Decatting a few questions.
Discussion
Got a few queries about removing the main cat.... have done a search but no conclusive answers, not that I am expecting one now.
I have a spare Y piece which has had the main cat removed and welded up which will be put in place of the original one which I will swap in for MoT's questions are;
I intend leaving the pre cats in the headers in, purely for convenience, firstly, and drawbacks in that ? Secondly the tune resistor... I was intending in leaving alone (ie set to the cat fuel map) mainly as the pre cats were still there and didn't want to do damage to them, and also not sure if the standard ECU's had all the mapping changed by TVR or just the one relevant to the car ie just the cat map, and didnt want to risk under fueling and the problems that may cause.
As I understand it running the ECU on CAT map with no main cat will cause it to run a little lumpy in low revs until open loop is reached at 3500 rpm, however I have also read somewhere that if the ECU runs in cat and no cats are present then it will think its over fueling and eventually go to limp home mode ? any 1 running like this who can advise on what may or may not happen ?? Also any mods needed like removing lamba sensors etc needed ?
BTW its a 93 pre serp.
TIA
Tim.
I have a spare Y piece which has had the main cat removed and welded up which will be put in place of the original one which I will swap in for MoT's questions are;
I intend leaving the pre cats in the headers in, purely for convenience, firstly, and drawbacks in that ? Secondly the tune resistor... I was intending in leaving alone (ie set to the cat fuel map) mainly as the pre cats were still there and didn't want to do damage to them, and also not sure if the standard ECU's had all the mapping changed by TVR or just the one relevant to the car ie just the cat map, and didnt want to risk under fueling and the problems that may cause.
As I understand it running the ECU on CAT map with no main cat will cause it to run a little lumpy in low revs until open loop is reached at 3500 rpm, however I have also read somewhere that if the ECU runs in cat and no cats are present then it will think its over fueling and eventually go to limp home mode ? any 1 running like this who can advise on what may or may not happen ?? Also any mods needed like removing lamba sensors etc needed ?
BTW its a 93 pre serp.
TIA
Tim.
The only thing the ECU needs for the catalyst map is the feedback from the lambda probes. The physical catalysts (precat or main cat) make no difference to this signal ECU wise , so you can still run the catalyst map without probems. The non cat map can be risky as it needs manual tweeking on the AFM for the correct mixture, it cant cope with engine modifications that alter the fueling requirements outside the fixed part of the fuel map, it could overfuel the cat'(if still fitted), and the fuel ecomomy can suffer. There is also a risk the the non cat map, that it is still the basic Range Rover one, so you would need to check the mixture at full power and its thought the Rangie map runs a bit lean for comfort at full power. The cat map does appear to snatch slightly worse however as there is a tendency for the engine to run little too lean at low rpm as the mixture cycles rich /lean to maximise the efficiency of the catalysts (they work best this way).
Mark
Mark
daz the plumber said:
can you put that in english, as my manifolds are coming off tomorrow.im having the face machined as i still have a leak after the first atempt.will it be ok to remove the pre cats and just drive it with out tweaking thanks.
If you have the opportunity to remove the precats while they are off then go for it 
If you are still using the main cat then you will not have to tweek anything if its just the precats your binning.
blitzracing said:
The non cat map can be risky as it needs manual tweeking on the AFM for the correct mixture, it cant cope with engine modifications that alter the fueling requirements outside the fixed part of the fuel map, it could overfuel the cat'(if still fitted), and the fuel ecomomy can suffer. There is also a risk the the non cat map, that it is still the basic Range Rover one, so you would need to check the mixture at full power and its thought the Rangie map runs a bit lean for comfort at full power.
Mark
That's interesting - does anybody know the answer?Mark
I have a spare TVR 5 litre non-cat ecu (chip marked '500 non-cat'). Obviously it's not the same as fitted to factory 500 cat cars. I assume it was developed for racing, or aftermarket decats, which would suggest the default non-cat map is not specifically for the 5 litre and may be standard Range Rover.
