DISCLAIMER: Do this at your own risk. I do not advocate it. In fact, I discourage you from trying it, as I suspect that there are better approaches to achieving better high-RPM airflow. Nonetheless, my respect for you, my fellow gearheads, forces me to share what I know.Okay, disclaimer aside, it turns out that I have these pics with me on my laptop, so I am loading this infor up now, rather than having to wait until I get home this weekend.
Converting a fixed-timing cam pulley to an adjustable one is mechanically simple, but it can be somewhat challenging to calculate just where to drill the holes.
Take a look at the first photo. Notice that there are three sets of holes drilled in the adjustor plate, plus three slotted holes. At lower center, the holes are marked in degrees advanced, 2, 3 and 4 degrees. At upper left three more unmarked holes are for the same degrees retarded. On the right edge, three holes mark 1 degree retarded, straight-up, and 1 degree advanced. Notice that the set-screw is in the straight-up position.
Now take a look at the photo below. This is of the backside of the pulley, and shows the OEM timing notch. One can also see the set-screw in the straight-up hole.
To advance the cam to, for example, two degrees advanced, loosen the three slotted screws and remove the set-screw. Rotate the outer ring until the hole behind the 2-degree hole is perfectly lined up with the 2-degrees advanced hole, and reinstall the set-screw INTO THAT HOLE (you can see that the hole is not prefect aligned - you will only have to move the outer pulley, well, 2 degrees!
). Tighten the slotted screws. [You will now have to re-time the injector pump, but we are only discussing how to adjust the cam's timing.]
To actually make the pulley, my friend Tom Nelson, who originally built this engine, started by drilling and tapping the three sets of 3 holes a convenient distance apart on three of the pulley's flanges, plus the holes for the slotted screws (yes, I know the screws are not slotted, but you know what I mean...
) Next, he prepared the .050" aluminum plate by cutting a round piece the right diameter and cutting the center hole and the 6 mounting holes (near the center). Tom then drilled the hole for the zero, or straight-up position.
By mounting the plate to the unmolested cam pulley, Tom reached in through the holes to mark the backside of the plate. This gave him the positions of the holes. He then offset the plate the correct distance and drilled the 8 remaining timing holes. To do this you can use a straight edge and protrator, or calculate the distance using some basic high-school trig, or eyeball it by dividing the number of teeth on the pulley into 360 to discover how many degrees per tooth there is on the pulley, then swag the distance to offset. I don't recommend this technique, but it has been used by DIYers since time-immemorial.
Finally, insert a hacksaw blade into each lightening hole and cut through the flanges. Debur the cuts 9see lower photo above). Now the cam pulley's outer section is floating with respect to the inner and you are ready to bend valves...er uh, I mean achieve new heights in diesel RPM and power!
Alternatively, if this all makes your head hurt, print out this description with the photos and take it to your favorite machinist. Explain to him what you want and be prepared to pay him for several hours of shop time. Yet another potential source is to talk to one of those places that makes adjustable sprockets for gassers and ask them if they would modify your diesel cam pulley in the same way. Sorry, Tom is not available.
Sorry this is so rushed, but I have to dash to the airport. By the time I get home I trust that some of the 40-pound brains here will have spotted and corrected the errors I no-doubt made.
Topic: DIY Adjustable Cam Pulley (Read 5868 times)
