I've had 2 sets of big block, 3 sets of small block, Indy SR and -1s, and MP products (Stage VIs). I've also been privy to builds using TFS, Brodix, Pro Action, and Dart on various "offbrands". I'd say overall, the best castings are factory ones. MP is not factory. On a scale of 1-10...
Edelbrock rarely has casting flaws, but they do occur. Their valve jobs are average, the exh guides typically are too tight on a few on each head. The valve springs are "rated to .600 lift" but dont have the open pressures most modern cams need. I've also seen a Pontiac set get shipped with no inner spring on 2 cylinders. The chamber is smaller, but like Chrco says, isnt a "high swirl or tumble inducing design. It's just smaller, with the plug near the exh valve. These are meant as an option other than factory castings, so maybe that's why. I havent seen them yet, but the new BB Victors have a better chamber, can flow lots more air, and come with no valve springs, so I'd say Edelbrock is listening to the shops' feedback in that regard. Edelbrock - 8 out of 10
Indy has had various line changes since the first MW port -1s were released. They have a design for almost everything size wise. But, they suffer from the same issues the others have. typically, Indy seats are out of round. Not all, but most. The same guide issues, and quality control issues are there. Any machine work they do should be scrutinized closely. I've also found aluminum chips over 1/4" in size lodged in the assembly oil inside the port of "ready to run" heads. And yes..the intake port, it would have been ingested if I hadnt taken them apart and seen it. The chambers are more modern and smaller, and have the same plug near the exh valve. Indy is very difficult to talk to unless you are paying them large amounts, and letting them design and do everything. (ego brat mindset) But the heads are a decent bigger alternative. Indy - 7 ot of 10
MP heads have a ton of engineering behind them. Modern chambers, correctly shapoed and sized ports, and in some cases, a good supply of support parts if needed. Teh castings are some of the worst you'll find. Porosity, misaligned cores, lots of uglies. The W5 heads were a huge mess because of that, and the early Stage VIs too. The blocks get no better. Apparently they cant get a good foundry. Teh machining on them is equally miserable. Chamber sizes are way off most of the time, as are some of the rocker shaft support heights. The quality of valve work is about on par with Indy. A set of iron Stage Vs I've seen just cant run the numbers, and I think the shop didnt correct everything on them. A set of Stage VI aluminums I know of took an extra $600 in work to correct and equalize the chambers, and then fix everthing else. Pretty bad for a head that could be very good. MP - 5 out of 10, but working on it from what I hear. (of course, those waiting years for a block may disagree)
Mopars are not alone. all aftermarket stuff needs work before bolting on. "Ready to run" or "Just bolt them on" means they will attach to the block. That's it. Most heads should need work. I know many people who run things out of the box. Some are impressed with the results, but I'm sure they could be running better if the stuff was checked first. Not all problems result in damage or a non running condition. All problems lead to shorter lifespan, and less power production. I havent seen really good valve tip patterns out of the box with much. But, in about 70% of the cases I've been able to correct things with a pushrod or rocker adjustment. Many cheaper lines of rocker have problems with QC, and you will get them longer and shorter. I have seen a couple of heads that did need the rocker shaft supports modified to get good geometry because of poor tip alignment accross the whole head.
