The Neglected 3.9 V6

Thanks to Vince Spinelli, Gabriel Couriel, Dan Stern, Vincent Roberts, and Mark P.

Thus, the 3.9, with a 3.91" bore, a close relative of the 318 V8 - with the same bore and stroke. It was developed because the new Dakota needed a V6 option, and buying Mitsubishi V6 engines was not an option. Developing a new V6 from scratch would have been expensive and time consuming. The slant six was gone by 1988, and in any case would have been too long for the space.

Though the 3.9 ended up as a truck-only engine, it served its purpose well in 1988, though by 2001 it had been outclassed and was replaced (consistently) by a 3.7 liter version of the 4.7 liter "Next Generation" engine which had better gas mileage and more power.

The 3.9 was only used in the Dakota (base motor), Ram 1500 (base motor), and Ram 1500 van ("economy" motor available mainly for fleets; the 318 V8 was standard).

One engineer wrote, "I remember they had an awful time with F/A (fuel/air) distribution. We were all asked to help out if we had a glimmer. I don't remember how they fixed it but I do remember it was acceptable in the end."

In 1987, its first year, the 3.9 used Holley 6280 two-barrel feedback carburetors. This setup may have lasted into 1988 on Dakotas. However, after that first year, Dodge slapped an EFI 2-barrel intake and induction on the 318 and the 3.9. They shared a Holley throttle body, at least until their conversion to the Magnum system, and used a non-vacuum advance distributor. The fuel injection from 1988 to 1991 was single-point, central fuel injection; gas was injected into the throttle body. This system was relatively inefficient, but still easier to control and maintain than a carburetor; replacing it with a modern multiple-port injection system in 1992 resulted in a power boost (though since the heads and other parts were upgraded at the same time, partly for more interchangeability with the 318, it's hard to isolate the effect of the better injection system).

In 1987, the 239-cubic-inch V6 pushed out 125 horsepower at 4,000 rpm, and 195 lb-ft of torque at 2,000 rpm, with a 9.2:1 compression ratio, far exceeding the single-barrel-carbureted 225-cube 1987 slant six’s 95 hp and 170 lb-ft of torque. Fuel injection did not change the power rating but greatly improved driveability. The engine was given lower-friction roller hydraulic tappets, which were used on the 318 as well starting in 1987.

The Magnum versions, with multiple-port injection, were rated at 180 hp. Torque jumped from 195 to 220 lb-ft as well. Gas mileage was roughly 2 mpg better than the similarly set up 318.

In 1994, the exhaust manifolds were shrunk to 1 5/8" and the exhaust was reduced to 2.5" from 3", eliminating 5 horsepower but no doubt saving some money. In 1996, EGR was eliminated.

In 1997, the 3.9 received sequential multiple-port fuel injection, where each injector fires as you'd normally expect anyway - as the cylinder is drawing in air (in standard multiple-port injection, the injector often fires against a closed valve). That and a larger spark plug gap helped to increase responsiveness, though horsepower and torque ratings remained the same as in 1996 - at 175 hp @ 4,800 rpm and 225 lb-feet of torque @ 3,200 rpm. At the time, the 318 was producing 230 hp @ 4,400 and 300 lb-ft @ 3,200.

Willem Weertman, the head engine designer, remembered, “They wanted to have an upgrade power plant from the 4-cylinder so the V-6 was designed as a way of furnishing a V-6 for the least possible tooling costs. Because of capital investment, we didn’t want to get into a whole new engine. We just wanted see what we could do with what we had and that caused us to look at the V-6 version of the Mound Road Engine.  That was the way it was done.  The engine had to be shorter than the V-8 in order to fit into the compartment.   It was only in later years that enough space was found in order to be able to put the V-8 into place. ... We had a challenge on the V-6 because the crank-pins had to be split in order to get away from the very unequal firing if we had only 3 crankpins, each crankpin having two of the connecting rods as is V-8 practice.  The reason is that the engine would be rather badly out of balance and would have not been acceptable even in a truck engine. So we had to do some redesigning of the bottom end in order to split the crank pins and make the firing order a little more uniform and it seemed to have worked out ok.”'

The other Chrysler V-6 engines for the 1980s and 90s (through 1998) were the 3.3 and 3.8, which used an aluminum alloy head and pistons with an iron block and cam; the 3.8 was a bored, stroked 3.3. (Click here for those engines).

Engineer Pete Hagenbuch remembered, “The 3.9 which was built in Mound Road, on the Mound Road machining equipment which included a 90 degree bank angle and it was another one of those boom-boom, boom-boom type engines. I had two of them, both automatics and it didn’t bother me a bit but the manuals were awful, especially if you lugged them down in speed.. It set off all kinds of sympathetic vibrations, just an awful way to build an engine. And Chrysler at that time had principles; we didn’t build engines that way. I’m confident that it went through the top and came back down with the message that ‘no way, you can’t do that it’s not commercial.’ Then years later we had already admitted that automatic Tempests and F85s were acceptable as long as you didn’t watch them idle under the hood where they were just thrashing around. With the vibration absorption you get in a torque converter they were okay as far as driving was concerned.”

Description	90 degree V6
Displacement	239 cubic inches / 3906 cc
Bore and stroke	3.91 x 3.31
Valves	Overhead - 12 valves, roller followers, hydraulic lifters
Fuel injection	1987: None, Holley carburetor 1988-1991: Single-point electronic 1992-1996: Multiple-point 1997-end: Sequential, returnless multiple-point (returnless means only one fuel line, going in)
Construction	Cast iron block and heads
Compression ratio	9.1:1 in 1997
Redline	5,250 rpm
Fuel	87 octane unleaded regular
Oil, coolant	4 quarts oil, 14 quarts coolant
Emissions	3-way catalyst, two heated oxygen sensors
EPA mileage (1997 Dakota)	16 city, 22 highway (2WD, manual transmission) 15 city, 18 highway (4WD, automatic transmission)