E30 Zone

hey

I want to modifiy my exhaust by fitting cherry bomb mid box and decat but im not sure if i need a cat for mot

i have a 316i 1990 plate

No you don't.

My 1991 plate 316i has never had a cat.
You don't need one on any pre-'93 car where that model was already in production.

You don't need a cat. for any pre 08/92 E30.
You also don't need a cat. on any E30 first used after 07/92 if it has a replacement engine first used before 08/92, OR it has an engine on which the engine number is unreadable (so could be an earlier engine), OR it is fitted with a type of engine that BMW never fitted, such as a M30, M5x, or engine from a different manufacturer.
Removing the cat. will also remove the lambda sensor, which will have an adverse effect on idle CO emissions.
On a M40 engine there are two wires which need plugging together under the bulkhead wiring cover, which connects the ECU to a CO adjustment pot. built into the air meter housing. This pot. will then need adjusting with the car hooked up to a gas meter.
Late M40s were supplied with an air meter that doesn't have the pot.

awesome, thanks for the help