There are no hard and fast rules, as its purely dependent on who did the origional remap, as you can modify the fueling on a map by map basis. As the standard mapping has also changed over the years, even if you look at the individual maps data, there is no real way of telling if its been modified or origional. I dont think things like dates and revision numbers are held the in chip, normally just lables on the outside. Not that much help. The basic warning is, if you switch to the non cat map, to just make sure the car runs rich enough under full load / full RPM for the engines safety. The stock map does not provide any more fuel after 5400 rpm, but I dont know if this is on a map by map basis, or this is a "trim" value that effects all the maps. This value is obviously altered on the TVR mapping.
And yes Daz- you dont need to tweek anything if you remove the pre cats.
Mark
And yes Daz- you dont need to tweek anything if you remove the pre cats.
Mark
sideways dan said:
evening all! i've just removed the precats and main cat myself just last weekend. should i now be getting the old girl remapped then???
You need to change the tune resistor to Green if my memory serves me right 
ETA I did change the chip thinking about it now which gave extra fuel at higher revs
Edited by Pasco on Friday 16th May 17:07
No No No. Dont do it unless you know the non cat map is modified for TVR use, it is more likely to be Range Rover mapping as already said. You dont gain any power either, you are just likely to loose MPG. The only gain is you MAY be able to reduce the snatching at low RPM by making the engine run a bit richer with the setting on the AFM. If its not snatching leave it alone, it works just fine.
Mark
Mark
The AJP Griff said:
Ditch the cats altogether,so you can run the happier map.Your car doesn't have to be tested any different to a pre-cat griff.This applies to all TVR's registered before 1 Aug 95.
Is this true?????I've got a 94 4ltr Chim!! I'll be having the precats removed in the next few weeks, but if i can get rid of the dustbin cat too, without any issues at MOT time, i'd be happy (not one for changing back and forth).
Just to clear up aswell....if you have both pre-cats and main cats removed you don't need the car remapped???
Is it just a case of having a slightly lumpier idle or will the car run lean and damage the engine???
Thanks,
Dave
I've removed all 3 cats and silencer and am running with a cat tune resistor
Car runs fine - not lumpy at all - but quite noisy !!
I would say that it smells a bit rich - although I may be imagining it.
Can anyone clarify the MOT statement.
Mines due next month and I really dont want to have to put the old Y piece back in
Cheers Andy
Car runs fine - not lumpy at all - but quite noisy !!
I would say that it smells a bit rich - although I may be imagining it.
Can anyone clarify the MOT statement.
Mines due next month and I really dont want to have to put the old Y piece back in
Cheers Andy
Thanks for the input Mark, kind of confirmed what I thought.
My understanding of the issue is....
If you remove the cats, and change the fuel resistor on a standard ECU there is a possibility that the non cat mapping was not altered by TVR in which case under heavy load you will under fuel and this my case a large hole in the pocket !! If you leave the tune resistor alone then you will possibly have a lumpy idle/low rev range up to about 3.5k revs when the map go's open loop. If you change the resistor to the EU no cat map you need to check the fueling etc, which would prob mean a rolling road session. I believe there are also chips which can be replaced in the ecu which will alter the mapping without the expense of a totally new ecu. There are also AFM issues which I am not familiar with on the non cat map.
As to MoT's i was under the impression it was down to the year the chassis was registered and I thought the cut off date was 1993 but I may be well off the mark. If you intend changing the Y and dont want the faff of doing so each year then best talk to your local friendly MoT inspector. Pre cats are no problem out for mot as my understanding is that they only contribute while the main cat is warming up,and as such, don't get tested as the car is warm for the MoT emissions test.
My understanding of the issue is....
