Altitude does affect the performance of Roots type blowers. Although I cant quantify it, there is a difference. My Regal has the M90, little brother to the M112 in the Lightning and Cobra, and I travel from home (2750 ft) to about sea level every day, and there is a noticable difference in the seat dyno.
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Take Care,
Dave
95 Mustang GT Convertible - SOLD
2000 Buick Regal GS (3.4 pulley, gutted airbox w K&N, U-bend, DHP PCM, TOG Headers. Pre-headers PB 14.81 @ 2700ft elevationw 2.25 60 ft)
1967 Electra 225 convertible (360hp)