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Thread: Timing & vacuum advance, SBC HEI.

  1. #1

    Default Timing & vacuum advance, SBC HEI.

    Installed the Sniper and every thing seems to be running OK only 40-50 miles or so on it so far. I was reading some stuff about timing and it sound like most people think you should run the vacuum advanced on manifold, not ported. So I decided to switch mine to manifold. I have a pretty much stock 1970 SBC 350 other than Edelbrock performer EPS and 50,000 volt HEI. Have about 19-20 inHg of vacuum at idle. Idle is set at 15° with vacuum disconnected and plugged. At 3000 RPM it’s at 32°. With the vacuum line plugged in the idle jumps to 40°, which seems way too high. My IAC was down to 0%, so readjusted that back to 6-7 ish.

    The engine has a slight stumble idling now. Not horrendous likes big cam, but noticeable and the numbers jump around a little bit. Is this something to be expected and will smooth out with some learning or is something funky. Learn at idle was at 20-23 now after changes it's up to 28.

  2. #2
    Join Date
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    I did the same thing with my MSD distributor and it killed my Learn Table and increased the fuel usage. I barely got it back to the house. Rather than mess with bushings and vacuum advance canisters, I switched to the HyperSpark. You can try to get a different vacuum advance for your distributor (some are available in different advance increments).
    Last edited by Leon82; 05-20-2020 at 08:56 PM.

  3. #3

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    So the 40° at idle is way too high right? The vacuum can may be adjustable what would be a better degree at idle? 30°? And would that help the stumble? Or should I just move it back to ported and call it a day?

  4. #4
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    They say 20 or 25° for an aggressive cam, but you could also use a vacuum gauge or the MAP sensor readings to find what the engine likes also.

  5. #5

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    Mine's pretty much stock, so 20-25° would be high for idle. My problem is idle is 15° and when I plug in the vacuum line it jumps to 40°.

  6. #6

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    So use ported vacuum.

  7. #7
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    1st, you can't just change the vacuum advance from the (ported) timed port to full manifold vacuum; the entire timing advance curve should be revamped for it.
    2nd, it shouldn't matter that the ignition timing retards a little in gear, because full manifold vacuum advance significantly increases the amount of idle timing anyway.
    3rd, the (ported) timed spark port isn't for performance engines (unless it has a big lopey cam). Even before emissions/EGR regulations, OEM engines used ported vacuum advance.
    4th, the Base Timing Tables for an EFI engine have the additional vacuum advance timing incorporated into the idle area (since there's obviously no distributor based advance).
    May God's grace bless you in the Lord Jesus Christ.
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  8. #8

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    There is a lot of information on setting timing parameters for an HEI: http://outintheshop.com/faq/hei.htm. Give this a read and verify your timing ranges and adjust accordingly. Your vacuum advance needs to be limited in order to keep your maximum timing in spec.

  9. #9

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    I was able to mess with it a bit tonight. My eBay HEI special did have an adjustable vacuum canister on it and I was able to turn it down To 14°. So now the car has 15° base, 32° WOT (no vacuum) and 14° of vac. It did smooth out the slight stumble a bit, so you can hardly see it now. Readjusted IAC again and I'll let it learn a bit to see what happens. I guess I can always go back to ported later.

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