Still from my own experience and from different articles PECO is the way to go.
Here is an extract:
As good as the OE exhaust manifolds are, the rest of the exhaust system can be improved upon, and for this the Peco exhaust system is a high-quality choice. There are plenty of aftermarket exhaust systems on the market, but those made by Peco seem to be the ones that live up to its manufacturer's promises and hence has become increasingly popular. It's the only header whose design takes into account the critical fact that the head uses a siamesed port for the exhaust valves of the middle two cylinders and hence has an oversize center branch pipe. Their quality control is very tight for that of an aftermarket manufacturer, so their system always seems to fit without a lot of bending, hammering, and cursing. It also performs well on both modified and stock engines, which usually can't be said for many of the others: they either work well only on stock specification engines, or work well only on engines that have been modified according to a specific recipe which will consist of other components made by the same company (which, of course, the advertisers never get around to pointing out before you spend your money on the #@!! thing!). The Peco system is street legal. Often a performance exhaust will sound good at idle and while accelerating, but then turns into a howling monster while cruising on the highway and literally drives you out of the car, ears ringing. This might be acceptable in a race car, but not in a street machine. At highway speeds the Peco system is actually quieter than an Original Equipment system, emitting a rich baritone sound rather than the ear-pounding basso profundo or the rasping tenor of some other systems. The gray system will fit the Original Equipment 1 3/4" pre-1975 exhaust manifolds without modification while the red 2" Big Bore system will require the use of the Peco 2" Big Bore header. The Big Bore system is actually intended for use on larger bore engines (1868cc or larger) or smaller-bore engines fitted with flowed heads and hot camshafts such as the Piper BP285. When fitted to smaller bore engines with Original Equipment camshafts they will result in a bit more high-RPM power at the expense of some low-RPM tractability. This is due to the larger bore of the exhaust system reducing gas velocity which in turn reduces scavenging effect in the combustion chambers. In addition, radiant heat from the tubular steel of the header is much greater, exposing the air in the engine compartment, the intake manifold and carburetors, and the fuel system to more heat, thus reducing fuel/air charge density and hence reducing power output. Jet-Hot coating of the header is therefore highly recommended. The lack of any middle resonator simplifies the Peco system and allows you more ground clearance (something every 'B' can use!) and allows the fitting of a slip-and-clamp American-made performance catalytic converter where the middle resonator used to be and thus satisfying the emission laws of many localities. However, if local regulations require it, or if you simply choose to rebuild the engine to Original Equipment specification, an excellent quality replacement twin muffler system is made by Falcon and can be had from Brit Tek (Part# FES001, MGB 1962-1974; Part# FES002, MGB 1975; Part# FES003, MGB 1976-1980). Brit Tek has a website at
https://www.brittek.com
Full article at
www.sterlingbritishmotoringsociety.org/ files/mgbtunings.PDF
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