What to Do - CAT Questions
 

So I am plowing around under my once-kid-maintenanced FD cause I hear this rattle at idle. I see the CAT heat shield is about to fall off because the two front bolts are broken free. Parts of the exhaust are new and parts appear to be replaced. I think the cat is quite old, but someone has been screwing with it. The line that runs back up to the engine from the CAT is loose and the clamps are not set. Ahh....the joys of home maintenance.

So there have been some running signs that the CAT needs replaced. Sort of acts like it is plugged, but not bad.yet. I am thinking that it is best to address it before any problems surface.

I need to know if there are bolt in replacements. Can someone point me in the right direction?

I also want to know which one to use that will have the least impact on the turbo function. The car still has the stock twins. I think back pressure has an effect on timing, correct?

The heat shields seem prone to breaking loose. See this thread for my experiences:
https://www.rx7club.com/showthread.p...at+heat+shield

Definitely inspect it for clogging, but if it's good then it's good. Exterior condition can really be different than the actual miles and functionality.

Bolt in replacements that can withstand the elevated exhaust temps of this engine are limited and always of the 'high-flow' variety. This usually results in higher peak boost - which you can figure out later and adjust quite easily and affordably with a wastegate manual boost controller. If the rest of your exhaust and intake is stock, it's possible the boost won't change at all, but in any case you need to have something to display it so you don't pop the engine from overboost. The reason this happens is because the back pressure reduction causes higher boost, and the ECU cannot adjust to sufficiently richen the fuel supply. I believe timing has little to do with it.

I'd start with a real good inspection of the heat shields and cat element. If it's truly shot, I'd try to find a low-mileage replacement. The for sale section here should yield plenty of options for stock and high-flow cats. The overall cost of a high-flow cat and manual boost controller is more appealing to me than buying a new cat from Mazda (MSRP > $1000). If you have aftermarket cat-back and downpipe, then I recommend the high-flow since they usually have 3" piping that will match the other parts at the flange. Fitting the stock cat flange to 3" piped downpipe and cat-back has given me fits since neither gasket fits well.

Regarding the Gotham Racing high-flow - some of those claims strike me as needing a few grains of salt. Compared to a midpipe, it will have a much smaller effect on running lean, and it won't boost creep. So compared to a midpipe it's the safe choice. I don't see how it can change fuel economy unless the previous cat was really bunked up. But to say it's completely safe is not true - it can increase boost levels, and elevated boost (say, above 11psi when stock is 10psi) must be tuned back to avoid damaging the motor. But after all that, Gotham is considered a reputable seller that sells only good products. So :bigthumb: for that.

Dave

Ok, so I am learning something here. It is obvious to me now that higher exhaust flow equates to higher turbine speed, thus higher boost pressure. I stuck a 10 psi boost gauge on the intake port and went down the road and it goes off the scale in both stages. There is no boost controller in the car and no changes from stock that I can see, except the precat is gone for sure.

My guess is that the cat has been replaced since there are many miles on the car. So, there may be a high flow cat installed. Not sure what the peak pressure is, but my guess is that it is better than 12 psi.

If I bought an aftermarket one, what are the chances that things would be exactly the same? Sounds like they would be.

Please tell me what is involved in putting in a boost controller? I will try and search this out...

TM

Aftermarket cats don't look much like the stock one. Compare to my pics, or pics in the FSM, etc. The canister is much smaller on aftermarket models and the piping is bigger and uninsulated.

Your boost gauge is missing half of its purpose if it pegs out. I strongly recommend getting one that can exceed 10psi so you monitor your peak levels. I think even the $25 Autometer gauges go to 15 or 20psi, and those are found for cheap on the for sale forum. Hopefully you'll find it's running at 10 or 11 psi - more than that would be unusual for a car with only a downpipe mod unless the previous owner changed the boost pills or already installed an MBC.

I don't know the odds of adding a high-flow cat and having the same boost level, but it's possible. I had a couple of flow mods before I saw an increase in boost. In any case, on your first road test after a mod, fill up with fresh 94 octane, and go WOT with your eyes on the gauge - if it is still rising at 11psi, lay off. 12psi is getting awfully close, IMO if you don't have tuning hardware set up. Then you can go MBC or swap in a larger wastegate boost pill. Again IMO, it's better to adjust boost levels working your way up rather than the other way around. This is why MBCs are so popular.

Dave