Dont hollow your cat (not your kitty dummy)

INFORMATION Articles / Teg Tips
1

I posted a while back that I read somewhere thatif you hollow out your cat it will hurt hp by creating turbulence (sp). I was half right. I came across this in Mike Kojima’s book “Honda/Acura Engine Performance.” I had origionally read it in SCC but it had been taken from this book.

“Hollow Cats- Whatever you do do not remove or hollow out the catalytic converter of your street car. The monolithic, straight-through design of Honda-Acura catalytic converters is usually quite free flowing, producing only a pound or two of extra back pressure. A hollowed cat can actually hurt horsepower as the empty box can cause flow stagnation, which effectively shortens the length of the moving gas column in the exhaust pipe. The empty box can also reduce important flow velocity. This is usually felt as a loss in bottom end power, although power throughout the range is affected. Oh, and it’s also illegal.”

Remember high exhaust flow velocity is what we like, along with low back pressure of course.

Ben

2

I can vouch for that, my cat was hollowed out cuz it was bad anyway, but then i got my midrange back!!! I only felt a minute (sp?) difference in the top end, but midrange=POWER!:clap:

3

ya and at first i thought my emissions were great and it didnt smell of rotten eggs nor does it now, but there are still fumes that get to the cabin if the windows are down at a stop. its tolerable, but im getting very annoyed and concerned about my health (ya and you guys too i guess).

so yes, i kinda regret it. i do like the gain and it isnt that much louder (in my case), but now im looking for another cat anyways so i shoulda just got the carsound cat in the first place. it gets rid of the smell, has just about the same gains (if not more due to the turbulence), and is good for the environment. think smawt. =D

btw, if i dont get the carsound and end up just getting a stock replacement for cheap, ill only used the gutted one for the track.

4

I have a stock ungutted cat for sale if anyone wants it for a decent price e-mail ,me at kennysp106@aol.com
But I love my gutted cat I felt a bigger increase doing that then with my full exhuast but I guess its up to the driver to decide

5

So in terms of HP and back pressure, would it be a bad idea to get a test pipe?

6

no in terms of hp and backpressure a test pipe is good better flow and less backpressure means more power

7

update- since i got a header and had to replace the o2 sensor, i havent noticed the smell. theres still a light breeze of fumes, but not as bad as before. probably the old o2 sensor, even tho it seemed to work properly, was letting too much fuel pass by (by reporting a lean mixture too much i guess).

8

So you guys feel gains with the hollowed cat? Weird.

ben

9

think about it all that high pressure having to stop and go through a foot of honey comb and most of our cars has 100000 plus miles on then so that contributes to the clogging. I dnt care what that guy says i have that same book and I couldnt believe what he said about that. Hollow is always going to be better than a honeycomb. straight pipes would be better because a hollowed cat there its a slight bottle neck.

10

Ok, I’m going to assume we all know what velocity is right? Ok, so lets assume that you’re running, but someone is behind you, pushing you. If you stop running, but that person keeps pushing you. You still are moving pretty fast. That person behind you is velocity, and backpressure equals velocity. I hope that clears some stuff up for the n00bs that don’t know what backpressure is, and are too lazy to search. BTW this applies to NEAR-STOCK CARS!

11

Originally posted by tegman17
think about it all that high pressure having to stop and go through a foot of honey comb and most of our cars has 100000 plus miles on then so that contributes to the clogging. I dnt care what that guy says i have that same book and I couldnt believe what he said about that. Hollow is always going to be better than a honeycomb. straight pipes would be better because a hollowed cat there its a slight bottle neck.

That guy knows what he is talking about. a hollow cat is not always going to be better. Think about it, a cat is a box. If you gut it out, the exhaust gasses are hitting the back corners and creating a little swirl that may end up trying to push back up the manifold. That is not good for gas velocity. I think the only reason peoples cars make power is because their cats are old and have a bunch of crap in them.

Ben

12

Think about it, a cat is a box. If you gut it out, the exhaust gasses are hitting the back corners and creating a little swirl that may end up trying to push back up the manifold.

this happens if the cat is full of honey comb any way that may happen at low throttle but at full throttle the gasses are being pushed out way to fast for that to happen once the cat is full of gas it does not matter but everyone has their ideas i felt more power with mine gutted and it was not all in my head and i did say straight pipes are better there are no corners on a cat its a cylnder so any gas that gets pushed right back into the main stream

13

I felt gains with my hollowed out cat on the civic, but mine was all clogged up. I can live with the smell, but there’s also a hole in it, so, I get a lot of backfiring.
~Lance

14

Originally posted by EasssHide502Teg
I felt gains with my hollowed out cat on the civic, but mine was all clogged up. I can live with the smell, but there’s also a hole in it, so, I get a lot of backfiring.
~Lance

My civic backfires almost every time I shift, sucks.

