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My first car was a '77 Cordoba with the 400 Lean Burn engine. It was a nice car overall but still not a Challenger. Though it was way better optioned and probably more comfortable. Coupled with 2.42 rear gears, it was by far a performance machine. It did ride and drive nice, under 100mph. Over that it floated like the boat it was. Lean Burn went out, swapped to points. It was still a dog but was reliable at least. At 60mph I got 15mpg. At 75mph I got 17mpg. Unfortunately, 55mph was the speed limit then. No amount of tuning made it any better. Low compression, high gears and a weak cam just couldn't be over came.
Until the 1974 440 Police Interceptor got installed. More fun to drive but the best gas mileage then was 12mpg. Most likely 10mpg. Speed was never a problem, keeping it between the ditches was. It would cruise 100mph at 2200rpm, if you dared to drive it that fast. A slight breeze, wind gust or bump in the road could make things really interesting.
 

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First off, back then Detroit was coping with new emission regs and mileage standards. The engines all had wimp cam shafts, low compression, small carburation and 2 something gearing. A lot of people blamed the Lean Burn system on the vehicles low performance. In reality, probably 75% of the LB system was dedicated to cold starts where the most pollution control was needed. But off it went and replaced by conventional electronic ignition. Cars didn't all of a sudden wake up and become capable performers from the switch. The LB systems in reality worked well but they had to be maintained. A lot of dealer mechanics were poorly trained on LB as well.
In my case, mine started backfiring so bad it would sound like a cannon going off. It's a wonder it didn't blow the muffler apart or off.
My dad had a 78 New Yorker, 440 car. Both cars had the Thermoquad carb so it's not like they were lacking fuel. His Lean Burn finally died in the late '80's. It was anemic at best. It ran fine but was no beast by far. Couldn't even power brake it on gravel. Swapped in electronic ignition, used no advance distributor from Lean Burn. Set timing at ~12*. Ran amazing after that! Could do a burn out on asphalt and got 24mpg. Up from maybe 17mpg before. Nothing else changed as far as setup.
 
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