Block Splitting - Rhyme Or Reason

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by jozsefsz, Aug 23, 2013.


  1. jozsefsz

    jozsefsz Active Member

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    Hey guys,

    I see lots of good examples of split 5.0 blocks from some unexplained phenomenon at some undetermined and apparently variable level of horsepower. I'm probably pretty close to the informal 500 crank-hp "limit" myself so the backup plan is probably a Dart block. Some say it's RPM-related, some boost-related, some think certain castings are more prone than others.

    The only thing I've noticed (analyzing every picture I can find online, I've not seen one in-person) that seems fairly consistent is that the cracking is universally in the lifter valley, in some cases (like the sticky on this forum) resulting in a cam that cracked in half -- or, resulting from a cam that cracked in half. I've seen very few cases where the cracking seems to originate at the crank, and girdles are reported to have almost no effect.

    So my question is... for those who've managed to split their block, what kind of cam / springs were you running? Understanding I have a very limited sampling-base available, your feedback would be helpful. My hypothesis is that heavy springs or above-stock lift / rocker ratio is the best indicator of potential block failure and NOT horsepower or RPM. Horsepower is correlated and not a causal factor (i.e. those making 500 horsepower probably run an above-stock cam).

    Hoping to put some predictive structure and simple engineering analysis to this fairly common problem by posting this question broadly with the goal of providing more than just loose guidance for the blocks' limits and trying to understand the most destructive forces by analyzing existing failures (as I have neither time nor resources to properly test the hypothesis). Thanks!
    #1
  2. NIKwoaC

    NIKwoaC 中國製造

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    I would be highly inclined to believe that the forces applied to the crankshaft are nearly exponentially higher than the valvesprings on the cam. Think about how much force is required to change the direction of a piston and rod assembly at speed, compared to changing the direction of a valve. If you talk to Woody over on the Corral, failure goes like this- you make a bunch of power, the main caps start to walk, crack starts in the main cap bolt holes and runs to the cam bores.

    I think the variance in hp level that these blocks are breaking at has MUCH more to do with the obvious stuff- machining, balancing, what components are used for the moving parts, and the quality of assembly. Heavy pistons on a long stroke that's been balanced poorly and then with poorly torqued main cap bolts is probably going to fail before a better built engine... Probably.

    But this is all stuff that's been talked about over and over again before. The production 302 block is just frail by today's standards. Buy a better block.
    #2
  3. 91TwighlightGT

    91TwighlightGT Active Member

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    MFE busted his stock block with GT40 stuff. IMO, the block issues are mostly RPM related. Go to corner-carvers.com and search, there is a big thread about it.
    #3
  4. HotFox

    HotFox Active Member

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    Mine not broked!o_O

    Was on Woodward just last week 7600rpm on the street. Been together going on 3 years:shrug:.
    #4
  5. 91TwighlightGT

    91TwighlightGT Active Member

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    I should have said sustained RPM with regard to block failure. Not that 7600 RPM isn't a stress...
    #5
  6. 15austin

    15austin New Member

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    How about stock rev limiter (6250) still in place and 500hp?Ive seen the lifter valley braces, how much would that help? Im about ready to go supercharged on a already 340rwhp n/a motor no plans to disable the stock rev limiter. maybe ill put the lifter valley brace in??
    There has to be a weak point somewhere if they all split down middle
    I doubt its the cam, it prolly breaks after the block splits.
    #6
  7. revhead347

    revhead347 I have face herpes.

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    I think it's a crapshoot with a stock block. Some last forever, some crack with basic parts. My last 347 last 10 years and 85,000 miles with 490/635 to the wheels. It finally went when the thrust bearing wore out. It never even really had a stress related crack. I got real lucky. I think RPM is the big killer.

    Kurt
    #7
  8. jozsefsz

    jozsefsz Active Member

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    Thanks guys. The examples I've seen posted show there are many different failure-modes and theories about them (high-rpm, high compression / forced induction, imbalance-related, stroker weaknesses) which could potentially all fail in different ways and which muddies up the conversation. They all seem to be lumped into general "block splitting" and probably shouldn't be -- many appear to be ordinary mechanical failures where the block was the victim and not the aggressor. No doubt the stock 302 isn't as strong as it could be, and that many failures occur on the bottom-end.

