426 Hemi Timing Specs: Initial and Advance Degrees

The Hemi engine timing is a topic often asked by Mopar owners. The following are the 426 Hemi timing specs, initial timing, for each year.

• 1966: 12 1/2° BTC set at idle speed.
• 1967: 12 1/2° BTC set at idle speed without the Cleaner Air Package (CAP)
• 1967: TDC set at idle speed with CAP.
• 1968: TDC set at idle speed with CAP.
• 1969: TDC set at idle speed with CAP.
• 1970 Auto Transmission: 5° BTC plus or minus 2 1/2°. Set at curb idle (with Cleaner Air System CAS)
• 1970 Manual Transmission: TDC plus or minus 2 1/2°. Set at curb idle (with CAS)
• 1971 Auto Transmission: 2.5° BTC plus or minus 2 1/2°. Set at curb idle (with CAS)
• 1971 Manual Transmission: TDC plus or minus 2 1/2°. Set at curb idle (with CAS)

This article will dive more into detail about each year including initial, advanced vacuum, advanced mechanical and total timing. I’ll also explain what the CAP and CAS systems are and how they affect the timing each year.

426 Hemi Timing Specs

The timing and tune-up specifications mostly differ each year. They are taken directly from the Chrysler service manual.

Even though the Hemi basically remained the same throughout its production run, the timing changes drastically.

The timing changes are a result of the Cleaner Air Package and the Cleaner Air System which are explained next.

Obviously if your cam or engine has been modified in any way, these specs may not pertain to you. Either way, it’s interesting to see how things were back then and how they changed during the Hemi’s six year production.

Cleaner Air Package

The Cleaner Air Package (CAP) is designed to reduce engine emissions to acceptable limits. It helps to control both the crankcase and exhaust emissions.

Emissions are at their highest at low idle speeds and deceleration.

CAP cars consist of a modified carburetor and ignition timing, plus some basic engine design refinements.

A CAP carburetor delivers a leaner idle mixture and idles faster. The basic timing of CAP engines, including the Hemi, has been altered to provide less timing at idle.

The mechanical and vacuum advance curves are also different. On some models a vacuum control valve is used to provide maximum vacuum advance during deceleration.

The vacuum control valve connects to the distributor, intake manifold and carburetor. You’ll find it above the intake manifold towards the rear passenger side. It’s colored green.

In addition to the carb and distributor, changes were also made to the intake manifold heat control and choke calibrations.

In 1966 the State of California Air Pollution Control Board ruled the Hemi exempt from CAP. This is why the timing specification for 1966 and some ’67 non-CAP carbs and distributors are higher at idle.

It also explains why the idle speed is lower on Non-CAP Hemi cars and higher on CAP cars. From 1968-1971 all street Hemis are CAP or CAS equipped.

Cleaner Air System

In 1970 Chrysler changed the name of CAP to the Cleaner Air System (CAS). CAS is basically a continuation of the CAP system with some refinements.

Some of the CAS changes, like lower compression, thankfully did not affect the Hemi. Although idle and timing changes continued as shown in the specifications below.

The 1970 and 1971 Hemis had a throttle stop solenoid and throttle stop adjustment screw. The throttle stop solenoid allows the throttle to close below the idle setting when the engine is shut down. This prevents after-run or dieseling.

The throttle stop is part of the idle adjustment procedure.

For a list of all the idle specs for each year of the Hemi, which also changes most years, check out my article, 426 Hemi Idle Specs.

Distributor RPM and Degrees vs Engine/Crank

Typically, initial engine timing and total advance timing are expressed in crankshaft degrees read at degree marks on the harmonic balancer.

A potentially confusing part of engine ignition timing is distributor vacuum and centrifugal advance curves are often described in terms of distributor degrees and distributor rpm.

The Chrysler service manual does this as shown below in the photo. The distributor rotates at half the speed of the crankshaft with the engine running.

Therefore, to get the crankshaft degrees or rpm, simply multiply the distributor degrees or rpm by two.

For example, let’s say at idle you set the initial timing to 12° BTDC at the crank/balancer. Let’s also say the distributor has 11 degrees of centrifugal advance. The total advance would be 34° with the advance fully in [(11 x 2) + 12 = 34].

Most stock Hemis seem to be happy and develop the best horsepower somewhere between 32 and 36 degrees total.

1966 Hemi Timing Specifications

In 1966 the Hemi was exempt from the CAP system. Therefore, the service bulletin published for the engine only showed the standard timing specs.

Distributor Part No./Prestolite

2642482/IBS-4006P

Timing

• 12 1/2° BTC **
• ** Set at idle speed

Firing Order/Rotation

• 18436572/Counter-clockwise

1967 Timing Specs

In 1967 there were two different timing settings, one for CAP, and one without CAP. CAP equipped cars had less initial and more advanced degrees.

Without Cleaner Air Package (CAP) Automatic or Manual

Timing

• 12.5° BTC**
• **Set at idle speed

1968 Timing Specifications

Starting in 1968 the specs are for the CAP or CAS systems only.

1968 Vehicle Emission Control Information Decal for Automatic or Manual Transmission

• Timing = TDC

1969 426 Hemi Timing Specifications

1969 Vehicle Emission Control Information Decal for Automatic or manual Transmission

• Timing = TDC

The 1969 decal indicates a different idle rpm than the service manual. Check out what the manual actually states in my article, 426 Hemi Idle Specs for Auto or Manual.

If you have any questions about timing specs, timing light, vacuum hose or more information you’d like to contribute, send us an email found specifically on our contact page.

Other Mopar Engine Timing Specs

440 Six Pack Timing Specs

For the past 40 years, I’ve been studying and researching Mopar engine timing specs like the 426 Hemi. I’ve set the timing and idle on Hemis, read books, articles, magazines, watched videos, attended seminars and spoken to other Mopar experts about the topic.

Read More Hemi Engine Articles!

Hemi Specs

Why the 426 Hemi Distributor Hold Down is Different

426 Hemi Distributor Part Numbers and Date Codes

Hemi Head Torque Sequence and Specs

426 Hemi Camshaft Specs

426 Hemi Intake Manifold Torque Specs

Article Resources

• Chrysler Reference Book: An Introduction To The CAP System
• Chrysler Reference Book: Evolution Of The Cleaner Air System
• Chrysler Service Manuals: 1966-1971
• Google Books: Challenger and Barracuda Restoration Guide, 1967-74