Magnum Rocker Arms

[JMB]

Has anyone succesfully switched the stock steel magnum rocker arm to an adjustable Chevy type steel rocker arm? The Late Model class we run will allow Magnum heads but not a roller cam or roller rockers. We would like to run a solid lifter cam. Can anyone help?

[340king]

I successfully converted a set of Magnum heads to the shaft style rockers of the LA engine. I can't claim the idea, as it came from a gentleman in Oklahoma City. I personally don't like the Chevy style of valvetrain, especially on the super wimpy Magnum studs.

The setup that he came up with, and I copied with a lot of help from my machinist, is a CNC'd piece of tool steel that bolts to the head and accepts the stock style adjustable ductile rockers. There are several steps required to get this deal together. The total cost for all new parts including the rocker arms is around $550 give or take a little. I haven't gone into a production run on the shaft adaptors, as I didn't see enough demand for them. We did save the program and can tool up and build several sets if the demand looks good enough.

I can send you a copy of a recent photo showing what they look like. The adaptors alone would cost around $200. You then need the AMC solid lifters, T&D 7/16" cup end adjusters, ductile iron rocker arms, shafts and hollow tube pushrods. There are a few misc. items and machine work required to complete the installation.

Mopar Performance now offers a cast iron intake for the Magnum heads. We adapted the bolt angle on the Magnum heads to LA style before the intake was available. Buy the new Magnum intake if it is legal, as it is sure to be less work.

[DYNO360]

I haven't seen the cast iron intake for the magnum heads. I would guess it is a low rise, dual plane? Is it a 2bbl or 4bbl intake? How much do they cost? This sounds like a good item to have, because you have a lot of work to do in order to run a pre-magnum intake on magnum heads. Other than upgrading the rockers on the magnum heads, is there any other concerns when running the late model, magnum heads on a pre-magnum block. I understand the magnum heads are better than "J" heads. It might be worth getting the intake and upgrading the rockers in order to run the magnums. I think someone said they would be worth 50 horsepower on a 360.

[340king]

From what I have learned, there is basically only one advantage with the Magnum heads. Compression. That is why I chose them for this limited late model/sportsman class engine. Here is why.

When you are dealing with a restricted engine, getting the most air into the cylinder is the key to making more power. We are restricted to a 2bbl carb, Holley 4412, 500 cfm. This acts like a restrictor plate for all practical purposes. Increasing the compression does two things for us. It gives us more low end torque, which is needed since we can't wind this thing up like a 4bbl engine. The other thing it does is increase the draw into the engine. This is accomplished in two ways. First, the higher the compression ratio, the faster the pistons movement creates vacuum. This is because the final compressed volume is a smaller percentage of the total swept volume. As the piston moves away from top dead center, there is a bigger change in volume, when compared to a lower compression ratio engine. More compression is always better in restricted carburetor classes! The other way is it allows us to use a larger cam. Subjecting the intake to a longer duration of vacuum.

The class the engine is built for has the rules bent around the sb Chevy. The maximum cubic inch is 360 for Chrysler, with a maximum overbore of .040", flat top(zero deck) pistons, factory heads(except Chevy), etc. The zero deck deal was put in to kill the Mopars, due to the lack of a decent quench head in stock form before the Magnums. With the 360 Mopar you have to add 40 lbs ahead of the engine/trans mid plate, just to make it fair for the 350+.060" Chevies.

As for the new intake, I haven't seen it either, execpt in the catalog. It is cast iron and a dual plane if I remember correctly. I don't know if we can run it yet.

We are toying with the idea of building a 340 Magnum. This way we can shed the 80 lb penalty(40 up front, 40 out back to balance it out). We are usually dry slick from about this time of year on. The 340 has plenty of zip anyway. The other reason for the 340 is that it is not as sensitive to overevving. By this I mean that the 360 is more sensitive to intake reversion when revved too high. The high vacuum draws the intake charge back up into the intake, pulling exhaust with it on overlap. This kills the HP. Also, the power band of the 360 is narrower than the 340, making gear selection more critical. We plan to use the Magnum R/T heads if they will let us.

As far as potential for HP, they are not any better in my opinion than the "J" heads. Both heads flow at or just under the 200 cfm mark in stock form. I think Mopar lists the stock Magnums at like 195 cfm. The "J" heads flow about 203 cfm with good valves. There is not much HP difference on a restricted engine with either of these heads, everything else being equal(compression).

Sorry for the long explanation. I didn't know if you would just take my word for it if I said "they were about the same" and then left you wondering why we went to so much work.

I almost forgot, there is a concern with the late model Magnum heads. There was a casting problem for about a year and a half. Heads were cracking on a regular basis. The problem has been resolved, but I would look very closely at any used head before sinking a bunch of money into them. We used new MP castings.

[lon]

As for the mopar performance cast iron intake.....they are not available. Ma mopar has them in the catalog but they have never been produced. I talked with a couple of the larger mopar performance dealers and the part number is good -just none to be found on their computer network. It is a dual plane, 4bbl with the magnum head bolt pattern.

Just have to find another way to attach the old style intake to the Magnum heads......

Lon

[340duster1]

I have converted two sets of magnum heads to work with an old style intake. I have a jig that attaches to the magnum head intake surface and is set up to allow the holes to be drilled in the correct position and angle. Initially we redrilled the heads for 3/8" bolts, but welding was required to put enuff threads in the heads----very messy. So the last set we did (which are on my car now) we drilled for 5/16" bolts and tapped out the holes..no welding required. I am very careful not to over torque the bolts as there are not as many threads as I would like. I have had the intake on and off four times since we did the conversion and no problems. Also note that the magnums use only 5/16" bolts anyhow.

As a side note you can run the old style valve covers on the magnum heads as well, if using the steel valve covers all that is required is that a small lip on one corner of the gasket surface needs to be tweaked a little, that way the chevy boys will never know what is under there!

[JMB]

Back to the stock magnum rocker arms, This might be a crazy idea but what if you cut the base off of the pedestal and added an adjuster nut to the rocker arm stud above what now becomes the fulcrum. We would add 3/8" rocker studs & proper guide plates of course. The reason we are considering this is that Crane makes a kit to convert Ford V-8's to an adjustable rocker arm from a pedestal mount type (p/n 43540). That rocker arm looks similar to a Magnum rocker. Any thoughts?

Again, We want to use a solid lifter cam & magnum heads, rules say we must run stock rocker arms.

Thanks!!

[JMB]

I should add that we will only turn this engine 6100 - 6300 rpm.