Serpentine or V-Belt?
Either solution will work, and this decision will largely depend upon what parts you already have. The most important thing to keep in mind is that serpentine and V-belt setups use different timing covers and water pumps. The serpentine setup uses the stock Magnum timing cover, but you will need the LA timing cover and water pump for V-belt use.
Chrysler V-belt setups can be tricky because each accessory configuration uses different pullies. For example, vehicles with AC and/or power steering have different crank and alternator pulleys, in addition to power steering/AC pulleys. If you have the brackets and pulleys from your existing setup, you can use those as long as you keep the same accessory configuration.
On the Magnum side, some of the later-model engines came with the crankshaft pulley integrated into the harmonic balancer. This would have to be replaced with an earlier non-integrated balancer to swap to V-belt.
*NEW* Pro Products has finally started offering their harmonic balancers with the correct counterweight for Magnum Engines! This is big news as the factory non-integrated balancers are getting harder to come by, and are expensive. Pro Products offers them in Street and SFI Race versions.
As stated elsewhere, you need to use a balancer specifically for the Magnum engine. DO NOT use a balancer for an LA 360.
Important! Gotcha Warnings
On our shop car, we decided to make the engine look as LA as possible, which necessitated a V-belt arrangement. Unfortunately, the original 318 was a power steering/AC setup, and we wanted to delete all accessories except the alternator and water pump. This forced us to scrap all of the original pulleys and bracketry and hunt for the correct parts at swap meets.
During this process, we made every possible mistake. The good news is that our experiences are detailed below to save you the trouble!
Water Pump Pulley Sizes
Mopar made two different single-groove water pump pulleys. These pulleys are radically different in diameter. The larger pulley spins the water pump much more slowly, and for us, resulted in cooling issues. The larger pulley will free up a little horsepower, but didn’t work well for us on the street. Replacing it with the smaller pulley addressed this problem.
There’s an unusual issue to watch out for with regard to the V-belt style water pumps. When using a parts store pump, you have two choices — regular or heavy-duty cooling. Turns out, the heavy duty cooling pump moves less water per RPM than the standard pump! This is due to the pulley ratios on the OEM setups with heavy duty cooling, and can cause overheating issues for the unwary (like us). For all setups, we recommend the standard duty water pumps.
Using stock pulley ratios can mean problems for alternators at sustained high RPM use. The factory crank pulley is HUGE. It will spin the alternator approximately 12,000 RPM at 6,000 engine RPM. We can say from experience that parts store alternators do not appreciate this kind of abuse, and have a couple of destroyed units to prove it. We opted to use a smaller underdrive crank pulley. It lowered the alternator RPM enough to solve the problem. The smaller crankshaft pulley can generate cooling issues if your system isn’t up to par, so make sure you follow the tips above.