[95nine68]

Anybody have any issues with power loss after a cat delete a little above 5k rpm? My guess is the O2 sensor is not getting hot enough to perform properly since scavenging is better now or that my O2 sensor is faulty now due to the local welding that was done. Alternatives I suppose would be to run a race chip that control fuel air mix by mass flow or I hear the computer does that automatically when the O2 is disconnected. For the rest of you not running a CAT what did you do? Running a weltmeister chip, factory headers and 3" straight back with a resonator and yes its loud! And how are you disconnecting the O2 if you do? I checked the wires out of the O2 sensor and there wasn't a convenient harness to disconnect, cutting the wires?

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[Cloud9...68]

Why did you remove your cat? How do you pass emissions?

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[gbarrier]

the connector is at the top of the firewall near the center.

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[ds968]

No idea about over 5k, but with a straight 3" exhaust pipe you don't notice some significant torque loss in the low to mid rpm range ?!

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[95nine68]

was told by a local qualified porsche mechanic they pass without it plus i just got it e checked good for a couple yrs. was told a race chip would be the cure to this since o2 is for smog. will run a lower fuel air mix ratio (rich)to maximize power. my cat was really restrictive, could barely see a flashlight thru it. still running pipe all the way to rear with a couple of bends at 2.5(non mandrel), throttle response improved with little torque loss at low rpm

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[flash]

that mechanic is dead wrong, and i would say grossly unqualified. it will not pass without a cat. it won't even pass with an aftermarket cat.



the first thing i would do is stop going to that mechanic. he doesn't have a clue what he is talking about.



the O2 is not for smog. that is one of the sensors that tells the ECU to add or subtract fuel to return back to stoich. without it you will be in open loop all the time.



running richer does not necessarily increase power, and in many cases DECREASES it. it all depends on where and when.



you will see about a 15 lb/ft torque loss at a bit over 3k rpm with no cat, and a fair amount of loss below that too.



the system is carefully designed to slow down the gasses to maintain torque. when the gasses are allowed to flow too freely, while you might gain some upper end hp (perhaps 4hp) you lose torque in places you need it for street driving. without a cat, the O2 will not get accurate readings, and will send the wrong signals to the ECU.



i've played around a LOT with the exhaust systems on this car (having spent over $11k testing and trying things). cat delete does not gain you anything anywhere useful on a street car, and actually costs more than you gain.

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[95nine68]

I should have mentioned this car is mainly for track and will see mostly the upper end of the rpm range for most of its life. Race Chip manufacturer says optimal performance (power) is acheived at [size="2"]A/F ratio between 12.5:1 to 13.5:1[/size]. O2 regulates to 14.7:1 ONLY to optimize the performance of the cat. Appreciate the input though, I have the cat still on the shelf, if it does work out after further diagnostic I'll have welded back in. There was also future plans to add a turbocharger in which the cat restriction would significantly impede performance on a turbo'ed system.

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[flash]

when i say upper end hp, i mean over 5k rpm. below that, you lose torque. torque is what gets you around the track, and most importantly, out of the corners. the cat delete torque loss is exactly where you don't want it.



as for mixture and power, it really depends on the timing too. the variocam plays heavily into this as well. the OEM maps use a lot of timing to get the power they do, and they do it with 14.7:1. yes, you can gain some power by fattening things up here and there. you can also lose it by fattening it up where it shouldn't be. been there done that. many chip tuners make the mistake of only messing with the fuel of the WOT map, and just doing a broad sweep of fattening. the best chips have both fuel and timing adjustments, and "on the road" tuning to get there. it takes quite a bit of time, but it results in the best programs. there are 142 maps in the 968 chip. that doesn't get done easily.



there are far better ways to gain power than a cat delete



turbocharging is an entirely different story. best of luck with that. it has not been terribly successful on this engine, and has always resulted in a lot more time, effort, and cost than anyone ever has expected.



by the way, you cannot see through even a brand new OEM cat. the cells are very close together. restriction is very low too. if it's clogged, that's another story, but in good working order, there is no issue.

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[flash]

it also dawned on me that if you disconnect the O2, or it sees readings out of it's anticipated range, the ECU will fault and go into "limp mode" and reduce timing advance so as to protect the engine

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[Eric_Oz_S2]

Flash, have you tested a cat delete on a supercharged car - say replacing it with a 2.25" stock size resonator?

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[flash]

oddly, yes, just out of curiosity as to how clean it burned - pretty much the same problem as running headers - too fast over the sensor, and the pulse timing is wrong - it didn't scavenge right, and part throttle and idle went to crap. might be different if i ran an S2 manifold though.



it never ceases to amaze and confound me how touchy this engine is about exhaust. i am hoping i don't run into more problems tomorrow

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[Eric_Oz_S2]

I was thinking about putting in a magnaflow resonator like this to replace the cat:



http://www.ebay.com....984.m1438.l2649



It is a 2.25" same as stock with a 2.5" internal pipe. I'll keep my current Dansk cat back (which is an awesome exhaust - quiet cruise and NO DRONE).



