1962 Impala 283 12 MPG - Impala Tech
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post #1 of 15 (permalink) Old 07-20-2014, 09:46 PM Thread Starter
Join Date: Mar 2014
Location: rapid city
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1962 Impala 283 12 MPG

I bought this '62 hardtop in March and finally able to drive it: it had numerous oil leaks like back of intake and incorrect spin-on conversion, , had a loose pilot bushing due to over-size hole in rear of crank, had old floppy center driveshaft support, had truck bell housing with too big center hole, had leaky pinion and tranny rear seal, had heater valve drooling antifreeze, ect ect ect.
Engine stamping F0205E and casting 375619 so a generic 283 but don't know current displacement. Runs strong and smoke free so had to be overhauled recently. Probably a mild cam. 3774682 casting on heads. headers and glass packs. Edelbrock 3X2 W/Rochester carbs a professional setup obviously with anodized throttle hardware, no-idle bases front and back.
I wanted to drive it to Impala convention in Iowa next week but it always gets 12 MPG and that's the best it gets. The rear (I VERIFIED 3 1/3 turns) is 3.36 as it was originally a 3 speed on the tree. Currently i installed wide ratio Saginaw 4 speed ( 3.5:1 1st gear so i am looking for a 3.08 gearset).
I have front and rear carbs blocked off, i have re-jetted center carb from #55 to #52 as this is what my old 1965 283 from high school still has. Now i disassemble again and notice the power valve has a nipple only 1/2 as long as typical, and the 'nail' pusher has a spring with only about 23-25 turns instead of typical 31-33 turns. Obviously someone has been here before me... The elevation here 57703 is 3200 feet, the RPM at 65MPH on the tach is about 2900. front tires 225/45Z18, rear tires 275/40Z18. all at 32 PSI. Stock dist with Pertronix III rev limiter set to 6200 just to eliminate it, initial advance at about 6 and i run direct vacci to the dist not ported vacci. I know i wont get 22-24 MPG but come on its just a 283 that feels and sounds like a 250 HP 327, if i can't get 15-18 it stays at home. Any help or suggestions appreciated...
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post #2 of 15 (permalink) Old 07-21-2014, 09:52 AM
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Not sure about a re-worked 283 like you have, but 283's from that era were known to get poor gas mileage. Probably carburetor related though.

I may have a 3.08 Rear available to sell, but shipping would be expensive to your location.

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post #3 of 15 (permalink) Old 07-21-2014, 12:03 PM
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Denny you are fighting two issues. Lack of an over drive that could be fixed by replacing the manual four speed with a modern five or six speed for only $3,600. Or a Gear Vendor OD auxiliary electric shift tranny for around $2,400.

The other problem is your Edelbrock 3x2 manifold with three 2GC carbs on it. Chevy never had a 302 SBC set up. The 348 and Pontiac Big Blocks all used a 3x2 with dedicated end carbs without an idle circuit. The carbs on your manifold are regular two barrels off of a 283 so you are tripling the amount of gas and air entering the motor at idle. The idle circuit on the end carbs have to be professionally disabled (requires a carb rebuild) to get only the center carb having an adjustable circuit.

Aside from trailering the car I have no other suggestions. on improving mileage. You don't want to go lower on the gear ratio (2.73) as the 283 barely has enough grunt to move it now.

Big Dave
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post #4 of 15 (permalink) Old 07-21-2014, 12:56 PM Thread Starter
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First: I have the front and rear carbs disabled presently by plugging the fuel supply to them.
Second: the setup is a professional setup with the front and rear carbs having a new cast base, special wound throttle return springs, and NO idle circuits.
Third: I have plenty of power even going up a LONG grade...I will put 3.08 gears in and help the RPM issue and keep my wide ratio 4-speed tranny.
Someone besides me has tried to get this setup to run efficiently and i think it should get 15-18 mpg! I will re-post my inquiry under 'engines' to get more opinions.
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post #5 of 15 (permalink) Old 07-21-2014, 02:17 PM Thread Starter
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BIG DAVE: a 3.36 stock (manual tranny) rear gears with a .8 overdrive calculates to 2.69 so why is that better than my choice of 3.08 since you say i don't have enough power for 2.73 gears? (i never seen 2.73 gears available for a 62 chevy anyway)
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post #6 of 15 (permalink) Old 07-21-2014, 10:48 PM
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You are correct the only gears currently being sold for the old 8.2 Salisbury drop out rear end are a 3.08, 3.36, 3.55, 3.73, or a 4.11.

