5.0L FORD BLOCK SPLITTING-FACT OR FICTION? 

I built and refreshed a .040 over 289 for a guy years ago. Every year his budget got bigger, but he wouldn't have a 302 or 351. Cast crank, side ground and shot peened stock rods with 5/16 ARP rod bolts. Got the compression up past 12.5 -1, roller cam etc. I begged him to keep it under 6500 rpm. Engine lasted 2-1/2 years until the stock oil pump drive broke at around 7800 rpm. Replaced the crank and 2 rods. Back at the track in 6 days. Car still has same block today, 38 years later.

I have been racing small block Fords for almost 40 years .l have had 302 blocks with close to 600 hp and never split a block but back then there was no internet maybe that's why I never split a block.

Probably casting lottery. Imperfections in the castings.

Well.... didn’t we just talk about this? Thanks Richard 💪 keep the RPM low and good harmonics

I’ve been running a 95 roller block for 2 yrs now. It’s on a 200 shot of nitrous, heads cam intake carb set up. It’s at 602 hp, no issues at all. About 120 10lbs bottles thru it, still strong.

I saw a 600hp+ 347 stroker split. The crank was in great shape after the failure and was measured to be over 100g out of balance. With the short deck, I think it is because of high RPM without having an internally balanced rotating assembly.

being a machinist my guess would most likely be an inconsistency in the casting material/block, things like the way its cooled after casting can effect and impuritys in castings, hard spots, air pockets,softer than normal blocks,core shift ect there are many reasons a block could split and it might just be the block at fault as mass production some times cuts a few corners to make production numbers.people used to let blocks bake in the sun once to harden them up not sure it works thou

This guy is awesome been watching all his videos lately. awesome work and so much time gone in to all this

Mickey Thompson ran many different blocks at high RPM. He said changing rpm changes the load on an engine, which adds the most stresses. He said keeping an engine at a constant rpm, even high RPM, the engine will live longer than up and down RPMs. He took 2 mustangs to the salt flats in 1969. His comments on engine's is priceless.

Being one of the old farts around here, and being a small block ford guy, I was always told via magazines that the hydraulic roller blocks could develop cracks in the lifter valley out past 550/600hp and could for short term run 750+. And that early non roller blocks could take more sustained loads and rpm. I've never seen a test showing that problem, I have seen split blocks, but not the circumstances why they were split. what I've never understood is how a 2 bolt main block could keep the mains in it long enough to split the block.. I've always figured it was detonation that killed the block, not the actual hp number. And this probably stems from 1 guy with a bad block on a dyno and a couple turbos not retarding timing, and people perpetuated the myth.

It’s when the stock harmonic balancer comes apart and the motor “ shakes the baby “ until bad stuff happens.

My stock roller block 347 with twin turbos made 700 to the tyre and ran 9.3’s at 150 for years and never split a block, broke plenty of gearbox’s, I always only ran race gas and meth injection at the track so I think it has a lot to do with detonation or bad tune.

WFO for 32 minutes!? Dude you are a hero :-)

Running 13lbs on my explorer p-head block with only aftermarket pistons. The assembly was balanced with the lighter pistons. This video gives me some confidence to upgrade to AFR heads and shoot for the 600 mark!

So cool , Love when myths are explained and sometimes busted , great video learning so much with every video thanks

Detonation and RPM. Another great video Richard!!!

back in 95-98 I ran true street in my 90 coupe 'junkyard refugee' you're old school, friends with jim from mmff so you may remember it. won best sleeper ie ugliest car award in baytown in 96 for its 5 color paint hah. anyways I was running a north texas performance 347 stroker short with tfs twisted heads with a 750 double on it, was using a nos 200 wet kit. Between all the true street events and the dallas/ft worth street racing I probably ran 150 bottles through that motor. It trapped 127, no idea on power we didn't have access to dyno's back then. In 98 I won a texas turbo twin turbo kit for winning an event and swapped over to that with a 1050 dom, I did get on their dyno for that and put down 720 to the wheels through a tremec 5 speed. I ran it for just over a year like that until I wrecked it in a race. The guy I sold the motor to had it split a few months later. As someone mentioned I suspect it had to do with balance, the "rattler" and "fluid damper" were just becoming a thing around then and most people were just slapping old balancers on their cars, not wanting to pony up 350+(in 1998 that was ALOT) for a silly balancer. I believe it was balance and harmonics from high rpm that splits blocks.

