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500, 472, 425, 368 Discussion, Gas Mileage of the 500 in Cadillac Engine Technical Discussion; That is one very strange combination. You are correct. That is a feedback carbonator on a 368. Looks like the ...
  1. #31
    The Ape Man's Avatar
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    Re: Gas Mileage of the 500

    That is one very strange combination. You are correct. That is a feedback carbonator on a 368. Looks like the distributor might have a vacuum advance diaphragm on it. Is that what I'm seeing? Take a look at the wiring from the TPS and the mixture control solenoid (the 2 weatherpack plugs on the carb).
    Does this wiring actually land up heading for the firewall bulkhead connector? Are you hearing the mixture control solenoid click inside the carbonator with the engine running or key on engine off? Looks like a CadOldsmobile. One way to get this combination together if it weren't factory would be to start with a 1981 Sedan Deville equipped with the Buick V6 and feedback carb. That would give the coachbuilder the correct body wiring and a computer that would work with a PROM change. Very interesting. Do you see an O2 sensor? Do you know which transmission the car has?

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    Re: Gas Mileage of the 500

    Yes, the dist has vacuum advance, and a harness that runs over to a box on the left (drivers) fenderwell. All the mess of wires do run to a bulkhead on the firewall, and there is a computer behind the glovebox.
    Yes, there is an O2 sensor.
    I'm assuming it has a TH400 tranny, I think that was all they put in the commercial chassis.
    If it helps you when asking questions, I've turned wrenches all my life, and 10 years for a fleet (I sit behind a computer all day now LOL!), so I am familiar with anything you might want to ask...

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    Re: Gas Mileage of the 500

    I didn't think the commercial chassis got the V8-6-4, wasn't that only the passenger cars?
    The Chiltons manual lists a 368 w/ 4bbl up through '85. Don't think Caddy wanted limos and hearses with the HT4100 or a diesel. Seems they retired the 368 in '86 in favor of the olds 307...

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    Re: Gas Mileage of the 500

    The thing that makes me believe that there was a visit from Dr. Frankenstein is the distributor has a vacuum advance. IIRC if the engine were factory CCC the timing curve would have been dictated by the ECM.

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    Re: Gas Mileage of the 500

    Not a very hard swap to put a mechanical HEI in place of the CCC HEI. I've never heard any good things about the super-lazy timing curve that the CCC gave, so a fair number of people have swapped the distributor but kept the feedback carb in other cars.
    Is there a spare harness connector laying somewhere around the distributor?

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    Re: Gas Mileage of the 500

    Nope, all wires are plugged in...
    First mod tho is going to be a standard HEI, then after that (or at the same time depending on availability) a early-mid 70's Quadrajet.
    I've rebuilt many quadrajets, haven't re-bushed one yet, and have all the "tricks" to mod them that I've found on the 'net.
    This car isn't going to be a daily driver, it's just a fun project. So sometime down the road it's going to have a 500/472 sitting in the engine bay anyway.

    After the Dist and Carb swap all the other "crap" will be leaving the engine compartment, leaving just that gorgeous caddy big block.

    Wonder what a hearse would look like tubbed...

  7. #37
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    Re: Gas Mileage of the 500

    My guess is that you already have a standard H.E.I. Have you ever had the cap off? Was there a mechanical advance?
    Does the carbonator make that clicking noise from the mixture control solenoids now that you plugged the ECM back in? If not then there is plenty of economy to be had by either repairing this setup or replacing it with the olde fashoned carbonator.
    The feedback Rottenchester has a cult following. I've heard several people say that it's the best fuel system ever built. If you do have one of those properly interfaced with a 368 then it could be a good thing. You can still do the hot rod tuning to the carbonator.
    More clues.
    More clues.
    Might be fun to check the ECM for codes and PROM I.D.

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    Re: Gas Mileage of the 500

    It's not a standard HEI. On a standard HEI there is one red power in, and optionally tach out. This has the one red wire, and a harness of (IIRC) 4 wires running to a control box on the left fenderwell.

    The problem with keeping the feedback carb is the clutter of wire all over the engine compartment. The engine is pretty much hidden. Even my '94 LHS 3.5 looks cleaner than this, and it has pretty much the same amount of wiring, just routed better.
    After I get the 500/472 it won't matter anyway.

