I am assembling an engine
for a dead-stock 1970
Swinger 340, 4-speed. The
machine shop that rebuilt
the short block says that
with today’s gasoline, the
total timing should probably
be 33 degrees or so.
I have seen where you
have written it should be
more like 50 to 55 degrees.
I looked in the service
manual, and it says, that
the timing should be 5
degrees, plus no more
than 10 mechanical and
10 vacuum, which would
be a total of 25 degrees,
not the 33 the shop told
me and even further from
your 50 to 55 degrees! I must be missing
something, so please help me understand
this.
Sure. The fact is, in this case, everybody
is correct—sort of! When the shop told
you 33°, they were referring to the timing,
in crankshaft degrees before top dead
center, that the engine will make maximum
power at (safely with today’s fuels).
That’s correct. If you look at the numbers
in the FSM, you find this data:
• “Timing” (i.e., base)–5° BTC (meaning,
before top dead center, more typically
written BTDC)
• “Advance—Centrifugal” (max) (Distributor
degrees at distributor RPM)–8° to
10° at 900 RPM
• “Advance–Vacuum” (max) (Distributor
degrees at inches of mercury) 7° to 10°
at 10.5˝
The error comes from your interpretation
of these numbers. See, for example, “Distributor
degrees at distributor RPM”? The
distributor rotates, in virtually every 4-stroke
engine, at one half the crankshaft speed—
but the crankshaft damper is where you
measure the timing. So, taking the max
of 10 camshaft degrees of mechanical
advance at 900 RPM, that’s 20 crankshaft
degrees at 1800 crankshaft (i.e., engine)
RPM. So that’s 25° total mechanical +
initial. Chrysler engineering was always
very conservative in their timing specs, the
concept being that giving up a bit of power
was preferable to having an engine rattle
itself to death. The number I have been told
repeatedly was that the FSM numbers were
6° retarded from peak power, bringing the
total to 31°, not far from the 33 the shop
suggested, but still far from the 50-55° total
you have heard from me.
But! Now add the vacuum number
from the FSM, but add it in crankshaft
degrees, which could be as much as 20.
The arithmetic: 31 + 20 = 51! Still how
do you reconcile this with the shop’s
33° recommendation? Easy. Virtually
every shop is looking to give you maximum
power, which is, in a stock 340,
usually obtained right about where they
suggested: 33° BTDC. That’s with no
vacuum advance, which they don’t give a
whit about. But running with no vacuum
advance will not burn all the fuel under
light throttle, high vacuum cruise conditions.
Where does this unburned fuel go?
Some will go out the tailpipe, polluting
the air, which you may or may not give
a darn about. Some will wind up on the
cylinder walls, where it will wash away
the oil film drastically increasing piston
ring and cylinder wall wear. Read: Drastically.
Some will get past the rings, diluting
the oil in the pan, increasing bottomend
(crankshaft and bearing) wear, and
will be pumped up top, increasing valve
train component wear, requiring very frequent
oil change intervals.
How can it not detonate with over 50°
of spark lead? Simple! The second you
crack the throttle open, placing any load
on the engine, the vacuum, and the attendant
spark advance, virtually disappear.