Hi all,
I'm rebuilding (really overhauling) my 3.9 liter.
The situation is that I was going to put in one of those Hughes Engines cams, but they aren't able to get one of those for me, so I'm going to put in the Harland Sharp 1.7:1 rockers to get a bit of a bump.

During my research I came across the specs for the 1991 TBI 3.9 liter cam.
So I have a technical question and depending on the answer I also want to solicit your thoughts about cam choices.

So first the tech question. Is there anything preventing me from putting the 1991 TBI cam into the 1993 Magnum block(such as bearing sizes, or radically different cam to crank timing or anything else that would cause damage to the engine or otherwise make it not run)?

If the answer to the above is that there is no problem putting the cam in, then let's move onto the tech specs and opinions.

So the two part numbers from Melling are mc1253 for the 1993 MPI cam and mc1258 for the 1991 TBI cam.

The timing numbers for the mc1253 (MPI cam) are:
Intake Duration at 050 inch Lift:
184
Exhaust Duration at 050 inch Lift:
193
Duration at 050 inch Lift:
184 int./193 exh.
Advertised Intake Duration:
254
Advertised Exhaust Duration:
267
Advertised Duration:
254 int./267 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:
0.413 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:
0.430 in.
Valve Lift with Factory Rocker Arm Ratio:
0.413 int./0.430 exh.

The timings for the mc1258 (TBI cam) are:
Intake Duration at 050 inch Lift:
191
Exhaust Duration at 050 inch Lift:
191
Duration at 050 inch Lift:
191 int./191 exh.
Advertised Intake Duration:
272
Advertised Exhaust Duration:
272
Advertised Duration:
272 int./272 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:
0.417 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:
0.417 in.
Valve Lift with Factory Rocker Arm Ratio:
0.417 int./0.417 exh.

Honestly the TBI cam seems like the better cam overall due to the increase in duration @0.050 and higher lift on the intake side.
If either one would work fine, which would you choose?

My goals are a bit more power in the midrange so that I can have the best throttle response and power at cruising speeds.

If I'm 4 wheeling I tend to use low range and second gear because I like the throttle response at that spot.

Your responses are much appreciated!

Sincerely,
Brian

 

Also note that the 1991 3.9 liter cam is non-magnum, but is a roller cam

 

Does the 3.9 use the shaft style rockers the LA engines did? If so, the H/S rockers won't work.

 

My 3.9 liter has the pedestals

 

I assume you have the 93, which is the magnum style motor.
On the V8 ie: 5.2-5.9 vs the LA 318-360.
Bearings are the same, etc. The main difference is on the older versions the early cams had provisions for the fuel pump where as the latter models do not.
I "think" being the 91 is a LA style engine, but it has an electric fuel pump, and it used roller rockers..... It would physically fit your 93, but be aware there is no tuning available for the 95 and earlier computers.
If you are set on wanting to change the duration on the cam, and you feel you can somehow deal with the tuning issues, you may want to consider having the existing cam reground. This would probably require different push rods....
Also keep in mind the springs really need to be upgraded with any additional lift. I "think" the lift is limited to .480 or so with the stock springs

 

wanted to add just a little more. If my calculations are correct the following would apply to the first cam listed above MC1253.
Lift is shown with stock rockers (this would be 1.5 ratio on a 91) to be .430 on the exhaust.
I believe the .430 lift with 1.5 rockers would have a camshaft lift of .2865
If you take the base cam lift of .2865 and multiply this by the 1.7 ratio of your rockers, you would have a lift of .487.
I believe this is too much with stock springs.

FYI the 91 and older use a rocker ratio of 1.5 and the 92 and up magnums used a ratio of 1.6.

For anyone reading this, if I am incorrect, please post, as im pulling from memory on a lot of this, and that itself is, well...... not that good!!

 

Ohh -Thanks!
I hadn't thought about the difference in rocker arm ratio in the LA version of the 3.9L!
I have some Mopar Performance springs but they seem to be the incorrect ones. They are like a half inch longer than the stock ones in the engine now.
I'm going to have to send them back to Summit I think. I'm not sure why they have those ones as being able to work in my engine.