FE Block Strength - Engine Block Stress Math Resource Question

[BigNate]

OK - so this is technical - but not strictly "FE" - so I'm not sure if it belongs here or in the non-technical forum (although the use of any info received will be applied to my FE specific plans...).  I wrote all the stuff below and my wife looked at me, rolled her eyes, and said "just ask the question...  So - I'll ask twice - once here and once at the end of my rambling...  Does anyone know of an established model / tool for calculating forces at the crank journal as a function of reciprocating mass, RPM, and cylinder pressure (as the core variables)?  In short I am trying to segregate the forces associated with reciprocating mass from those associated with cylinder pressure.


I asked this on another forum a few years ago - and the topic generally fizzled.  I figured I'd give it a shot here.  I've always wondered about the "limits" of the basic 390 block.  What surprises me is that the general consensus seems to be that the basic 390 blocks tend to split up the mains at about 600 HP.  I also know that the consensus amongst the "turbo mustang" crowd seems to be that a "good" early 351W block ('69 blocks are prized) can live indefinitely at the strip at ~800 HP under boost with guys pushing hard blocked units past that for a while.

When I compare the bottom ends of these blocks the FE has a distinct advantage in terms of the mass of material in the mains and I would think structural rigidity due to the block skirt, webbing, etc...  I have to assume that the difference in experience (in terms of the strength of these blocks) is either a function of some weakness in the FE bottom end design - or a difference in the how the forces / stresses are applied based on the way that the various crowds make their power.  My suspicion is that the latter is cause of the gap in experience - that the FE crowd tends to be "old school" about making horsepower (I know this is changing somewhat) - relying on RPM to pursue peak power while the "turbo mustang crowd" tends to rely on boost.  Given that the kinetic energy of an object in motion increases as the square of velocity, the stresses on the block are going to increase exponentially as RPM (and piston/rod velocity) increase - and as a result the portion of the stress on the block that come from cylinder pressures vs those that come from reciprocating mass change as a function of RPM.  Given that the N/A motor increases airflow and fuel burn by increasing RPM (among other things of course) and the super/turbocharged motor does this in a manner that is independant of RPM, the point at which the stresses on the block become great enough to see failure can occur at a very different level of power.  I suspect that the FE block may be good well past 600 HP if that power is made at 5800 RPM under boost.

I've started working through the math around the forces at the main journal in an internal combustion engine and while I have a decent math background this is pretty daunting stuff...  Does anyone know of an established model / tool for calculating forces at the crank journal as a function of reciprocating mass, RPM, and cylinder pressure (as the core variables)?  In short I am trying to segregate the forces associated with reciprocating mass from those associated with cylinder pressure.

[jayb]

I don't know of a model per se, but I'd think a trip through the physics book ought to yield the answers you are looking for.  I'll take a peek at this myself over the weekend and see what I can find out.  I'll bet that Bill Conley may have an answer, too.  But let's say that you can determine the forces acting on the block.  What then?  How will you know if the forces are too high?

As far as FE blocks splitting at 600 HP, the only issue is with the non-crossbolted blocks, and they don't split in half like a small block Ford does, they generally just crack along the oil gallery running from the mains to the cam journals.  So you will then have an internal oil leak, but usually you won't blow the bottom end out of the motor or anything.  Also 600 HP is a pretty conservative value; I know a lot of people who have run 2 bolt blocks harder than that without any trouble.  It depends on what you are doing with the engine.  If you have a 500 HP motor and you are revving to 6000 RPM and dropping the clutch at the line to get your 14X32 slicks moving, you are more likely to hurt the block than if you were running a 650 HP engine with street tires and an automatic. 

My general rule of thumb is that if you are going to exceed 600 HP with the engine it is worth the time and expense to cross bolt it. I'd recommend you just do that if you have any doubts.

