I never knew this. Well, had I ever been involved with building a Hemi engine I would have known.
Nothing against todays Hemi engines, but I have always thought that the TRUE/REAL Hemi engines were built from 1950-1971. The 426 being the KING OF THE HILL!
And, yes it's true, MOPAR/Chrysler still uses the same hemispherical cylinder head ("hemi-head") design for it's current Hemi engine equipped vehicles. Which I applaud very loudly for.
But, why is it so dirty? You state because of its big bore? You would think that with all of the history of the Hemi the engineers would have figured this out. Or, do they know and and just "go with it?"
The current (modern) HEMI's don't really have hemispherical chambers, they are somewhat closer to pent roof. They had to add quench areas in the chambers because a hemispherical chamber has very poor manners at low engine speeds, resulting in incomplete combustion and really not fun to drive (low on power, poor response...etc).
This is why the "street" HEMI engines, like Ford's BOSS 429, had quench areas added to the sides of the chambers, to improve drivability and give the engine good street manners.
The modern HEMI has always needed "help" in running clean, that's why they have dual ignition (16 spark plugs). because the chamber design just doesn't lend itself, even with quench areas, to clean and complete combustion at lower engine speeds, the dual ignition, which is phased, fires twice in order to try and clean that up. It's actually an extremely old technique used on big bore engines (I've seen 30's vintage stuff with it).
So, the 6.4L, which has a 4.09" bore (the 5.7L has a 3.92" bore) will suffer a bit more from these characteristics. It's also higher compression (10.9:1) and tuned for performance so it will fuel dilute. You put all that together and you have an engine that's going to generate more carbonaceous material than your average small bore high efficiency 4-pot or even a small bore engine like a Modular. The fuel dilution means that some of that is invariably going to end up in the sump.
Of course the HEMI isn't alone in being "dirty". The LSx engine family was hit with the same TSB from AMSOIL for avoiding extended drains with the EaO filters because they can apparently do the same thing. Of course they don't need dual ignition to pass emissions requirements, so I expect they aren't quite as bad.
Whole article on the heads here:
We take a very close look inside the G3 Hemi cylinder head. Get the details only at carcraft.com or the October 2012 issue of Car Craft Magazine.
www.motortrend.com
But some pictures:
The early (non-VCT) 5.7L:
This is the Eagle chamber (modern 5.7L, my wife's truck):
You can see the considerable quench areas on each side, making it less round. The top picture with the head gasket really shows this well.
This is the 6.4L Apache chamber:
More round (more hemispherical), less quench area. Means it is going to be dirtier running at low speeds. It also has absolutely huge valves for a stock engine, 2.14" intake, 1.65" exhaust (as noted in the article).
