GJD Posted May 1, 2009 Share Posted May 1, 2009 (edited) I've heard both ways on this question so I'm seeking any more opinions that might be out there: J-spec TT. Currently both cats removed. Question is, if I put one cat back in, do I still need the restrictor ring? I get the impression the answer might be: Front cat only fitted = probably no need for restrictor ring - does the answer depend on the size of decat pipe in place of the second cat? Second cat only fitted = maybe no need for restrictor ring. Thanks Gavin Edited May 1, 2009 by GJD (see edit history) Quote Link to comment Share on other sites More sharing options...
mikeyb10supra Posted May 1, 2009 Share Posted May 1, 2009 I have my first cat in, 2nd cat removed and cat back exhaust and it sits at 1.1bar perfectly....the cat is acting as your restriction Quote Link to comment Share on other sites More sharing options...
tony tt Posted May 1, 2009 Share Posted May 1, 2009 If the first cat is in place and second cat removed then no need for restrictor ring. If the second cat is in place and 1st removed im not too sure in that situation if restrictor ring is needed as i dont know how much back pressure to the turbos the 2nd cat provides. Why dont you leave the first cat in so theres no issue with overboost? Quote Link to comment Share on other sites More sharing options...
GJD Posted May 1, 2009 Author Share Posted May 1, 2009 Why dont you leave the first cat in so theres no issue with overboost? That's what I'm after - a one cat solution that avoids overboost. Just want to be confident that the first cat on its own gives me that solution before I spend the money. Quote Link to comment Share on other sites More sharing options...
hadyn Posted May 1, 2009 Share Posted May 1, 2009 I dont see more than 0.9 bar boost on mine with the 1st cat still in place Quote Link to comment Share on other sites More sharing options...
Homer Posted May 1, 2009 Share Posted May 1, 2009 That's what I'm after - a one cat solution that avoids overboost. Just want to be confident that the first cat on its own gives me that solution before I spend the money. It's not a good solution though, the 1st cat will vastly increase temps, it's far better to keep the 2nd cat in place have have a small restrictor ring after it, it'll allow for quicker spool and help reduce gas temps. Quote Link to comment Share on other sites More sharing options...
GJD Posted May 1, 2009 Author Share Posted May 1, 2009 It's not a good solution though, the 1st cat will vastly increase temps, it's far better to keep the 2nd cat in place have have a small restrictor ring after it, it'll allow for quicker spool and help reduce gas temps. Presumably, in terms of temperature the first cat on its own wouldn't be worse than both cats (i.e. as Mr Toyota intended) so the issue is one of higher temp reducing power rather than actually damaging turbos or the engine or anything? Quote Link to comment Share on other sites More sharing options...
SupraGirlie Posted May 1, 2009 Share Posted May 1, 2009 I have have 1st and 2nd decats, I put a restrictor ring in and I was only making around 0.9 bar so I binned the restrictor ring and it sits just under 1.1 bar, it's because I only have 2.5" decats which seems to restrict it. Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted May 1, 2009 Share Posted May 1, 2009 Presumably, in terms of temperature the first cat on its own wouldn't be worse than both cats (i.e. as Mr Toyota intended) so the issue is one of higher temp reducing power rather than actually damaging turbos or the engine or anything? What boost are you planning to use? The temps go up a LOT if you up the boost over stock when using the OEM cats. Quote Link to comment Share on other sites More sharing options...
Homer Posted May 1, 2009 Share Posted May 1, 2009 Presumably, in terms of temperature the first cat on its own wouldn't be worse than both cats (i.e. as Mr Toyota intended) so the issue is one of higher temp reducing power rather than actually damaging turbos or the engine or anything? 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. Quote Link to comment Share on other sites More sharing options...
Carl Posted May 1, 2009 Share Posted May 1, 2009 I have removed the first cat only and do not have any restrictor ring in place. I have not seen any boost higher than 1bar & certainly not had fuel cut yet so all seems well. I gather the second cat is acting as my restrictor untill I remove it. Quote Link to comment Share on other sites More sharing options...
Homer Posted May 1, 2009 Share Posted May 1, 2009 I have removed the first cat only and do not have any restrictor ring in place. I have not seen any boost higher than 1bar & certainly not had fuel cut yet so all seems well. I gather the second cat is acting as my restrictor untill I remove it. Correct, but the results do vary a great deal car to car. The cat back system will have a big effect on the boost too. Quote Link to comment Share on other sites More sharing options...
pistonbroke Posted May 3, 2009 Share Posted May 3, 2009 Correct, but the results do vary a great deal car to car. The cat back system will have a big effect on the boost too. Any idea why it varies so much from car to car.? mine has both decats and had a restrictor ring. Yet when I first picked her up, she would boost to nearly 1.3 bar. I've since turned the boost down on program A so it never boosts over 1.2. Program B is my safe wet weather 0.9. Quote Link to comment Share on other sites More sharing options...
