daver

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ORiG1NaLKhMeRsTA &raquo;</TD></TR><TR><TD CLASS="quote">2.25" sounds about right for d-series</TD></TR></TABLE>

he'll still have a weak low end with that diameter.

d

rexizbest

so my best bet would probably be to throw on the stock midpipe for the time being, right? Thanks for the input

Terrible One

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by daver &raquo;</TD></TR><TR><TD CLASS="quote">

he'll still have a weak low end with that diameter.

d</TD></TR></TABLE>

his low end with a 2.25" will be better than 2.5"

rexizbest

in the mean time, should i throw on my stock midpipe? Thanks for the replies guys

daver

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ORiG1NaLKhMeRsTA &raquo;</TD></TR><TR><TD CLASS="quote">

his low end with a 2.25" will be better than 2.5"</TD></TR></TABLE>

and even better with a 2"

i don't know what kind of hp the d15 vtec engine can make, but there's no point in allowing it to flow more air than the engine is capable of pushing.

For reference the stock gsr dia. is around 2", and the itr is 2.25".

d

daver

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by rexizbest &raquo;</TD></TR><TR><TD CLASS="quote">in the mean time, should i throw on my stock midpipe? Thanks for the replies guys </TD></TR></TABLE>

that's entirely up to you. If you can get it back in there you'll get some low end back, but it'll rob your topend. With the auto on there, you may want all the low end you can get.

d

rexizbest

Well i'll throw on the stock midpipe for now. Anyone know where I can get a 2 inch midpipe?? All im finding is 2.25 & 2.5

builthatch

to quote myself on d-series.org-

"you are losing torque, which is what you feel; in a world where we fight for 2-3 whp and pay hundreds of dollars for a few hp, it's best to get the right size exhaust....

the exhausts job is to scavenge exhaust; get it out as quickly as possible. If you have a relatively low power engine, it will take a long time to fill that big *** volume of 2.5 inches, and the process of expediting the exhaust is slowed, especially at low rpm, that is why you lose torque.

if you have 12:1+ compression, a wild cam, a fully ported head with race intake mani and header...then you might consider a 2.5"+ exhaust."

rexizbest

I realize this now, thanks for the input. Im putting the old midpipe on tomorrow with the new axle back to see what happens. Thanks guys

ninor

I'd also loose that catalytic, when you take it's guts out it kills the flow. You will make more power with intact one. I know I tried.

daver

ninor, did you do that cat hollow out mod all by itself at the time and notice the decrease in performance? The other issue is that when you hollow out a cat, you create a little expansion chamber in there that can hurt performance. I test pipe would be the best route.

As for a 2" pipe, rex, i don't konw who makes them, you may have to go custom for that. I used to have a 2" Stromung catback for my ls integra 10 years ago (they're gone now i believe). They told me they went with 2", as opposed to all the other 2 1/4 inch exhausts, in order to preserve the low end torque. You may also consider the posibility that you may yank that engine out in a year and get somthing decent put in.

I had read somewhere on this site a chart that listed the max whp each exhaust pipe dia. was capable of flowing. I can't find it now and the only figure i remember was 2 1/4" exhaust can flow up to 180whp. I can't attest to the veracity of this tho.

Until they invent continuously variable exhaust tube diameters pipes, we're stuck with compromise .

ninor

Yes I did, hurt performance big time, low-mid loss. For the original post-er stock cat with 2" is the way to go. (2" might be too big for that motor as well I ran fully built D16A6 11.5:1 , cams engine management, and got best power with 2" all the way)

Test pipe is the way to go if you insist on cat out, but keep that diameter small.

JunkyardDog

Regarding this statement { I'd also loose that catalytic, when you take it's guts out it kills the flow. You will make more power with intact one. I know I tried.}
Has any one done a Dyno test on this or are you going by the seat of your panths?

Evo-man@20psi

Dude with that setup all you need is stock piping and a welded on muffler

rexizbest

rippin on d-series are we?