Ms = 6567.18; (* mass 7H *) 
Mb = 7482.538; (* mass 8He *) 
Mp = 938.272; (* mass proton *) 
Mt = 1875.612; (* mass 2H *) 
Mr = 2808.391; (* mass 3He *)

Kin = 1785.15; (* 8He lab momentum *)
K = (Mt/(Mb+Mt))Kin; (* 8He CM momentum *)
Kspf = (Ms/(Mt+Mb))Kin; 
Q = Mt+Mb - Ms - Mr;

Ebal[Kpar_,Kperp_]:= ((Kin^2)/(2 Mb))+ Q - (((Kpar^2)+(Kperp^2))/(2 Ms)) - (((((Kin-Kpar))^2)+(Kperp^2))/(2 Mr))

ContourPlot[Ebal[Kpar,Kperp],{Kpar,500,2000},{Kperp,0,500},Contours->10,Epilog->{Line[{{1350,40},{1500,20}}],Line[{{1500,20},{1570,70}}],Line[{{1570,70},{1400,150}}],Line[{{1400,150},{1350,120}}],,Line[{{1350,120},{1350,40}}]}]