I finally got under the car and checked the exhaust system front to rear. Got out the handy micrometer and did some measuring. This is what I found.

The primary cat dumps into a cast iron exhaust pipe that has the second 02 sensor a few inches down from the top. It looks like this is a major restriction as it is no more than 1.75 inch inside diameter. The piece is about 1 1/2 feet long and comes down from the cat and bends under the oil pan. There is a short piece of exhaust pipe that runs from the cast iron piece back to the second cat. This is only 1.9 inches outside diameter (about 1.75 inside allowing for .060 wall thickness). Pisses me off because it's not stainless steel and the dealer told me the whole exhaust was stainless.

The second cat has an inlet that looks larger than the short exhaust pipe and an outlet that is the same size. The pipe is stainless at this point and is 2.125 diameter so figure 2 inch inside diameter. The exhaust is this diameter to the resonator, and stays that way until it connects to the rear part of the system. For some reason the rear part of the system is back to 1.75 inside diameter from the flange to the muffler, and then the tailpipe out of the muffler.

Seems to me if we could replace just the first cast iron piece and the short intermediate pipe between it and the cat with 2" inside dia. pipe we would get rid of a lot of flow restriction. Then replace the stock muffler with the sport muffler and you'd have a 2" system front to back. You could probably replace the cast iron piece and the short intermediate pipe as a single piece and have a bung welded onto it to put the 2nd 02 sensor in the stock location. Has anyone tried something similar? If not, it's on the list of things to do to my car. Should be real inexpensive for just that short pipe and we know the sport exhaust is about $220 list so I think the whole thing could be done for less than $300.

 

that's great information. it doesn't seem like it would be hard to get an exhaust shop to make a piece to go between the cats.

 

now heres a good question, if you can get away with it (like here in nebraska) what reasoning is there to keeping the cats at all?

just a thought.

 

I wish man. Emissions testing for inspections kinda puts a damper on some things though. At least here in NC. But a couple, or one, high flow cats and some 2.75 - 3.00 inch pipe should do the trick!

 

Hey guys, here's an update. I took the exhaust off the car today from the 2nd cat forward. Man, there is some serious room for improvement. First thing is the diameter of the pipe from the short manifold off the #1 cat to the #2 cat. It's only 1.7 inside diameter even though the #2 cat inlet is 2" inside diam. They neck it down right at the inlet going forward.

Next is the short manifold from the bottom of the first cat to that short pipe. It's barely 1.75 at the cat end, and is about 1.7 at the pipe end. It's also cast iron. Get rid of it and lose an immediate 5 lbs. I went to a high performance exhaust shop today and had them bend me a 2.25 OD replacement pipe to go from the #2 cat forward to where it hooks up under the engine. will finish fitting it and weld it up on Sunday.

They mandrel bent a 90 degree bend in a 2.25 OD pipe for me also so I can replace that short manifold piece. I have an 02 bung also, so will weld that in at the factory location. I'll retain the stock type spring loaded joint from the back of this pipe that attaches to the short pipe going to the #2 cat. This will maintain the factory exhaust joint that allows the engine to move around without breaking exhaust joints.

I HAVE FOUND OUR MAJOR RESTRICTION IN THE EXHAUST. It's the #1 cat, but it's not the cat itself. The cat is approx 9 inches in diameter and doesn't look that restrictive. BUT the outlet of it is. It necks down to a measely 1.7 inches. There is a large amount of cast iron at the mounting flange and the gasket ID is 2.1". I'm in the process of opening up the cat exit to about 2.1 or so. This will take it from 1.7 to 2.1 ID and that represents an approximate 35% increase in total area (2.2697 square inches increased to 3.4635 square inches). The neat thing is, the gasket surface is still huge. They really used a lot of steel on it. I'm polishing the inside of the lower part of the cat and blending the transition also. I've already got the opening enlarged to 1.95. I'll do the rest tomorrow.

I found a place locally that has a chassis dyno. I'll dyno this when I get it all put back together. Didn't have a before graph, but they are all approx. 98-102 wheel HP from the factory so I'm sure we can get a ballpark idea.

 

Actually, what I think you are shooting for here is an increase in velocity with the stepping up. Stepping up uses the expansion of the exhaust to help scavenge the cylinders. The thing is with our systems, keeping the initial size of the system too small. If you step up it needs to start at a size that will still flow well at peak horsepower. I'm going to keep the whole thing right around 2 inches since that's the diameter of the stock stuff all the way back to the last section of exhaust with the muffler. I'll put on a larger system from the cat back eventually. Right now I'm just trying to get rid of restrictions. The stock muffler and the piece of pipe that runs from it to the connection forward of the axle is only 1.75 ID also (1.9 OD). This will be the next thing in line for mod. I'm going to use the sport muffler (which is 2.25 OD I think) and see how it works. If I can get one that is. Everyone keeps saying they are back ordered.

 

I'll take pics of the cat when I'm done with it, and then take pics of the stock cast iron manifold that comes off it for comparison. I'll take pics of the stock cat forward pipes as well as the piece I'm replacing it with. I'm planning on welding on some tabs with nuts welded to them on the manifold replacement piece so I can maintain all the factory heat shielding. Haven't pulled the manifold off yet and not planning to at this point. Just trying to alleviate some of the factory exhaust restriction while I'm waiting to get my valve cover back. I'm putting the intake cam in this weekend also.

