Catalyst location on lean burn Honda cars
 

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. :thumbup:

Integral!

:)

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.

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. ;)

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).

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?

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.

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.

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)

That will never work :p

Also, if your concern is really having the manifold crack, then that's not much of a concern. Even if it does, what does one of those cost? I used to have 3 or 4 laying around the house and I sold them to homemadeturbo guys for like 20 bucks each. They're also incredibly easy to take off and put on.

Furthermore, the o2 is where it is to help get it heated and because it needs very precise o2 measurements. It's not going to work well in the normal o2 position on a header and it won't work well bunged up on the 3rd running 6 inches from the exhaust port, either, in my experience.

-I thought the VX O2 was a five wire and therefore heated?
also this will depend if you have a VX or HX. The HX searches for the "edge" via the knock sensor not the O2 reading.

we'll see lol. . . As far as I can tell the ECU is deciding which mode so it doesn't have to switch back and forth frequently.

The other reason I think its possible is because pre-FI, it was pretty commonplace to switch it up so you could get high speed one day and the FE the next(The test car was a dodge charger).

The only problem that car ran into was the valves got toasted, but since then valves have gotten alot better in performance. Valves on that engine would go periodically anyway whereas I have yet to have one go belly up over 180-250K miles. So if I have to swap a whole set out from the get go and its worth it(previous owner had not replaced them from 100K-180K before that I have no data or knowledge how the car was taken care of.)

Yes, that's true, but even a heater 02 sensor is heated more quickly by being 5" from all of the exhaust ports. These engines go into closed loop operation very quickly.

Sure, the ECU is deciding, but it's incredibly complicated. Artifically forcing it is going to either not work or not work properly. You could force a chicken to fly but it wouldn't do that very well either.

Anyway, a good driver can do it with their foot.

Number 3 is the best cylinder to have the O2 sensor on, as it gets short changed on cooling and will run the hottest, and also be the first to fail typically. I've found that it's more common for 02 sensors to fail on the VX and CX models because they get too hot, where the standard converter versions seem to last the life of the engine.
The O2 sensor needs to be at around 600* to operate properly, which can easily be attained by mounting the bung in the upper 1/2 of the header. Without the cat, however, warm up would be very slow and would effect performance.

True you could do it with your foot. . .but if you eliminate the throttle body, you can very likely accelerate under leaner mixes that eliminate hydrocarbons. If you can do that you can dump the cat altogether and swap in for injection of water up front or something else(ammonia, urea, sometimes water) in back.

I can accelerate at 2000 rpm and very slight depression of the throttle. . .With the same amount of fuel, alot more air and alot less pumping I can accelerate more. So either you can have faster acceleration at the same mpg or you can get same acceleration and greater mpg.

Then a good driver can get lean burn, less emissions and less fuel all the time when otherwise a good driver can only get lean burn under very low acceleration.

Perhaps. I don't think it's practical. At a certain point of complexity a modification becomes more difficult that doing something like putting an EV fifth wheel on and making a hybrid, so I'd always prefer that.

I'm just talking about switching the throttle to WOT, and then controlling speed by a variable resistor and misting water into the intake(which would also run on a variable resistor run along the same potentiometer the fuel control is on). You of course have to bypass the O2 but thats pretty simply as you just have to make sure it outputs a steady .45-.5 volts.

Then as far as the OP is concerned he can fulfill the statement of if emissions are to be ignored in cat placement, because the cat becomes superfluous.

I still don't fancy it being worth enough of a fuel economy gain to be worth the effort. The new R series engines run WOT and they're not impressing anyone :p

The Honda R series? Like 2006 civic?

Those are still throttled. Its just controlled by a servo instead of by a cable.

Actually people usually do make a pretty big deal when one of the manufacturers does say they are going to make something throttle less(as in no throttle body not as in no way to step down the fuel delivery, as it could be taken either way). Throttleless engines can come close to matching DI(diesel) FE. The reason they don't happen is emissions.

I can make my 93 more emissions friendly than it is with lean burning and a little water, but manufacturers can't get inside ULEV and do it without costs. Also no manufacturer would be able to sell a consumer car that requires a water tank to be filled on a regular basis. How could you market a car that had "extra maintenance." It would be like having to market a car that less HP. Those issues are irrelevant to hypermilers since we go to great lengths for little drops of gas.

So what exactly makes the car go into lean burn mode? Can a tuner get into your ecu and permanently turn it on or change the times it does? I know they can alter vtec engagement and even boost by gears so why not this?

PS: if you have a d16z6 you dont have a lsvtec motor.........

I won't answer your question until you explain your avatar, Drunk.

I couldn't think of anything clever and I was drunk when I set it up so wha-la.

Ah.





The ECU decides when it's time for lean burn mode. My engine can only do it under very light loads, and it won't do it until it thinks the catalyst is hot. But if conditions are right, it will automatically go in to lean burn, and stay there until I ask for more power than it can provide while lean burning.

Unless you have a Civic VX, HX, 02-05 Civic Hybrid, or MT Insight, you don't have a lean burn Honda. AFAIK, the folks on this forum who have converted their cars to lean burn have installed an engine and ECU from one of those cars. Lean burn models use different heads, pistons, ECUs, and emissions systems.