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Thread: Looking to put GM EFI on a 1955 Chevy, 235 cu. in. engine.

  1. #1
    Carb and Points!
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    Looking to put GM EFI on a 1955 Chevy, 235 cu. in. engine.

    I have a TBI and harness off of a 1987 Camaro 305 cu. in. engine. I do not have a ECM. Looks like there is lots of info on the GM 1227747 ECM. Would it be a good ECM to use or is there a better one?

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    Welcome.

    The 7747 would be an easy one to use. There are technical advantages to some later ecm's but they are also more difficult to tune.

    Is the '55 a car or truck?

  3. #3
    Carb and Points!
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    The engine is in a 1955 Chevy Belair. Probably now closer to a 255 cu in. Mild Schneider cam. Strong smooth idle at 450-500 rpm. Vacuum is good no problems with vacuum booster on brakes.

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    Putting TBI on an old I6 is still a pretty common project. For the best results I'd recommend allowing the computer to control both the spark and the fuel delivery.

    For either method you'll need to get an rpm input into the ecm. A modified HEI distributor from a late 250 might be a good choice although I've read they are gitting difficult to locate. The distributor needs to be updated to install a a 7 pin HEI module and the mechanical and vacuum advance mechanisms need to be disabled in order to use the ecm to control timing. Most of the sensors will be straightforward but you will want to do a little thinking about your throttle body selection. The original intake used a single throat carb and putting a single barrel TB on the top seems like a sound idea, but you will run into a few gotchas because the chips in most of the 7747's will be set up to fire two injectors. If you can find an old single barrel TB and an ecm from a 4 cylinder truck you may be in luck. The two barrel TB's can be mounted above the single opening manifold with an adapter and quite often they run well after tuning. Building your own wiring harness is a great example of investing time to save $$. Many folks these days opt for a custom built or semi-finished harness but if you can find a full size van with TBI as a donor you'll get a harness with more than enough extra wiring to DIY for your car. You'll want to plan your fuel system conversion as the stock single line system does not work with TBI. Options range from modifying a stock tank so it will accept a fuel pump (most work) to replacing the tank, (less work if planned properly) to installing an inline pump (can result in problems). These solutions will also include installing a new 3/8" supply line from tank to engine. There is the option of installing a fuel reservoir with pump which would allow the stock mechanical pump, tank, and lines to remain intact but you will need to plan carefully to make this solution work well. Tuning will likely require some investment in equipment on your part although there are plenty of tuners who will work with you to get a base tune set up through the mail. It sounds like you have a good engine in reasonable condition so the odds of success are in your favor.

    The swap from carb to EFI on a 235 is not new so there is plenty of reading available. Inliners.org and stovebolt.com as well as the 47-59 chevy truck forum on 67-72chevytrucks.com will all have some examples of this swap. Folks here will be happy to answer any specific questions an offer suggestions as well so feel free to ask away.

  5. #5
    Carb and Points!
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    Thank you for the input. I really appreciate the help. I did find a wiring diagram for my harness and OBD diagnostics, Inc. has information on their website for GM OBD1 Cables. They also sell the ALDL cable to perform diagnostics. Their website is loaded with sources for scanning and tuning software and other cable setups and interesting links. BTW Tom Langton at Langdon Stovebolt.com sent out a notice a month or so ago that he was looking to retire and was hoping to find an interested party to take over his business.

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