Any experience with the Magnaflow resonators?

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[flash]

i have been running that model in an 18" x 2.5" for a few years now. it knocks down some noise, but does almost nothing for the flow resistance needed for torque retension on a 968 engine.



the factory cat does an excellent job of doing what is needed. there is a whole technical paper on the "why" it works.

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[gtolou]

So - a cat back system is lighter than stock. I have a dynomax muffler and 2 1/2 pipe. 

 

I've thought about going the header route - in otherwords, 4-1 header, cat delete and smallish 2 1/2 exhaust for torque, etc. BUT, that's a pretty big deal on these cars, headers, etc. 

 

What happens if I leave the stock headers on (two 2 into 1) and do a cat delete with that. Will there be a hp increase? Waste of time?

 

I've raced trans am/drag racing and the stock porsche 968 headers seem pretty well designed. I've never seen an exhaust that performs best leaving the cat on. lol. 

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[banditsc]

Deleting the cat on a street car is a waste of time, you will reduce power in the car, this motor needs some back pressure. If you're going to be racing it and running wide open most of the time then cat delete maybe ok for you. Your best formula for better power is a cat back, chip and the airbox mod and call it done.

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[94SilverCab]

Deleting the cat on a street car is a waste of time, you will reduce power in the car, this motor needs some back pressure. If you're going to be racing it and running wide open most of the time then cat delete maybe ok for you. Your best formula for better power is a cat back, chip and the airbox mod and call it done.

 

Plus the header...

 

JMHO,

 

Jay

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[gtolou]

Deleting the cat on a street car is a waste of time, you will reduce power in the car, this motor needs some back pressure. If you're going to be racing it and running wide open most of the time then cat delete maybe ok for you. Your best formula for better power is a cat back, chip and the airbox mod and call it done.

 

thanks for the response - I've done those. Cat back, stage 1 chip and airbox mod. Some on the site have suggested that headers, cat delete, and a stage 2 chip is the next path to ?? 10-15 more HP. 

 

I guess my point was is it worth going to factory headers/cat delete/and cat back with a fairly small tube cat back system to promote back pressure. Appreciate the response and that IYHO - would not be worth trouble and could hurt power. 

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[gtolou]

 

Plus the header...

 

JMHO,

 

Jay

B nice to see someone dyno the air box/stage1/cat back vs the header/cat delete/cat back and state 2 chip.

 

Jay - question, the aluminum porsche head on a 968. The exhaust threads have helicoil inserts or are those studs in straight aluminum?

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[RS Barn]

The OE cat is awesome. It has a very high flow rate and spreads the exhaust over a large surface area. It really doesn't have back pressure (unless clogged) . I always tell people not to cut it out and expect more power. Plus the tuning gets very wacky.

When you go to a 4 to 1  Header the story changes. You can run a High flow cat or straight pipe, but the tune is different and will be very lean unless you have a chip for that set up. 


Studs are in straight Aluminum.

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[flash]

i cannot stress this enough.  removal of the cat on a car to be used on the highway is illegal in all 50 states.  this is a federal law, not governed by the individual states.  in 37 states, where they have inspections, you may get caught, or you may not.  it really often comes down to whether or not the guy doing the inspection knows what the heck he is looking at.  in the other states, you likely won't get bothered, but it is still illegal.

 

the same goes for aftermarket headers.  it's a roll of the dice when it comes to inspections.

 

the aftermarket cats available for this car do not do a very good job of cleaning things up, and i have not seen anything but the big double biscuit unit pass california emissions, and even then it had to get really hot first.  it also crushed top end power.  the OEM cat does an excellent job of cleaning things up, and flows extremely well.  gas velocity is a very big deal on this car, and speeding it up too much means real low end loss, and slowing it down too much means upper end loss.

 

on this car, exhaust modifications, whether it is a cat-back or a header, do not add big power.  they can contribute to making other modifications work better though.  be aware that the system is highly sensitive, and exhaust modifications tend to focus on specific areas in the curve.  you will generally see things like gains in one area but often losses in others.  it is largely governed by cam overlap, and the dual resonant intake manifold.

 

i found that i could get almost all of the gains of a 4 to 1 header, all the way up to almost 6k rpm, by merely cleaning up the anomalies on the insides of the OEM header, a good ceramic coating inside and out, and modifying the collector a bit, at a fraction of the cost, though it did take the car off the road for a couple of weeks.

 

what to do is really up to your wallet, and the intended use of the car.  the guy who likes to rev high will like one setup, whereas the guy who shifts earlier will like another.  but, no matter what you do to the exhaust, the difference will only be a few hp, so temper your expectations accordingly.

 

been there done that.  14 different cat-backs and 3 different headers before i figured out what i wanted.

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