E8*E8 has 496 gauge modes, with each E8 having 248 of them. It is possible to get the Standard Model out of E8 and its fundamental multiplet, 248. It is also the E8 adjoint multiplet, the multiplet where each member corresponds to an algebra generator. Something only true of E8.
E8 -> E6*SU(3)
248 -> (78,1) + (1,8) + (27,3) + (27*,3*)
I've also seen
E8 -> SO(10)*SU(4)
248 -> (45,1) + (1,15) + (16,4) + (16*,4*) + (10,6)
The possible paths from E6 to the Standard Model have been abundantly explored by GUT model builders. Here are some:
E6 -> SO(10)*U(1)
27 -> (16,1/3) + (10-2/3,) + (1,4/3)
27* -> (16*,-1/3) + (10,2/3) + (1,-4/3)
78 -> (45,0) + (1,0) + (16,-1) + (16*,1)
SO(10) -> SU(5)*U(1)
16 -> (10,-1/4) + (5*,3/4) + (1,-5/4)
16* -> (10*,1/4) + (5,-3/4) + (1,5/4)
10 -> (5,1/2) + (5*,-1/2)
45 -> (24,0) + (1,0) + (10,1) + (10*,-1)
SU(5) -> SU(3)*SU(2)*U(1) (Standard Model)
5 -> (3,1,2/5) + (1,2,-3/5)
5* -> (3*,-1,2/5) + (1,2,3/5)
10 -> (3,2,-1/5) + (3*,1,4/5) + (1,1,-6/5)
10* -> (3*,2,1/5) + (3,1,-4/5) + (1,1,6/5)
24 -> (8,1,0) + (1,3,0) + (1,1,0) + (3,2,1) + (3*,2,-1)SO(10) -> SO(6)*SO(4) = SU(4)*SU(2)*SU(2)
16 -> (4,2,1) + (4*,1,2)
16* -> (4,1,2) + (4*,2,1)
10 -> (6,1,1) + (1,2,2)
45 -> (15,1,1) + (1,3,1) + (1,1,3) + (6,2,2)
SU(4) -> SU(3)*U(1)
4 -> (3,1/4) + (1,-3/4)
4* -> (3*,-1/4) + (1,3/4)
15 -> (8,0) + (1,0) + (3,1) + (3*,-1)
SU(2) -> U(1)
2 -> (1/2) + (-1/2)
3 -> (1) + (0) + (-1)E6 -> SU(3)^3
27 -> (3,3*,1) + (1,3,3*) + (3*,1,3)
27* -> (3*,3,1) + (1,3*,3) + (3,1,3*)
78 -> (8,1,1) + (1,8,1) + (1,1,8) + (3,3,3) + (3*,3*,3*)
SU(3) -> SU(2)*U(1)
3 -> (2,1/3) + (1,-2/3)
3* -> (2,-1/3) + (1,2/3)
8 -> (3,0) + (1,0) + (2,1) + (2,-1)So while one can get from E8 to the Standard Model, doing so requires some rather complicated symmetry breaking. In E8*E8 heterotic-string models, this is done by the compactification of six of the ten space-time dimensions. Their compactification's topology then induces symmetry breaking, and one can get some approximation of the Standard Model with suitable topology.