    I'm really just hoping to gather data on valvetrain configurations of those who have split the valley to see if there's any possible correlation. It's evidently the weakest part of the casting, and my curiosity was spawned by the sticky showing the cracked cam along with the split block.
    #8
  9. 84Ttop

    84Ttop They make new pistons every day, so why worry? SN Certified Technician Mod Dude

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    Most of the split blocks that I have seen seem to originate from the mains and travel up through the blocks. I have seen several with broken mains cracked into the cam journal and nothing up in the lifter valley. Like the others have said, I do not believe it is a valve train problem but more cap walk and stress throughout the bottom end.
    #9
  10. revhead347

    revhead347 I have face herpes.

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    +1 on the above post. Most cracks originate at one of the main cap bolts and travel upwards. They generally crack because the 2 bolt main caps are weak and move around too much. More power along with more rpm cause them to move more.

    Kurt
    #10
  11. Rick 91GT

    Rick 91GT SN Certified Technician Site Sponsor

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    +2

    The crank walks first and then all hell breaks loose

    Sent from my XT907 using Tapatalk 2
    #11
  12. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    so, how does one prevent crank walk?
    #12
  13. Rick 91GT

    Rick 91GT SN Certified Technician Site Sponsor

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    DART block..

    What I'd do to a stock block..

    Good machining on the align hone, good cap register (how they snap in), quality studs and I do believe a main girdle helps....

    Sent from my XT907 using Tapatalk 2
    Last edited: Aug 24, 2013
    #13
  14. Bullitt347

    Bullitt347 man bewbs please...

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    I would think that if you had a Dart block you would not need a main girdle...........:shrug:
    #14
    madspeed likes this.
  15. Rick 91GT

    Rick 91GT SN Certified Technician Site Sponsor

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    I'll modify above...the second was a stock block

    Sent from my XT907 using Tapatalk 2
    #15
  16. GroverDill

    GroverDill GoldMember

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    Some hack built mine
    [​IMG]


    hi Rick :)
    #16
  17. jozsefsz

    jozsefsz Active Member

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    Thanks guys for all the replies. Hopefully you can forgive me for being a pain in the rear, but I see a lot of theories being stated as fact. All of the explanations sound reasonable -- crank walk, mains shifting, etc., and there is some reasonable evidence that the failure begins at the mains. There's also some evidence that the reasonable explanations might not be the case -- i.e. that a main girdle does "nothing" to help. If that's what was actually happening, it would most definitely help. My theory is that high-lift cams or high-ratio rockers with stiff springs can possibly transmit some ferocious harmonics to the crank snout causing the initial cracks which just snowballs from there. There is no doubt that the combustion forces on the bottom end eclipse what's happening in the valvetrain. But harmonic forces can cause just as much destruction as raw power (especially when the destruction begins as a crack and not a shower of metal fragments).

    Consider a couple of pieces of data which I've come across:

    - Turbo cars (at similar power & rpm levels) are less likely to split the block than supercharged, N/A, nitrous applications. Turbo cars will typically have mild (often stock) cams.

    - Zero-balance cranks can substantially increase the power threshold before cracking occurs. Harmonics, and not raw power, is the explanation here.

    I realize this topic has been beaten to death with hundreds of threads on this site and others, but I haven't come across a single thread that resembled an engineering analysis -- the end result always being "it's power-related, get a Dart." I'm trying to get some data to start one, and to see if it's worth the effort of doing some 3d vibration analysis as well.

    - I split my block N/A making 500hp at 6500rpm with cam abc, 1.7 rockers. I split my turbo block making 600hp at 6000rpm with stock cam and rockers. That would be very useful data for my effort! If you have that type of data, please shoot me a message. Thanks again!
    #17
  18. GroverDill

    GroverDill GoldMember

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    The problem is no one wants to (or should) build a zero balanced set up in a stock block. My stock 93 lower end held up to 15 psi of S trim boost and made 540 rwhp I do think a good balancer helps as well as keeping the rpms under 6k. I could not in good conscious put a couple thousand bucks of high end rotating assembly in a stock block, the risk was not worth it ( the juice wasn't worth the squeeze ) So I had Rick build me my Dart motor and now I have no worries about block breaking.
    #18

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