Getting pinging under load. Is this timing too aggressive?

Discussion in 'Engine Topic' started by Cue-Ball, Jun 28, 2009.

  1. Cue-Ball

    Cue-Ball Veteran Member

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    Jun 24, 2009
    Kirkland, WA
    I just picked up my new-to-me '70 Camaro on Friday. On the drive home the car started pinging under heavy load (really hot day, heading up a mountain pass). I pulled off at the first rest stop and retarded the timing a few degrees. I didn't have a timing light, so I just used the calibrated eyeball method. The pinging went away, but I wanted to verify the timing numbers anyway.

    Today, I checked/set the timing using an adjustable timing light and digital tach. I set initial timing (no vac advance) at 10* at about 700 RPM. Centrifugal advance comes in at around 950-1000 RPM and is all in by 2000 RPM. Initial+centrifugal timing is 25 degrees at 2000+ RPM. Plugging the vacuum advance back in gives me 29 degrees of timing at idle and 45 degrees of timing at 3000+ RPM.

    These numbers don't seem overly high to me, but I'm still getting pinging under load (not WOT), though not as bad as the day I brought it home. It's been a while since I've worked on a car that didn't electronically control its own timing, and this is my first Chevy, so I'm a bit foggy about what ballpark timing for this engine should be.

    The engine is a GM 350ci crate engine (part #10067353). It's got a small cam and ~8.5:1 compression and uses HEI. It's got a two barrel carb and, I think, the stock 307 manifold that this car came with originally. Tranny is a TH350. The engine has about 70,000 miles on it. Despite the low compression, I'm running 92 octane.

    I would think that on an engine with this small of a cam, this low of compression, and running 92 octane that it would take a LOT of advance before it would start pinging. Am I wrong and simply running too much timing for this engine combo, or is there something else that could be causing the pinging under load? Is 10* initial too much? Is 45* total too much?
     
  2. SexyTransAm

    SexyTransAm New Member

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    Apr 20, 2009
    E-town, Kentucky
    45 seems real high to me, but im far from a pro. maybe like 35 total
     
  3. Damon

    Damon Veteran Member

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    Nov 16, 2000
    Philly area
    Something's a little odd there. Centrifugal starting at 950 or so sounds around right for stock but it's all in by only 2000? And only 15 degrees at that point? Sounds like not enough, but also in too quickly. Most stock HEIs supply around 20* of centrifugal advance and you typically shoot for that number to be all in around 3000 or so.

    ~20 of vacuum advance on top of that is typical for a stock HEI. And its also typically too much once you have the initial and centrifugal dialed in for maximum WOT power.

    So, 45* max total is about right, but the way you're getting there sounds a bit funky.
     
  4. markw

    markw Veteran Member

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    Jun 6, 2009
    Illinois
    What temp was the motor at? Is it using any oil? Oil has very low octane. For a real world street curve I would run 10-12 initial + 24 centrifugal all in at 3500-4000. Having the timing all in later lets you 'lug' the motor up a hill at 2500 rpm without full advance. An adjustable vacuum advance unit let's you tune away any additional part throttle problems. That motor would like a 4 barrel.
     
  5. hhott71

    hhott71 R.I.P 11/19/18 Lifetime Gold Member

    14,377
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    Mar 30, 2001
    Joplin Mo. 64801
    10,000 threads on ignition timing in this section alone. Search works.

    To understand ignition timing better, read the information on MSD's website.
    Your vacuum advance is supposed to be plugged into the ported vacuum port.
     
  6. Cue-Ball

    Cue-Ball Veteran Member

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    Jun 24, 2009
    Kirkland, WA
    The weather was hot both on the drive home Friday and yesterday after I'd set the timing (hot for this area being mid-80s). I don't know how hot the car was running since it only has an idiot light and no gauge, but it doesn't seem to be running hot and it's got a huge radiator. I think it's actually a 4-core designed for a big block. It was changed when the 307 was pulled and the 350 was installed.

    It burns no oil and doesn't seem to use any, though I've only put on about 250 miles so far. I'm planning to put on a Performer RPM and an Edelbrock carb, but I wanted to make sure that the timing was sorted before I start messing with anything else.

    I've read everything I can find about ignition timing, but it's all rules of thumb. Half of the articles say 50* total timing is fine, half of them say it's too much unless you have a huge cam.

    Hott71 - You're one of the only people I've ever heard recommend using the ported advance connection. Most of the engine builders say to use manifold, most of the carb gurus say to use manifold, and cars didn't even come with ported vacuum connections until emissions became an issue (the retarded timing allows leaner idle jets, burns hotter and lowers emissions). In any case, ported vs manifold only affects idle timing, so it wouldn't have any effect at 2000-3000 RPM steady state cruise anyway.

    Damon and Mark - I think you guys are right that the centrifugal is all-in too quickly. I'm not sure what springs/weights are in there now, but I can pull the cap later today to check. Hopefully I can find a combo that still starts coming in around 1KRPM, but that doesn't hit full advance until around 4000. If I need to, I can get an adjustable vacuum advance, but I think recurving the weights might do the trick.

    Thanks, guys!
     
  7. tom3

    tom3 Veteran Member

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    Aug 1, 1999
    ohio
    Car been sitting a while before you got it? Funky gas? Run the tank low and fill up with brand premium gas. Timing on that engine should not be the problem unless it was really advanced.
     
  8. J007

    J007 Member

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    Apr 27, 2009
    Las Vegas, NV
    Maybe pull the distribuator cap and rotor to make sure someone didnt change out the weights and springs. Might have to put heavier springs on. Maybe try disconnecting the vacuum advance.
     
  9. z28rod

    z28rod Veteran Member

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    Apr 30, 2009
    hopewell junction,ny
    set at 35 degrees total and check springs and weights in dist. run 93 octane sunoco or 94 if you can get it.
     
  10. Bikefixr

    Bikefixr Veteran Member Lifetime Gold Member

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    Mar 13, 2006
    Hott71 is right. On modern cars use ported vac to the distributor. It is NOT an idle-only advance. There is NO advance at idle. Manifold vac is highest at idle, that is why you jumped to 29 at idle with it hooked up. Ported vac has been around since the late 60's. It allows a little more static advance to get a cleaner idle emission. As the throttle opens, then the vac begins to advance the timing, and it builds proportionately with RPM untill WOT when it starts to drop off again due to carb flow restriction. Full manifold vac is highest at idle, then it goes away reducing your timing as RPM builds, then it gradually comes back to 1 or 2". In the days of hgih octane fuel, it really made no difference how you got your advance. Using ported vac just times the advance differently in the RPM climb. I've seen test mule engine pulls using both methods and there wasn't more than 3-4HP diff on a 350HP 350Chev across the RPM band, but the ported vac engine had better fuel consumption (BSFC). If you have ported vac, and a mechanicl curve for manifold vac, you have waaay too much advance at midRPM under load, just at the point you want advance to either drop back a little or at least not advance very much. Stock smog heads need more overall advance because of poor chamber design, but they also want a slower curve as well. Vortec heads need less overall advance, and they like the advance to come in fast. I always found that it is worth every penny of the $75 to get my dizzy dialed in by a guy with a Sun-pro distributor machine. Tell him what you have, and he'll dial in the curve, reset the stops (some smog-era dizzys had as little as 12deg of mechanical advance because the slots were so short), set the vac advance with an aftermarket can (again, many stock cans allow very little advance and only at very high readings that a modified engine might seldom see). I drop in the dizzy, set inital at about 14 and let it rip. I always run a 160 t-stat as well..high temps hurt HP and also cause pre-ignition earlier in iron heads.
     

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