It's gotta be the flowmasters.....

That's why your channel is way better than other people trying to do the same or forget to mention why they think stuff don't work and the PROOF... Great info Richard that's why I can't wait to see the max HP and Trq on the 455 Buick on boost 💪🏾💪🏾💪🏾

As a seasoned engine builder, I've seen the 302's hold together very well if tuned properly. Improper timing, incomplete combustions events, or improper parts will destroy an engine. Nitrous kits have come a long way in the last 20 years. Most racers had kits that were just "smacking" the engine with 150hp or 250hp kits. Progressive Nitrous controller was a great addition to the drag racing world. I have actually seen these blocks crack or split completely. Great video

I just can’t wait until one of those Ford “Godzilla” engines Lands in your possession. 😉

I don't think that the "splits at 500" should be taken literally, but rather it's contextually relevant advice for the "AVERAGE" guy building a stock block 5.0l in his garage. Ford is notorious for their thin block castings in their pushrod V8s - no, not because people claim this, but because *Ford themselves* HEAVILY invested in this from the '60s on. And, they did it for reasons why you would want to - to reduce weight per displacement! Unfortunately, as a self proclaimed gearhead/car guy first and a Ford guy second, for us HP junkies, this leaves us with blocks that often can't be bored out as much as competing brands and, in the case of the 5.0l, what I believe is a just plain fragile block. It isn't 'weak', it isn't 'poorly designed', as shown in this video and by many other builders, a thorough detail oriented build can push these engines pretty far with stock iron, but I believe that they are fragile. BUT, the problem is that there just isn't a lot of margin for error with these blocks that you typically get with other engines(including other Ford pushrod V8s; 351s can reach up around 1000, 460s can handle WELL over 1000). And, with so many guys throwing these things together in their garbages, you're more likely to issues arise, and HP/torque only magnifies these issues. With other engines, they'll put up with some of this before failing, while, IMO, the more fragile 5.0l is prone to failure in these instances. And, it just so happens that right around 500HP is where these issues start to become too much for these blocks. It isn't the HP output that's splitting them, but it's issues(mild detonation for instance) AT these HP levels that start splitting them. ...that's my complete and utter conjecture anyway!

I really enjoyed this video and I hope this comment finds you! To answer the question at the end of the video "It just can happen" I am logged in under a personal account but well know in the tuning market for small block fords with foxbody's in general EEC-IV or standalone. The beauty of understanding is from well respected engine builders that breakdown a motor and tell you why it let lose. Detonation is usually a easy give away because it is seen. As we never want it to happen, but honest advice I can say when a block splits or internals just dissolve you will know if tuning is correct or not. Funny, though Turbo combos are very efficient set up correctly and usually more forgiving then a blower combo at the same power level in some cases. When the tune is spot on then it all depends on the engine staying happy from its birth =) all factors considered.

I’m thankful for this channel

I've heard about the small block splitting but never personally witnessed one and often wondered why ,what and how ? Now that I've seen this I have same questions as you ,interesting!!!!

Glad to see all this ford content, building a 347 for my little 84 flareside right now and wanting about 500 hp

Always the most informative videos nice job!

My guess is detonation. I've been curious of your combination for a long time. Awesome! I run a stock block 347 w vortec s trim.

Proper tuning is a big factor. My Incon twin turbo system on a basically stock 306 on 12 psi was 513 at the wheels. Had a Crane 2031 and kept the shifts below 6500. Ran this setup for 6 years and thousands of miles. Then sold the car.....the current owner is still driving it with the same setup.

