Question...CS and Pre-Cats

Sorry to sound ignorant, but what is a "pre-cat"? I have a euro CS and it has Cats but nothing else....

 

A pre-cat short for pre-catalytic converter is a three-way catalytic converter that is installed immediately after the exhaust manifold, and in the case of 360s it is integrated at the collector of the headers. The installation of pre-cat near exhaust manifold helps the substrate reach operating temperature much quicker than main cats (installed downstream), hence reducing cold start emissions significantly until downstream main cats have reached operating temperature to eliminate emissions. Once the main cats are warmed up there is absolutely no need for the pre-cats.

In the case of 360s, it appears there is a problem with pre-cat system design since their inclusion in the exhaust path was an after thought. I’d say after thought because 99s and early 2000 did not have them and later in 2000 and beyond they showed up in the US spec cars. There are two issues:

1. There is no O2 sensors installed just before pre-cats:
There are two O2 sensors installed on each bank one before and one after the main cat. Since there is no O2 sensor prior to pre-cats, there is no actual measurement of A/F mixture at the combustion chamber by ECU to see if mixture is rich/lean...ECU adjust A/F mixture based on the difference in Oxygen reading from the two sensors before and after the cats. If the two numbers are similar ECU is not instructed to make any changes to A/F mixture. A rich fuel mixture will dump fuel into the exhaust, and a lean mixture will cause valves to exceed operating temperature and eventually burn.
2. Pre-cats operate in open loop:
There is no thermocouple to make sure pre-cats don't exceed operating temperature. If A/F mixture is overly rich and undetected by ECU, unburnt fuel will be dumped into exhaust and when it reaches glowing substrate the fuel will ignite...If this continues, substrate’s temp will exceed beyond its operating temperature and will eventually break it up into pieces w/o any warning.

 

Out of curiosity, how does air injection fit in? It sounds like that's an "exclusive" to US spec cars as well.

 

If you mean from the system design, Essentially an emissions "add-on" installed by the automakers to help further clean up emissions, the air injection system supplies air to the exhaust stream to promote additional burning of exhaust gases such as hydrocarbons (abbreviated as HC) and carbon monoxide (abbreviated as CO). Some systems also supply air to the catalytic converter to further reduce HC, CO and oxides of nitrogen (NOx), a major contributor to photochemical "smog."

 

That sounds like a pretty integrated "add-on"...I'm guessing that even if you removed the pre-cats, you'd still need to maintain the air injection set-up on a US spec car. Is this a correct assumption, or could you by-pass the manifold portion of this? Ultimately my goal is to remove the pre-cats while keeping the car smog legal once running.

 

all newer 360 CS, Modena & CS have three oxygen sensors – one before the pre-cat – if you look into the engine bay it is sticking out of the headers to the side

also

If you do not have a pre-cat you don’t need the air injection system – air is injected during start-up into the exhaust stream to get the cats up to temperature faster – the ECU at the same time sets the fuel mixture to rich – air & fuel burns and heats cat – one can identify the phase by listening to the rattling of the air pump shortly after startup for a couple of seconds

the system consists of an air pump, two valves, the pipes, and the injectors for each exhaust pipe located at the point where they are connected to the engine block – so it should be pretty simple to disconnect the pumps & valves and take the pipes out if you are not using them

obviously the ECU will still run through the air injection phase and modify the mixture and one would take chances with what will happen

 

I guess I was correct in assuming poor system design in some 360s with precats. Thanks for the post, Ferrari must have added the third O2 sensor later in the design cycle as mine does not have the O2 sensor in the manifold prior to the pre-cats.

Edit: there is somthing sticking at the corner of the manifold which is plug to insert gas analyzer but there is no sensor attached to it, at least not in my car.

Edit: My mistake, there is an O2 sensor prior to the pre-cat in my car as well, but from what I understand there are still two per side. In any case makes my statement in earlier post regarding A/F mixture wrong.

 

I don’t think that you can have a cat without an oxygen sensor before it – cats are very sensitive to heat & air/fuel mixture – they do not work outside of their operating range – I assume you do not have a pre-cat; can’t remember the model year when they have been introduced 2003?

Yes - all cars have the sampling port

 

Yes, my car actually does have a pre-cat and just checked the manual and it shows where the O2 sensors are, there are only two of them. Does yours actually have three O2s?

There is still an issue regarding thermocouple reading from the pre-cat to show if the temperatures are exceeding safe levels.

Removing the pre-cats now in an engine designed to have one will become more difficult since the ECU will sample the difference in oxygen sensors prior and after pre-cat. If pre-cat is removed the numbers are essentially the same and will cause CEL.

 

You are right I had forgotten about the locations of the O2 sensors relative to the precat. The sensors would have to be relocated to before and after the main cat just as on the 99.