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    Re: Gas Mileage of the 500

    those four wires sound like the wiring for the controll module. Usually it is bolted in the distributor body, but if they switched a CCC HEI to standard, they might have wired in the controll module externally.
    Most of that clutter is probably emissions stuff, not the feedback carb. Personally, I love the idea of te feedback carb since it takes all the fuss out of tuning the carb. You only have to play with the secondaries to get WOT right and the computer keeps everything happy the rest of the time.

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    Re: Gas Mileage of the 500

    GM had several timing controller happy boxes over the years. I've seen them on early (79) Sevilles but they usually show up on GM or
    Chevy trucks. Some are used with a standard H.E.I. (one with vacuum and mechanical advance). There were also spark control modules on some 4.3 truck V6's. IIRC there will be an extra plug on the ignition module going to that 4 wire plug and nothing else in the distributor.
    Let us know if you open up the ECM and get the PROM number. Many of these numbers show plenty of interesting application and service info when simply put into a Google search.

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    Re: Gas Mileage of the 500

    1981 commercial chassis had the DEFI 368 WITHOUT V8-6-4, I could be wrong, but IIRC the commerical chassis kept the base 368 until the 307 was used.

    maybe somebody did a little swapping?

    That air cleaner riser looks exactly like the one on the 1981 V8-6-4 I got all the, well, V8-6-4 stuff from...

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    Re: Gas Mileage of the 500

    Sorry I haven't posted anything for awhile.

    Is there any definitive way to tell the difference between the motors?
    Like serial numbers on the motor, and if so could you point me to this resource?

    Today I lit up the rear wheels, very impressive, since I had a '70 fleetwood that would hardly turn them. I know the hearse had lower gearing that the old fleetwood, but I keep remembering the factory power specs on the 368 and can't believe that it's performing like this.

    And consistant 10mpg, no matter how I drive it (well, other than today I haven't hammered it, or really got on it... Gas is just too expensive!).

    Thanks for all your help!

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    Re: Gas Mileage of the 500

    difference between which engines?

    what is the car in question?

    a '70 Fleetwood should lay a whole lot more rubber then an '81.... something was wrong there....

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    Re: Gas Mileage of the 500

    Quote Originally Posted by Chrome
    Is there any definitive way to tell the difference between the motors?
    Like serial numbers on the motor, and if so could you point me to this resource?
    Serial numbers are all the same. Only difference is the stroke. Best way I know how to tell is the measure it.
    Take a coat hanger and straighten it out. Remove a sparkplug and stick it in the hole. Rotate the engine untill the wire starts to reverse direction. Mark it right where it goes into the plug hole. Then keep turning the engine untill it just starts to reverse direction again. Again mark the wire next to the plug hole. Remove the wire and mesure the distance between the two marks. About 4" means a 472, about 4.3" means a 500.

    I agree, your Fleetwood should have smoked the tires hard. My SDV will just sit still and spin the right rear if I snap the gas down hard at a stop, no footbraking required. I've got 2.93 gears, while a Fleetwood should have 3.15's for even more tire shredding.

  15. #45
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    Re: Gas Mileage of the 500

    Quote Originally Posted by Night Wolf
    difference between which engines?

    what is the car in question?

    a '70 Fleetwood should lay a whole lot more rubber then an '81.... something was wrong there....
    It's an '81 hearse. Superior completed the car in '81, but it's an '80 model from Cadillac.

    Engine tag says it's a 368, but it sure doesn't feel like it in the seat of the pants... Plus I figure, even in a hearse, that the 368 should get better than 10 mpg, and that's what I get consistantly pretty much no matter how I drive.

    This thing pulls harder than pretty much anything I have owned (car or truck):

    '71 charger 318 (weak)
    '73 cutlass 350 rocket (not bad)
    70 Fleetwood (should have stomped all over it, must have been something wrong, engine must have been worn out... LOL!)
    76 chevy truck 350
    78 ford truck 300
    79 Grand prix ( early 400 swapped in, really nice)
    and a '70 camaro SS 350, with a 454 (out of a truck, 4 bolt mains) nothing I have ever driven pulled like this car! But somebody had the right amount of money at the right time, so like an idiot I sold it.

    The hearse gets worse gas mileage than all of the above with exception to the Fleetwood and the Camaro.

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