"The internet isn't always accurate" that should be in the Bible or something. Maybe in the Pledge of Allegiance, or the National Anthem.

That's awesome to hear about you racing!! Thank you for sharing

The time and effort that goes into these videos is insane!

I have always thought, that simply over torquing the intake bolts on this type of configuration would contribute to splitting a block, the intake would act like a huge wedge,since the bolts are vertical . and the intake is a wedge.

5 block 5 splits ,just after the 600 hp mark, 2 where drag cars ,and the others where street cars, the one happen in a road race, it is not the balance, and they where all around the same year blocks late 80s. We think it is a load to take off, and the last block we had on the dyno, did not split but was cracked. So there are even more people in my area that this has happen . It will be interesting to hear your view.

Thank you for putting that information out there. Great video! I've always thought that drag race environment is the hardest on vehicle's and equipment.

Great video as always this gives me confidence with my turbo 5.0l plan to keep it under 6200rpm and eventually 10lbs of boost🤞🏿🤞🏿

I think it's torque and tuning has a big part of it. My buddy has one with 650 horsepower stock block and he launches that car at 5,000 RPMs with slicks and it won't die. He does have AR fasteners in the bottom end and the head studs. You should do more ford videos

Consider the intense angular momentum of a three foot long 70 lb crankshaft spinning at 6-7000 rpm. Then change its rotational axis relative to where it started spinning, say from a launch that causes the front end of the car its in to pitch up, or a sudden right or left turn. The gyroscopic forces are immense. Also, i always heard that the early S.B. fords did this, they tended to crack though, through the crank and cam journal at the front of the block.

Nice Video, I've been telling people all the time that I'm making over 600 to the ground at 16 lbs of boost on a stock bottom end 302 with trick flow top end kit. And my engine is a 95 none forged pistons.

I just recently watched a roots blown 351W split right down the middle! And it was still idling, although rough, after the split. The mounts and the accessories held it together!! But once unbolted it was absolutely two inline 4 cylinders with a common crank!!

Back 20+ years ago, my 100,000 mile stock block 302 h/c/i single turbo fox body was probably making 600 hp or so, judging by 110 mph in the 1/8 @3300 lbs. This was with old-school backroads tuning too, no idea what was really going on like with a wideband o2. It never went over 6k rpm, maybe that helped it live. Many people say turbos are easier on motors too. I know if I had to do it all over again, it would still be turbo and low-ish rpms for the theory of making a stock block live.

Since the block cracked during street testing, it was probably a fuel octane issue that caused detonation which transferred those harmonics to the crank and cracked the block.

Afternoon Richard 😎 . The only block I ever truly split was the stock Buick 350 in my 71' Skylark. I was out in the desert Flats east of Boise hammer down in 3rd when I bumped the ratchet shifter cresting a rise a little faster than expected and it slipped into 2nd and the VDO tach hit 7k and it scattered to Four Points of the compass. I got it towed home and got a better look at the damage and for all intents and purposes there wasn't much left of the short block. There were a couple large holes in the oil pan. It pretty much split the Block in half at the center line from One bank to the other. The passenger side of the short block was completely gone as were most of the cam journals and the only pieces of the rotating assembly that were in one piece was the crank and 2 rods & pistons. The rest of it is out there in the sagebrush somewhere. Granted it really sucked blowing that engine but accidents happen and that's the way the car was when I bought it. Luckily I had already started building a nasty lil' 30 over Buick 350 to drop in it, it just got finished a lot sooner than expected 😋 ...

I've seen one split due to crank breakage back in the day on a heavily nitroused high compression 302 with super heavy antiquated TRW domed pistons in it. I remember the crank was broken in 3 pieces and there were rods and pistons still attached to the pieces of the crankshaft. The middle piece still had the complete thrust bearing, cap, journal and part of the cam tunnel attached. I think the casting lottery theory applied to both the block and the crank. The car was an automatic equipped, full weight 65 mustang that went like 10.20s in the quarter with a 302 with dart iron Windsor heads and a shit ton of spray. I dont know exactly how many runs it had on it but It had quite a few and was street driven occasionally.

