Watch “ David Vizard “ on youtube he’s an 80 yr old top engine builder. He explains Ford blocks better than anyone i’ve ever seen. He actually did do the dyno tests. He claims the blocks split due to harmonics , He started using main cap support from DSS racing and a lifter valley support. They have been testing over 1000 hp for 10 years and not 1 block has cracked split or failed. i took his advice and i’m running both supports on my 408wI don't think anyone has really put them to the test, nor would any test really be that valid... grab any two blocks out of the junk yard, and one might survive 600hp at 7500rpm, the other might split at 450hp at 6000rpm... it's kinda impossible to test. Of course, this works to their advantage in regards to advertizing, because they can say "We installed these and made 800hp on 10 runs on a dyno", but do they ever say "when we took them off the block spit the next dyno run"... I've never seen that test, but that would probably be the most valid. Most people here don't think they are worth their weight in scrap iron because just looking at the construction and the means of attaching it to the block, there's not really any way it adds any significant strength compared to the amount of iron that is already holding the block together, and also the intake manifold would appear to have at least 3 times the strength benefit that the valley girdles would (three times as many bolts holding it, same location of support).