Poor or decent mileage? - Impala Tech
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post #1 of 6 (permalink) Old 06-23-2009, 09:50 AM Thread Starter
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Poor or decent mileage?

My 1970 wagon with 350 V8, 2bbl carb and TH350 makes about 12-13mpg. I do about 70% highway and 30% city driving but I rarely go over 60-65mph on a highway which keeps my RPMs at about 1900-2000. I also do not have lead foot because I can't even spin the tires with my 250HP engine (well, I really did not try).

Recently I had the distributor cap, coil, wires, plugs, air filter, fuel filter changed and the car got tune up including new carb. I am filling her up with regular gas (87). Car has 73K miles which I was told are original ones. It doesn't smoke, runs smooth and transmission shifts fine.

So based on that do you think that this mileage is ok or poor? To be honest, I was expecting something in 16-18mpg range.

Thanks.
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post #2 of 6 (permalink) Old 06-23-2009, 10:03 AM
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Mileage is normal. You won't believe this but you will pick up two to three more miles per gallon by going to a Rochester QuadraJet four barrel carburetor. It has tiny primaries and barn door sized secondaries that only opens when under a load (the engine needs it, not when you want it to open to go faster, which is why most people bad mouth the carb). If you bumped up to the middle grade of gas you could run a point or two more ignition which would improve power and gas mileage (makes the motor more efficient). You have to bear in mind when this motor was built the average regular grade of gas was 96 octane with 102 to 106 being the average for premium back in 1970. The motor is a high compression motor by today's standards and in order to run the cheapest grade of gas you have to retard the ignition to keep it from going into detonation. That means the motor is already past peak engine pressure before the plugs even get the mixture lit off.

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post #3 of 6 (permalink) Old 06-23-2009, 10:44 AM Thread Starter
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I got her filled up today but I will get octane booster and add that to see if it will make a difference.

I was thinking of getting 4bbl carb but that would mean new intake and carb. Then there is also OD transmission but I was told that with 1900RMPs at 60mph it wouldn't make sense. Plus that would be $1500 upgrade so 1-2mpg savings would take years to brake even.

Would upgrade to HEI make any difference? Simple Hypertech upgrade is only $70 at Summit (link).
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post #4 of 6 (permalink) Old 06-23-2009, 10:49 AM
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I don't have a city/highway comparison for you BUT I only knocked down 15mpg average on a full tank of gas this past weekend on a Highway-only drive. I didn't hot-rod it, and stayed right around 65-70 with only a little time around 75mph.

I was expecting 17-18 on the highway. I have new plugs, air filter, condenser, good tire pressure, but old plug wires. Runs good though. I have no tachometer and do not know the rear-end gear ratio. Seems like a 'highway gear' though.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #5 of 6 (permalink) Old 06-23-2009, 01:34 PM
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The 350 was never renowned for it's high mileage. EFI helps (and an HEI would help with starting and yielding a hotter spark to fire the plugs). That is why GM brought out the 305 to get their CAFE numbers in line without loosing too much in terms of torque (uses a very small 3.670" bore with the crank out of a 350 at 3.480" stroke). In comparison the small 283 has a 3.875" bore which allows it to breath deep with a hot cam and big heads so it can rev up to 10,000 RPM making much more horsepower than the larger 305.

This is an excellent example of the difference between horsepower and torque in automotive use. Having a 340 horse 302 Z/28 motor compared to the 160 horse 305 is an excellent example of this (both are 5.0 liter motors). If you put both in a pick up truck with a stock automatic the 302 would stall as soon as you put it in gear. The 305 would drag not only the 5000 pound pick-up around but another 6500 pounds of trailer. (though it wouldn't set any land speed records).

Big Dave
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post #6 of 6 (permalink) Old 06-23-2009, 01:52 PM Thread Starter
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Thanks for replies. I guess there is nothing to worry about then.

I am working on a small list of upgrades that I want to do in the next month or so. At this time I will go thru the whole engine bay to see if there is anything that needs attention. For some reason looks like parts of the engine were maintained/replaced but others neglected. Hopefully with the maintenance and upgrades I can squeeze additional mile or two.
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