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Originally Posted by jknowles87
I have a 1981 Chevy Impala sedan with the stock 267 V8 engine, TH-250C transmission, unsure of gear ratio in the rear. I want to rebuild this engine, with a better camshaft, and a 4bbl Q-Jet to replace the 2bbl. Any thoughts on the potential power and mpg gains, if any?
Also, I want to replace the TH-250C with a 200-4R so I can get better MPG. What would be the best gear ratio in the rear for this setup? (I want to say it has either a 2.73 or 3.08 based on how it feels on the highway, but I am not sure).
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Do not bother rebuilding the engine. It is definitely not worth it. The 267 was built for 3 years only. It was a low-powered (110hp, I think), low-revving economy engine, intended for buyer who just wanted good fuel economy, but did not want the extra vibration associated with the GM V6 engines of the day.
I am not sure how you feel about performance, but the 267-equipped cars I have driven were so slow they were hazardous in todays traffic. You simply have no extra acceleration to safely cope with merging, lane-changing etc.
Pick up another carbureted V8 Chevrolet engine and rebuild it, like a 350 cubic inch version. Rebuildable cores are still pretty cheap. A 350 will cost the same to rebuild as a 267, but will be MUCH better for power and performance. It will also boost the resale value of your car more than a rebuilt 267.
Even if a 350 core is too much money, a 305 cubic inch engine would be better. 305 cores are still very inexpensive and easy to come by.
Either the 350 or the 305 looks the same on the outside as the 267, and will bolt in place perfectly. You can easily get one with a factory 4 bbl carb.
If you go with a stock rebuild, your mileage will be very similar to a 267. This may seem counter - intuitive, but a Quadrajet carb will actually get better mileage than the 2 barrel you have (when driven normally) because the 'primary' venturis of the 4 barrel are a more efficient design than the 2 barrel that came on the 267's (that I have worked on, anyways). Also, a more-powerful 350 or 305 can get your heavy car up to cruising speed more quickly than a 267, so you can get the benefits of top-gear mileage sooner.
For an overdrive transmission, I would suggest installing one with the rear end you have and see how it performs. You can go with a 3:23, 3:54 or 3:73 rear end and still get good highway mileage. But the 2 rear end ratios you list will work fine with an OD transmission, unless you really want a race car.
BTW the 267 was underpowered, not just because it had less displacement than a 305 or 350 but also because the cylinder heads had (comparatively) very small valves and small ports. The heads did not flow very much air. You cannot bolt-on better heads from a larger 305 or 350 engine because the valves will hit the cylinder walls.