If you remove the cats, and change the fuel resistor on a standard ECU there is a possibility that the non cat mapping was not altered by TVR in which case under heavy load you will under fuel and this my case a large hole in the pocket !! If you leave the tune resistor alone then you will possibly have a lumpy idle/low rev range up to about 3.5k revs when the map go's open loop. If you change the resistor to the EU no cat map you need to check the fueling etc, which would prob mean a rolling road session. I believe there are also chips which can be replaced in the ecu which will alter the mapping without the expense of a totally new ecu. There are also AFM issues which I am not familiar with on the non cat map.
As to MoT's i was under the impression it was down to the year the chassis was registered and I thought the cut off date was 1993 but I may be well off the mark. If you intend changing the Y and dont want the faff of doing so each year then best talk to your local friendly MoT inspector. Pre cats are no problem out for mot as my understanding is that they only contribute while the main cat is warming up,and as such, don't get tested as the car is warm for the MoT emissions test.
Heres the exact difference between the two maps:
Non cat. This is fixed, and the mixture is fixed at tickover by the AFM setting. Beyond that it blindly follows the fixed map. You can easily alter the overall amount of fuel going in with a fuel pressure regulator, but in doing so may cause overfueling at a lower RPM if the pressure is set for maximum power at full RPM. Like wise you could end up with lean spots if you set the optimum mixture at a lower RPM.This really depends on how close the origional mapping is to the engines requirements. Fuel economy can be really bad if the overall mixture is rich.
Cat map. Automatically sets the mixture at its optimum point to keep the catalysts clean, by using the Lambda feedback. This involves firstly setting the overall fueling to the correct point at idle (like ajusting the AFM manually) and then constantly cycling the mixture around a mid point of about 14:1, although on mine it swings between about 12:1 and 15:1 (and leaner). Above 3400 rpm, or about 3/4 throttle the system goes open loop, and the mixture stops cycling, and becomes fixed at around 12:1 for maximum power. With my experiments it appears that altering the fuel pressure (within limits) simply get compensated for at idle, by the overall fuel map being a richened or leaned off. I think it may have a little effect during the open loop range, as there is no overall corrections now. Open to debate on that one. The plus side of this is the fuel economy is much better as the system is adaptive, and it stops the engine running over rich at light loads plus you dont loose any power once the map is open loop.The only down side is the possible snatching at very low airflows as it runs too lean. This is more a function of the AFM and 14CUX and cam timing that cannot make the mixture rich enough at this point, so it drops so lean it stops cycling around 14:1.
Mark
Non cat. This is fixed, and the mixture is fixed at tickover by the AFM setting. Beyond that it blindly follows the fixed map. You can easily alter the overall amount of fuel going in with a fuel pressure regulator, but in doing so may cause overfueling at a lower RPM if the pressure is set for maximum power at full RPM. Like wise you could end up with lean spots if you set the optimum mixture at a lower RPM.This really depends on how close the origional mapping is to the engines requirements. Fuel economy can be really bad if the overall mixture is rich.
Cat map. Automatically sets the mixture at its optimum point to keep the catalysts clean, by using the Lambda feedback. This involves firstly setting the overall fueling to the correct point at idle (like ajusting the AFM manually) and then constantly cycling the mixture around a mid point of about 14:1, although on mine it swings between about 12:1 and 15:1 (and leaner). Above 3400 rpm, or about 3/4 throttle the system goes open loop, and the mixture stops cycling, and becomes fixed at around 12:1 for maximum power. With my experiments it appears that altering the fuel pressure (within limits) simply get compensated for at idle, by the overall fuel map being a richened or leaned off. I think it may have a little effect during the open loop range, as there is no overall corrections now. Open to debate on that one. The plus side of this is the fuel economy is much better as the system is adaptive, and it stops the engine running over rich at light loads plus you dont loose any power once the map is open loop.The only down side is the possible snatching at very low airflows as it runs too lean. This is more a function of the AFM and 14CUX and cam timing that cannot make the mixture rich enough at this point, so it drops so lean it stops cycling around 14:1.
Mark
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