Small bore exhaust is strangling the 1275cc engine

[ColinN]

Hi
i have a 1275cc Marina engine fitted to my Series II convertible, but it has the original downpipe and small bore exhaust system - albeit stainless which would have been appropriate for the original 803 or 948cc. I feel the exhaust is strangling the larger engine and not allowing it to perform as it should. What bore of exhaust should the 1275cc have and presumably i also need a larger exhaust manifold? who supplies them?
Also , while i'm upgrading the exhaust and manifold, i'm thinking of replacing the single 1.75" SU with a twin carburettor arrangement but which set up would suit it best and which car should i source the inlet manifold from - presumably with 1.25" SU's? Mini or Midget or other?
Many thanks
Colin

[chrisryder]

Best carb is the one you've already got, the single 1.75 inch.

Presumably you've got separated manifolds to allow the use of the big carb?

Many minor suppliers do large bore exhaust kits. With a bore of 1.5 inches, possibly a little more. They'd be perfect, or you can go direct to maniflow, who make large bore and small bore performance exhaust systems for minors.

[bmcecosse]

Forget the tiny twins! Useless.... If you have a 1.75" carb then get it on an alloy unheated inlet manifold -and a 3 branch exhaust. If it's an otherwise standard 1275 - an LCB is marginal - and tie it up to a 1.5" bore system.

[JOWETTJAVELIN]

Twin carbs are a nightmare to set up and keep running evenly. I think Weber did a conversion for the 1275 which gave you a few extra horses, but it was pricey.

[mike.perry]

For that classic 60s look you can't beat a pair of twin SUs

[bmcecosse]

But not for performance....

[Alec]

Hello J.J.,

" twin carbs are a nightmare to set up and keep running evenly. I think Weber did a conversion for the 1275 which gave you a few extra horses, but it was pricey."

S.U.s, single or twin are easy to set up and do not need frequent tuning or fiddling with,set up and leave alone.

In my view S.U.s are better than Webers in virtually every instance; their racing carburettors (DCOE and the like) give a few more peak horsepower on tuned engines for racing, that's all. Their conventional compound carburettors offer no benefit and are very complicated. The deceptively simple S.U. is a very good design and seems to be under rated for some reason. Certainly, DCOEs look the part but looks aren't everything.

Alec

[bmcecosse]

Actually - a well set up and 'Vizardised' pair of 1 3/4 SUs are BETTER than a 45 DCOE weber, provided the SUs are fitted on a straight-in long runner manifold - with N O balance pipe. The weber has max choke of 42mm - the SU is 45 mm, both have a throttle plate, and the Weber has an additional obstruction in the airflow where the primary choke sits..... There is indeed no great difficulty setting up twin SUs and absolutely no reason why they should 'go out of tune'. The twin 1 1/4" " SUs are just too small for a 1275 engine ! Remember -each cylinder breathes individually through it's inlet port. There is no worthwhile crossflow through the standard BMC inlet manifold balance pipe - indeed - where it intersects the inlet runner it actually disrupts good flow from the carb, and so in effect with twin 1 1/4" carbs - each cylinder only sees a single 1 1/4" SU! The 'benefit' comes from the fact that the inlet manifold is NOT heated by the exhaust manifold (or by silly water jacket) - and the extra power comes purely from the cold charge entering the engine. A big single SU is a better bet - each carb then sees that bigger carb - slight disadvantage of course is the curvy path of the inlet runner - some manifolds are better than others in this respect.