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Old 10-08-2017, 03:44 AM
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I want to shed a few 'truths' on this subject.
Pick an engine... Let's say it's a 390 just for example. Ford put the 390 in heavy cars, trucks, marine, construction equipment, industrial generators, etc. My point here is, although the engine is the same, the service is quite different and so are the corresponding torque curves.

A heavy vehicle w/390 designed to haul trailers and equipment (like a tow truck) will have a different torque curve and a different cam than a Mustang w/390. Next, there are altitude and climate variations where one 390 needs more jetting and the other needs less. Along with air/fuel variations, weight and the whole torque curve band needs to match the type of service this engine is designed for.

Distributor (ignition) timing can ONLY be accomplished while the engine is under a load. Those SUN distributor machines cannot simulate a load, like the vehicle climbing a mountain.

On a lighter car built for speed, I advance ignition timing much sooner than the same engine in an F-150 truck for example. In a hauling situation, engine RPMs climb much slower. So at any given speed, the engine will spend more time at that advance setting to deliver more precise torque peak than a race engine that spends most of its life in the upper rpm band.

Knock and ping are bad at any speed. If you are getting this at all, back off on your ignition timing and consider using more octane in your fuel to slow the burn.

The timing numbers on your engine are simply a general place to start. Ford has no idea where their engines will live, the climate, altitude and service the vehicle will be used for. Tuning is up to you.

I use the 'piston stop' to verify my cam settings as well as finding TDC. I do the hand rotation, find TDC, then I put the straight-edge across #6 rocker arms when they are dead even (during scavenging, between exh closing and int opening). Why? Consider this:
  • The crankshaft keyway may be off,
  • the crankshaft sprocket keyway may be off,
  • the camshaft sprocket keyway or dowel pin may be off,
  • the camshaft keyway or dowel pin may be off,
  • new camshafts may include some advance/retard ground into them from the factory.

These variations can stack up in OEM and aftermarket parts. I want to see the true cam position on my degree wheel and verify exactly where the cam is before replacing the timing cover. Checking takes no more money but a little more time.

I tune my street engines to deliver more torque at low-to-mid rpm range, and race engines to deliver the goods at mid-to-top rpms.

Ignition timing has little to do with cam timing. Even though the cam drives the distributor, 'spark' is referencing the crankshaft position, not valve timing. So, it's possible for the cam to be one tooth off but ignition timing 'right on'. In that scenario, this engine will never run right.

The same holds true if the distributor gear is off a tooth. Simply moving the spark plug wires ahead or behind will NOT fix the problem because the rotor will not be directly in front of the correct distributor cap tower when the points open. The rotor will be between towers at that time which will direct spark to either of two spark plugs. This is evidenced by looking at the inside of a distributor cap. A white residue will show on one side of the electrode and on one side of the rotor tip.

Over many decades, I've seen engines that 'never ran right' according to their owners. Many of them were simply timed incorrectly either at the cam, distributor or both. We have the tools to verify and correct everything before or during final assembly but it simply never happened. - Dave
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