426 Hemi Compression Ratio: Street and Race Hemi Engines

Many questions popup overtime about Chrysler’s most iconic engine ever produced. One of the topics asked about is the 426 Hemi compression ratio.

The 426 street Hemi had a 10.25:1 compression ratio. This engine was produced by Chrysler with the same ratio from 1966 to 1971. The 426 Race Hemi had a 12.5:1 compression ratio and was produced by Chrysler for racing applications only.

This article will explain why the compression ratios are different between the two engines. In addition, how the ratio affects performance and the fuel type.

426 Chrysler Hemi Compression Ratio

There were two different 426 Hemi engines produced by Chrysler, the Street and Race Hemis. The street version was produced from 1966 to 1971 and is the one most people are familiar with from the muscle car era..

The race Hemi was produced by Chrysler and provided to racing teams starting in 1964 for Nascar and the NHRA. Since the race engine came first, I’ll start there.

426 Race Hemi Compression Ratio

The first 426 Race Hemis were invented, designed, engineered and produced by Chrysler. This racing engine had much more power than the typical street wedge engines as its main purpose was for racing applications only.

For this reason the internals of the Mopar Hemi engine are different to produce more horsepower and torque. One of the differences is the compression ratio of the engine.

The 426 race Hemi was produced with a 12.5:1 compression ratio. Within reason, the higher compression ratio the more horsepower and torque generated.

A higher compression ratio allows for more energy from the combustion process due to better thermal efficiency. This occurs in the combustion chamber of the engine where spark, fuel and air combine.

A higher compression ratio requires higher octane or racing fuel to prevent detonation or the knocking noise you hear from the engine. Higher-octane fuel can resist self-ignition at a higher temperature and pressure than a lower octane fuel.

So, for these reasons, the racing Hemi has higher compression, horsepower and torque but also requires higher octane racing fuel.

426 Street Hemi True Compression Ratio

When Chrysler decided to develop and produce a street Hemi for sale to the public, changes to the race engine needed to be made.

One of the changes required was the compression ratio of the engine. The 12.5:1 ratio requiring racing fuel didn’t make sense for the average motorist buying a Hemi cars.

Racing fuel is expensive, but it’s also not available in the average gas station. To make the Hemi streetable, the 426 street Hemi was produced with a 10.25:1 compression ratio.

They accomplished this by installing lower compression pistons. This also helped to lower horsepower.

It was recommended by Chrysler to use premium gas which is the highest octane available at the gas station. This enabled a Dodge Charger to proudly pull up to any gas station and fill up.

If you’re interested in other differences between the race and street engines check out my article, The Difference Between a 426 Street Hemi and a 426 Race Hemi.

Thankfully Chrysler kept the ratio as is in 1971 and didn’t lower it like other manufacturers did. Even the robust 440 Six Pack saw a slight reduction that year.

Check out the Hemi pistons and combustion chambers in this 10.25:1 street engine.

Any questions or if you have more information you’d like to contribute, send us an email found specifically on our contact page.

More Mopar compression ratio articles:

For the past 40 years, I’ve been studying and researching Mopars and engines like the 426 Hemi. I’ve read books, articles, magazines, watched videos, attended seminars and spoken to other Mopar experts.

Learn all about their cams in my article, 426 Hemi Cam Specs: Street and Race.

Read More Hemi Muscle Cars Articles!

What Problems Did the 426 Hemi Have?

Transmissions a 426 Hemi Used

426 Hemi Specs

Hemi Oil Capacity (Each Year and Model)

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