someone explainthe RR v's decat to me :)

[paul_h]

after reading a number of posts about decats when going BPU i am very confused about the EGT's

 

from a variety of sources the general consensus is if you remove the first cat and leave the second cat in it may or maynot cause the boost to reach too high a level for stock turbos 1.2+ bar, if it doesnt and boost is increased using a boost controller then with the second cat in place this will cause dangerously high EGT's.

 

if you remove both cats which will definately cause over boost then "restrict" the exhaust flow using a restrictor ring and raise the boost with a controller this wont cause high EGT's

 

can someone explain to me why using a cat to restrict flow causes dangerous EGT's when using a restrictor ring to restrict it to the same levels doesn't as i can't figure it out

 

thanks

 

paul

[Ian C]

It's leaving the first cat in that causes the problems. The second cat is under the car and very simple to take out, so that tends to be the first one to go. The first cat, though, is on the turbo downpipe and tricky to get at, plus the nuts tend to get heat welded on and snap the studs when you try and remove them.

 

I think the reason a restrictor ring doesn't cause EGT problems is because it's sat almost at the back of the exhaust system, where the back box pipe joins the mid pipe. The first cat, however, is about 12" from the turbo. The exhaust gas gets pressurised somewhat due to the restriction, but in the case of the restrictor ring the volume of gas being pressurised is several times the size of the volume bumping up against the first cat if it's still in place, and on top of that it's got all the extra time and metal surface to dispose of its heat before reaching that restriction. Makes sense to me but it might be completely wrong

 

-Ian

[paul_h]

thanks for that i understand why leaving the first cat in would cause a problem but there are many posts saying running with either cat will cause problems, this is where my confusion lies

 

paul

[pistonbroke]

Basically, I would say running higher boost with either of the stock cats in will cause high EGT's. It may be due to the CATS being even more restrictive than a restrictor ring.

[paul_h]

this is what i mean people keep saying the egts will be higher but i want someone to explain why this is said?

 

if it runs 1.1 bar with the second cat in or i remove the cat and run a 1.1 bar restrictor ring why would there be higher EGT's if the restriction is the same?

 

paul

[pistonbroke]

Because you probably won't hit 1.1 with a cat in without a boost controller raising the boost. So raising the boost would cause huge back pressure against the CAT. A proper RR will reign it in to the roundabout prescribed level with less restriction, so you could reach 1.1 by turning the said boost controller down. (BPUing is not an exact science and every supe seems to be different.)

[Homer]

this is what i mean people keep saying the egts will be higher but i want someone to explain why this is said?

 

if it runs 1.1 bar with the second cat in or i remove the cat and run a 1.1 bar restrictor ring why would there be higher EGT's if the restriction is the same?

 

paul

 

Ian has it right above. The way I understand it is to do with placement of the restriction, the RR placed at the cat back/2nd decat point gives a significantly larger area for exhaust gases to cool.

 

Or to put it another Restriction is not equal to exhaust gas temperature.