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Thread: 76 sedan deville 500, MPG question

  1. #1
    500deville is offline Cadillac Owners Member
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    76 sedan deville 500, MPG question

    Hi, I'm from south america and I want to buy a nice 76 sedan deville that I found but I have a question about fuel milleage. I know that a 76 cadillac 500 and MPG are antonyms but what If I tune it to run on CNG?

    I was thinking on a setup like this: 12:1 or 13:1 compression ratio (only CNG not gas), new HEI ignition system, replace the TH400 with a six speed 4L80E and a rear end gear of 3.53, 195 thermostat, electric fan, electric water pump.

    Don't worry about the CNG tanks weight, maybe I can found lighter tanks and use two or three on that big trunk

    I was hopping to get something like 10 -12 MPG town / 17 - 19 MPG highway with this set up. Can I get this or I just only dreaming?


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  3. #2
    drmenard is online now Cadillac Owners Connoisseur
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    Re: 76 sedan deville 500, MPG question

    The motor can have the compression ratio raised to 12 or a little more just by putting on early heads. The 472 and 500's all have the same block and heads. Early heads from 68 to 73 are small chamber , from 74 to 76 are large chamber.. So just get some early heads and put some new SS valves.. And when you have it apart put a new timing chain and gears.. Make sure its the good set, $80+, and not the cheap set, $40 or less... I would not take out the TH400.. A 4L80E is a 4 speed transmission and needs a controller ...

  4. #3
    500deville is offline Cadillac Owners Member
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    Re: 76 sedan deville 500, MPG question

    Quote Originally Posted by drmenard View Post
    The motor can have the compression ratio raised to 12 or a little more just by putting on early heads. The 472 and 500's all have the same block and heads. Early heads from 68 to 73 are small chamber , from 74 to 76 are large chamber.. So just get some early heads and put some new SS valves.. And when you have it apart put a new timing chain and gears.. Make sure its the good set, $80+, and not the cheap set, $40 or less... I would not take out the TH400.. A 4L80E is a 4 speed transmission and needs a controller ...
    But TCI offers a 4L80E six speed including torque converter, lockup and the transmission TCU. Is a expensive setup but if that could help the milleage maybe worth the expense or maybe a new six speed 4L80E (from silverado, tahoe, etc) with the TCI Transmission TCU

  5. #4
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: 76 sedan deville 500, MPG question

    It should be able to get 10 mpg city 13-15 mpg highway as it is. Some people claim higher.

  6. #5
    500deville is offline Cadillac Owners Member
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    Re: 76 sedan deville 500, MPG question

    Quote Originally Posted by cadillac_al View Post
    It should be able to get 10 mpg city 13-15 mpg highway as it is. Some people claim higher.
    That mileage with stock set up and just tuning work? thats the same of a mopar 440, I was expecting something like 7 - 8 MPG town / 10 - 11 MPG highway.

    So If the tiny 500 can do that with stock set up, what can do with the set up that I proposed? Maybe 12 - 13 town / 17 - 19 Highway?

  7. #6
    BRUCE ROE is offline Cadillac Owners Fanatic
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    76 sedan deville 500, MPG question

    Quote Originally Posted by 500deville View Post
    Hi, I'm from south america and I want to buy a nice 76 sedan deville that I found but I have a question about fuel milleage. I know that a 76 cadillac 500 and MPG are antonyms but what If I tune it to run on CNG?

    I was thinking on a setup like this: 12:1 or 13:1 compression ratio (only CNG not gas), new HEI ignition system, replace the TH400 with a six speed 4L80E and a rear end gear of 3.53, 195 thermostat, electric fan, electric water pump.

    Don't worry about the CNG tanks weight, maybe I can found lighter tanks and use two or three on that big trunk

    I was hopping to get something like 10 -12 MPG town / 17 - 19 MPG highway with this set up. Can I get this or I just only dreaming?
    Those are the gasoline figures I get with a 77 Olds 88 and 403 engine. Expect using 20% more fuel on the Cad and 500. I expect that CNG will be worse, though higher compression will help. Getting cruising rpm down near 1500 rpm at 60 mph will help too, only possible with a lockup trans. Bruce Roe

  8. #7
    the APEMAN is offline Cadillac Owners Enthusiast
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    Re: 76 sedan deville 500, MPG question

    The Cadillac engines of that era have very mild camshafts and make a lot less horsepower than what you might expect for the engine size.

    One reason they last so long.

    They get exceptional economy for what they are when everything is working right. This includes have a working EGR.

    14 MPG with mostly 50 MPH cruising and some city is what I got with my last 500 in a 5000 lb Cadillac.

    That's on regular fuel.

    Same engine with Cadillac fuel injection on a 1600 lb lighter car with overdrive once got 24 MPG on a steady all highway 60 MPH roll.

    After I launched the overdrive trans I swapped a TH-400 with cheap high stall converter and went down to 10 MPG but it sure cooked.


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