Straight through exhaust/mot regs

[Tom]

Right i went to see my mechanic mate over the road today about him welding up a straight through exhasut for me he said yehyeh no worries you clamp it up and i'll weld it no problem Theres me thinking yes great he'll do it for free aswell
Buuuuuuuuuut then he started banging on about i'll loose power and that i need back pressure and theres no point and it'll fail mot for emissions i was like no its just smoke for diesel buuuut no no its emissions aswell :o
So any of this true or should i just tell him to shut up and do the welding?

As far as i was aware(Been told) diesels dont create back pressure, thats why a dump valve doesnt work on them in the traditional way. Correct me if im wrong ofcourse.

I realise how old the posts are but i was intruiged.

All engines create back pressure, just in a petrol you need to get rid of it as quickly as possible to make more power (i think) In a diesel the pressure helps to spin the turbo and after the turbo we dont need it at all because its only getting in the way. Again (i think).