The 327 was introduced in 1963 to provide more power to move the bigger cars that were getting heavier every year.

As a personal aside I had a 350 in my 4.11 geared 4x4 three quarter ton Suburban. It got 11 miles to a gallon towing a 5600 camper trailer. I swapped out the 350 with a 454 and the mileage improved to 14 miles per gallon. This is because I didn't have my foot in it all the time to get it to run.

The 350 came out in 1967 to move even heavier cars, and in 1970 the SBC 400 was introduced to move the heaviest cars made which was the 1972-'73 model year. After that they started to get lighter, then smaller to loose weight.

There are four different cfm sized Rochester two barrel carbs. Not all will bolt to the Edelbrock manifold which has the wide base plate bolt pattern to use a Chevy 2GC. The Pontiac used a smaller bolt pattern to match a smaller sized carb. If your Chevy 3x2 was modified to accept the smaller Pontiac Rochester two barrels and you have disabled the carbs so that none on the end carbs can open or admit air or fuel then yes you are running off of the middle carb only. It probably is spending all of it's time at WOT to get the car up on plane to use a maritime metaphor for these big boats.

Brake specific horsepower at 100% volumetric efficiency yields the amount of gas that is consumed to generate one horsepower. Multiply that by the number of horsepower to move your car (air resistance rolling resistance, weight and gearing) should indicate your potential best MPG. Weight becomes an issue whenever you climb a hill as you are literally lifting the car. Theoretically you regain that energy as you go down the hill but Entropy says don't hold your breath as it will not be available to recover all of it. That is why we buy gas to move these cars around.

Big Dave

Last edited by Big Dave; 07-22-2014 at 10:11 AM. Reason: Had called the front carb, but it is the middle carb running
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post #7 of 15 (permalink) Old 07-21-2014, 11:43 PM
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I don't have any great advice but I do agree that you should be getting more than 12mpg. (I presume that is highway mileage)

If I were in your shoes, I'd buy an A/F gauge and O2 sensor combo so that I knew 100% at all throttle positions what my A/F ratio was.
I think I'd even spend the $30 - $50 to get a rear-wheel dyno so I could see the A/F with the torque chart.
It's pretty clear you want to maximize what you have without spending a lot. I'd do a gear ratio swap focusing on picking a gear that would put my cruising RPM in as low a spot of that torque curve as seems decent, just trying to keep it out of a dip or dropoff. (Ideally, I'd rather have a 5th gear or OD auto)

I'd also measure the headers. They might be 1 5/8" and I think that's too big for a 283, I think they'd be hurting you. I think 1 1/2" is better suited for such a small engine.

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post #8 of 15 (permalink) Old 07-22-2014, 05:04 PM Thread Starter
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Thanks for the good info and ideas from all that reply...I do have a spare ORIGINAL 1962 Belair 250HP 327CI engine that needs a typical overhaul (it smokes), I probably will rebuild it this coming winter...as i even have .040 pistons in hand. I agree that the 327 may get better gas mileage just because it isn't working so hard. Meanwhile this summer i just hate to give up on the trusty old school 283... It isn't a slug by any means! However factory torque of the 327 is easily 60 Ft-Lbs higher than the 283.
The A/F ratio meter is an excellent suggestion, i see they are only $20 on ebay if i can round up my own OX sensors...right now i have the poor man's gauge, a vacuum gauge. It shows about 15" at cruse, 20" down a grade, 10" up a grade.
Today i re-adjusted the FLOAT LEVEL as there are numerous instructions cover foam and brass floats some measuring the whole float from gasket and others measuring at the brass float at the parting line. I opted to adjust for 3/4" at the closest parting line. Also i swapped the power valve with a new one and replaced the top gasket as it seals around the vacci passage feeding the piston for the power valve actuation rod. Ready for another run down the highway to test the results.
PS the conversion kits for building Rochester 3X2 carbs are in the speedway catalog now and i suspect these are the same parts the professional turn-key suppliers use.

