Cats.....do They Make A Difference?

[simpletool]

OK so I installed a 3" ballistic cat from Sureflo with 4.5" 100 cell body.

Came with flanges and bolted straight in. Even had a spot perfectly located for mounting the temp probe.

Previously I had a 300cell high-flow ceramic cat (3" pipe and didn't have the temp probe bung).

Sound:

At idle it sounds slight crisper but not noticeably louder.

Onto boost and full noise it is also crisper sounding and probably 3db louder. (ie. definitely noticeable but nothing overly dramatic to the ear).

Performance (objective):

Well objectively I can say the car makes 0.5psi more max boost on all 3 manual settings with my IDIII controller.

12.2 became 12.8

14.1 became 14.6

15.0 became 15.5

It also reaches boost 200rpm earlier in 2nd and 3rd (yes I can plot this accurately).

These were all average measurements at similar night temperatures (yes it varies a little with temp). But this is repeatable and probably conservative as it was warmer than the previous night.

Performance (subjective):

Well here it gets difficult. It does feel faster even at the same boost pressure as before.

Overall:

Well obviously the cat reduced back pressure in the exhaust which means extra performance. how much? I'd say the the 3-4% figure seems about right which means another 7 to 9kw at the wheels. This would entirely come about from the reduced pressure differential between intake and exhaust. With typical turbine pressure ratios of 1.8 to 2.0 I'd estimate I have reduced back pressure after the turbine by 1psi

1psi drop??? Does that matter?:

My car would produce ~900lb/min exhaust flow. Taking bench testing results this would mean ~2.5psi for a 300cell cat and 1.5psi for a 100cell 4.5" body. Then I'd add 1psi for the down pipe and 1psi for the rest of the exhaust. giving a previous total of 4.5psi, and a new total of 3.5psi.

the pre-turbine pressure would then be:

now, (3.5+14.5)*1.8 = 32.4psi (17.9 relative)

before, (4.5+14.5)*1.8 = 34.2 (19.7 relative)

I have intake pressure of 14.5 so the ratio of intake to exhaust pressure was 19.7/14.5 = 1.36

but now it is 1.23

And that ratio is efficiency and explains both the extra power and the boost creep.

In the end I have made a shed load of assumptions in the above and am probably now qualified to write a Today Tonight expose. But really I just like the crisper and slightly louder sound plus a bit more power (all while being a bit green).

I also have a spare cat in working condition should the emissions testing get more severe (but atm I think I'd pass pretty comfortably).

Edited June 20, 2011 by simpletool

[Pal]

^ so what did that cos ya Daniel. and you lost me after ' does that matter'

[SargeRX8]

As said above, it sounds crisper. I cant judge on performance. My car was stock running stock cat. I upgraded the turbo and cat at the same time so I am in no position to draw a line on performance but all I can say is if it can flow better it should perform better. General rule of thumb, to an extent.

[simpletool]

Yeah when I post from work I can get a bit technical...bad habit.

Essentially less back pressure is multiplied by the turbine housing to be even more back pressure in the exhaust manifold. The exhaust air and sometimes (valve overlap) intake air must work against this back pressure somewhat when filling the engine. Less back pressure is a good thing.

[simpletool]

Cost was $290, but the prices might vary a bit. This was with flanges.

I figure $290 for slight gain, and more response. And I'd pay $870 for triple the benefits in a heartbeat.

[lilcrash]

I've run a single cell for over 4 years in the stagea and 3 years prior in one of old cars and never had any issue with the police. But they did both shoot flames quite happily on down shifts.

[simpletool]

I've run a single cell for over 4 years in the stagea and 3 years prior in one of old cars and never had any issue with the police. But they did both shoot flames quite happily on down shifts.

I wouldn't ever post that on a public forum that is easily searchable..with a picture of a conspicuous car....and will still be here in years to come. Especially with so little to gain from writing it.

Edited June 21, 2011 by simpletool

[lilcrash]

I doubt that anyone would be bored enough to search forums for illegally modded cars and I'm not worried about the police anyway, It's those damn purple lighted cars I'm scared of, I see them about once a year driving around and they can actually defect a car. All police written defects I've gotten have been removed without touching a thing. But the rta guys know what is and isn't legal so they always get me.