What affect does cutting the pipe from the exhaust manifolds to the base of the carb have? I notice a lot of the racers do without it, but why, and what would problems would arise from having one like that on a road car?

Atomisation of fuel is poorer without the preheat pipe in place, so the mixture needs to be enriched to compensate.
The racers dispense with it assuming/hoping that the cooler charge entering the cylinders will give more power, however the engine will be prone to carburettor icing.

When the UK mob went to Spa some years back to do a 2 hour 'warm-up' race prior to the main event, the majority of them were back in the pits at the end of the first lap, bonnets up and chipping away at the chunks of ice which were blocking the carburettor throats.

I've seen carb icing affect the racers at Oulton late in the season, also cars doing faster lap times on a cool weekend at Croft once some gaffer tape was planted over the grille...

Kkn

That all makes sense, is there any evidence of the inlet manifold bolts stripping the threads in the head, as I'd have thought the weight would put undue stress on them, not being supported at the front?

Dunno what happens nowadays,
but I always left the pipe in place on the offside to give support for the outlet which carries the alternator.
Blanking off the hole on the nearside meant that the exhaust gasses weren't pulsing to and fro through the pipe, so the heat input to the inlet was reduced considerably.

ken

( Should have read the question properly.
Nope, no greater occurrence of stripped inlet threads, iirc )

That was my concern with the alternator so I welded in a bar.

I never had a problem with the other side and my crossbox was only supported on one side.

Harley

Norry Taylor had a way of welding the NS exhaust manifold with a protruding stub which was a tight fit in the cut pipe as well as plugging the OS so that the manifold retained its support. But Norry being Norry had various combinations for various plans of attack.

If infact the tube is for heating to avoid icing then this would slighly reduce power. In most light aircraft engines there is a carby heat control to disapate ice and when applied causes a reduction power. It would not be difficult to incorperate such a device to allow use only when in icing conditions and more power at other times??
Geoff