bro as far as my knowledge goes... it think its the wiring of the plugs (wiring ti the plugs) but let some expert give u a better description!
yes there are many types.... there are a few which work better on cng. i guess they;re specialized for it... the size cud depend on hoe much space u got in the engine... in city idsi.. the engine is a bit compacted and the stock intake is on the bottom right. but it does affect your mileage! mtlB fuel consumption increases plus oil jaldi ganda ho jta hai!
i hear ky jo plugs + wire change hai... it also increases economy of the car