AFR affect of adding high-flow cats
Thread Starter
Burning Brakes
Joined: Mar 2001
Posts: 1,228
Likes: 16
From: Mansfield Texas
AFR affect of adding high-flow cats
I have a 06 ZO6 with an ERL short block, CNC heads, cam and ARH long tubes 3" X-pipe offroad and Borla exhaust. Custom conservative tune by reputable performance shop putting out an estimated 580rwhp naturally aspirated.
Question, I am going from an ARH x-pipe non-catted mid-section to an ARH x-pipe high flow catted mid-section. I know a re-tune is advisable to verify fuel, air and timing are correct but my question is will adding cats raise or lower my AFRs and if so by how much?
Question, I am going from an ARH x-pipe non-catted mid-section to an ARH x-pipe high flow catted mid-section. I know a re-tune is advisable to verify fuel, air and timing are correct but my question is will adding cats raise or lower my AFRs and if so by how much?
Drifting
Joined: Oct 2011
Posts: 1,684
Likes: 73
From: Orlando FL
Should get slightly richer.
Less air is passing through cylinders meaning less oxygen but same amount of fuel is being injected.
Make sure you have plenty of room between cats and anything you dont want to burn. They get stupid hot with a cam.
Less air is passing through cylinders meaning less oxygen but same amount of fuel is being injected.
Make sure you have plenty of room between cats and anything you dont want to burn. They get stupid hot with a cam.
Thread Starter
Burning Brakes
Joined: Mar 2001
Posts: 1,228
Likes: 16
From: Mansfield Texas
Thanks guys so far so good it is running great at idle, under a load in high gear no surging or bucking, and it seems a have a choppier idle out back in terms of more compression pop just from the slight increase in backpressure and biggest thing of all no more sulfur smells out back once the cats heat up so I am happy with my decision. No wideband btw.
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 326
From: CA.
St. Jude Donor '13-'14-'15
I think the AFR's will remain the same---The key issue with your change only applies if you were reading the AFR in the back with a tail-pipe adapter---If you used an 02 bung PRE-CATS to read the AFR's then no issue--But if you read the AFR's with the tail-pipe adapter then those readings will change---as with cats the AFR's will read leaner as the cats clean up the hydro carbons somewhat---With NO cats it will read the ACTUAL AFR's and will tend to be richer than with cats
Thread Starter
Burning Brakes
Joined: Mar 2001
Posts: 1,228
Likes: 16
From: Mansfield Texas
O2 bungs are in the header collectors forward of the cats and there are no 02s being read after the cats. Still seems a little rich at idle and under 3K RPM part-throttle cruising, over 3K WOT it comes alive very quickly and I feel a significant transition of power.
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 326
From: CA.
St. Jude Donor '13-'14-'15
The richness is not caused by having cats or not having them----The richness is a simple byproduct of LT headers----That is why they are illegal in many states-because the front 02 bungs are moved further aft with LT's often as much as 3 ft---The re located 02's will make them NOT read correctly and not heat up correctly--giving your front 02 readings incorrect data--Always richer----There is no 100% fix for this--again why they are illegal---However there are a few fixes that tuners you can do to help--1st may will install the rear 02's onto the front locations----as the rears heat up better and are more sensitive--2ndly your tuner can LOWER the 02 switching points to trick the ECM into commanding a leaner P/T or closed loop AFR---to get it back to the stock stoich AFR of 14.68
PS: after my LT install on my 02 my PT AFR's went way rich to like a 14.0--After moving the rear 02's onto the front and lowering the 02 switching points by 150--The P/T fuel went somewhat leaner to a 14.4----This was the best I could get it and I lived with it--- You'll never get it back to the Stoich of 14.68 with LT's
PS: after my LT install on my 02 my PT AFR's went way rich to like a 14.0--After moving the rear 02's onto the front and lowering the 02 switching points by 150--The P/T fuel went somewhat leaner to a 14.4----This was the best I could get it and I lived with it--- You'll never get it back to the Stoich of 14.68 with LT's
