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1970 |
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Ranchero Squire |
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429 Cobra Jet |
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1970 |
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Ranchero Squire |
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429 Cobra Jet |
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Don't underestimate the effect of correct ignition timing. To much advance and you'll blow up your engine with preignition. To little and your giving up lots of horsepower. The common rule-of-thumb is between 5 and 8 horsepower is lost for each degree your engines timing is retarded. Ouch!!!
The distributor controls when the spark plug fires.
Surprisingly you don't want the plug to fire when the piston is exactly at the
top of the cylinder. Because the flame that moves from the ignition point
- the spark plug tip - spreads out slowly, not instantaneously. (I seem to
remember the flame front is fast but subsonic, e.g. around 500 MPH). If
the
plug ignited at TDC by the time the flame had engulfed the entire fuel-air
mixture the piston would be well on its way down the cylinder. The
solution is to fire off the plug just a little early. Yes, that means just
a little of the effect of the ignition works against you to push the rising
piston back down, but more of the ignition occurs at TDC so you get more
power.
Here's the trick. At higher RPM the piston is moving up and down faster but
the flame front is a chemical reaction moving at a constant speed, so the faster
the engine is running the earlier you need to fire the plug to optimize the most
combustion at TDC. Old distributors - before computers - do this by
attaching some spinning weights to the dizzy shaft. As the engine RPM picks up
the spinning weights shift a little lever that advances the timing.
Eventually the weights have advanced the lever all it will go and you are at max
advance. This is called mechanical advance.
(For optimum fuel economy and emissions, you also want to advance the timing
even more when the engine is lightly loaded. Old distributors do this with
a little vacuum actuator that increases advance even more when the vacuum is
high. This is called vacuum advance. Vacuum advance is a good thing
if you drive on the street. It will also keep your engine cooler.)
Here's the best note I've seen about getting the ignition timing "right" for older cars. Credit goes to "strangedude" who hangs out in alt.hi-po.mopars for this clean explanation.
Screw the specs - with cars this old it's likely the distributor characteristics are completely different than when the car was new (not to mention gasoline quality). Install a timing tape or get a dial-back timing light. Disconnect the vacuum hose from the vacuum advance and plug the line. Hook up the timing light, start the engine and rev it until the timing stops advancing. For a big block, it should read around 36 to 38 degrees. If it's off, remember how much it's off and reset the initial timing at idle to give you the proper amount of total mechanical advance. Remember the timing at Idle. Now hook up the vacuum advance canister to a mity-vac. Leave the engine at idle and pull a vacuum on the canister until the timing stops advancing. The canister should add somewhere around 15 - 20 degrees more timing.
What this means is that the engine will run around 36 to 38 degrees total timing at high speed with the throttle floored (low vacuum) and around 51 - 58 degrees total timing at high speed cruise on the highway (high vacuum). If your car pings with premium gas at full throttle with these settings, rotate the distributor to pull out some mechanical advance until the pinging stops. If the car then feels sluggish around town at low speeds, weld up the slots in the mechanical advance plate so you can run more initial advance and still have the proper total timing. If the car is OK at full throttle but pings at part throttle (like going up a hill at part throttle), either swap vacuum advance canisters for one with less advance (the amount is stamped on the arm) or use an allen wrench in the vacuum nipple to increase the stiffness of the spring inside the unit. This will increase the amount of vacuum required to get a certain amount of vacuum advance. Good Luck!
Using this technique I picked up 3 precious extra degrees of advance with no preignition. I ended up at 37 degrees of total advance, one short of the factory spec. Not bad with 11.3 compression on 92 octane pump gas.
The only thing missing from strangedude's note was a discussion of how too much initial advance can make you motor hard to start when it is hot. You'll know you are at the limit of maximum initial advance when your hot motor sometimes has trouble cranking. Ideally, you'd like to get your initial advance right to the edge of not being able top start the car when hot.
This might also be the right time to get rid of the crummy points in your distributor. It is cheaper than you think.
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Page lasted updated April 06, 2008
