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madman wrote:To put a hole in the outside of the narrowest area (and I don't suggest anyone has done this) would cause the low pressure area to suck air into that hole and therefore slow down the air upstream of the hole.

It's nearly 20 years since I studied fluid dynamics but a restriction will increase the speed of the air only through the restriction, won't it? Once the csa of the pipe increases again the flow will slow down and the pressure will return to normal, with or without holes (venturis?) at the restriction.

Edit: Ah, just read your post again and I see you've said the same thing:

madman wrote:I would have thought that a narrowing of a pipe of fixed length would have made no difference to the airflow

And, let's face it, accelerating the air before it reaches the filter is going to have no useful effect on engine performance since the filter then screws up any smooth air flow you might generate. It's flow after the filter which has the greatest effect - this is the theory behind the R1150GS having smaller intake pipes between the filter housing and throttle bodies than the other 1150s - it accelerates the air more at lower engine speeds and improves low-down response at the expense of high-rev power.

I read a bit about, the 1150 I think, chopping off the the extended air supply pipe in the plenum that feeds the throttle body; but only on the right side!? The R1100S has such extentions, anyone tried this chop mod? I remember older Kawasaki fours had a baffle tube that extended down into the throat of the air-filter. The recommended mod was to cut it off. On the R1100S, the whole plenum would be at a given pressure, what is the purpose of such a pipe?

The 1150GS has 'trumpets' that extend into the airbox. I'd guess the purpose of these is to make for a longer inlet tract which should make for a less turbulent flow into/through the throttle body.

BB Power recommend chopping the trumpets off (both sides, not just on the right - that would be plain odd ) as part of the 'conversion' process when fitting their chip. Made bu**er all difference to mine but was a pain in the a*se to do.

Incidentally fitting R1100RS intakes (which don't have the extra trumpet bit anyway) between filter housing and throttle bodies, in conjunction with the extra fuel that the BB Power chip chucks in to the engine, makes for a HUGE increase in stomp at the top end. Most un-GS-like but great fun [GS intakes are stepped down to a smaller diameter - RS ones are straight].

I miss that bike.......