DE-CAT PIPE = MOT FAIL!!!!

Today we did a little experiment...

We took an 850R with RICA software WITH a catalytic converter. MOT passed. We then took the same car and had the catalytic converter replaced with a straight through pipe. The result was a FAIL. The emissions test failed on the CO reading, indicating that the ECU was overfueling as a result of the catalytic converter not being present. Interestingly, it also indicates that it is highly likely that the car will be LOSING power with a de-cat pipe as a result of the overfueling during closed loop fueling.

So there you have it. An MOT test and a road side emissions test will both result in a fail, and your car will be deemed as not suitable for the public roads!

Adam.

ps. The test was performed on site using our own testing station.

One other point, the DE-CAT pipe causes a boost spike in the higher RPM, but the ECU brings it back under control. It seems that the DE_CAT pipe allows the turbo to spool up faster, and the ECU has to reign it back in. So it won't be producing any more power! It may appear for a brief second to produce more power, but the ECU detects this and everything soon settles back down to the power that it should be!

On driving the car, it doesn't feel faster at all. If I had forked out money for a decat pipe in search of performance, I would be rather disappointed.

Adam.

[sholtby]

Adam,

Did that car have 2 lambda sensors with one before and one after the cat? Otherwise I cant see how the ecu would be able to detect a fault with the converter or that one was missing?

Simon

It only has one, but rest assured the ECU still detected it. If it didn't detect it, then the CAT is not converting the CO and hence the MOT fail. Maybe it was due to the different air flow as a result of the de-cat. Bottom line is that the result would be an MOT fail.

Adam.

[Peter Milnes]

I suspect that if you had adjusted the fuelling with the de-cat in place you would not have failed the MOT on emissions. If it is overfuelling with the de-cat then surely you have a means of reducing the fuelling to give the same result as with a cat. It is a case of slight re-mapping to suit the altered exhaust conditions. You may have noticed a fairly significant power gain with the de-cat.

All the best, Peter

The car felt the same with de-cat. In fact, if I had bought a de-cat pipe in search of performance, I would be VERY disappointed.

In order to get it through the MOT, you would need to lean the mixture of about 40%.

Adam.

[Cypher007]

i ran an 850 2.5 20v for about 18 months de-cated, and went through one MOT no problem. i found the car would accelerate quicker and on power enginering's road i acheived 174BHP. i did notice a slight drop of about 1-2mpg in fuel consumption, probably due to the fact i was getting fuel/air through the engine quicker. i suspect if your ECU was returned to its original settings it would pass.

No it wouldn't because the idle map is not modified. Having said that, I'm gonna give it a go to see if it can be done. I'll post results here!

Adam.

[sholtby]

I have removed cats in the past and noticed a good improvement in acceleration. I suppose there is less restriction of exhaust gasses for when you put your foot down.
Whats the CO limit for an MOT? 0.5% rings a bell. When I had carburetter and Bosch K jetronic cars most were set at 1 % idle but didnt seem to run excessively lean at 0.5%. Just got to keep an eye on the spark plug colour and make sure they dont go white.

simon