When I have done some failure analysis on damage it is normally a combination of things and not straight one thing because it will be the obvious "normally", but let's put aside that balancing is good either internal or externally balance, in internet seems that the fracture starts at the mains, and if I see the pictures this is at the threads which is a logical part because it is a stress raiser >

My little 302 which is almost a fully stock bottom end and it’s around 600 on the engine , key is tune and rpm

Great video. Love all the info. Best channel ever. Richard you should do your own version of MYTH BUSTERS for engines!

Thank you Richard great info 👍🏻

I was under the impression it was walking a fine line with rpm, and the 50 oz. imbalance crank. The cracks start at the main journal, work it's way up to the cam bearing journals.. Some break in the 4xx's. Some take a beating well beyond that. (If I'm starting with nothing) Comparing to LS motors, I can't see spending $2800.00 just for an aftermarket block to be on par.

I was always told it was high rpm. Like what happens in a missed shift. Never heard of an auto 5.0 splitting so that would be my guess. But then again I had a friend with a solid roller 347 and he would spin that thing 8k every pass and he never had a problem and it was a stick car that made 500ish wheel na. I think it has more to do with if that crank starts walking around and proper balanced bottom end.

To get a good starting point you need to NDT a stock block to ensure no hidden failure points lurking under the surface that wouldn't matter in a stocker but bound to surface when you step on it...same as you would with used rods if you want to reuse them intending to beat it. Ford uses good content of nodular iron in everyday blocks and I guess a little better in export parts. Would be interesting to do fracture mechanics analysis in the leftover corpse.

Think it's the tune. Never broke a block. Cracked pistons and blew both head gaskets when I first tried nitrous. Too much timing and not enough fuel! Learned a valuable lesson. Talk to people who already learned the hard way!! Later we were shootin 4-5 passes a night 3-4 nights a week. Laughing at us for shooting 150. Didn't laugh when they picking peices of their stuff off the track. "Nitrous doesn't kill motors,we do." Doc. from street outlaws!!

From what I have seen and heard from the real world experience around these engines. It seems to me the magic split is likely due to lean out detonation, (or a similar shock event) combined with a weaker webbing cast. When you catch the cracking early you can see it starts from the main bearings and walks up to the cam. I have heard of guys doing sonic testing on the webbing and being able to pick out a block that will fail at the 600 number. But that may just be myth.

Harmonic's going through the crankshaft vibrate to destroy the crank and that's what splits block

Stock roller block 347, 260ish 670 lift mechanical roller, ported Canfields, 200 shot..... Ran mid 9s in a small tire Ranger. Engine lasted about a year before the factory block cracked from the mains up to the cam tunnel. Didn't hurt anything else. We put all the parts in a new FMS Boss block and its been good for many yrs now.

I’m a diehard Ford guy. I watch your and other videos to learn. My gut says a casting flaw or balance issue.. but again for me if I were building for hp I don’t want to worry about failures so I’d start with a Dart 4 bolt main block..

I'd have to know where it cracked before I could speculate on the cause.

I literally asked him to do this yesterday and what do i see today? A test! Thanks richard! Now if you could only find me a real parker funnelweb lol.

Love that shirt. We should all be thankful. I've always heard that the real block splitting HP number is 600. Of course alot of guys run girdles and better aftermarket harmonic balancers and stretch that number out a bit.

