Thread: 20+mpg small block with carb.

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OK this post is supposed to answer a general set of questions by 1972Chevy regarding setting up a SBC for higher mpg. While we have Tech1 available for discussion maybe I can learn something and help 1972Chevy at the same time? The STALL rpm has to do with how high the rpm can go against a locked no-go situation but I am not aware of a rpm rating for when a locked-in gear selection will either slip at the converter or stall the engine. I am sorry to say my brain was frozen in the 1950s relative to a "road test" site near my home. It was/is a short (about 100 yards) steep hill (about 40% grade) approached by a sharp left curve so you could not accelerate much at the bottom before attempting the grade. The "test" was would your car be able to reach the top of the hill in high gear and many a Model A could make it due to the heavy flywheel and low rpm torque. Later model 59AB flatheads would barely make it and the engine would "buck" and almost stall near the top of the hill unless the clutch would slip. Today most automatic transmissions just downshift on that hill and few notice the problem except that the hill is steep. So with a 700R4 in OD locked in either electrically or by an internal hydraulic condition about what rpm would the transmission downshift? A 305 SBC with a Crower 000902 cam suggested by Tech1 is a good idea but the question is whether the shift point for the OD will be low enough to shift into OD at say 55 mph where higher mileage is desireable? I have sweated this out trying to match tire size to my 3.55 rear ratio and 700R4. My engine (350 sbc) now runs but I have yet to set the shift points. A local transmission guy says my 3.55 rear and 235/75R15 tires are in a good range with less wheelspin than with a 3.73 rear but still good traction and potentially reasonable mileage with the 700R4 shifting into OD somewhere around 55 mph but I have been checking the rpm values at various shift points and it looks close to me whether the shift point into OD will be low enough to help the mpg. I will only know after the trans shop sets the shift points for me. Perhaps my worrying and the formula I have provided will help 1972Chevy? At the very least 1972Chevy might want to worry over tire size as well as the rear ratio.

Don Shillady
Retired Scientist/teen rodder