Decat fitting and fuel mixture MB6

[edwardm]

When i fit my MB6 decat, are the lambda sensors to be considered?

For example, if they need removing and refitting, can someone tell me where they disconnect in the engine bay please?

Also, my friend told me that decatting with a full exhaust system and NOT remapping the car may cause it to run lean. This doesn't sound likely to me, but what do you guys think?

Thanks

[tontvti]

The O2 sensor is in the exhaust manifold so you wont have to touch it unless you're changing the manifold aswell.

Yes the car will run leaner for a few minutes but the ECU adds more fuel as the O2 sensor tells it that the mixture is lean. Resetting the ECU is the easiest method

The way I did it is half way down the below link, the MB6 has both a back up fuse and an ECU fuse, most other civics seem to have one or the other.

viewtopic.php?f=62&t=46&p=33453&hilit=reset#p33453

[edwardm]

Ok thanks for the info.

So why on earth does it run lean when you touch the exhaust, especially if there's no sensors on the cat/exhaust?

Surely it would only run lean if you affected the cams or inlet side of things?

Thanks

[tontvti]

A free flowing exhaust increases the VE ( volumetric efficiency) of the engine, the exhaust gases travel faster out of the engine; less back pressure by removing the cat. More air out of the engine means more air is sucked into the engine, more air = more power.

Many mods increase the VE of the engine, all which will alter the air fuel ratio. Since the ECU targets stoich at part throttle and idle it knows it needs to add more fuel because the O2 sensor tells it that its a bit lean.

http://www.hondata.com/techclosed.html

[edwardm]

That's a great help thanks

[prezes]

Although Not enough back pressure = less torque

[jalesi2001]

I dont think this is correct, it would be if we had 2 lamda sensors, but we only have one before the CAT, so the ECU doesn't have any idea wether you have a CAT fitted or not. So in theory the fuel/air mixture and the emissions sould be the same coming out of the engine, but the CAT will not be there to filter/clean the gases, so it will fail the MOT emissions test.

The ECU has no way of knowing if there is a CAT fitted, and no way of 'sniffing' the gases once its left the exhaust manifold, so it shouldn't make any difference to how much fuel/air the car will use.

Now if you have 2 lamda sensors, (like EP3) etc, then the ECU will run the car lean as it will detect the CAT isn't doing its job, because the second sensor will detect excess fuel in the exhaust gases.

For us though, its not a problem....(apart from MOT time!) Hope this helps.

[tontvti]

I dont think this is correct, it would be if we had 2 lamda sensors, but we only have one before the CAT, so the ECU doesn't have any idea wether you have a CAT fitted or not. So in theory the fuel/air mixture and the emissions sould be the same coming out of the engine, but the CAT will not be there to filter/clean the gases, so it will fail the MOT emissions test.

The ECU has no way of knowing if there is a CAT fitted, and no way of 'sniffing' the gases once its left the exhaust manifold, so it shouldn't make any difference to how much fuel/air the car will use.

Now if you have 2 lamda sensors, (like EP3) etc, then the ECU will run the car lean as it will detect the CAT isn't doing its job, because the second sensor will detect excess fuel in the exhaust gases.

For us though, its not a problem....(apart from MOT time!) Hope this helps.

But no CAT means less back pressure (the cat is a restriction) and since the internal combustion engine is essentially an air pump; if more air can be pumped out with less effort then at the intake end more air is drawn in (vacuum effect) resulting in a slightly leaner mixture. A ported inlet manifold/P&P cylinder head/headers etc all have the same effect.

less restriction > better combustion - > - gases easily pumped out - > - leaner mixture - > - more air drawn in -- v

^-------- < -------- < ---------- < ---------- < -------- < --------- < --------- < -------- < ---------- < -------- < ----- < ----

Dual O2 sensors is to do with emmisions and to see if the CAT (or secondary air pump) is actually scrubbing the exhaust gases.

Although Not enough back pressure = less torque

kinda, if you move the power band to higher rpms by removing a restriction you'll get less torque at say 3000rpm but more torque than before at 5500rpm Peak torque will be less but thats because

Horsepower = (Torque * RPM) / 5250

A flat torque curve is optimal in the upper rev range, its just a function of maths

[james]

so when mapping an ecu , the engine has all the above mods exhaust, de-cat, port and polish, better intake, air induction kit,

they will turn up the petrol mixture to run it right again??? (less lean)

[tontvti]

so when mapping an ecu , the engine has all the above mods exhaust, de-cat, port and polish, better intake, air induction kit,

they will turn up the petrol mixture to run it right again??? (less lean)

Most modifications tend to shift the power band higher up the rev range so yeah fuel will added where its needs.

Fuel is only half of the equation; ignition timing will be changed aswell, this is where you get the major power gains.