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03:54:47 - 12/24/2024
General Dakota Board
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DaKarl
Dodge Dakota
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6/11/2005
21:44:47
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 Subject: 3.9L just chopped off 5.2L?
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I am wondering if my 3.9 L is just a "chopped off" 5.2L. There is very little performance parts in my area for the 3.9 and a fair amount for the 5.2. In a nutshell, I want to put 5.2 L rocker arms and throttle body on my 3.9. The headers are available but the internal parts are more rare.
Thanks, DaKarl
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IntenseDak39
*GenIII*
6/11/2005
22:28:08
|  RE: 3.9L just chopped off 5.2L?
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it is a short 5.2
but there are rocker arms for the 3.9 and aftermarket throttle bodies.
there are even headers, ignitions, nitrous kits, plug wires, chips, computer programmers.. etc etc etc
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DaKarl
Dodge Dakota
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6/11/2005
22:45:46
| RE: 3.9L just chopped off 5.2L?
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I read on another thread on this site that cranecams.com has 1.6 lift ductile iron shaft rocker arms, however I can't seem to find ones for a V-6. There are only ones for the V-8s. I'm most likely just going to get the 5.2 rocker arms and pushrods.
If it's all fine and dandy, I'll have a daily driver with a little more oomph.
Thanks for the confirmation, IntenseDak39, and nice truck. Mine is an '89 and all that's wrong is one of the rocker arms is clicking!
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gen1dak
Dodge Dakota
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6/11/2005
22:53:26
| RE: 3.9L just chopped off 5.2L?
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This is a simplified way of looking at it, but essentially the 3.9 is a 5.2 with two cylinders lopped off. In reality, half of the two front cylinders on one end were lopped off, and half the rear two was lopped off, then the crankshaft was offset to compensate, and the 6 cylinders were reconstituted around the new crank location. The bore/stroke and parts dimensions match the 5.2, but certain parts like the crank, valve covers, intake manifold, exhaust manifolds, cylinder block, distributor, and rocker arm shafts (in pre-Magnum 3.9's), but not the rockers themselves, are V6 specific, as well as Magnum V6 flexplate/flywheel because of V6 crank sensor requirements (six "windows" as opposed to eight).
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DaKarl
Dodge Dakota
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6/11/2005
23:10:55
| RE: 3.9L just chopped off 5.2L?
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Thanks, gen1dak
I guess I'll have to get a set of 5.2 pre-mag rockers. I'm thinking of 1.75 to 1.8 ratio with corresponding pushrods. If I can't get ahold of pre-mag rocker arms, I'm praying that there is a set of shafts to adapt.
Once all is said and done I'm going to have it tuned up... it's running a little rich. Maybe the computer will know how to compensate with the 5.2 TBI...
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DaKarl
Dodge Dakota
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6/11/2005
23:40:38
| RE: 3.9L just chopped off 5.2L?
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Just out of curiosity, I think the stock 3.9 has 125 HP. Am I correct? I am really new to the Dakota world and I have 3 different stories... 125 HP, 175 HP, and 195 HP. This is the pre-magnum engine, mind you.
Also, since there is no computer or TBI-friendly cams, I am wondering if there is a maximum rocker arm ratio before adverse effects occur.
Thanks, DaKarl
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gen1dak
Dodge Dakota
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6/12/2005
00:25:37
| RE: 3.9L just chopped off 5.2L?
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Ummm, well, I haven't seen anything beyond the 1.7:1 ratio offered for LA-style rocker setups, and that's pretty rare. Even with 1.7's, your max lift with a stock cam will be in the .436" range at the valve (.410" with 1.6). Your stock computer should have no problem with either since the same capacity PCM/TBI was used on the 5.2.
125 measely HP is correct for the pre-Magnum 3.9 V6. Now, while Mopar never offered any real performance stuff for the pre_magnum injected units, they do offer a couple mild cams for the 3.9 LA engine.
P4452806 (.405" lift) and P4452808 (.410" lift). Both are mild, but every little bit helps, and when coupled with some 1.6 rockers, add up to some decent lift numbers. 1.7's even better. It also states that when replacing the OEM roller cam, P4286808 pushrods are needed, so these are flat tappet cams.
Invest in a new double-roller timing gear and chain. Makes a world of difference when the chain is fresh and tight.
Keep in mind that the 175HP rating on the Magnum V6 was due primarily to 3 things: Hotter cam (.432 lift), better cylinder head flow, better exhaust manifold flow. So, get your V6 heads freshened up with a mild port and polish, open up your exhaust with a good muffler, and do anything you can to get more air into that engine.
Something else to consider for a cam..... Why not use a Magnum V6 cam? All you need to do is to make sure your tappets are oilers (look for an oil hold in the pushrod seat of the tappet, and be sure to use hollow pushrods. Use the Mopar Performance HD LA-style rockers (reinforced for hotter cams, oil hole in the pushrod cup of the rocker-so the oil can get to the rocker shafts). The Magnum cams do not have oil holes like the LA cams, so it is necessary to have the rockers oiled via the pushrods....just like the Magnums do it. Then, you can use the Magnum V6 Hipo cams. Just remember to factor net valve lift against the 1.5 ratio. So, if you were to find a cam with .480 lift, your net would be .450", which would make a sweet street cam, and since it's gound for computer-friendly operation....bliss can be yours. :-)
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slopehead
Dodge Dakota
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6/12/2005
09:28:13
| RE: 3.9L just chopped off 5.2L?
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DaKarl?
the story you have is a bit off. the 3.9 pre-mag or LA based engine is 125 hp the magnum is 175 hp and no-where is there a 195 hp magnum six. there is a 210 hp 3.7 but that is another animal and not related to the magnums. you must be a bit confused and are thinking of chevy's 4.3 with 195 hp.
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DaKarl
Dodge Dakota
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6/12/2005
10:17:32
| RE: 3.9L just chopped off 5.2L?
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When I got the truck I was told it was 195 HP. When I got the new exhaust, more modding needed to be done ;). Upon more research, I found the 175 and 125 HP ratings. Unfortunately, the info wasn't specific. So here I am, learning from the pace-setters!
PS: I had an 1986 Buick with 2.8L and it was rated at 160 HP... so I figured a 3.9 should have 200 HP like most 3.8s these days.
DaKarl
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DaKarl
Dodge Dakota
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6/12/2005
17:03:43
| RE: 3.9L just chopped off 5.2L?
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Thanks to all who chipped in on my little "project" Depending on the local cost, I may use all the suggested parts. I'll see about the costs tomorrow and keep you posted.
Cheers
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