Based on internet research, and not my own testing, 3 primary points relating to longevity, and some smaller points. - Rpm, keep it lower than 6k - Harmonic balance. The closer to zero balance the better - Tuning, a safe tune as you approach high rpm is crucial. Too much timing and a sketchy A/F ratio can be the formula to a kaboom. Other things like manufacturing tolerances, wear on the block (age/ mileage), cheap valvetrain parts, e.t.c. That said, I believe the consensus is 500whp though, not crank hp. Good info all around. I've dyno'd at 570/540 on a reman Cobra motor with a safe tune. We took 2 degrees out and it's currently running at 540/520. I'll run it this season and find out how it holds. Btw, it's EFI, controlled by Holley's Terminator X.

I think it’s a shock load issue. 4-5000 RPM clutch dumps at 400 WHP puts quite a strain on the engine. The 302 Windsor is a thin wall casting to begin with. When they made the Hi-Po and Boss they made the blocks thicker (along with 4 bolt mains). On a drag strip it’s hard on an engine to launch and then go from torque to HP peak 3-4 times in the span on 12 seconds or less.

Rpm is the main cause of cracked stock blocks as well as being improperly balanced.

Our shop saw two 289 / 302 splits. First one was an 89 5.0 H.O. stock bottom end that got a set of SVO N-351 Sportsman heads and a .600+ lift roller cam. It was in a NA alcohol Hilborn injected sprint car that the owner had run out his engine budget for the year blowing up his two 351C's bottom ends and couldn't afford another 700 HP 351C build; but needed to run the remainder of the season with a new engine for under $2k. The H.O. made 460+ HP on the dyno, but the block split completely down the center while running warm up laps at the track on it's first race. Only the motor mount plates and the main caps were holding the halves together. A year later, a late 289 block in a street car cracked through the valley at just over 400 HP at 7000 RPM NA. I think it is the RPM - combined with a less than perfect casting stock production 8.200" SBF block - seems not up to the stress of the high RPM to make over 400 HP NA. Yet I know of many stock block 8.200: SBF's making up to 650 HP supercharged (not turbo) at up to 5500 RPM with no issues.

One of turbo John’s friends has the stock bottom end 1/8 mile record and he says 700 hp is the magic number. You can get it to live if the tuning is spot on, but he doesn’t even spray his to 700. But then you Mike Murillo. That dude pioneered high horsepower 5.0’s back in the day and dominated. I watched him make his first 7 second passes in the Texas car like 20 years ago, and he ran several in one night, then drove it onto the trailer and left in one piece. I personally think it’s the shock load of dynamic compression that does it. The guys that sprayer 100% of the line broke blocks. The ones that used progressive controllers could get 3 or 4 seasons out of a block before needing a refresh. This is just what I saw and experienced though.

Im loving those techno intros. It’s like sex, but with turbos and superchargers

High torque at low to mid rpm. Like big hits of nitrous at lower rpm.

All I had to do was put a B-303 in my Mustang and it broke the CAR in half. :| Great video Richard, I'm blaming the internet.

A good balance job and quality rotating assembly has never let me down... ALWAYS replace the stock fasteners in the bottom end.

I think its imbalance, or detonation that causes the main caps to walk, but I'm no expert

This is a 351W that split in two. I paused the video to examine it. Notice that there really isn't much material thickness above the crank saddle. Once you go beyond the crank saddles look there is really nothing reinforcing the block. So if one saddle cracks there isn't much there to prevent the others from cracking. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-MJLS9xFXYAQ.html

Great video. Could you test a remote turbo kit to see how much of an effect exhaust length and charge pipe length has on a motor. Also it would be neat to move a air to water cooler from close to the motor then back near the turbo to see if that effects the density of the air ect. Thanks again!

I have a 302 with edelbrock heads and intake RPM & bigger valves Paxton supercharger 71/2 lbs boost I believe your numbers are spot on thank you

Fords aren't the only ones. I work at the remanufacturer that does GM'S stuff. In the early 90's I uncrated a 403 Olds core that was split just like that. I don't know what the magic number is, but I think the weak point is the oil gallies between the mains and the cam. Fords are pretty big, so were the Olds. In a sbc they're small comparatively. This is in an area where the pistons and rods are trying out tear the block in half.A side oiler is the answer in my opinion.

