Timing Advance and Retard Primer

Date: Fri, 9 Oct 1998 11:51:04 -0400
From: Tomislav Marincic
Subject: Advance/Retard examples (long)

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 change 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 change 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 change 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 change 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=3D12 deg BTDC + 22 deg + 0 - 0 =3D 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)

Addendum:

Date: Fri, 9 Oct 1998 12:01:41 -0400
From: Tomislav Marincic

Jim,

RE:"I am willing to accept that many of you know more about this than I do, but on the 69 the retard connects to the manifold. The later ones go to the carb, but not the 69."

That's because the '69 and early '70's are the only ones (except the TR250) that have a vacuum on/off switch under the front carb that is opened when the throttle linkage is in the idle position. Later TR6's used tuned vacuum ports to achieve the same effect.

Best Regards,
Tom Marincic