Timing vs. economy
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From: Kansas
Timing vs. economy
In the FAQ, it is suggested running the timing at 10 to 12 BTDC for improved throttle response instead of the factory/stock 6 BTDC. How will this affect fuel economy? Also, set around 6 (not sure) BTDC, I get some popping through the carb when I change from acceleration/steady to coasting (ie coming down a hill, taking foot off gas to slow down for speed change). I know this can be cured with timing, but which way?
Engine is a 390, 2bbl, stock except bored +0.030
Thanks for the help.
Matt
Engine is a 390, 2bbl, stock except bored +0.030
Thanks for the help.
Matt
Last edited by MAHinton; May 8, 2004 at 11:38 PM. Reason: Forgot to include engine specs
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Within reason, it should give you better fuel economy. 10-12 sounds reasonable to me.
If you take it too far, you'd probably reach the point of diminishing returns. Or of course start the engine knocking. But, even if you put aviation gasoline in it... Advancing beyond that, it's possible the economy could get worse, but I'd guess you'd have to take it pretty far.
If you take it too far, you'd probably reach the point of diminishing returns. Or of course start the engine knocking. But, even if you put aviation gasoline in it... Advancing beyond that, it's possible the economy could get worse, but I'd guess you'd have to take it pretty far.
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From: Kansas
By "too much total timing", I'm assuming you are adding the vacuum advance to the timing of 10-12 BTDC, correct? How do I check that to see how much the advance changes the timing? I have the stock/replacement breakerless ignition system (replaced at O'Reilly's when I did the engine 8 years ago), nothing upgraded or replaced on it except cap, rotor, and wires when needed.
Also, engine was originally a 360 until the rebuild. The original sticker is still on the valve cover. According to that, plug gap should be between 0.052 and 0.056, so I have also gone with 0.054. Anyone I tell this to is shocked at the huge gap. In my Haynes manual, it calls for 0.044 gap. How much difference will the gap make in power/mileage/life of plugs/coil/wires?
Thanks again...
Matt
Also, engine was originally a 360 until the rebuild. The original sticker is still on the valve cover. According to that, plug gap should be between 0.052 and 0.056, so I have also gone with 0.054. Anyone I tell this to is shocked at the huge gap. In my Haynes manual, it calls for 0.044 gap. How much difference will the gap make in power/mileage/life of plugs/coil/wires?
Thanks again...
Matt
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Run as much timing as you can get away with without detonation. As far as economy, one has to balance the price of higher octane gas these days against any gain you might get from advancing the timing to the point where you need it. 92 octane is $2.59 in my area.
There is a limit to how much you can advance the timing no matter what fuel you're using.
As far as plug gap, around .045 is what I have always seen for electronic ignition. So the .044 sounds correct.
There is a limit to how much you can advance the timing no matter what fuel you're using.
As far as plug gap, around .045 is what I have always seen for electronic ignition. So the .044 sounds correct.
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From: iowa
well after I switched over from a 2bbl to 4bbl on my 390 there was 55 total degrees and that had caused the carb to pop and backfire at times. I had to much total timing since I had bought the truck. A way to check it is to mark the balancer at 38 degrees but it only goes to 30. Guesstimation time. Then check the timing with the vac advance hooked up and apply the throttle. Watch to see where about it maxes out at.
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Originally Posted by MAHinton
In my Haynes manual, it calls for 0.044 gap. How much difference will the gap make in power/mileage/life of plugs/coil/wires?
Thanks again...
Matt
Thanks again...
Matt
Of course, there is always the opposite argument... A larger gap will enable you to run well with leaner mixtures withouog misfire. The greater gap gives you a bigger spark volume, creating a higher probability of good ignition under marginal conditions. BUT, when the automakers do this, they use fatter plug wires, and better designs and materials for the rest of the system to handle it.
Last edited by mlf72f250; May 9, 2004 at 09:06 PM.
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