Decat

[Ian C]

Name:
Decat

Also known as:
Double Decat
CAT removal
Straight through exhaust pipe

What is it?
The stock Supra has two catalytic converters. These do all manner of wonderful things for the emissions of the engine, but to do so they get in the way of the exhaust stream, and this reduces the flow and increases backpressure. In a turbo car, backpressure is an evil, and once you are past the turbos you need a little as possible. The restriction also increases the heat in the engine bay because one CAT is on the downpipe.

Decatting simply replaces the exhaust pipes with the CATs in for exhaust pipes without CATs in. The diameter and quality of material etc. varies but they all do this one simple function. The bonus factor is that freer-flowing exhaust means a small increase in the volumetric efficiency of the engine. That means more power. Double bonus is the the turbos can flow more air so they produce more boost. That means *lots* more power. People who get decats done regularly report of the car feeling like a new one afterwards, the difference is that big.

However, the increased boost is a double-edged sword, you have to watch out for boost creep, running lean on fuelling, and fuel cut kicking in. Running lean after a decat is a very real phenomena, the J-spec fuel pump isn't up to the job most of the time, and the ECU can't fuel for more than 1 bar of boost anyway, especially once the fuel cut is removed.

Pros:
Increases the volumetric efficiency of the engine, thus giving you more power
Increases the boost of the turbos, giving you more power
Removes heat from the engine bay

Cons:
Apart from some success with the first (1993.5) MkIV Supras, you will need to swap one CAT pipe (usually the centre section one as it's easiest) back in order to pass an MOT
Boost creep may occur
Fuel Cut (http://www.mkivsupra.net/vbb/showthread.php?t=35932) may occur
Your fuelling may not be adequate for the extra boost
Stock j-spec ceramic turbos appear to go pop faster under the higher boost levels
You will need a colder grade of spark plug with a 0.8mm gap (instead of stock 1.1mm) as the higher boost will cause a Misfire (http://www.mkivsupra.net/vbb/showthread.php?t=35931)

[Animal]

What about N/A Supras. I'll be decatting mine this weekend - is there anything else I need to do/watch/listen for. I'm presuming it'll possibly just run lean, but I know nothing about the mechanics of anything with 4 wheels.

Cheers
Animal

[Dash Rendar]

I don't know much about engines either. But I seem to recall reading that you need a certain amount of backpressure on an N/A because this aids in 'scavenging'. If the exhaust is too free-flowing, it can be counter-productive from a performance point of view. (This doesn't apply to turbo systems under boost.)

Hopefully I'm not talking complete rubbish! :read:

[jusie]

By accident last year my garage put my striaght through decat pipe on but forgot to put on my 1.2 bar restrickter ring on which every one said I needed. I was having fuel cut and boost creep problems and took it to Thor where they put a ring on. However since then i feel a real rumble coming from under the drivers seat is this normal?

[Kranz]

I don't know much about engines either. But I seem to recall reading that you need a certain amount of backpressure on an N/A because this aids in 'scavenging'. If the exhaust is too free-flowing, it can be counter-productive from a performance point of view. (This doesn't apply to turbo systems under boost.)

Hopefully I'm not talking complete rubbish! :read:

Not exactly true.

Scavenging is about tuning the exhaust system length and diameter (Helmholtz resonator theory), not about back pressure.

The less back pressure in all 4 stroke engines the better. In 2 strokes the back pressure needs to be matched to the port timing at the power band that is needed. 4 strokes, however, suffer from exhaust reversion and contamination of the fuel/air mix in the cylinder if there is back pressure. Also pumping losses are higher.

The NA will respond to a low back pressure free flowing exhaust. Unlike turbo engines (which aren't fussy because of the huge amount of air being forced throught them) there is an optimum size (diameter) to achieve max power without being too restrictive or without being too big to loose gas veocity.
2.5 inches should be fine.

[carl0s]

the ECU can't fuel for more than 1 bar of boost anyway, especially once the fuel cut is removed.

Am I correct in thinking (remembering) that the ECU just goes full-out for fuelling above 1 bar of boost? i.e. it runs the injectors at 100%, resulting in (hopefully) the car running rich. So, as long as the pump is OK, and you're not going *too* high, it shouldn't be dangerous, right?

[Jads]

the ECU can't fuel for more than 1 bar of boost anyway,

Just before I ask: 'The only dumb question is the one you don't ask' =]

Whats does that mean exactly that it 'can't fuel'?

[mellonman]

you get fuel cut/boost cut at 1 bar or more after 3 seconds which is the ecu telling there is to high boost for standard ecu you have to fit fcd to over come this problem or new ecu.

[den1]

The Horsepower on a stock TT is around 325 hp ( i think:innocent: ). Does anyone know what the hp would be after a decat ( stage1)...and then a bigger intercooler was fitted :)

[TurboBrett]

every car is different mate, do a search for BPU , basic performance upgrade, this can bring upto 400BHP but totaly transforms the way the car drives, huge power gain on both tubbies (post 4k rpm) fmic's are not generaly need at this state of tune the stock smic is a very understated intercooler, however once they have passed there best it is cheaper to replace with a fmic. Over 400bhp is when you would see gains from having a fmic but that is another story, search "single turbo convertion"

Brett

[carl0s]

The Horsepower on a stock TT is around 325 hp ( i think:innocent: ). Does anyone know what the hp would be after a decat ( stage1)...and then a bigger intercooler was fitted :)

It has been suggested that you can equate each 1 psi of raised boost above stock to a 10hp gain.

Stock boost is 11psi I think, so if you raise boost to 17psi then you can expect an extra 60hp if this simple theory is anywhere near true.

Sounds reasonable enough to me.