6G Celicas Forums

I'll be running a 3" high flow cat with 2.5 reducers on each side, I just noticed that the stock Gen III 3sgte downpipe has a cat built into it. I know some downpipes will kill drill bits and others you can pound out with a hammer if you spend a few hours..lol. I want to hollow it out though, are the downpipe internals hard to gut?

Yeah, I'd be really interested if anyone's done this with a 3rd gen 3sgte as well, I've been contemplating doing it to mine. On my old 5sfe the bottom part of that forward cat off the exhaust manifold broke apart for whatever reason, clogging the exhaust, and so I just gutted the rest of it, and it was pretty easy. Still even passed IM afterwards (since it still had a rear cat I'm guessing).

cool, that's good enough for me to start mine. If the 5sfe was easy the 3sgte should be the same. I think the newer cats might be a problem since they are getting more high flow. The newer nissans use high flow cats on the stock cars I think..don't quote me though.

I've hollowed out the downpipe cat on my MR2's Gen2 3S-GTE, and it wasn't too bad. A drill with a long 1" blade bit (that you don't mind mangling a bit) and some long pliers did the job for me in an hour or two. Just be careful, there is a metal mesh between the ceramic cat matrix and the downpipe wall, and it tends to wrap around the drill bit. Drill out the ceramic, then use the pliers to pull out the mesh. Also wear eye protection and a dust mask at least, you do NOT want to be breathing that stuff in.

But if you've got the money, I'd recommend trying to get a aftermarket downpipe. Way easier to deal with.

Why not buy an Ebay downpipe or something? they are fairly cheap.

my upper half of the dp is a blitz cast iron one, lower half is stock. And I would rather save the $60 and hollow it out.

Apparently you don't know how much a stock cat costs...

Yes I do. And we never mentioned buying stock cats, he mentioned buying an ebay downpipe instead of hollowing out the stock one. I looked it up and the cheapest ebay downpipe is $60. Plus, try buying a stock gen III cat that isn't used.

unless I read your comment wrong and you are talking about how hollowing out the cat is a bad idea because of how much they cost. If that's the case...stock cat = restriction, old cat = more restriction...if I had 3" piping from the DP exit to the muffler tip and if the stock cat only flows the equivalent of a 2" pipe because it's stock and/or old then my exhaust will only flow as much as the smallest point in the system. Since I don't know how much flow the stock cat is capable of either as a new cat or an old cat I would rather just hollow it out and use only my aftermarket high flow cat, so I know what the flow of my exhaust is. And I would rather take the time and hollow it out than pay the $60 for a new ebay one.

It's not very hard, i took my cheap ryobi drill with an irwin auger bit and drilled 5 holes in a straight line from one side to the other. Then separated the sides from the housing with a long screwdriver and hammer. The two middle halves came out with a little force after that. Chiseled out the rest of the pieces that were stuck and wirebrushed it. All in all it took about 1.5 hours. Now I have a hollow DP! yay!!

that bottom half of the downpipe that contains the cat is typically worth 60-85$ to your local steel recycler. (with the cat material intact.)
could have cashed it in, threw in a few bucks, and got a real downpipe.

+1

a real downpipe weighs a hell of alot less and looks tidier in the engine bay too

The factory outlet of the DP is 2.75 inches, this one is more than enough for me instead of paying more for an aftermarket DP. The DP's that "look tidy" are horrible on performance. DP's and headers need to hold in the heat, which is what cast iron does best. Stainless and chrome dissipate too much heat making the exhaust cooler and more dense. And the DP is right in front of the radiator fans with outside air being blown over them as you're driving. Once the exhaust gets cooler it expands and gets harder to push, this directly affects performance. If I have a 3" exhaust system with a chrome header or DP the exhaust might flow like a 2.5" system or less. This is why you see drag cars with a slow flaring individual exhaust setup. The headers are the exhaust piping and they start out as a couple inches and gradually flare out to 4 inches or so. This allows the exhaust to flow smoothly as it expands and gets more dense coming out the pipe, and they didn't have to pay enormous amounts of money for a cast iron one.

