Initial and total timing with an MSD distributor

Discussion in 'Engine Topic' started by DesmoEd, Mar 21, 2021.

  1. DesmoEd

    DesmoEd Veteran Member

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    Finally got around to swapping out the advance stop bushing on the MSD #8361 distributor in my Camaro.

    Silver 25* bushing came out and the 18* black one went in. Bumped up the initial to make up for the 7* mech loss and what a difference in how the car idles and runs off idle. I rolled in the initial as high as 22* while at warm idle and she really liked it. To be safe I settled on 18* initial with the mech advance providing another 18* for 36* total all in by 2800 rpm courtesy of two blue springs.

    Looks like in order to get the initial up any higher I need to order some different stop bushings.

    After everything was finished I hooked up the vacuum advance and took a peek at the timing, MSD says their can delivers 10* advance at 15 inches manifold pressure but I'm showing 36* at idle which means the vac can is supplying around 18* advance??? As soon as I unhook the vac can the timing at idle drops from 36* back to the 18* initial I set...

    At 22* initial I did cycle the car on and off several times and it showed no sign of being difficult to start.

    After settling on 18* initial for now, I did have to lower the idle slightly and also noticed I picked up 2 inches vacuum - from 13.5 in at idle to 15.5 in.

    I will say changing the bushing with the dist in the car requires much patience and precise channeling of ones inner zen. I did find that using grease to hold the bushing and washer in place works pretty well while you try to get the locknut started. Had to go buy a short 11/32 open end wrench as it was about the only thing that I could see working to get the damn nut off.
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    I've had good luck with this MSD vacuum advance limiter kit, let me bump up the initial, keep total mechanical, and limit vacuum. https://www.summitracing.com/parts/msd-84281
    I have an Edelbrock ProFlo 4 now, so it's just a touch on the screen.
     
  3. biker

    biker Veteran Member

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    Could the extra timing you see be coming from the mechanical advance kicking in? If your idle comes up when you plug in the vac advance, maybe it is high enough that the mech advance is starting to come in.
     
  4. G72Zed

    G72Zed Veteran Member

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    Good to hear this DesmoEd. The proper timing curve with a reduced and "limited" centrifugal and vacuum advance works great with the proper initial timing. Cleans things up, allows better carb calibration and not chasing your tail getting the AFR's and circuits all tuned in.
     
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  5. flybye70ss

    flybye70ss Veteran Member

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    You need to give this guy a call and he will drop so much knowledge on you about how to understand the timing and get you the CORRECT bushing. Also, changing in the car is one of the most difficult things. I pull the dizzy everytime for my sanity.

    http://4secondsflat.com/MSD Distributor Tuning.htm
     
  6. DesmoEd

    DesmoEd Veteran Member

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    Thanks, thats my next step in limiting the vacuum advance.

    Once I see just how much initial she really likes I may just try running without it.

    I hear ya about the ease of tuning EFI, if I was doing a car today I'd no doubt go EFI on top of a modern LS type motor
     
    Last edited: Mar 23, 2021
  7. DesmoEd

    DesmoEd Veteran Member

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    thats a good point but I dont think so as the idle is only changing 100-200 rpm so it should not be triggering the mechanical advance.

    Here is MSD's info:

    Vacuum Advance (PN 8361/83613 Only): The vacuum advance will advance the timing up to 10° during partial throttle driving (with 15 inHg of vacuum). The vacuum line should be routed to a ported vacuum outlet above the throttle plates

    So I call Holley (MSD) and talk to their tech guy and he tells me that this vacuum can is supposed to provide 15* advance at 10" manifold vacuum....
     
  8. DesmoEd

    DesmoEd Veteran Member

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    Thanks, just ordered their 14* and 10* bushings

    Gonna try the 14* first along with 22* initial timing and see how that works
     
  9. rchydzik

    rchydzik Veteran Member Lifetime Gold Member

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    I thought vacuum advance cans were stamped with degrees of distributor rotation, no crankshaft. If it is stamped with 10 degrees, that means distributor degrees, or 20 degrees crankshaft.
     
  10. DesmoEd

    DesmoEd Veteran Member

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    I think I have it figured out now after talking to someone different at Holley - his info is the can provides 8-16* advance at 8-11 inches vacuum - this makes much more sense as the can has to transition as it deploys vs the stated fixed values on the MSD paperwork

    Also, after looking at the timing charts they do show around 3* advance coming off the mech weights at a 1000rpm idle.

    So my initial 18* + 16* off the vacuum can + 3* off the weights = the 36*-37* I'm seeing at idle
     

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