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Thread: Idle Speed

  1. #1

    Default Idle Speed

    What is the lowest practical idle speed with an Atomic TBI system? I have put a TBI system on an old Cadillac engine before and the problem I ran into was trying to get a relatively low, stable idle speed. The old GM Hydramatic transmission stops slipping at a relatively low speed, much lower than modern fuel injected engines typically idle. The factory idle speed for a 50s Cad engine is 375-400rpm or so to accommodate this fact. If you are idling much above 500rpm, the transmission wants to creep forward and changing gears from D to R and back is difficult because the engine is transmitting too much power through the transmission (not slipping enough). So my question is: if one is to use a stock Hydramatic transmission on a mid-50s Cadillac engine (say a 331 or 365ci Cad engine), how low can the idle speed be set using the Atomic TBI system? In my prior experience with a TBI system on a 1949 Cadillac engine, the engine started instantly, gave good fuel economy, had great throttle response, etc. It also was completely impervious to vapor lock, which is the NUMBER ONE problem with cars of this vintage (IMHO). Fuel injection, like the Atomic TBI system, could be a good approach to solving vapor lock and hot soak issues for good. But if you can't make it idle low enough to work with the stock transmission, few will adopt it. Pictures of my former 1949 Cadillac with fuel injection attached. Attached files.


  2. #2
    kentiv Guest

    Default Idle speed

    I got my 1959 Ford 352 FE to idle at 550 RPM without stalling. I did have the same concern as you and the answer from MSD was at that time at least 800 RPM. However, it required that the Atomic took control of timing.

  3. #3

    Default Idle speed

    That is a pretty engine bay. I love the old Cadu's. As stated, the Atomic must manage timing to get it that low. It'll actually have to pull timing out of the engine to achieve such a low target but it can. The only issue I think you might have with that engne is water temp sensor. If I remember correctly there's only one port in the water jacket and the Atomic must have a temp sensor to function correctly. If you don't need the one on the dash you should be fine, but if you want both, this could be problematic.

  4. #4

    Default Phase-Adjustable Rotor Required

    For the Atomic TBI system to manage timing, a distributor with a phase-adjustable rotor is REQUIRED. No such distributor exists for the Cadillac 331 or 365. Holley/MSD do not offer one. The only widely available aftermarket distributor for these older Cadillac engines is the GM small cap HEI series. That distributor eliminates points and adds vacuum advance, but it does NOT have a phase-adjustable rotor.
    Last edited by whitecrow; 09-30-2020 at 10:08 AM.

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