Only One Cat - Restrictor Ring Still Needed?

[Homer]

yeah my car is stock apart from blitz tt-r realise cat back exhaust, not sure if its 2.5 or 3" tho. how many o2 sensors on a jap supra TT then? i can see a sensor from the engine bay in the first cat?

 

Please take a read of the BPU guide Ryan, it will answer your questions (click the blue text).

 

The jspec has a single o2 sensor, the UK spec has two. They also have a cat temp sensor so don't get those mixed up with o2 sensors.

[supraGZaerotop]

cheers fella, which is which tho. havint been under car for a while, i know theres a sensor in the first cat, is there another in the second cat? the sensor in the first cat is temp sensor right?

[R Black]

cheers fella, which is which tho. havint been under car for a while, i know theres a sensor in the first cat, is there another in the second cat? the sensor in the first cat is temp sensor right?

 

Yes thats correct iirc, also the

temp sensor can be tied away,

i have a decat that you reconnect

the temp sensor to, but i dont

think this is required, a good

quality decat like Chris Wilsons

would suffice.

[4packet]

As toyota intended it will be running 0.76 bar with both cats in, if you remove the 2nd it'll reduce the restriction and therefore increase the boost pressure. The result of this is an exponential increase in exhaust gas temps which in turn drastically increases the EGT's. This risks engine, rather than turbo damage.

 

When raising boost the restriction should be moved further down the exhaust to allow gas temps to reduce and spool to be optimised. This is why the 1st cat should always be removed when raising boost pressures.

 

I have not measured my exhaust gas temperatures, but I struggle to fully follow the logic of this. I can understand how an increase in boost will raise temps and also for a given gas flow (same boost) a rise in back pressure will also raise exhaust temps due to the extra work on the gas. I can also comprehend how leaving a restriction in the exhaust and raising boost with an EBC will compound matters. What I cannot follow the logic of is how placing the restriction further down the gas stream will make a difference to engine out exhust temps. To reduce the boost you are altering the pressure differentional across the turbine so no matter where you place the restriction, the pressure backs up to the engine out exhaust gas. I do agree the spool will increase, because it will take longer to pressurise the larger volume in the exhaust pipe.

 

My other confusion is why the 1st Cat should be any worse of a restriction for heat than a Restrictor Ring. A lot of people use the RR to hold boost down to 1bar, then raise with an EBC. With the 1st Cat left in and a straight through system you can also see 1bar. So what is the difference? If you were to measure the temps post Cat you may see a rise due to the exothermic reaction in the Cat, but this would not affect gas temps upstream and hence affect the engine.

 

This is my understanding of the system, but am happy to accept a logical explanation to the contrary.