Steviekid Posted May 3, 2009 Share Posted May 3, 2009 (edited) mine has both decats and had a restrictor ring. Yet when I first picked her up, she would boost to nearly 1.3 bar. Mine does the complete opposite. With double decat and 1.25bar RR I only got 0.8 - 0.9bar or thereabouts before I fitted an EBC! Edited May 3, 2009 by Steviekid (see edit history) Quote Link to comment Share on other sites More sharing options...
pistonbroke Posted May 3, 2009 Share Posted May 3, 2009 What exhaust system do you have.? Mine is a straight through with no silencers 5zigen fireball. Quote Link to comment Share on other sites More sharing options...
Steviekid Posted May 3, 2009 Share Posted May 3, 2009 What exhaust system do you have.? I've got 2.5 inch decats and a Nur Spec R. Quote Link to comment Share on other sites More sharing options...
Mike B Posted May 3, 2009 Share Posted May 3, 2009 I ran a stock system with one cat, and an fmic, while it was fine in summer I got regular overboost creep in cold weather; 10 degrees and lower. Quote Link to comment Share on other sites More sharing options...
Homer Posted May 3, 2009 Share Posted May 3, 2009 Any idea why it varies so much from car to car.? mine has both decats and had a restrictor ring. Yet when I first picked her up, she would boost to nearly 1.3 bar. I've since turned the boost down on program A so it never boosts over 1.2. Program B is my safe wet weather 0.9. They vary due to the huge amount of variables in the system... cat back exhaust restriction & size, decat size, turbo condition, engine compression, age of fuel, intercooler effciency, air tempurature, air pressure, gear type & condition, etc, etc, the list is endless The boost controller is allowing the car to bypass it's natural boost restriction. Turn the BC off and that will be your "natural" boost level. On a BPU car with both cats removed and a restrictor fitted that should be around 0.9 bar. You then use the BC to set your desired level. I ran a stock system with one cat, and an fmic, while it was fine in summer I got regular overboost creep in cold weather; 10 degrees and lower. That's normal though Mike, cooler air means higher charge density hence more boost and more power. In an ideal world the restrictor ring should be sized for your maximum desired level during a cold morning, this will ensure that it doesn't creap higher than intended. During summer months a BC would be needed to get the lost boost (due to higher intake temps/lower charge density) back again. Quote Link to comment Share on other sites More sharing options...
GJD Posted May 4, 2009 Author Share Posted May 4, 2009 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 see what you're getting at. Thanks for the advice - I see this thread has got a few people thinking. I guess second cat is the way to go. Probably easier to fit too. Quote Link to comment Share on other sites More sharing options...
RobD Posted May 4, 2009 Share Posted May 4, 2009 Alot easier can do it on the ground with a jack! Quote Link to comment Share on other sites More sharing options...
bodilx6 Posted May 5, 2009 Share Posted May 5, 2009 I can tell you that with a full 3" system with the 1st can still in, my car put down 354bhp. I knocked a hole through the 1st cat for it to act like a RR, and hitting 1.2bar max (maybe a tad more in really cold weather) it puts down 390 bhp. The big difference has been the power delivery though. The well known "BPU kick" when the 2nd turbo hits was very subdued when running with the full 1st cat. But more exciting after getting the hole knocked through. Quote Link to comment Share on other sites More sharing options...
supraGZaerotop Posted May 14, 2009 Share Posted May 14, 2009 so for a standard TT j-spec is it safe to take out the 1st cat, and will this give me any more power and not risk the car. Quote Link to comment Share on other sites More sharing options...
pistonbroke Posted May 14, 2009 Share Posted May 14, 2009 so for a standard TT j-spec is it safe to take out the 1st cat, and will this give me any more power and not risk the car. It probably wouldn't do a great deal, not unless you put a boost controller on. Quote Link to comment Share on other sites More sharing options...
bodilx6 Posted May 18, 2009 Share Posted May 18, 2009 so for a standard TT j-spec is it safe to take out the 1st cat, and will this give me any more power and not risk the car. Hard to say. Depends on the rest of your exhaust system. Do you have a cat-back system? 3"? If you do have a full 3" system and remove the 1st cat only to leave the 2nd cat as a restrictor, you will boost past 1.2 bar and risk your turbos. So, first of all, get a boost gauge. Then you can do and test as you like and keep an eye out for not overboosting. If your car is 100% stock, you will hit fuel cut @ 1 bar... Quote Link to comment Share on other sites More sharing options...
supraGZaerotop Posted May 19, 2009 Share Posted May 19, 2009 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? Quote Link to comment Share on other sites More sharing options...
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