Precats are silly!
Discussion
I mean the ones in catted exhausts, not the cars that don't have any at all... 
Why have a cat before the lambda sensor? How are you supposed to tell anything about the mixture with any accuracy? Why do it this way at all, given cats don't really like being in an uncontrolled environment. It's the only engine with this configuration I've seen.
Can I just use an offset to the lambda target on a wideband positioned where the old lambda sensor was? Probably not, as cats work over a range of conditions...
Maybe I should just be rid of them? I assume it'll pass a MOT without precats if the lambda target on the wideband is 14.7:1 and the main cat is fitted...
Why have a cat before the lambda sensor? How are you supposed to tell anything about the mixture with any accuracy? Why do it this way at all, given cats don't really like being in an uncontrolled environment. It's the only engine with this configuration I've seen.
Can I just use an offset to the lambda target on a wideband positioned where the old lambda sensor was? Probably not, as cats work over a range of conditions...
Maybe I should just be rid of them? I assume it'll pass a MOT without precats if the lambda target on the wideband is 14.7:1 and the main cat is fitted...
Had my pre-cats out on both of the griffs i've owned.I'ts much nicer for noise
,its much better for the underbonnet temps and all the things that suffer from this.There isn't one good reason to keep them imho,so BIN THEM!!.Oh and yes it'll pass the mot with ease,as long as everything is in good shape of course.
,its much better for the underbonnet temps and all the things that suffer from this.There isn't one good reason to keep them imho,so BIN THEM!!.Oh and yes it'll pass the mot with ease,as long as everything is in good shape of course.
Apparently they're there for cold idle emissions.
Yes, I understand the cold emissions thing - the Impreza had 2 cats for that reason too. The fundimental difference is that the lambda sensor is looking at the gas BEFORE the cats. I can't get my head around the lambdas being AFTER the cats 
They're just going to tell me lies!
>> Edited by dnb on Tuesday 24th January 20:09
They're just going to tell me lies! >> Edited by dnb on Tuesday 24th January 20:09
dnb said:
Yes, I understand the cold emissions thing - the Impreza had 2 cats for that reason too. The fundimental difference is that the lambda sensor is looking at the gas BEFORE the cats. I can't get my head around the lambdas being AFTER the cats
In theory, it doesn't particularly matter where you put the lambda sensors. These 3-way cats convert hydrocarbons and carbon monoxide into water and C02, and reduce nitrous oxides to nitrogen and water. So they create oxygen, but only when there is a significant amount of nitrous oxides, which only occur when the engine is running lean. The net result is that the only time you should get oxygen in the exhaust upstream OR downstream of the cat is when the engine is running lean, or there is a misfire or incomplete combustion. If this happens it will leave unburnt oxygen and fuel in the exhaust which will probably burn in the cat, but that's a simple combustion reaction and not what the cat is for.
(Some exhaust systems inject extra air into the cats to help the HC/CO oxidation and if you have this sort then it is important to get the lambda sensors in the right place, but the TVR RV8 engines don't use this.)
If you're taking the pre-cats out, they may be loose and just fall out of the manifold when you tip it up, or it may require brute force with a chisel. If have a welding and grinding gear you might find it's quicker to cut the manifold open rather than chisel it from the end, but you should be able to get the pre-cats out without resorting to this.
Got welding gear - so not too much of a problem.
Remember that I'm putting wideband sensors in place of the narrow band (I'm told the Bosch LSU widebands use partial gas pressures or some such thing to tell what's going on rather than the usual way oxygen sensors work.) So having cats in the way will have an effect say during hard acceleration (or any non-ideal conditions...) when I want to know how rich the engine is actually running, not how rich it is after stuff's potentially burned in the cat. Or have I missed something here?
The grand plan is for automatic VE table optimisation in the new ECU.
Remember that I'm putting wideband sensors in place of the narrow band (I'm told the Bosch LSU widebands use partial gas pressures or some such thing to tell what's going on rather than the usual way oxygen sensors work.) So having cats in the way will have an effect say during hard acceleration (or any non-ideal conditions...) when I want to know how rich the engine is actually running, not how rich it is after stuff's potentially burned in the cat. Or have I missed something here?
The grand plan is for automatic VE table optimisation in the new ECU.
dnb said:I dont know about your ecu Dave,but on my Omex,you can choose when the sensor feedback is in use,including the throttle position.So it isn't a problem to take it out of the equation on full throttle(100%)acceleration etc.
Got welding gear - so not too much of a problem.
Remember that I'm putting wideband sensors in place of the narrow band ........ So having cats in the way will have an effect say during hard acceleration
The grand plan is for automatic VE table optimisation in the new ECU.
At the mo. im running completely open loop,but thinking of going closed loop on mine(only one sensor though),once the completely de-catted system is in
dnb said:
So having cats in the way will have an effect say during hard acceleration (or any non-ideal conditions...) when I want to know how rich the engine is actually running, not how rich it is after stuff's potentially burned in the cat.
I believe that the lambda sensors are feedback devices which enable the ECU to modify fuel quantity to achieve complete fuel burn during steady throttle conditions. Under acceleration conditions no such modification is performed because the engine performance would be rubbish.
Your point about the fuel igniting in the pre-cat (before the lambda sensor)is interesting - that would seem to affect the sensor measurements and thus cause inaccuracies.
Norbert500 said:
That post is incorrect dnb. You have the passenger footwell filled with battery, fuse board, ECU, wires, an LCD display, a PS2 keyboard, a laptop and assorted random parts. This means there is no space for you! I haven't driven MY car for ages because of your twiddling....
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