Comparison: 3.8 Vs 3.9 V6 Mustang Engines

Posted by on February 8, 2010 - more

Article Credits

Author: sonicpony03

Editor: Brent

People often ask what the differences are between the Essex 3.8 liter v6 engine and the Essex 3.9 liter v6 engine. This article is here to explain the differences between the two engines, reason for replacement of the 3.8 liter v6 mid year 2004, and whether modifications for the 3.8 liter engine apply to the 3.9 liter engine.

The Essex 3.8 liter OHV v6 engine was used in the model year 1999 thru mid 2004 year base mustang. It utilized the split port cylinder design originally used on the Ford Windstar. For 1999 and 2000, the 3.8 did not use intake manifold runner control (IMRC). It left all 12 intake runners open at all times, producing 190 horsepower at 5250 rpm and 220 foot pounds of torque at 2750 rpm.

In 2001, Ford decided to add IMRC to the 3.8 liter bringing total output 193 horsepower at 5500 rpm and 225 foot pounds of torque at 2800 rpm.

Starting October 7, 2003, the Essex 3.9 liter was installed in model year 2004 base mustangs. The reason for this switch was due to the model change from the Windstar to the Freestar minivans. The Freestar used the 3.9 v6 just as the Windstar used the 3.8, so it is not odd that it was handed down to the v6 mustang. Ford felt that using the same engine would avoid manufacturing complexities and potential complications of building similar, yet different engines side by side.

The only significant difference between the 3.8 and the 3.9 liter v6 engine is the piston stroke and reduction in the pin to dome length, resulting in an increase in displacement. The piston stroke increased from 86.00mm to 88.00mm, and the pin to dome length reduced from 38.83mm to 35.83mm. These changes resulted in increased displacement from 3.79 liters to 3.88 liters. (as with many engines, the manufacturers rounded the displacement number up) The cylinder bore, compression ratio, horsepower and torque went unchanged for the 3.9 liter v6 engine.

Many have asked questions related to whether 3.8 v6 engine aftermarket parts will work on 3.9 engines. Several engine modifications will work for the 3.9 engine that are only labeled to work on the 3.8 engine. For example, the American Muscle CAI Kit is only listed as working on 1999 to early 2003 v6’s, but it will work just fine on late 2003 3.8’s and all 3.9’s. Most vendors and manufacturers relay whether their parts will work for both engines.

So the bottom line is don’t be afraid of the 3.9 engine. The only difference is internally, which doesn’t affect mods outside of the guts of the engine. They are very compatible with the 3.8 engines, and are just as mod friendly.
There is no difference in power output or efficiency between the two if you have a stock 99+ 3.8 and your buddy has a stock 3.9, he won’t beat you due to any extra engine power, it’ll simply be a driver’s race!


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  2. Great article.  Thanks for the information.  Do you know where I can get information from ford that may explain the engine change in greater detail?  For example, I have to get new piston rods, will one piston rod work for whichever engine I have?

  3. 68countrysedan says:

     “The only significant difference between the 3.8 and the 3.9 liter V6
    engine is the piston stroke and reduction in the pin to dome length,
    resulting in an increase in displacement.”

    It is?

    How about the 3.8L V6 is an OHV configuration, while the 3.9L is an OHC configuration.

    The 3.8L is a 90-degree block, which makes it an odd-fire engine, which generates unwanted NVH issues. Ford invested a lot of money in reducing NVH via split rod journals balance shaft, cast aluminum oil pan, etc.

    The 3.9L is a 60-degree block, so it’s even-fire. The block is a totally new casting, not to mention the heads. Without additional research, I would guess the crank, rods and pistons are totally different from the 3.8L.

    In other words, the 3.9L is much more than just a reduction in the piston compression height.

    • Joss Whedon says:

      The 60° Essex block is a completely different engine from the one described in the article. The 90° Canadian Essex block is the only block used in the Mustang. I know this because my Mustang has a 90° 3.9L Essex block sitting in it right now. And it’s an even fire engine BTW. :)

      I know I’m responding to a 3 year old post but someone has to put out some good information to make sure future people don’t get confused by the above very confused comment that has mixed up the Canadian 90° essex block and the 60° UK Essex block (Which ended production 4 years before the 3.9L Canadian Essex block was even produced BTW)

  4. devin says:

    So, it’s a smoother engine. How does it not make anymore power though? I’m sorry if it is a stupid question, I just found out about this 3.9L engine. Would it be better to mod than the 3.8L?

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