It is 1970. You own a brand-new TR6. Everything works perfectly,
nothing has been modified. The distributor is a Lucas 22D6, part #41306,
equipped with both vacuum advance and retard. The ignition timing was set
by the factory and dealer at 12 deg BTDC static, 4 deg ATDC dynamic.
All Lucas type 22D distributors have mechanical (also called
"centrifugal") advance. Yours was set by Joe Lucas to provide 0 deg advance
at idle, and 22 deg at 5000 RPM. The vacuum advance capsule is stamped
4/7/8; it can provide a max 16 degrees of vacuum advance. The retard
capsule is stamped 3/10/8; it can provide a max 16 degrees vacuum retard.
In the examples that follow, bear in mind that:
TOTAL ADVANCE = STATIC ADVANCE + MECHANICAL ADVANCE
+ VACUUM ADVANCE - VACUUM RETARD
EXAMPLE 1. You just started your fully warmed up engine, which
is idling smoothly at 850 RPM.
-Static Advance: 12 BTDC. The Triumph mechanic set this by
rotating the distributor and clamping it. It won't chance in operation.
-Mechanical Advance: Zero. Mechanical advance is a function
of engine speed only, and your distributor is "curved" for zero advance
at this engine speed.
-Vacuum Advance: Zero. Although the manifold vacuum is very
high, the vacuum advance unit doesn't sense it. The vacuum pickup, on
top of the carb, is masked by the throttle butterfly.
-Vacuum Retard: 16 degrees. Manifold vacuum is high, and the
vacuum "signal" runs from the bottom of the carb, where no "masking"
is occurring, to the vacuum retard capsule, which is providing its max
retard value of 16.
TOTAL ADVANCE=12 deg BTDC + 0 + 0 - 16 deg = 4 deg ATDC
And behold: this is why you set static timing to 12 BTDC with the engine
off, but when you idle the engine with a timing light hooked up you see
4 ATDC.
Example 2: You are driving downhill in top gear at 65 MPH,
stuck behind some pogue in an MG Midget wearing open-knuckle
driving gloves and a tweed cap. Engine RPM is 3000. The throttle
is partially open, obviously.
-Static Advance: 12 BTDC. The Triumph mechanic set this by
rotating the distributor and clamping it. It won't chance in operation.
-Mechanical Advance: 14 deg. Mechanical advance is a function
of engine speed only, and your distributor is "curved" for 14 deg advance
at this engine speed.
-Vacuum Advance: 4 deg (est). The manifold vacuum is
moderate, and the vacuum "signal" runs from the top of the carb to
the vacuum advance capsule, which is providing "some" advance. This
increases both fuel economy and CO emissions.
-Vacuum Retard: Near zero. Although the manifold vacuum is
moderate, the vacuum advance unit doesn't sense much of it. The vacuum
pickup, on the bottom of the carb, is masked by the throttle butterfly.
TOTAL ADVANCE=12 deg BTDC + 14 deg + 4 deg - 0 = 30 deg BTDC
If someone could hook a timing light to your engine right now, they
would see 30 deg BTDC. That's a lot of advance for such a low RPM, but the
engine can tolerate it without pinging because it is only at part-throttle.
Notice that the Vacuum Advance is not providing any "power".
Example 3: You've *just* pulled out into the passing lane and
mashed the accelerator pedal. The engine is still at 3000 RPM, it has not
yet reponded.
-Static Advance: 12 BTDC. The Triumph mechanic set this by
rotating the distributor and clamping it. It won't chance in operation.
-Mechanical advance: 14 deg. Mechanical advance is a function
of engine speed only, and your distributor is "curved" for 14 deg advance
at this engine speed.
-Vacuum Advance: Zero. (Surprise!) The manifold vacuum is
low, hence no advance.
-Vacuum retard: Zero. The manifold vacuum is low, hence no
retard.
TOTAL ADVANCE=12 deg BTDC + 14 deg + 0 - 0 = 26 deg BTDC
Notice that you now have less advance than when you were "just cruising"
at the same engine speed. That's because the engine is now under load
and can tolerate less advance without going to higher-octane fuel. To
avoid pinging, that "nice-to-have" vacuum advance economy feature is
automatically cut out.
Example 4: Victory at last! 110 MPH through the hills, god bless
you. Engine RPM 5000 plus, pedal to the floor.
-Static Advance: 12 BTDC. The Triumph mechanic set this by
rotating the distributor and clamping it. It won't chance in operation.
-Mechanical advance: 22 deg. Mechanical advance is a function
of engine speed only, and your distributor is "curved" for its maximum 22
deg advance at this engine speed.
-Vacuum Advance: Zero. The manifold vacuum is low, hence
no advance.
-Vacuum Retard: Zero. The manifold vacuum is low, hence no
retard.
TOTAL ADVANCE=12 deg BTDC + 22 deg + 0 - 0 = 34 deg BTDC
Your engine is spinning fast now, and can tolerate more advance even though
it
is under load.
Points to remember:
-Vacuum retard is an idle emissions feature. It does little, if
anything, at off-idle RPM
-Vacuum advance provides part-throttle economy, not high-
load or high-RPM power. It was deleted on later cars due to CO emissions,
and power did not suffer a whit because of it.
-Vacuum advance and retard can never really operate at the same
time.
Best Regards,
Tom Marincic
(NOMEX on, let me have it)
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