of each boss is skewed with respect to a centerplane 54 dividing the passageway longitudinally the bosses have an arrangement such that the terminal of each spark plug will project onto a point on the trochoid wall 10 preferably offset a distance 60 from the centerline 61 of said trochoid wall see fig1 the combined offset distances are less than the diameter of either of said bosses the bosses are arranged so that looking at them along the passageway they show frontal or upstream portions 40a and 41a which are substantially non overlapping whereby fluid flow of a high velocity may scavenge such surfaces and prevent the collection of vapor generated at such hot surfaces vapor generation tends to collect and develop a vapor binding film 50 along the upstream side of each boss in an 34 in line 34 situation see fig2 ideally the positioning of the terminals 27 and 29 of each spark plug for this invention approach an in line arrangement on the trochoid surface 10 gas side of wall 34 while the bosses are arranged to effect a very definite and noticeable offset arrangement in the passage e the bosses are packaged in the housing in such a manner that the boss centerlines 44 and 45 will each form an angle 70 with respect to the plane 54 dividing the coolant passage longitudinally and an angle 72 with respect to a plane 55 dividing the coolant passage transversely the range for such angles is as follows angle 70 is preferably about 25 u00b0 65 u00b0 and angle 72 is preferably about 15 u00b0.