Got the time
?
Timing seems simple enough ... pull out the book & the timing light & set the timing to the spec listed
But is this really where the engine runs the best & makes the best power & or fuel milage ??
First lets go over the pricipals for timing an engine
fuel burns at a given speed & takes time to burn so we want to start the fire early so maximum pressure from combustion happens when the piston is at TDC
If we start the fire too soon maximum pressure is reached too early & trys to stop the piston from reaching TDC ,
If we start the fire too late the piston is past TDC & the pressure in the cylinder is starting to drop as the area expands before the maximum pressure is reached
which wastes fuel & power
Another factor is the fuel quality ... contrary to popular belief low octane fuels burn faster & more easily , while high octane fuels are harder to light & burn slower
Also heat is a factor, so actual compression as well as altitude , & cam profile all have an affect !!
Cam profile is rarely considered when timing an engine .. for example a short duration cam with low overlap will build a lot more compression at lower rpm increasing
the heat in the cylinder , conversly a longer duration cam with more overlap traps less pressure at lower Rpm reducing heat in the cylinders , this effect reverses at high
rpm as the exhaust leaving the cylinder will help draw in a lot of air/ fuel mix effectively supercharging the engine often well above100% volumetric effeciency
on long duration cams while while shorter duration cams tend to lose this effect & starve at higher rpm
so how can a book determine the fuel quality & dictate a correct timing position for your engine
Lets start with the base timing setting at idle
A book may state that you need 6* BTDC of timing at idle so the fuel now has ^8 to complete combustion & maximise pressure in the cyl to get the best use of the fuel
But is this what the engine really needs ?
the Easy way to test this is to advance the dist at idle [clockwise for Big blocks , counterclockwise for samll blocks] & either listen to the engine or watch the tach to
see if RPM increases , generally it will
most engine like 14-18* initial timing [at idle ] or more , generally just making this change will increase fuel milage a& throttle response dramatically so
now the fuel is more completely burned when the piston is at TDC & more pressure is created , with long duration cams advancing the timing is virtually a must just
to get the engine to idle
Now we have a problem you probably have a 32* curve built into the dist so with the idle set at 6* btdc you had a total of 38* advance ,But now total timing may be set at
48* this is where the advance curve in the dist needs to be altered to still work correctly . In reality the total timing is far more important then the timing at idle , first how much
time is spent at idle & how much power is needed at idle ? Not really much , most of the time the engine is at 2500 Rpm or more & this is where the power IS needed
You can time any engine with a light by making a mark on the damper 2 3/8" clockwise on the damper from the TDC line , this will give you a mark at 36* & you can use the
scale on the timing cover to adjust from there , most engines like between 36-42* when set at around 3000 rpm
Understanding the curve
the curve is present so the fuel has the correct amount of time to burn even as the time to burn becomes shorter as the rpm increases so the fire has to be started earlier to
maximize pressure in the cylinder at TDC the same as at idle
Ther are 2 curves in the dist Mechanical using weights & springs & vacuum which rotates the pick up plate inside the dist
the vacuum advance is designed to advance the timing as rpm increases to add time for the fuel to completely burn at TDC , a very common mistake is too hook the vacuum
to the manifild vacuum source which is high at idle & decreases with RPM , this is incorrect as it give more advance at idle & & decreases it with more RPM which creates
the burn Far too late & will kill off power & milage so make sure the vacuum is hooked to a Ported / Venturi vacuum source
Shroteneing the curve ... there is a allen screw inside Most Mopar dist so turning this screw adjusts the length of the curve , turning counter clock wise decreases the length
of the curve by approx 1* per each full rotation of the screw , the vacuum pot allows 14 turns max & no damage is done if you turn too far you will hear a click at the end of
the threads but turning clockwise again re-engages the threads , Some times using no vacuum advance by either unscrewing the allen screw all the way or just diconnecting
the vacuum hose yields the best results
The other way to shorten the curve is to decrease the Mecahnical advance curve , thi sis more difficult to do , s the dist needs to be disassembled & the slotted advance plate
needs the slots welded in [ the new Mopar performance Distributors come with an allen screw to adjust this as well ]
to set total timing I set by ear , you can hear the rpm rise , then it goes flat & quits rising & if you continue to advance it will start to pop into the exhaust basically you want
the timing set just at the point where the RPM quits rising for best performance , you can also experiment with a light &or a tachometer adjust the setting higher until you
feel no power or rpm increase
The other consideration is the speed of the curve , there are 2 springs on the mechanical advance plate , by using a heavier or shorter spring you can slow the curve so the timing
is advanced at a slower rate , conversly using a lighter spring speeds up the curve which generally increases power unless you encounter pinging which nessesitates slowing the
curve
When pinging is encountered [sounds like rocks in a metal tin being shaken ] it really is not a timing problem , it is a fuel problem , pinging , really known as detonation is a
condition where the fuel is burning from heat Before the spark plug can start the fire , or the fuel is burning too fast , the real solution is too use better fuel , too bad 104 octane si
not available , other solutions are to use a colder spark plug , octane booster , or a bigger cam to decrease cylinder pressure . Detonation is generally only a problem at lower
rpm so lenghtening or slowing the curve or decreasing initial timing can cure the problem while still maxinimzing power at higher rpm but this is really a band aid approach as the
engine will make better power & milage with the timing advanced
Another factor is oil consumption , oil burns at a lower temp than fuel so if you have an oil consumption problem , detonation is far more likely
Pix to follow
