In regards to pack and the effect of hole size and oil weight in the shock, there is lots of bad info on the internet and no clear explanation as to what the difference truly is other than to say combo X has "more pack" while combo Y has "less pack. Here it is in layman's terms and keep in mind that this may go against what is stated somewhere else online.

Pack is based on the speed of the shock oil passing through the holes in the pistons. A smaller amount of hole area will lead to more pack and a larger amount of area will lead to less. There are some places online that get this backwards.

Let's say we has a shock that has 2 holes in it and it has 40 weight oil in it. Now lets say we have another shock just like it that has 1 hole in the piston but 1 hole but 40 weight oil in it. Lets assume the holes in the piston are all the same size. Keep in mind this is an example only and the numbers chosen are for illustrative purposes only so don't get overly caught up in if this is completely accurate or now. It's the mental image I want to convey.

We would logically think that twice the oil thickness going through half the area would equal the same amount of resistance. However the shock with the single hole and the thinner oil will pack faster. It all has to do with the hole area. The movement of fluid through this smaller area has to happen twice as fast as the shock that has 2 holes. I know the viscosity of the oil is different but it still doesn't change the fact that the oil still has to go through a certain place in a certain amount of time. You'll hit a certain point where the shock needs to move so fast that the flow through this hole ends up being turbulent and can't move fast enough. This will cause the shock to just not keep up with the demand and it will pack. It won't compress as far as necessary in the time demanded of it. This may or may not be a problem though.

The shock with the 2 hole piston and the 40W oil however may be able to keep up just fine. The speed of the fluid through the larger area is slower. At higher shock speeds it will be able to keep up better and will pack less. The difference in hole size will not be made up with the thicker oil at these speeds. The average speed of the oil through the holes in the piston is over a smaller range than that of a smaller hole/area in the piston. It will pack less as velocity increases. What if we used 3000W oil but no piston? We'd only have the resistance of the shaft moving back and forth and would have zero pack. The oil can't make up for it. This is the extreme example of no pack. The larger the total area of the holes in the pistons, the less the fluid needs to "move" as the piston compresses.

There are a couple of ways of looking at pack. Most people will say that on bumpier tracks with smaller jumps you want less pack and smoother tracks with large jumps you want more pack. In general this is true. What about a bumpy track with large jumps? It's a compromise.

Now remember how shock location on the arm and it's angle affects spring rate, it also does the same for dampening. I stated at the beginning that it wasn't uncommon to run a thicker oil up front than in the rear. Using the same pistons it's easy to see why. A local guy here swears by running more pack in the front and again it's easy to see why. If you've got a setup that requires a 3 lb spring in back but a 4.5 lb spring up front to balance due to arm mounting points, you've got 50% more static spring rate up front to equal the same wheel rate. Pack will respond in a very similar way. A higher shock oil may work up front or more pack. They each behave a bit differently on the bumps and I may change back and forth depending on what I need to car to so. I've gone more pack in the back but if the track is rough the car will get unsettled in the rear and won't take the bumps as well at that end. My current setup is using the true balance method with balanced springs, the same shock oil all around and more pack up front but again this changes with the track needs and the difference between them in terms of handling can be quite extreme even though on the bench it all balances.