408 Magnum vs LA

[chargd72]

I know there are some key differences when building a magnum vs LA block but can someone tell me why there are different stroker kits?  I thought rotating assemblies would be the same.  There is a nice price difference between the two kits as well.

[chargd72]

Are block heights different?

[Cooter]

Difference in rear main seals, difference in how the cranks are balanced. (Yes, an LA and Magnum ARE balanced differently). Etc; etc.

[chargd72]

Difference in rear main seals, difference in how the cranks are balanced. (Yes, an LA and Magnum ARE balanced differently). Etc; etc.

That's right.  So you couldn't have a machine shop balance an LA assembly for a Magnum block?

[Cooter]

Difference in rear main seals, difference in how the cranks are balanced. (Yes, an LA and Magnum ARE balanced differently). Etc; etc.

That's right.  So you couldn't have a machine shop balance an LA assembly for a Magnum block?

Sure, it's called an aftermarket stroker kit. How you gonna put a TWO PIECE real main seal crank in a ONE PIECE block??

[chargd72]

I'm looking at the Hughes site and they have kits that are priced differently for Magnum and LA blocks but on their 426 kit is says works for Magnum and LA.  Weird...

[Troy]

That 426 is likely internally balanced (haven't looked at it in a while). Any externally balanced kit will be based on the original engine's balancing so you can use the balancer and flywheel/flex plate that was already in the car. The Magnum cranks - while physically the same - are drilled differently then the LA cranks due to the lighter pistons. However, I don't see how this makes any difference when buying a whole kit. The Magnum pistons are shaped differently on top due to the heads and have thinner rings (not sure if this is still true on the stroker pistons but I don't see why not). Perhaps that's it? There is a difference in rod ends but, again, with a whole new pile of parts that doesn't matter (it might in pricing though). The LA assembly is slightly taller which also calls for different lits/prices. I'd bet the 426 expects the block and heads to be precisely machined. There's also a difference in oiling between the blocks but that affects pushrods and rockers if I'm not mistaken. The rear main is dependent on the block itself - not on the stroker components. As far as I know, all Magnums (5.2 and 5.9) use the old LA oil pan. There is a one piece seal for the Magnums.

In my opinion, I'd rather have the Magnum since I like the heads better AND the large majority of them came with a roller cam/lifters. It's inexpensive to regrind a factory Magnum cam while putting a roller cam into an LA engine will cost $$$.

Troy

[chargd72]

That 426 is likely internally balanced (haven't looked at it in a while). Any externally balanced kit will be based on the original engine's balancing so you can use the balancer and flywheel/flex plate that was already in the car. The Magnum cranks - while physically the same - are drilled differently then the LA cranks due to the lighter pistons. However, I don't see how this makes any difference when buying a whole kit. The Magnum pistons are shaped differently on top due to the heads and have thinner rings (not sure if this is still true on the stroker pistons but I don't see why not). Perhaps that's it? There is a difference in rod ends but, again, with a whole new pile of parts that doesn't matter (it might in pricing though). The LA assembly is slightly taller which also calls for different lits/prices. I'd bet the 426 expects the block and heads to be precisely machined. There's also a difference in oiling between the blocks but that affects pushrods and rockers if I'm not mistaken. The rear main is dependent on the block itself - not on the stroker components. As far as I know, all Magnums (5.2 and 5.9) use the old LA oil pan. There is a one piece seal for the Magnums.

In my opinion, I'd rather have the Magnum since I like the heads better AND the large majority of them came with a roller cam/lifters. It's inexpensive to regrind a factory Magnum cam while putting a roller cam into an LA engine will cost $$$.

Troy

Thanks Troy.  I was aware of most of these differences you mentioned minus the piston heads shape being different.  That's why I mostly thought the kits would be interchangable.  I already bought a magnum block for the roller lifters.  But regrinding the stock cam never entered my mind.  Is that really cheaper than just buying a repro with the grind I want?  I also just bought a pair of Eddy RPM alum heads.  

[Troy]

Regrinding a cam is about half the price usually. However, and this is important, you are limited in the lobe shape and amount of lift because you're working with an existing cam. They can only "tweak" the shape - not make it totally different. If you want to go all out you'll have to buy a new one. If you're going with a carb you have a lot more choices (and can go a lot bigger) because the EFI requires a strong vacuum signal. I forget the names of most of the places that can do it but you can always check Hughes for pricing and specs. You'll definitely want a cam to take advantage of those heads. Did they come with springs and retainers? That's where the Mopar dealer screws you! Remember that if you're using a carb and mechanical fuel pump you'll need an LA timing cover, pulleys, and a brackets to mount a factory style pump. Since the stock cam doesn't have a fuel pump eccentric you either need to buy a bolt on one from Hughes OR buy a new cam that has the extension. If you end up buying a new cam anyway then solve both problems in one shot.

There are some low budget approaches here that don't require all new parts. You can keep the original cam and add 1.6 rockers. This makes the engine feel like it has a bigger cam without the expense of a cam. You can regrind the cam and add 1.6 rockers for even more performance. At some point the PCM chokes (can't handle out of spec data) but using a carb gets you around this. The 385 HP crate engine was basically a stock truck block with a better cam, intake, and a few minor tweaks. The bottom end wasn't touched. Magnums respond very well to just intake and exhaust system upgrades (headers and intake typically net 40-60 HP). Since you've got the heads I'd concentrate on the top end to make the most of it.

Troy