My honest thinking about big intruders is they act like a knife to the combustion gas force, and direct the main force away from the centre of the piston and create a sort of splitting swirt effect, but I’m only guessing on that one, I haven’t seen any data to support it, other than the fact that big intruder engines never make the power that a small intruder engine makes, I have seen this happen of BTCC, WRC and S1600 engines on several occasions…….am I making sense now?

try to imagine a wide valve layout, say 50 degrees included angle, the space created in the combustion chamber would be massive, in order to get a sensible compression with sensible valve openings, you would need a big intruder, with quite deep fly cuts, don’t forget the deeper the fly-cuts, the less the compression – not a good situation to be in!.now consider an 18 degree included valve angle, the combustion chamber shape will be a lot shallower, requiring a much shallower intruder and fly-cuts to get the same compression as a 50 degree head……. In my mutterings about the compromise between intruders and fly-cuts………. which normally restrict flow, but it could easily be argued that this bend in the ports could help the tumble effect of the incoming charge, more tumble is a good thing.obviously F1 type installations dont have the bend packaging problem though! I’m not saying the guys wrong, but the more upright the valves are, the more bent the ports need to be, for under bonnet packaging reasons. Hmmmmm…….decreasing the valve angle makes the gas flow better. The combustion chamber is therefore practically contained in the piston cavities, such cavities becoming bigger as the stroke/bore ratio decreases, which makes it hard to obtain the high compression ratios required by high specific power engines.' It should be borne in mind that aspirated racing engines require rather extreme valve lift and overlap angles, therefore, cavities are made in the piston crowns to prevent contact with the half-open valves. The 'shape factor' is a critical synthetic value to check a combustion chamber's good operation, and a good indicator of its compactness and 'thermal efficiency'. This stems fundamentally from the need to augment the injection advance and from the worsening of the 'shape factor' of the combustion chamber which, with the reduction of the bore/stroke ratio, becomes ever broader and flatter. Unfortunately, such a large bore currently causes combustion problems with dramatically decreased efficiency. 'The Testastretta engine fitted to the Ducati 998R 2002 version, the bore is 104 mm. I think that the decreasing valve angle has contributed to good gas flow and this is where the power is coming from not necessarily the combustion chamber shape.

Flycut on the piston