GSY300

Sorry SkylineDave but, with respect, you are incorrect. As Trevz quite correctly states, by removing the obstruction in any exhaust pipe, any engine will breathe more easily and therefore produce more power. In Guernsey, moped type bikes are sold to 14 year olds for road use and are restricted to 30mph by placing a washer in the exhaust at the manifold thereby reducing the airflow through the engine. Take the washer away and it will do 40mph. Simple. A Catalyst works in an identical way. Remove the obstruction and maximum boost will increase by 2-3psi. On any non-turbo car, petrol or Diesel, performance will be improved without cats also, or if you drive sensibly, a fuel saving is to be had. Other advantages are an improvement in torque, quicker spool-up of the turbo's and as mentioned by Trevz there is less likelihood of setting fire to the scenery when parking off-road. There will however be no noticeable increase in noise as the turbo's, in recycling all of that waste energy into usable power, makes the turbo motor an inherently quiet engine. You mention 'boost jets, or a manual/electronic boost controller' as the only way to increase boost. Don't know what boost jets are as i'm old school, but I can assure you there are many ways to increase boost charge pressure including ram air, intercooling, NOX injection, tuned inlet and outlet manifolds, bigger turbo's, bigger valves, more valves and a host of other methods. However the cheapest and simplest single measure any engine can benefit from is an unrestricted exhaust, and by removing the cats, you have created a bigger hole for the exhaust gases to disappear through. For the smoke issue re-read my post above. This is my experience and without meaning to offend I hope this helps readers. GSY300

Hastings, Removing the cats is effectively a boost upgrade. You will benefit by a 20% increase in power or 20% decrease in fuel consumption, you choose. There is no real reason for the turbo's to be in any way effected by the removel of this back pressure except that I suspect the seals will take a few thousand miles to re-profile themselves due to the reduced back pressure and may smoke for a time. This happened to mine but after 18 months there was little smoke and a further 18 months no smoke at all. Due to the fact that your engine will able to produce more power and therefore be able to burn more fuel, the engine will be running hotter, especially around the turbo's. This will encourage oil residue burn off to occur throughout the exhaust system and may produce some oil smoke in itself. All turbo's should be treated with care before engine shut off by simply allowing the engine to run at low boost for the last few miles. Allowing this to occur will help prevent hot to cold thermal damage across the seals extending engine life. Use the highest spec engine oil available, fully synthetic is expensive but will break down at a much higher temperature than cheaper oils. With oil you get what you pay for. If I had little money and were able to make one modification to a 300 it would be to remove the cats. GSY300

I had a straight through but well silenced Blue Flame stainless exhaust fitted three years ago after which there was some oil smoke, particularly at idle' for 18 months. Now smoke is rarely seen. The turbo's are fine after 90k miles so just take care on the last few miles before the end of a journey and allow the turbo's five minutes of normal driving to cold soak them. I suspect that the turbo seals reprofile themselves over a period of time due to the decrease in effective back pressure. I suspect your turbo's are fine. Give them a few months yet. GSY300