A DC9-30 WAS DISPATCHED IN NON COMPLIANCE WITH THE WRONG ENG RAM AIR TEMP ENG PRESSURE RATIO INDICATOR INSTALLED IN #1 ENG POS.

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I WAS INFORMED ON MAR/SAT/01 THAT I FLEW ACFT XYZ ON MAR/THU/01 WITH THE WRONG RAM AIR TEMP EPR GAUGE INSTALLED. ACFT IS EQUIPPED WITH JT8D-7 ENGS. THE GAUGE INSTALLED WAS A JT8D-9 GAUGE. THE PREFLT OF ACFT WAS VERY HECTIC -- LATE INBOUND; COPLT WAS LATE (I DID ENTIRE PREFLT/SET-UP); MAINT ON BOARD DEALING WITH ELECTRICAL PROB; NUMEROUS FLT ATTENDANT REQUESTS; FLT RELEASE AMENDMENTS; ON AND ON. WHEN THINGS SETTLED DOWN I NOTICED RAM AIR TEMP/EPR GAUGE WAS READING ERRONEOUSLY HIGH (+28 DEGS C WITH OUTSIDE AIR TEMP AT +4 DEGS C). THIS OF COURSE GETS MAINT BACK ON BOARD TO CHANGE OUT GAUGE. THAT COMPLETED; WE PUSHED OFF GATE. I SIMPLY DIDN'T NOTICE THAT MAINT INSTALLED A -9 RAM AIR TEMP/EPR GAUGE. THE DIFFERENCES BTWN A -9 AND -7 GAUGE ARE VERY SUBTLE; COMPOUNDED BY THE FACT ONLY 3 OF OUR DC9-30'S HAVE -7 ENGS -- IT IS VERY COMMON TO SEE THE HIGHER EPR SETTINGS OF THE -9. NO WHERE IN ANY OF OUR MANUALS AND NO TIME DURING TRAINING IS IT STRESSED TO VERIFY THE PROPER RAM AIR TEMP/EPR GAUGE IS INSTALLED.