The closer it is to the engine, the greater volume of gasses need to flow through it. The mass of the flow at any point on the exhaust is the same, obviously, but when it is hotter up near the engine, the volume is greater. So, putting it closer to the engine is more of a restriction than putting it further from the engine.

It might be okay at the end of the downpipe, but I would put it as far away from the engine as is reasonably possible. My plan is to put both the cat and a 14" muffler in my midpipe with the cat first.

-Max

 

I hate the smelly exhaust, too, so I switched back to my N-Tech high flow cat. I don't have an air pump, and it got rid of the smell completely.

I'm switching to a metallic cat now and expect the same odor-defeating results.

-Max

 

please let me know / post the result after u switch cat
i dont think i can buy the cat now coz my money is stuck somewhere
but i'd definately want to try that combo

 

I emailed Dynatech, asking about the suitability of their cat for a turbo rotary, running rich. This is their reply:
"Our cats are used in our late model SuperMAXX systems. We have not used them in a turbo application such as yours. At this time we do not have any data on how they would work for you. They do not have a provision for air injection."

 

^^ It was responses like the above that made me feel more comfortable buying Random's cat for a few bucks more. At least the Random guys have done this on a turbo and non-turbo rotary and can comment in a more knowledgable manner.

 

Did you get the car smogged yet? And get any pics yet?

 

Reply from Random Technology:
"The direct fit converter for your RX-7 is our part number 903018. It has 3" diameter inlet/outlet and bolts in place of the stock converter. It does have provisions for connecting an air tube.

As for the issue of rotary engines running rich, that's always a problem for any converter. We have been selling RX7 converters for about 8 years and have had an unusually low failure rate, considering the reputation that rotaries have as converter eaters. However, any converter will fail if an excess amount of fuel enters the exhaust system. The problem isn't so much a matter of handling a rich mixture, but of handling unburned fuel, which causes internal temperatures to skyrocket. "

 

Just to keep things clear - I am NOT using a direct fit converter. I'm using a metal substrate universal fit that I had welded into a midpipe.

 

I have asked them which flows better- the direct fit ceramic, or the universal metallic. So far, they haven't replied. They did say the ceramic is "Super High Flow" and does have an air tube.

 

hehe. email isn't always the best form of communication

The metal substrate cats flow better would be the correct answer. Check the graph below
http://www.secureperformanceorder.co...e/powercat.cfm

 

And, their reply:
"The 9000 series doesn't flow quite as well as a metal matrix converter, but unless you're running an excessive amount of boost, should be more than adequate. Also, the metal matrix converters don't have a provision for an air pump, so something would have to be plumbed in in front of the converter."

 

Taking a look at this thread: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/highflow-cat-will-work-without-airpump-last-415169/

The guy did NOT pass smog with the Knightsports metallic cat without the air pump hooked up. He passed fine with the air pump connected.

Now, this guy's car may not be running perfectly, but this tends to indicate what we feared -- that even the metallic cats will not pass without the air pump connected.

In my situation, I'm willing to run the metallic cat to get the benefits of high flow without the smell or noise of a midpipe and just put my stock cat back on every 2 years for emissions testing.

 

Would any cat pass without the air pump? I don't think this is just a metallic-versus-ceramic issue.

-Max

 

Well, there was some question (and hope) that the new metallic converters could pass smog without the additional air being injected. It looks like those hopes might be short lived. I guess if people are getting their hi-flow cats fabricated using universal converters, then welding an air pump fitting won't be that big a deal.

 

That's my line of thinking. Considering the excessive amounts of unburned hydrocarbons that an FD engine produces, trying to convert the exhaust gases to anywhere near legal levels requires additional oxygen to assist with the conversion process. Metallic substrate cats without any O2 hookup apparently work just fine with many of the latest motorcycles, but those engines are also tuned pretty lean from the factory at those lower rpms anyway to help them out.

 

could there be room to add a resonator into a midpipe with this random tech metal substrate?


Oh, PureMdMa in the SE section did pass ATL metro area emissions with a reman and a stock cat w/o an airpump.

 

Any more input on this? Anyone have a dyno comparison? Anyone using their airpump with the metallic substrate cats? (In PA, on a 1994 model, all you have to do is pass a visual inspection that the original equipment is still on the car. I suspect some inspectors might catch the lack of air tube to the cat.) Max, did you get your muffler/cat combination installed?

 

I ran out of time to get my random cat + muffler midpipe done before my dyno tuning session tomorrow. I'm going to get it tuned with the SR resonated midpipe and then get my new midpipe made next week. I expect that I will have to lean it out a bit, but I've got a wideband and will make the necessary adjustments. I will probably get it dyno'd again after the exhaust changes (wg into dp, cat+muffler in midpipe) to see how it turns out. I expect to lose some power, but unless it's 50+ WHP, I'm going to stick with my current plans.

-Max

 

Max, what cat are you running? are you running the cat without the airpipe/pump?

 

I have an N-Tech high flow on the car right now. It has an air tube, but it isn't connected to anything (except for the check valve, so it won't leak).

I bought a Random metallic cat with no air tube that I will be installing in a midpipe with a 5x8" oval 14" long center/offset Magnaflow muffler (also in the midpipe -- it will be a tight fit ).

-Max

 

sorry, no dyno yet. no smog check either. haven't even seen the car in 3 weeks.

my initial impression of just part throttle low boost action was that it wasn't too different from running a midpipe. haven't gotten a chance to drive it much though. too busy with life.

yes, there should be enough room to install a resonator in the mid-pipe along with the cat. the cat is actually pretty short - 9" tip to tip and you could probably chop off 1" on each side if you wanted to. if you plan on doing that, you might need to mount the cat closer to the front of the midpipe around where it bends and the resonator toward the rear of the midpipe. we'll see what max comes up with

looking forward to seeing max's dyno sheet

 

Any updates?
Pics would be nice too (If that's not asking too much)

 

...

 

I just want to know does it smell a lot less or you can still smell the exhaust? I don't care much for sound but the smell is killing me.

 

Sorry guys. I just finished up finals for grad school and am in the process of moving. I'll definitely take some pics this weekend.

It definitely helps the smell!! I would say it gets rid of that nasty stench completely. It quieted the car down somewhat transferring the noise more to the exhaust tips than just being loud all around. Pretty nice overall.