cat-back exhausts

[brian snapes]

Could anyone please tell me what is a cat back exhaust and what would be the cost and the advantages of fitting it. At present I have a cat fitted with a big bore AMEMIYA  rear box. I have thought about fitting a decat pipe but do not want to sound like a aeroplane going down the road. Would ideally like to gain power but without excessive noise.

[Glenn Butcher]

Hi Brian,  I would be careful about fitting a cat back if you don\'t have an aftermarket ecu of some sort.

You will run into problems with boost spike/creep etc.

Do a search on Boost Spike or Boost Creep and you should find some hits.

[Mart160plus]

From my understanding, and I\'m not going to go against what our resident experts say as I\'ve had my RX less than a year, you can have one decent mod on your car before you upgrade the ECU, whether it be induction or exhaust, but not both. I have a Dragon 2.75" cat back on mine, which is otherwise standard, and it\'s been fine so far under all performance conditions. When the decat, induction and other stuff gets added before the Nurburgring onslaught in June the Apexi PFC I have bought will be going on though.
If you have a different back box, surely that joins the existing exhaust close to the back of the cat anyway

Mart.

[Glenn Butcher]

I always lean (pardon the pun!) on the side of caution.

12-13 years ago (well before we knew anything about boost spikes/creep, lambda, tuning cars properly on a dyno etc. etc.), we use to put FC 13B turbo engines into older rotors, and of course changed the exhaust.

I personally went through around 4 standard engines, all had apex seal failures.  These all had Microtech ECU\'s on.

Of course knowing what we all know now, they were simply leaning out and detonating as the boost couldn\'t be controlled through the standard wastegates, as there was not enough restriction in the exhaust (we ran 3 inch straight through exhausts - no baffled mufflers).


For FD\'s I know of a couple of people that changed exhausts (removed cats) and could not control the boost, a few months later they had to rebuild the engine.

Installation of a Power FC, or some other type of boost controller can help.  But when boost is high the standard wastegate on and FD simply isn\'t large enough to vent the gasses it needs to.


Therefore I strongly believe that removing the cat causes these issues.

I\'m not sure about the one decent mod. theory, as that one mod. like the removal of a cat. could kill the engine.  I personally would even hesitate at changing a backbox - without really knowing what is going on by having it dynoed and checking lambda properly with a wideband lambda sensor, but that\'s me.

[Oedipus Rex]

I\'ve got a straight through exhaust and am having trouble controlling boost, even with an Apex\'i.

I\'m having to keep the revs down to about 5k until I get round to fitting a boost controller, even with a bigger pill fitted,

[20B_boy]

Well said Glenn.

Is say 20hp worth risking a motor for? If you do it, get it checked on some ones wide band at least. Preferably buy a programmable EMS (the Power FC is so cheap now) and get it looked over and tuned - then you\'ll actually get the full benefit from it in addition to probably a smoother ride and slightly etter economy!

And if the exhaust is causing spikes or creeping, fix it. If that means a boost controller, wastegate porting or adding restriction back into the system so be it. A lot cheaper than rebuilding before time.

R.

[brian snapes]

I get the rough idea how the exhaust works, the less obstructions  e.g cat, silencers the more gain in power. You probable gain 15-20 bhp by de-cating, question is what do you gain from a cat-back, and how does it work?

[Glenn Butcher]

Brian,

My understanding is that a Cat Back exhaust - is replacing everything from the Cat back - in other words it stays in place.

I don\'t think there would be much of a performance increase, but it would certainly change the sound.  But if you have already replaced the rear muffler there might not be that much change - depending on how large the pipe is compared to the original etc.

Rich could probably tell you a little more about this.

[Glenn Butcher]

Just to make it a little clearer - which I think you know already Brian.  

There is a distinct difference between a Cat Replacement pipe (in other words removing the cat and replacing with pipe) - as to a Cat Back system which is replacing everything from the Cat Back.

This is a Cat Back system on an RX-7:

http://www.2kracing.com/product_info.php/products_id/3734


Cat replacement pipes aren\'t strictly legal on late model cars of course.  But they just look like a piece of pipe in place of the cat.

4 inch high flow Cats are available out there, but again I refer back to my original post - with boost issues etc.

[Brett]

I\'ve only ever uprated my exhaust when I\'ve uprated everything else.
 For each mod you do you create a problem elsewhere in the system.   At the moment I suspect your car pulls nicely throughout the rev range and feels pretty \'together\'.  Some exhaust changes can create odd power curves throughout the rev range etc.

 I would get an aftermartket ECU with a decent boost controller FIRST, then look at what else can be changed.

 Better safe than 20+hp but -20k engine life!

[brian snapes]

To give you a little historyof myself and the car. I probably not a typical rx7 owner as 60th birthday is approaching at a alarming rate . I bought the car with a blown engine it is 93 auto,the car came pretty much as standard but with abig bore ameyiya back box and mazdaspeed wheels. At present I am gradually going thru the car getting rid of all the rattles and making sure everything works as it should do. I have just rebushed the rear suspension and intend to have the car 4 wheel aligned. I purchase 2nd hand a stock rear box but am unsure about fitting it the car is not excessively noisy, from the information given here I will not de-cat the car. I will probable take it to some one to  fettle the engine when I have done about 3000miles at present mileage is about 1200miles. I am a little concearned fuel consumption is so high probable getting 15 mpg and also have a strong smell of exhaust and fuel, I can hear a blow from somewhere near the manifold either a crack or gasket leak. Thanks for the information.

[Brett]

15mpg sounds like average mpg to me, I think most people get between 12-20mpg depending on the type of driving. 5mpg is not unheard of if you\'re \'in a hurry\'.