Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Regarding a slightly richer target lamda with a cat

Understanding AFR

Forum Posts



Tech Articles

Discussion and questions related to the course Understanding AFR

= Resolved threads


This video explains suggests that a slightly richer target lamda should be considered at wot if there is a cat. I understand that this controls EGT and cats don’t like high EGT. At the same time however I’ve always been under the impression that excess fuel in the cat quickly heats it up. So I feel like one point contradidts the other. Just looking for some clarity or a bit deeper look the factors here and pros and cons.

Hi joe there is a difference of excessive fuel and slightly richer the window of tuning afr is fairly broad but lets say we are normally targeting 12.0 under wot with no cat, i would target 11.8-11.9 with a cat and in this case i would call excessive being 11.3 and richer

hope this clarifies your questions

Regards Ross

So the effect on the EGT by adding fuel beyond stoich is more than the effect of the additional fuel on the cat but only to a point / at some point the gains on EGT cooling become not significant enough in comparison to the added heat generated in the cat?

I'm quite confused by this thread. I may well be mistaken, but my thoughts.

There are two things that heat the cat'up, one is the heat of the exhaust gasses and this is actually required to get them to catalyze properly.

The second is the catalysing of the unburned fuel and carbon monoxide in the cat' releases heat. This is also why a misfiring engine overheats the converter, the fuel is being burned in the cat'.

For correct operation of the cat' the fueling should be at stoichometric, or leaner, so there is sufficient oxygen to burn all the unburned fuel.

The cat', in and of itself, has no affect on the actual combustion, cylinder filling, or whatever, other than it's affect on back pressure - so I don't understand why one would run a richer mixture? Not to mention the excessive levels of CO that will be produced?

@Gord, your comments echo my confusion as well and why I’ve asked the question. The source of information to go a little richer in lieu of a cat is from the Understanding AFR Course from a module called “Special Considerations”. Here’s the link, don’t know if you can access it:


At 52 seconds Andre suggests that going a little richer than he would if there were not a cat at the top 1/3 of the RPM range to further control EGT to prevent damage to the cat.

This is a contradiction to my far less qualified understanding of what the cat likes and doesn’t and why I’m looking for further elaboratuon on the topic.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?