OOPS! Almost forgot. You need to have a flange made that fits the oddball bolt pattern on the #1 cat. I had that one and one that duplicates the flange at the end of the short manifold made today. They charged me for 45 minutes of shop time which came out to $40. The pipe only cost me $20 total including the bending. But, I have a MIG welder and will finish it myself. If the muffler shop did this, they told me they'd charge $60-70 for the work, plus I'd have to supply the flanges since they don't exist. I had a machine shop make mine out of 1/4" plate steel. So figure about $65 for the muffler shop, $45-60 for the flanges. About $110-$120 for the works mounted on the car. Then $220 for the Mazda sport exhaust. Cheaper than most cat backs and probably more effective. You will probably have to enlarge the cat opening yourself though. Just use a die grinder to get it close to final dimension, then use a cylinder hone to finish it perfectly round.

You can do that by removing the front manifold piece from under the #1 cat, doing the work, and then remount it. The engine will run just fine (same size restriction it had before due to the manifold connected to it) on the way to the exhaust shop.

 

Reversion only occurs in the first couple feet of a header. Step headers are for torque production on the street. They step up because the natural tendency for hot exhaust coming out of the cylinder is to expand. The step up in the header helps the engine to scavenge at lower rpm thus building torque. Even the best versions of this on V-8s are only making 10 lb/ft at 700-800 lower rpm and that's on something with a hell of a lot more cubic inches than we have. Also, we don't have hardly any valve overlap and have short duration cams. Reversion is probably the least of our worries. By the time you get 3 feet from the cylinder head, the exhaust is starting to cool. Thus, you don't need to keep stepping up as there is no longer a reason. At that point you just want less restriction so the engine can push it on down the line without having to push against a resistance.

 

Did a bit more work today. I've got the cat opening to 2.030 and will do another .020 with the hone tomorrow. Since the flange I had made is 2.1 ID, I'm going to take the cat outlet to 2.05. This will keep the flange from causing a restriction no matter how far you slide it side to side. The bolt holes allow for about .025 of slop, so even if I shove it all the way to one side during installation, it won't stick into the opening. The 2.05 opening is an area of 3.30 square inches which is still a 32% increase from the stock 1.7 ID of 2.2697. Should be good for something. I'm welding up the short manifold replacement piece tomorrow. Will try to do the rest also time permitting. I'm taking pics as I go, but since I don't have a digital camera will have to wait until I get them developed and put on CD ROM.

 

yeah Jesse you are right, cause my homeboy has an accord with headers and each pipe runs to bigger pipe,, know where could I get a custom header made for my car,, hmm will have tolook into that .. oh wait my boy is a welder
hmmmm
Bruce

 

All this information sounds great, but from talking to other owners, many already have a cat-back system.
I know that what I would like is for someone to make a mid-section that we can unbolt our existing 2nd cat, and put in a new high-flow cat with all the mounting. It should have holes for the o2 sensor, and size up with aftermarket systems.
I know we will still be restricted with the header/1st cat, but there are many companies in the midst of releasing one.

Traveller, are you planning to make a product?
If not, hopefully your research will help someone mass produce them.

 

I have the cat-back now. Want a header and a high-flow cat next. The idea is that the header eliminates the primary cat, and the high-flow eliminates the mid, the cat-back eliminates the last.
RIght now there are lots of cat-back systems, and there are headers in the works. Nobody is doing anything about the mid-sections.

Where I live there are no legal issues.
I just think the high-flow cat would reduce emmisions, and eliminate the ECU error with no cat.

 

Oooh, nice info guys ... keep it up
First I was looking at a catback system from Brullen.. local shop, 2 other guys in Toronto have them. They actually dyno'd it while they were developing the first one and did get some power gains .. maybe +6 Hp or something (can't remember exactly).

But now hearing about all the restrictions up in front of the cat is making me rething spending $700 on a catback system. I want to run in Solo 2 stock class so I won't be able to modify the headers or anything like that.

So maybe my money would be better spent for now on the Mazdaspeed air filter, and the Mazdaspeed exhaust.. it's so pretty too. hehe.

 

Hmm yeah .. if you want better performing headers get these from Mazdaspeed:

 

I believe there is a disclaimer with those headers that says "for offroad and closed course use only" correct? I've got the cat exit enlarged to 2.05 now. Got the short manifold replacement all built and ready to install. It's a bit of a project if you want to retain all the factory heat shields. Better get tight with a good exhaust guy if you want all the factory heatshielding. Takes quite a while to set up all the mounts for them plus having to make a rear flange that will accept the graphite impregnated 'donut' that serves as a pivot. It's done, it bolts up to the cat nicely, it fits the factory brace off the tranny and the heat shields all bolt up to it. Best part is when you set the old short manifold piece next to it and compare the inlet sizes!

I'll do the rest of it tomorrow. We had company tonight so I didn't get it finished. Have to pop in the new cam also. The old one is out and the new one is ready to go in.