426 Hemi Idle Speed: Idle and RPM for Auto or Manual

Many Mopar owners wonder what their stock 426 Hemi should idle at or was set by the factory. The following are the 426 Hemi idle speed specs for each year.

• 1966: 750 rpm (Automatic or Manual)
• 1967: 750 rpm with or without Cleaner Air Package (Automatic or Manual)
• 1968: 750 rpm (Automatic or Manual)
• 1969: 750 rpm (Automatic or Manual)
• 1970: 900 rpm (Automatic or Manual)
• 1971: 950 rpm (Automatic or Manual)

This article will dive into more detail including the fast idle speed specifications for each year. In addition, I’ll explain the discrepancy in 1969 and how the CAP affected idle speed, particularly in ’70 and ’71.

426 Hemi Idle Speed Specs

The tune-up specifications and idle speed differ each year. The fast speed idle increases more each year and the neutral idle starting in 1970.

At first it may seem like it was because of the cam changes. The main reason is the Cleaner Air Package (CAP) and the Cleaner Air System (CAS) 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 and Idle Speed

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

In 1966 the State of California Air Pollution Control Board ruled the Hemi exempt from CAP. The CAP cars had an increased fast idle speed and curb idle in later Hemi years.

Cleaner Air System and Throttle Stop Solenoid

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. It’s on the driver’s side above the intake.

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 solenoid is part of the idle adjustment procedure and is separate from the idle screw.

If you’re interested about how the CAP affected timing check out my article, 426 Hemi Timing.

Vehicle Emission Engine Compartment Decal: Tuning Adjustment

1968 Vehicle Emission Control Information Decal for Automatic or Manual Transmission

• RPM = 750 in Neutral

1969 Idle

The fast idle speed increases again and there is a slight discrepancy between the service manual and engine compartment decal.

• Idle Speed: 750 RPM (Automatic or manual)
• Fast idle speed: 2,000 (After approximately 500 miles if necessary)

Vehicle Emission Engine Compartment Decal

The 1969 service manual indicates an idle speed of 750 although the vehicle emission decal, shown below, indicates 800 idle speed.

1969 Vehicle Emission Control Information Decal for Automatic or Manual Transmission

• RPM = 800 in Neutral

If you have any questions about idle speed specs, fast idle speed, posts or more information you’d like to contribute, send us an email found specifically on our contact page.

More Mopar Engine Idle Articles

• 440 Idle Speed
• 440 Six Pack Idle Speed

For the past 40 years, I’ve been studying and researching Mopar engine idle speed 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

426 Hemi Camshaft Specs

Hemi Firing Order

426 Hemi Main Bearing Size

Article Resources

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