I saw this happen once with a buddy that raced 302 is in a Mustang he had a big nitrous shot on it 300 I think it running really bad after the last pass the next weekend he pulled the motor out when he took the intake off it was cracked straight down through the lifter Valley I always called the cracking of a 302 b******* but that was the only time I ever saw it

Maybe it’s just because I am a Ford fan or that I have a Fox but I a super impressed with all of these numbers. Not sure how much more you actually expect out of these small engines.

I believe it is a combination of everything together to make a perfect storm...From what I have seen most people just throw parts at the stock block without prepping the block.

I've seen one split at 520 to the tire..blown.. 23 lbs boost. I came up qhen the 5.0's were coming out.. I lived that life. Ive had all kinds of hotrods.. Honestly..Ford just holds up better to me. Hot rod or daily driver work truck. They all have issues.. chevy and dodge is the transmisions.. rearends.. ford is.. well ..a loose nut behind the wheel..😎☕🤣

I have always thought that the way the intake manifold bolts down on the SBF acts as a wedge on the blocks. The high power combinations exploit the inherent problem when combined with engine mount and chassis flexing. Just my 2 cents 🤷🏼‍♂️

A lot of great comments with experience here, in which I am not as educated in the engineering and mechanics as they are, but I do have common sense. I thought the testing of the engines in the video was very educational. The 5.0 is a strong engine and can handle a lot of HP and Torque no doubt. A couple of my questions were answered toward the end of the vid where you stated they ran on a "manual transmission" (stick) yet also blew out a few transmissions as well. When pushing 600+ HP and 580 lbs of torque, the transmission will always be a factor. In fact, I haven't seen very many 2015 and up S550's with an automatic that can handle that kind of power, unless it is built to do so. More factors.... Drive-shaft! I've seen many drive-shafts fail due to the power ratio that it makes me sick, yet if you have a fully built, strong D.S. and reinforced U-Joints, that would help. Couple of other factors I question, which probably needs more testing to confirm…. Rear-ends, axles and rear suspension. Are the test cars running solid, straight axle rear ends or independent suspensions? What gearing was used in the rear-ends, 3.55’s which are stock today, 3.73’s as an upgraded Performance Package or all-out monster 4.10’s? How did the rear-ends hold up too? I’ve seen many rear-ends blow out with only 400 HP, but a 9-Bolt Posi-traction was the answer. Yeah, that is old school, but all of this is a factor when throwing down high end HP/Torque to the pavement. Lastly, what about “frame-twist”? This is also a factor and I’ve seen frame twist under power blow out the windshield on several occasions. Is that why “Pro-stock” and above competition cars use polymer windshields? I’m not sure and like I said, not an expert, but learning. Sorry for all the questions.

The factory (Helms, I guess) shop manual has a special section on what to do with a porous block or block with "Sand Holes". The outside areas acceptable to cover with a Ford epoxy goo is-- oil pan up to bottom of water jackets, lower rear face of block. And return it to service.. Fresh from the factory with marginal block castings. But Ive heard people were knocking the main bearing webs out at the strip.

There would be so many factors that make them split. Torque, shock loading, core shift, age etc etc.

I was wondering about this, i think it's mostly people hating on ford products and talking out of their ass. Ford power baby :D

I have seen one split first hand, one of my close friends and I were at the drag strip and he split the block. The back story is it was a stock 302 block, was stroke to a 406 I believe, twisted wedge heads, don't remember the compression and also had a 200 shot of gas. Done burn out fine, life is good, pulls up to the line for 3500 clutch drop and boom it just shut off after the clutch drop and slamming the gas. Nothing came out of the block and acted like to locked and wouldn't turn over. We load it up, go home to tear it apart, then we saw the split dead center of the block right above the camshaft from the timing cover to bell housing bolts and the rotating assembly looked visibly fine but it was not reused. I suspect it was the shock to the crankshaft that caused some bad harmonics. I also have heard this block splitting rumor about SBC with stock block at some HP number as well. Hope it helps, just my thoughts.

