We've used AVGAS and a Super unleaded 50/50 mix for years and on many different engines with no issues, race gas is just so expensive over here in the UK. The 100LL is about a 1/2 of the price of real race gas. I once had to get some race gas at the track as I progressed through the rounds and ran out of my normal avgas/super mix. FWIW we didn't notice any difference in performance or any change in the AFR's just a lighter wallet 
'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials.
9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge.
RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03
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