Ben

15

what size piping do you have

16

TESTPIPE & Custom Piping 2.50

ok hey guys,

 I got a hollow cat when I first bougth the car (around 98K miles) now its on 103500K  and I had the car with the testpipe since then, but yesterday I got custom piping, from the catback (testpipe in my case) to the muffler, it is 2.50 all the way back, I got it for 50 bucks.  well I had my testpipe and my custom piping on, and at first I got it on drove it around a few miles, and my friend noticed that I had a exchuast leak so I took it back and they fixed it.  But anyways it didn't sound to good, I acutally sounded like a damn ricer, and I hated it, and I think that I didn't have enough backpressure from the testpipe and my piping that I got just got done, and I was smoking bad this mouring, I was scared shitless :uhoh: , then after school, I went home, and put my cat back on, and I acutally noticed more power when I put it back on, and I liked the sound ALOT more, and it doesn't sound like a ricer.  Now I agree with tegman17 if you have stock exchuast and headers, get a testpipe, cuz it works great! and sounds really nice, but if you decide to go with a aftermarket exchaust ESCAILLY a straight pipe, I really don't suggest doing it, you will be dissapointed and sounds like total ****.

Pretending to my car, I get a raddle noise coming from my muffler in first gear at low rpms because I have all this 2.50 piping and no where to go, the guy at the shop said a FLOWMASTER (2 chamber) 40’s series would make it sound much better and obvoious more powerfull. But last time I checked I’ve NEVER heard of a import on a flowmaster, but if the laws of phsyicals (or whatever) works, I think that it will make it sound a little more deep throat then it would raspy, but I HAVE NO IDEA what it would sound like. Has anybody ever heard of a g2 with custom piping on those kinds of flows, the dude at the shop says its the lowest quite piping they make for flows, so I would assume it wouldn’t be as low as a 1 chamber 80 series, and thought it would sound good, would anybody know?

BTW: if anybody wants my testpipe 92-93, $ shipped, just email me nosequipped@hotmail.com

and I think that custom piping is the best way to go, sleeper style. =)

17

when i hollowed out my cat, teh honeycomb was completely in tact. the color was a uniform white too. dont know how old it was tho. the carsounds and other cats ive seen had the honeycomb all stained and discolored when new…

i think we need to study more on the behavior of exhaust gases, cuz logically you would think that the honecomb obstruction is obviously a barrier, but in reality the performance in the low-midrange is the opposite and the gains in the upper range are not worth it unless you are really built or have forced induction. instead of thinking of it like a ball just being pushed out, consider it as a fluid and the dynamics of a fluid’s velocity vs. volume with a pump

18

I have headers, gutted cat, and a pacesetter cat back. The pacesetter has always been raspy thats why I have a thermal on order I would assume going from a straightpipe to a cat you would feel more torque, im assuming thats the power increase you felt. I cannot see in the high revs how that honey comb wouldnt affect the flow of gasses. I see why a straight pipe would make more power. in that honda/acura book it says that a muffler with perferations can make more back pressure than stock. Isnt a cat’s honey comb just like those perferations?

19

Originally posted by tegman17
I have headers, gutted cat, and a pacesetter cat back. The pacesetter has always been raspy thats why I have a thermal on order I would assume going from a straightpipe to a cat you would feel more torque, im assuming thats the power increase you felt. I cannot see in the high revs how that honey comb wouldnt affect the flow of gasses. I see why a straight pipe would make more power. in that honda/acura book it says that a muffler with perferations can make more back pressure than stock. Isnt a cat’s honey comb just like those perferations?

No, those perforations are kind of like the straight through mufflers but they stick up into the exhaust stream. cats probably flow better, the honey comb goes straight through, it doesnt pull gas into the side (like glass packs).

Ben

20

the only way to really settle this is on a dyno. People can talk about “feeling” more power all day, but numbers don’t lie. So, get to a dyno with a stock cat, a hollowed out cat, a test pipe, and a carsound “high flow” cat and get some numbers! :wink:

Steve