I said all of this to say...I now have a 2.75" cast iron DP. I paid jcbass7 $30 for the cat part on my trip home and brought it back with me, and I've had the blitz cast DP for a while. Since the factory DP has a 2.75 outlet, and the inside of the blitz DP is pretty roomy, I now have a "real" downpipe. So you can take your factory one to a steel place and get some money for it and then buy a "real" downpipe that is shiny and takes away HP, heatwrapping it will make the flow better, but there goes the nice looks. Or you can buy a more expensive aftermarket cast DP to get the same thing that you would for hollowing out the cast iron one.

This post has been edited by match220: May 12, 2012 - 1:38 PM

your car, your call, but i don't agree your smashed out cat is better for performance than a well crafted 3" downpipe (ive had both)

so choose what you like for your car, but maybe leave your personal theory as that instead of parading it as fact??

match, do some research.
the cast elbow and large opening for the cat material are horrible for flow.
all the velocity is lost in the cat section, which is what you want, velocity.
a well made 3"downpipe will make more power, everywhere, over the stock downpipe, blitz upper section + stock lower, or any 2.5" dp.
its been proven, over and over. there are dyno charts where the testing has been done, so before you get your panties all in a bunch, i suggest you know your facts.

PSA

Moto-IQ Article on Exhaust

/PSA

These are facts, it's thermodynamics, the foundation for a lot of what our physics are based off of....density. They aren't personal theories or things car nerds type up that others find via google. They are good things to keep in mind when building an exhaust system.

And as far as the DP flow, I have to suck it up and agree now that it won't be as great as an aftermarket one. When I was at the car yesterday I noticed all of the bends inside of the two DP sections(mainly the upper half), and how it gets larger and smaller and changes from square to round and round to square. This will disrupt flow and keep it from being as smooth as I would like it to be, but I will save the money and keep it since it's what I have. Thanks for the comments though guys. I'm not mad, and I hope that isn't how I'm coming across, I know that's how it seems sometimes when there is just text and no voice tones.

delusionz and pressure2,
I just read your comments and responded quickly before I left this morning. In earlier posts I wasn't saying it was better than a "well crafted" downpipe. So to recap this thread a little...Someone had mentioned buying a cheap ebay one, which was when I started talking about how they dissipate too much heat and add resistance as exhaust expands and slows down. The ones I was talking down about were cheap ebay ones, I didn't say my stock hollowed out one would be better than a well-crafted one, I said it would be as good as a real one with a 2.75 outlet. Pressure2 mentioned getting a real one for $60-$85, which I disagree with because the cheaper ones are cheaply made, it's pretty hard to find a nice one for that price. Anyway, when I looked carefully at the inside yesterday I saw the bends and different size diameters which sucks on performance.. and I actually can't find my blitz DP right now, which also sucks, so now I'm stuck with the factory one until the find the blitz upper. I thought the factory DP would be smoother and straighter inside than what it actually is.

Anyone can bend some metal to spec and put some flanges on it. They are usually chrome or thin stainless and I wouldn't put it on my car unless it was free. The "well crafted" ones that you mentioned will be thick stainless or titanium, maybe even cast which aren't as popular. These are the ones that hold in heat better than the cheap ones. Heat wrapping a cheap ebay DP would probably work out great, but on that note, heat wrapping your whole exhaust would work out even better. A ton of velocity is lost as the heat is lost, and also as pipe diameters change from being large to small. Bernoulli's principle starts to be factored in, as diameters change from small to large the velocity drops and as it changes from large to small the velocity increases. The more times we allow exhaust to expand and contract in the exhaust system the more energy is lost. I've done extensive study on exhaust flow, thermodynamics, and how the laws of physics affect our exhaust systems, I almost designed exhaust systems for a local exhaust place, that's actually the area of performance that I've been most fascinated with and wanted a job in for a while. I would be more than happy to write up an article with references if I have enough time if someone would pin it, or verify it first before it's pinned.

4 words...

mild steel & ceramic coating

that's what I need..either that or a titanium one if they make them for the 3sgte, none of that cheap ebay crap. I don't feel like dropping $300+ for a DP though, or more for a titanium one. However, I do think about heatwrapping a cheap ebay one from time to time, but I can never bring myself to buy one.