Which is better and why? the 2.5" or the 3" and why is the 3"

Without a big turbo it's not worth going to 3'' dp route.....for remap etc 2.5'' is fine.

CEL is the number of cells within the cat...the less cells the freer flowing the cat.

cheers bud...:cheers:

Also is there a limit or recommendation as to minimum cel number for still passing MOT without having to keep swapping cats?

I have a Blueflame turbo back exhaust system with a 200cel sports cat on my Octy vRS. At the last MOT she sailed through

A 100 cel cat should also be ok

100CEL is fine but you may get the Check Engine Light on which can only be mapped out...IIRC.

Mine went through MOT...with big turbo and 100CEL cat....no problems.

Just out of interest, Does anyone know how many cels are in the standard cat?

No idea what CEL a standard cat is but ill be goin for a 3" down pipe

sod payin £100 extra for a smaller pipe! No one ive spoken to has not said to get a 3" just that the 2.5" is quieter and that its rather for higher BHP (240+ i think) so ill be better of puttin the extra cost towards a remap

I have a Blueflame turbo back exhaust system with a 200cel sports cat on my Octy vRS. At the last MOT she sailed through

A 100 cel cat should also be ok

I have the turbo back Blueflame exhaust with the 100 CET sports cat, sails through the MOT and never had the management light on. Go for it.

Is there any downside to going for the 3"? apart from the fact it's just not needed without a big turbo, would it be detremental fitting it anyway as it is

Is there any downside to going for the 3"? apart from the fact it's just not needed without a big turbo, would it be detremental fitting it anyway as it is

Hi

As Adrian said I do believe that it is down to backpressure.

I would suggest the 2.5" downpipe to anyone who asked about this.

Also IIRC there are 400 cells in the OEM cat

Sarah

AIUI if you drop the back pressure too low on a turbo you can get an oil leak through the exhaust turbine seals, which gives you an interesting blue coloured exhaust emission, would give a problem with passing the emissions test anyway, and will clag up the cat, which will almost certainly cause an MoT fialure, accompanied by a bill for a new cat.