Do you like / love driving a motor vehicle? Vote now please!

[Bonefishblues]

What is remapping exactly? Does it conform to the word and the spirit of the law?

It is perfectly legal.You're basically telling the computer to change specific dynamics within the cars running system.
Most cars are 'de-tuned' to fall into certain categories.For example my car is 170bhp detuned from 230bhp.My insurance is happy for me to remap to 230bhp as it is safe to do so without having to modify parts of the engine.Obviously I need to inform them(and I do/will).Tyres/brakes etc are all OK as they are rated higher.My tyres for instance are 'Y' rated(186mph) the car is rated at 138mph.
There are generally two types of remap.One is out-and-out power/torque for acceleration/top speed the other is for extra power/economy.I go for the later.It cures the flat spots and turbo lag so as well as acceleration a bit quicker it's more economical.My last car averaged around 5-8mpg more after a remap despite 'gaining 30bhp.A big saving when you do 12000+ miles a year!

There are of course grey areas when it comes to remapping.Removing a DPF for instance.Although not strictly legal(or illegal) it is extremely difficult to detect as the ECU still thinks it's there and the shell is still visibly there but as it's gone it doesn't throw up a fault so never has to regenerate(or self clean).You are fooling the ECU in reality.
Then there's the extreme remaps/mods where people take engines to 3/4/500+bhp.This would massive increase insurance premiums so I would hazard a guess that not everyone who does it declares it!
I know a guy with an Audi RS4 Avant.It's a V6 440bhp as standard.He's had it tinkered with and remapped to 550bhp.He's eventually going to 600+.His insurance is already around £3k a year so he's not declaring anything and freely admits it!
Remapping will always happen.The more sophisticated engines and software get the easier it is for a computer nerd to get around.There will always be a way to get around the software just as there was always a way to 'soup-up' engines in the good old days.

Sorry to be boring but how do you get around the 'visible smoke instant fail' part of the mot that applies to Euro 4 and newer engines if the dpf has been gutted?

[Ride-sleep-repeat]

Sorry to be boring but how do you get around the 'visible smoke instant fail' part of the mot that applies to Euro 4 and newer engines if the dpf has been gutted?

You're not being boring.I've just read some back posts and see you asked this before.For some reason I'm not getting all notifications from all threads.
My apologies.
To answer your question:-
There is 'no visible smoke'.The readings are exactly the same on the MOT emissions unit.This begs the question "what does the DPF actually do"?IMO,and the(other)Mechanics I trust my cars with to do the remapping(Ie my Brother or one of his employees) not a lot other than cover the car manufacturers legally and make more money for them and their service centres.
It's beginning to emerge that 'Adblue' and the so-called 'Blue-motion' engines are along the same lines.We watched a very interesting video on Youtube where a mechanic/technician or whatever emptied the Adblue tank on a VW Transporter van and filled it with 100% tap water(AdBlue is 67% distilled water/33% Urea).The van ran exactly the same as you'd expect it to as the 'AdBlue' is sprayed into the DPF to further 'clean' the emissions.The thing is the emissions were very similar...BUT after 250 miles the ECU said the 15l AdBlue tank was empty.So the sensors obviously know there is no 67/33% mix.Other than that they noted no difference.So all it needs is some geek to find a get-around program to enter into the ECU to fool the sensors and you can use tap water.We are keeping an eye out as my new car is Bluemotion.
It will be interesting to see how it affects my emissions when we remap/remove the DPF with the Adblue still being sprayed on the exhaust emissions that have been through a non-existent DPF.My money is on no difference as with previous vehicles

[Stevek76]

The urea is there to reduce NOx (both in the literal and chemical sense) which is not measured by the MOT.

Similarly the DPF is there to remove fine particulates which are not well correlated with the smoke test used in the MOT. The present MOT wording is to fail any DPF that even appears to be tampered with, removing it is absolutely illegal and is socially selfish as well, particularly if driving in urban areas.