Last edited by denson1932; 07-22-2014 at 05:07 PM. Reason: more info
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post #9 of 15 (permalink) Old 07-22-2014, 09:54 PM
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Ran 3 two barrels a long time ago. You need to block the air coming in to the intake by making a base plate block off plate where the front and rear carbs sit on the intake. Remove the throttle plates from them also.
Sounds like the fuel supply is good, but you've got way to much air entering the intake.
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post #10 of 15 (permalink) Old 07-23-2014, 12:24 AM
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I have some old fuel records from my 64 Impala done back in 1986. I am pretty sure that was still the 283/powerglide. I saw a high of 17.8mpg and a low of 9.3mpg with an average of 13.3. Keeping in mind this is a 25 year old foot on the pedal, so I was not exactly driving for efficiency

Also, today's fuels with ethanol reduce your MPGs,

Now a days, I have a 383/700r4 and get 8-11mpg. Still not driving for efficiency, I guess my right foot has not aged much

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post #11 of 15 (permalink) Old 07-23-2014, 04:22 AM
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Hey Denny,
For additional reference, my '67 2 door fastback, stock 283 with Rochester 2 jet, Powerglide, don't know the rear end gears, got 16-17 mpg around town and 19 mpg on highway. That was in '87-'88. I was 20 years old then and either accelerating or braking. Lol
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post #12 of 15 (permalink) Old 07-30-2014, 08:57 PM Thread Starter
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I'm close to ordering a set of new 3.08 R&P and a master bearing installation kit...my only concern is setting the pinion depth right the first time so i don't have to worry about ruining a newly-installed inner pinion bearing ( if needed to pull it off) in order to re-do the number of shims....
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post #13 of 15 (permalink) Old 07-31-2014, 09:24 AM
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Originally Posted by denson1932 View Post
I'm close to ordering a set of new 3.08 R&P and a master bearing installation kit...my only concern is setting the pinion depth right the first time so i don't have to worry about ruining a newly-installed inner pinion bearing ( if needed to pull it off) in order to re-do the number of shims....
Research the procedure on line, and note the tools required to do it correctly. The good news is that the drop out third member (Salisbury style) is easier to work upon than the Spicer. I used an engine stand with an eighth inch thick steel flat plate cut out to mount the rear end welded to a second flat plate with holes bolted to the engine stand. Couple of forty five degree braces welded to it to hold it together makes it a cheap, but valuable tool for working on the rear ends.

I haven't seen that tool in a while and was probably left in my old garage when I closed it. It allowed me to build a lot of rear ends back in the mid sixties as I broke them frequently.

Big Dave
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post #14 of 15 (permalink) Old 07-31-2014, 11:46 AM
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I've got a 307 in my 64 impala, and I'm running an MPG test now, my results are going to be horribly skewed by the fact that I spend a lot of time in the car with it just idling, waiting on my fiancee (picking her up from work). I'd be surprised if I show less than 14mpg though.
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post #15 of 15 (permalink) Old 11-30-2014, 10:43 PM
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Not a apples top apples comparison but my 68 Fastback 307 two barrel and powerglide with 3.08 got 12 all day long. I swapped it for a tired 350 that smoked and a good quad and it pulled 17HWY. It could be a lack of power issue. It may pull hard...but its working the engine past its happy spot and using more fuel.
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