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Thread: MSD 6AL box question

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    MSD 6AL box question

    In the instructions for setting the rev limiter, it says "Setting both rotary dials to '1' will activate a special function explained below." Well below it tells you how to program the box to show you the rev limit setting on your tac before starting the engine. This can be done without turning the two dials to "1", so that's not it. Just what is the special function?
    Tony

    1955 Bel Air Sport Coupe

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    I have MSD 6AL boxes on my 55 and 3 of my Camaros and no rotary dials, they all rely on rpm chips. Are you sure you have a 6AL .

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    Registered Member chasracer's Avatar
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    Checking the rev limit is the special function. Once the function is activated, it stays on until you go through the procedure to turn it back off. Sounds like yours is activated.

    Mark - he has the digital unit. The rotary dials are under the cover marked Rev Limiter on the front of the box. We have one in our Camaro which replaced our aging analog 6AL - works really well.

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    Seems like something is missing. Checking the rev limit is turned on and off by the grounding of the tac wire procedure. It doesn't mention the two dials. I just turned that on without touching the two dials.
    Tony

    1955 Bel Air Sport Coupe

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    What got me looking is that I was messing with my timing and it seems like I can start the engine with the timing advanced really far with no engine bucking. I was looking to see if it has a retard function while cranking but I don't see that anywhere.
    Tony

    1955 Bel Air Sport Coupe

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    Registered Member chasracer's Avatar
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    Tony - there are multiple versions of the 6AL - I believe a programmable retard and a separate start retard is on the MSD 6AL - Plus version.

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    Maybe you can help with my other question, when not using vacuum advance, around what do you set your initial timing at?

    I had it way advanced till it pinged but it only did it immediately upon acceleration then stopped. I'm thinking that may be due to the vacuum advance period when it is *falling off* (manifold vacuum). If initial is often 18 advanced and vacuum is close to 12 more advanced, then since the engine doesn't buck set at 30° advanced with no vacuum advance, that may be a starting point. Or am I way off?

    Oh, and my box is a 6425 which Jegs and Summit lists as no retard function? So why doesn't it buck at 32 advanced initial?
    Last edited by 55 Tony; 07-05-2018 at 05:42 AM.
    Tony

    1955 Bel Air Sport Coupe

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    Today I set the initial to 25° and kept the vacuum advance on. The test drive went well, I didn't hear any ping at all. On the highway it chirped 3rd every time. Now when it gets really, really hot, I'm getting a random miss that I can feel when going up my steep driveway at 5mph. I may have to swap distributors again to see if the pickup in this one is also bad. It's nothing like it was with the RTR circuit board in it, much much less. Not noticeable at WOT. Temps around 90 and the engine as hot as it gets.
    Tony

    1955 Bel Air Sport Coupe

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    Back to my post about the "special function" when setting the switch dials to 1 and 1, I called MSD and luckily got someone who sounded intelligent. He said the special function is only used if you have a power grid setup. Something to do with the power grid limiting the rpm's.

    Asked if maybe the later 6425 box's might have a timing retard for starting and he said no.

    Also asked how much a pickup costs and it's about $98. He said it's cheaper to send the unit in for repair, but I really don't know about that when adding shipping both ways. And with that I explained how I removed the faulty circuit board and am running just the pickup, would they work on such a distributor and he said yes, a lot of people have done the same thing.
    Tony

    1955 Bel Air Sport Coupe

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    You are not treating the vacuum advance correctly when setting the timing. You seem to be trying to tune for WOT so let's start there. Your distributor has mechanical advance also which adds timing as rpm increases. Do you know how much advance and what rpm that it maxes out? Typical timing for a big block would be 32-36° advance total, all in by 2500-3000 rpm. When I say "total", this is the sum of initial advance + mechanical advance (no vacuum yet), at an rpm where the mechanical advance has quit increasing. So let's say for discussion sake that you have 16° initial timing and 36° total timing. The mechanical advance would be 20°. You can adjust the mechanical advance with springs and stops.

    Are you setting the initial timing with the vacuum advance hose disconnected and plugged? That's the only proper way. (I suspect you haven't been.) Also, your engine does not make vacuum at WOT, so you could actually disconnect it at the track and it should make no difference.

    Vacuum advance is only useful at part throttle - when the engine actually makes vacuum. More ignition advance at light loads/high vacuum is useful for throttle response and fuel economy. In an engine with high compression or marginal fuel, you may have to limit the amount of vacuum advance with a stop to avoid pinging.

    So again, to tune for WOT concentrate on initial and mechanical advance, and disregard vacuum advance entirely.

    I suggest you go do this - disconnect and plug the hose for vacuum advance, hook up the light and see what the timing is at idle - that's your initial advance. Then slowly rev the engine past 3000 rpm while monitoring the timing. When it quits increasing, that's your total timing. Report both back here and we can discuss.
    Last edited by Rick_L; 07-05-2018 at 10:48 AM.

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