Catalyst location on lean burn Honda cars

[RPM]

Hi everyone,

As you might have noticed, lean burn Honda cars like the Civic VX and HX have a very short exhaust manifold with the catalytic converter mounted very close to the engine.

Supposedly this will help it warm up faster and reduce NOx emissions. However I am told this manifold design constraints power and they do have a tendency to crack, I can only assume due to excessive heat generated by the catalyst.

My question is, do you think this manifold design is integral to the potential of the car in terms of FE and that if I change it for a more performance oriented setup my mileage will suffer? Or, on the other hand, do you think I'll gain power at low revs and thus be able to drive with less throttle and improve my numbers?

Thanks in advance for your opinions.

[SVOboy]

Integral!

[Ryland]

I think it is because your engine is not wasting as much energy and a lower mounted catalyst will not function because it will not get hot enough under normal usage, it's not just the lean burn engines, it's all of Honda's high mpg cars were like this.

If you want to get more power out of your car then sell it and get a more powerful car, don't rip it apart trying to make it something it's not only to get board with it and sell it to someone who is trying to get better mileage and force them to undo modifications that you did for speed.

[RPM]

That's interesting, thanks for your input.

Don't worry, I'm not going to ruin the car trying to make it fast, I have a proper sports car for that. But if the manifold were to develop a crack like it did with some of my friends, I'd like to know what would be the best course of action. Seems like an OEM replacement is the way to go.

[theunchosen]

I suspect the OP might have a minor point. It could be(and most likely is) that Ryland is correct and they are closer on all Honda's but I was talking to a couple of mechanics this weekend and they all agreed I would need to move the cat forward during lean burn mode.

The reason was the exhaust temp would not be as high.

On that note however they also pointed out(which I had assumed) that if you are in lean burn mode your standard cat is not really going to achieve anything because 3-way cats aren't designed to clean NOx and SOx(in O2 rich environs) but hydrocarbons and lean burn is not going to emit very many hydrocarbons(its the point).

[RPM]

Let me just rephrase the question so as to make it clearer: if emissions were not an issue and you only cared about FE, what would the best location for the catalyst be?

[rkcarguy]

I'm in the same boat, I'd like to enjoy the weight savings of a header and run the standard honda converter behind the engine in it's typical position. I think it was placed on the front of the engine to heat up sooner for lower emission. Aside from the hotter converter doing a better job at "cleaning" the exhaust, I think heat held in the engine is a bad thing.
The 92-95 VX and CX have the front mounted cat. The DX, LX, EX, and Si have the standard cat behind the engine. In 96-00 the DX has the front mounted cat, not sure about the others.
I've found the front mounted cat to be restrictive and have always switched my cars to the regular manifold or header with the standard cat instead, without noticing any substantial delay in warm up or increase in warm up stink. My experimentation with lean burn is yet to come though, but on it's way.

[theunchosen]

Originally Posted by RPM

Let me just rephrase the question so as to make it clearer: if emissions were not an issue and you only cared about FE, what would the best location for the catalyst be?

On your work bench at home. The catalyst does not increase FE. It creates backpressure which the engine fights everytime it goes to exaspirate the hot gases from the cylinder. If you ignore emissions the cat is worthless other than maybe avoiding causing some soot on the back of your car from extra hydrocarbons(I think this takes a while, but take a peak at some ricers that have that big black smear above their tailpipe.)

If you mean you are going to leave it on. . .then the place it generates the least amount of backpressure. This would be somewhere that the air is more or less cooling to ambient temperature if the cat were not there. In that place it will reheat the air to keep it moving along instead of condensing and slowing down(that said the cat is guaranteed going to produce more backpressure than the condensing gases). You also have a problem in that you need the cat to get hot to make it work. So you'll need some way of keeping it hot at that location.

simply the furthest you can get it from the engine and it work is best.

[rkcarguy]

Originally Posted by RPM

Let me just rephrase the question so as to make it clearer: if emissions were not an issue and you only cared about FE, what would the best location for the catalyst be?

IMHO, the garbage can.
But, honda's are designed and tuned to have them, so best keep it. Just get the larger one mounted further back and I'll bet you see a slight increase in economy and performance.

[theunchosen]

Originally Posted by rkcarguy

IMHO, the garbage can.
But, honda's are designed and tuned to have them, so best keep it. Just get the larger one mounted further back and I'll bet you see a slight increase in economy and performance.

I vote garbage can and find a way to make the ECU run lean all the time(at least when not accelerating)