Mufflers, crossover and gas mileage

[tyler]

Likely part, or most, of my problem. Intake manifold crack

http://www.gmcmhphotos.com/photos/data/500/medium/DSCN6854.JPG

Visible crack underside of intake manifold. Blocked off passage and re-installed per Paterson.

Haven't got it back to together yet to test, but would explain hot problems and poor mpg.
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1975 Glenbrook, 1978 Royale rear bath
Raleigh, NC

 

[john r. lebetski]

Fix that 2" section to 2.25" aluminized pipe. Area on 2" is 3.14 and area on 2.25 is 3.97. Enough to matter I think. With old crossover unblocked
enough to cook the intake long term. I think 2.25 is the minimum you want to allow a good compromise between flow and velocity (torque and
drivability). Though the long pipe at 2.5 is less than 2 x 2.25s combined, the gas volume becomes less as it cools with distance. For example on the
455 Olds 442 GM used 2.25 exhaust pipes and 2" tailpipes after the mufflers. Same on GS 455 and pre cat years Corvettes.
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John Lebetski
Woodstock, IL
77 Eleganza II