So why do 3.9's suck

-

HankRearden

Well-Known Member
Joined
Sep 11, 2017
Messages
1,365
Reaction score
2,899
Location
PA
Ive been pondering a v6 for lightweight project and being a mopar guy I keep coming back to the 3.9.
But in my experience the 3.9's even new were gutless and not great on fuel. I've even seen more blown up 3.9's than 5.2 and 5.9 together.
But why? Is't the 3.9 just based on a magnum 5.2? Are the heads comparable flow?
What is it that made them so disappointing compared to a gm 4.3 or 3800?
And how hard is it to make a 3.9 better?
 
3.9 is an excellent motor. I've owned a few, and my Dad bought one brand new in a dakota. These engines snicker at 200k, and 300k would be easy with any care. They weren't slouches in their day, 1992. I ran low 17's in the 1/4 mile with a club cab automatic Dakota, that was not bad for the day. Dakota's were a bit beefier and heavier than the S10's and Rangers.

And yes, the 3.9 is a 5.2 missing 2 cylinders. Heads are the same but shorter.
 
Depending on the year, they could be either LA or Magnum based.
Ones I've dealt with were always decent runners, they just didn't get the kind of mileage that you'd expect. Power output is about average, but even a 318 was considerably more powerful and only a couple of MPG lower...
Try to dig up some of the corresponding year DC books, they had similar buildups listed for the V6 as they did for the slants, LAs and big blocks. BUT good luck finding any of the parts, especially cams distributors and intake manifolds, that are in their recommendations.
I do remember some Dakota class racers that were laying down some good times with the V6, years back...
 
3.9 is an excellent motor. I've owned a few, and my Dad bought one brand new in a dakota. These engines snicker at 200k, and 300k would be easy with any care. They weren't slouches in their day, 1992. I ran low 17's in the 1/4 mile with a club cab automatic Dakota, that was not bad for the day.
weird. I worked on and drove a bunch of them in dakotas and my experience was not nearly as positive.

I even bought a couple first gen dakotas with less than 100k miles and blown up 3.9's. Both broken connecting rods.
Maybe Ive just never seen a good one.
 
I've had a few and they weren't bad, the one in my 99 Dakota was quite doggy compared to the ones prior to it, my 90 (pre magnum) and my 94 weren't bad. Never blew one up. And I used them hard for what they were. But when I could buy an otherwise identical truck with a 318 and have more power with no worse mileage (usually better), that kills my enthusiasm for them.
I know at one time Mopar performance offered cans and a 4bbl intake for the pre magnum version
 
Depending on the year, they could be either LA or Magnum based.
Ones I've dealt with were always decent runners, they just didn't get the kind of mileage that you'd expect. Power output is about average, but even a 318 was considerably more powerful and only a couple of MPG lower...
Try to dig up some of the corresponding year DC books, they had similar buildups listed for the V6 as they did for the slants, LAs and big blocks. BUT good luck finding any of the parts, especially cams distributors and intake manifolds, that are in their recommendations.
I do remember some Dakota class racers that were laying down some good times with the V6, years back...
Yeah, I found a bone stock roller cam,2v carb la 318 was a drastic improvement when swapped into a dak. Both in power and mileage. I did two of them. One with the stock 5speed and one with the 904 out of the same m body the engine came out of.
 
weird. I worked on and drove a bunch of them in dakotas and my experience was not nearly as positive.

I even bought a couple first gen dakotas with less than 100k miles and blown up 3.9's. Both broken connecting rods.
Maybe Ive just never seen a good one.
I've also seen them in the shorty vans with way over 200k on them. Actually, my sisters family had one and my brothers family had one. Those vans had more power than you'd expect from that small 3.9 in that mass. Neither had the OD tranny's, just 3 speed lock ups.
 
1992 the 3.9 had 175 hp
1992 the 4.3 had 193 hp.
18 more HP for the more cubic inches.
 
1992 the 3.9 had 175 hp
1992 the 4.3 had 193 hp.
18 more HP for the more cubic inches.
weird. I used to do a lot of work for a Chrysler dealer from about 97 to 02 and I drove a boatload of First and second gen 3.9 Daks and only one ever impressed. It was such a shock eeryone in the shop drove it.
A 2wd 2nd gen with a five speed. We suspected it snuck out of the factory with better gears.
 
Ive been pondering a v6 for lightweight project and being a mopar guy I keep coming back to the 3.9.
But in my experience the 3.9's even new were gutless and not great on fuel. I've even seen more blown up 3.9's than 5.2 and 5.9 together.
But why? Is't the 3.9 just based on a magnum 5.2? Are the heads comparable flow?
What is it that made them so disappointing compared to a gm 4.3 or 3800?
And how hard is it to make a 3.9 better?
I bought a 1992 Dakota Sport new 3.9 5-speed and it was NOT gutless! I put 95K miles on it in 4 years and towed quite a bit open trailer with a 69 Dart on it, plus the short bed loaded with parts. Only issue I ever had was blowing a trans seal bringing back a friends 68 Dart from southern Illinois after it broke down. Of course I heard the 92 was the best of them...
 
I bought a 1992 Dakota Sport new 3.9 5-speed and it was NOT gutless! I put 95K miles on it in 4 years and towed quite a bit open trailer with a 69 Dart on it, plus the short bed loaded with parts. Only issue I ever had was blowing a trans seal bringing back a friends 68 Dart from southern Illinois after it broke down. Of course I heard the 92 was the best of them...
'92/93's made more power, both 5.2 & 3.9's. The '94's and up had smaller cams, smaller exhaust. I did side by side comparison on the exhaust with my '92 Dakota and my Dad's '94.
 
I daily drive a '99 Dakota 4x4 with a 3.9 & 5spd. Gets around 18-20 MPG and has 250K on it and climbing every day.
 
Yeah, I found a bone stock roller cam,2v carb la 318 was a drastic improvement when swapped into a dak. Both in power and mileage. I did two of them. One with the stock 5speed and one with the 904 out of the same m body the engine came out of.
I actually had a '87 Dakota with a 3.9 2bbl carb, 5-speed. It ran good and all, but I think it only had about 125 hp and 195 ft lbs torque. Still, that was about 30 hp and 20 ft lbs above the slant in the same year.
 
Why would ya expect a 318 minus two cylinders would be a power house ?
3.9l got way more potential than a /6 but a 273 has way more potential than a 3.9l.
 
Why would ya expect a 318 minus two cylinders would be a power house ?
3.9l got way more potential than a /6 but a 273 has way more potential than a 3.9l.
these types of engines, be it a 3.9 or a /6, they have to be taken in the era they were created and appreciated within that context. A person can throw a turbo at either and have fun, but I just view them as little 6 poppers that keep going, get the job done the simple way.
 
Why would ya expect a 318 minus two cylinders would be a power house ?
3.9l got way more potential than a /6 but a 273 has way more potential than a 3.9l.
I don't. But when I was test driving these things all the time they did not perform commensurate to a jeep 4.0 or gm 4.3 or injected 3800.
 
I don't. But when I was test driving these things all the time they did not perform commensurate to a jeep 4.0 or gm 4.3 or injected 3800.
you say "lightweight project"... what does that mean? You want a lightweight but heavy hitter? How about a 2.5 turbo? Now there is a ton of opportunity for just a few pounds of engine!
 
-
Back
Top