Good breakdown of the side bet here
http://wizardofodds.com/games/war-blackjack/

Before you get your second card the dealer takes your side bet unless your first card beats their up card. Otherwise the dealer matches your bet. you can keep your winnings or parlay the winnings onto your blackjack bet before you see your next card.

Here are the expected values of parlaying wins


Expected Value Table after Two Cards
PLAYER
CARD DEALER CARD
2 3 4 5 6 7 8 9 10 J Q K A
2 Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss -0.477328
3 -0.067072 Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss -0.493467
4 -0.081435 -0.043845 Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss -0.509968
5 -0.094434 -0.056269 -0.013914 Loss Loss Loss Loss Loss Loss Loss Loss Loss -0.525235
6 -0.102975 -0.056679 -0.007415 0.042483 Loss Loss Loss Loss Loss Loss Loss Loss -0.534153
7 -0.065916 -0.021684 0.025496 0.074316 0.125486 Loss Loss Loss Loss Loss Loss Loss -0.543463
8 0.03422 0.070862 0.109038 0.148831 0.190361 0.162702 Loss Loss Loss Loss Loss Loss -0.486622
9 0.096481 0.128206 0.16138 0.196182 0.231257 0.185002 0.117605 Loss Loss Loss Loss Loss -0.390236
10 0.22864 0.252115 0.276744 0.302689 0.329076 0.301068 0.241753 0.160387 Loss Loss Loss Loss -0.226097
J 0.22864 0.252115 0.276744 0.302689 0.329076 0.301068 0.241753 0.160387 -0.006345 Loss Loss Loss -0.226097
Q 0.22864 0.252115 0.276744 0.302689 0.329076 0.301068 0.241753 0.160387 -0.006345 -0.006345 Loss Loss -0.226097
K 0.22864 0.252115 0.276744 0.302689 0.329076 0.301068 0.241753 0.160387 -0.006345 -0.006345 -0.006345 Loss -0.226097
A Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss Loss

i think at high counts your eV goes up a lot and the 7v3 and 6/5v4 become positive. Are aces even good for this side bet? It makes hands more unpredictable, reducing the benefit of getting to parlay while you have an advantage. The minimum adjustment from Hilo would be to ignore aces and deuces. I would guess aces and deuces are both pretty bad tho, maybe worse than 5s and 6s. So maybe use a balanced count of a-4 vs tens or else use a-6 vs cards 8+