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Thread: big RPM drop

  1. #1
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    big RPM drop

    I have a 383 blueprint engine (420 hp/450 tq} it has a TH350 trans with a 2600 - 2700 stall convertor , pertronix vacuum advance dist. the timing is where blueprint suggests (32 to 34 deg. total @ 3500 rpm ) I have to idle it at about 1350 rpm , so when I put it in gear it drops to about 900 rpm (it drops an additional 200 rpm when the dual electric fans come on . What can I do to lessen this rpm drop ?

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    Registered Member JT56's Avatar
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    What is the initial timing at?

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    One thought is maybe a solenoid operated idle compensator like they used in the 80's when the AC came on, that could be adapted to come on when the fans turn on to kick up the idle. Are you running ported, or non-ported vac advance? A vacuum gauge gage is a great thing to use also to diagnose what is going on too. That motor should be happy idling at 700 rpm I would think. Running full vacuum advance at idle can help a lot. If you have good vacuum at idle helps a lot too. 17+ inches is good.
    Last edited by 55 Rescue Dog; 11-03-2016 at 04:28 PM.

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    Look for a vacuum leak first.

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    I really wonder why such a mild motor idles at 1350 to start with, my SBC drag car idles about that with a .625 roller cam. Something else is going on, like as stated a vacuume leak. Not feeling the need for vac advance, I have some strong running street cars without it.

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    at 1st I thought I had a vacuum leak ( the rpm was not steady at all) changed to a new edelbrock 750... now steady as a rock . The engine gets about 11 inches of vacuum (needle on gage is very steady) I switched from manifold to ported vacuum and it didn't like that at all "banging out" the engine runs great , instant throttle response ,great power , just a big rpm drop. The distributor is the only thing not new ..... maybe a mechanical advance distributor ? A shop in town said they didn't think it had a vacuum leak .
    Last edited by N2RODDIN; 11-03-2016 at 08:11 PM.

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    Registered Member JT56's Avatar
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    Tell us more about the dist? I run my dist locked out at 35 degrees and my car idles 980 rpm in park and around 750 rpm in gear with two fans running and electric pump. Since its a low compression motor, have not issues with starting when hot.

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    Vacuum advance has been used since 1935 for a reason. The only time it is not needed is at wide open, when there is no vacuum.

  9. #9
    Registered Member chevynut's Avatar
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    Quote Originally Posted by 55 Rescue Dog View Post
    Vacuum advance has been used since 1935 for a reason.
    And what is that reason?

    The only time it is not needed is at wide open, when there is no vacuum.
    Not true...it's not needed with a modern ignition system.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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  10. #10
    Registered Member Maddog's Avatar
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    Quote Originally Posted by chevynut View Post
    And what is that reason?



    Not true...it's not needed with a modern ignition system.


    LOL, N2RODDIN ignore cnutroill he doesn't know shit about building an engine. He likes to copy stuff from the internet then post it like he knows something. Short man complex.

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