That legal?
Let alone does it do the engine any good? Probably not. Or improve its performance? Probably the opposite. Impress your other neighbours or anyone else? No.
I am surprised your neighbour even agreed to remove a silencer. Maybe he is not quite the highly-trained, highly-skilled motor-mechanic he claims.
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It would be highly illegal in Britain. Even if you have a car whose catalytic converter has been stolen, you are not allowed to drive it until it has been repaired. It would have be towed or lifted to the exhaust-fitters.
Instead we have these young clowns fitting exhaust resonators to make even a little Citroen Saxo sound like what... a racing car? ... a bulldozer with a broken silencer? Some are "designed" to be even worse, by creating very loud back-firing - normally a symptom of faults.
(I kid thee not about the Saxo. I once saw one with not one but two "boom box" exhausts. One must have been a dummy, rather expensively because these fittings cost around £150 each.)
What these antisocial petrol-heads must consider is that if their car is over three years old they would have to replace the ridiculous device with the proper exhaust for the annual MoT test (of meeting legal standards for roadworthiness and increasingly, environmental health).
They must also consider that if involved in an accident, even if the exhaust had nothing to do with it, if the insurance company had not accepted its fitment they have every right to refuse to entertain the claim and even to cancel the policy altogether.
Lose your policy and you cannot legally drive at all until you have found a company willing to take a risk with you - at inflated cost.