"Splits right in half and throws itself to the ground"🤣😂 And definitely driveline shock, but I always thought some girdling would help Also heard that older blocks had more nickel but I do know the "Mexican Block" has a thicker casting. I got the chance to do a side by side block comparison once and the difference was visible to the naked eye

A massive shock of torque at a low RPM. Mainly with my experience, nitrous!!!! Also out of balance engine and heavy internal parts!!!

I split one down the lifter valley . NO ANTIFREEZE 😜

This is interesting, I have heard the same thing about the 109 block in a Grand National or Turbo Regal in MY 86/87. My engine builder told me that if I wanted to make 600whp that I needed to go to a Buick Motor Sports stage II block or a TA Performance aftermarket block. There have been several guys that have made over a 1000whp with the stock 109 block, with forged internals. Not to mention that one guy ran the stock short block - stock crank, mains and pistons and he made over 10 minutes worth of 9 sec passes and when he took the engine out to check it out there was a main cap broke. So IMO, I think an excellent tune goes a long way. The guy had an aftermarket ECU, where he could fine tune the fuel map and spark. He is still running a stock block, not sure what he has done to the block though. I watched him go from 80 to 130 in about 4 seconds. Pretty good. Remember this is a V6 - 3.8L block, with a turbo on it. The block only has 4 main caps to distribute the load, not 5 like a v8 or 7 like an inline 6.

Great shout out to the silver state classic Richard

Search "302 split block" and take a look at nitro unions vid. I can be done but im not worried about my dads 289 leaving us anytime soon!.

Engine Harmonics like bad damper, or poor engine assembly like not balancing/ Checking the weight on rods and pistons before installing them together. Also bad tuning

Had a 375hp 306 crack in two of the main webs that support the crank. Both cracks were from the cam bearing journal to the main bearing journal of the crank. - I was running Flowmasters too!

Personally, I think that the weak point is the threaded holes for the spyder in a line along with the drain back holes in the lifter valley also lined up combined with sharp corners and a lack of fillet material giving a clear failure path. Deburring the holes would possibly help. The failure mechanism could be detonation, imbalance or any other anomaly looking for a relief path. I'm wondering if the failure rate also extends to the older flat tappet blocks?

Metallurgy & casting tolerances. Core shift that occurs during casting process that makes the valley thickness thinner. Would be interesting to sonic test that area in a cracked block vs one of Richard's blocks that's been wailed on hard & see if there's any difference. Then again, it could all be some fake Chevy fanboy conspiracy 😆

Personally I blame squirrels

I've saw it done once. 306 with afr heads good pistons & rods.I'm not sure of the rest of the setup. But had a 300 shot on it. Had a decent number passes on it before it split. Ran low 6s in the 1/8th....as you said it just cracked the block length wise. Didn't fall apart or anything lol

When using stock mass production blocks you have to take casting anomalies and voids in the cast-iron into account. Perfectly balanced rotating assemblies and main bearing girdles help a lot to mitigate vibration and harmonic frequency but you are still dealing with the stock thin wall casting which has it's limitations. But hey Windsor blocks are cheap and available so run them till they blow and build another one.

My 306 made 520 rwhp. Never a issue.

I split 5 roller blocks making 520 N/A. I believe it was rpm as I shifted the engine at 8200rpm. The blocks had DSS main girdles and ATI balancer.

I would say it's mostly rpm and balance related. I remember years ago doing connecting rod stress calculations. Throughout the operating range rpm had the most dominant effect, even with turbo boost. In fact the highest rod tension was at highest rpm and lowest boost. So a combination of rpm and imbalance, (or also crank straightness) are dominant in material stress. Boost is probably dominant for thermal stress.

I think it’s weakness of the valley webbing or lack thereof, combined with casting flaw at the foundry. And yes, saw this from pictures that were shared...before digital cameras were mainstream.