Cats

[Keith D]

Yes this old chestnut.

 

I got my hands on a set of Cobra HFCs today and thought it would be interesting to compare them to a set of OE cats and and a set of HFCs I couldn't get through an MOT despite a remap and getting them cooking. 

 

Firstly I'm not trying to pick an argument with anyone nor run down any manufacturer - please don't take offence it's not intended.

 

The 200 cel cat brick is ~4" dia and the substrate is ~4" long.  Assuming a substrate surface area of 75"sq/inch, the total surface will be 3605"sq. 

 

The 300 cel cat brick is ~4" dia and the substrate ~2.3" long.  Assuming a substrate surface area of 110"sq/inch, the total surface area will be 3170"sq.  About 13% less area.

 

I've never pulled an OE cat apart, and unfortunately mine are back on under the car.  If anyone happens to have a set kicking about perhaps you can let me know the dimensions, but if I recall the front down pipe bit has a 400 cl ~4" and ~2" long substrate, and the rear part has another bit of oblong substrate maybe ~4" wide x 2" tall and ~2" long.  Assuming a substrate surface area of 150"sq/inch, this makes the total surface area approx 5600"sq.  

 

I'm going to see if its feasible to get a local friendly garage to do back to back emissions tests (I'll swap the cats).

 

Seems to me there should be a linear graph of substrate area v performance that can be used to help choose an optimum substrate type and size.  I found I can can buy bricks online and I have a local custom exhaust fab who can cut and weld my HFCs with new bricks. 

 

I'll keep people posted.

Edited Monday at 18:53 by Keith D

[Keith D]

Has anyone ever experimented with welding second bricks into the y pipe?

 

That would give a substrate surface are equivalent to stock cats, but all the advantages of higher flow 

[Keith D]

Okay, so some interesting findings from my first test.

 

Standard cats, reset STFTs, warm engine, at fast idle the trims are 105% and AFR 14.6.

 

Then I put on the cobra 200 cel cats, and a radium oil catch kit I bought on black Friday.

 

Reset STFTs again, warm engine, fast idle, the fuel trims are now 95% and AFR 14.6.

 

I was expecting HFCs to reduce backpressure and let a bit more oxygen reach the upstream O₂ sensors at idle. That would bias the sensors lean, and the ECU would add fuel.  But I got the exact opposite.🤔