[Joe-jdc]

I personally think you are worried about something that is imaginary in real world usage.  All the 5.0 blocks that I am aware of that split were using the 50 oz imbalance crankshaft, damper, and flywheel.  If you take a roller cam block and install a lightweight crankshaft, internal balance, and use light pistons, you can get considerably more than 500 hp before block failure.  Also, many of those folks sidestep the clutch pedal at high rpm to get the maximum launch, and that is hard on everything.  In theory what you suggest of adding boost and keeping the rpm's lower should work, if, and a big IF everything is balanced perfectly, and made light as possible without sacrificing strength.  Many folks have raced 390 style FEs for years revving them to 7200+ without splitting a block.  Most of the failures were rod bolt related at those rpms due to heavy pistons.  I raced FE's since 1969, and I have never broken a block, even when rod broke, camshaft broke, etc.  I always went through the lights at close to 7400 rpm with a mechanical tachometer in my 427 FE.  Joe-JDC.

[66FAIRLANE]

Probably the biggest impediment to a mathamatical formula is all blocks aren't created equal. And neither are all builds.

[WConley]

Hoo boy!  Calculating actual loads in a multi-cylinder engine block is a major job.  It's a three-dimensional problem since loads / vibrational energy are traveling along the crank.  Throw in some external balance, like on a 428, and it gets crazy.

My resource on this matter, "The Internal-Combustion Engine in Theory and Practice", by Charles Fayette Taylor, devotes about forty pages of scary- looking equations to this subject.

Yeah I was a Ford Engine Engineer, and I have designed engine blocks, but today I wouldn't even attempt such a calculation.  This is why God invented finite element analysis    Yes it is a big FEA model you would have to build for an FE, but at least it would give you excellent data that points out where the weak areas are for your given load case.

FEA is how the big boys do it now.  It's relatively routine for them, and possible (but very difficult) for someone like me with a reasonably normal computer and software.  I would rather let somebody else do the heavy lifting 

Oh - and despite all of the computing power in the building at Ford, the first pre-production modular v8's were lining the teardown room with rods through the blocks.  There's still stuff going on in these engines that demands experience and talent to get right.

So my message would be to maybe try some calculations as a very rough baseline, but then ask the guys who are really building and racing the FE.  Here's another thing we got wrong with the Ford Modular V8:  All of the FEA analysis optimized the block so that the bottom end would withstand no more than 460 HP.  (It was felt that 100 HP/Liter at the 4.6L displacement was more than the engine would ever need to put out.)

Today that same block architecture, with some development mind you, is supporting over 1,200 HP in race trim.  No one back in 1990 would have ever believed it was possible, even if you made the block out of titanium.  Even if we had been able to study today's 1,200 HP Ford GT or Shelby version of the Modular, we would have figured maybe 500 or 600 HP tops.  That's where years of building, racing, and tinkering can make the formulas and FEA models look pretty bad.

 

[BigNate]

Thanks gents.  I guess my question is not as much about "proving" something - but rather trying to build a general understanding of the range in which the lines merge - and that mostly to satisfy my own curiosity. 

I agree with the assertion that there is a lot to be done to reduce the likelihood of this kind of failure and that balancing, lightening, and properly assembling the rotating assembly is all at a premium.  I also agree that I'll never get an absolutely accurate "number" at which failure occurs, for all of the reasons mentioned (inconsistency in the blocks, differences in driving style and uses, etc...)

I had done some digging and found models for estimating forces associated with rotating and reciprocating masses etc - and I know I can calculate force associated with pressure etc... I guess I was just hoping to find that someone geekier than me had modeled it on-line and saved me the work... :-)

I'll fiddle with it and if I get anywhere I'll post what I come up with in case anyone feels like tearing it up or validating...  Like I said - it really is more a question of curiosity. 

I am sure that most definitive option is to put something together that I can throw boost at and see what happens...

[WConley]

BigNate -

In my estimation it's pretty hard to break an FE block with boost, if you're not zinging it.  The inertia loads go up with the square of rpm, and quite often will vastly exceed the combustion loads at higher rpm.  If you're building power with boost at 5,000 rpm, you'll be able to support a lot more than naturally aspirated at 7,000 rpm.  As you mentioned, light, well balanced reciprocating components pay big dividends here.

If you encounter detonation though, all bets are off.  Higher rpm detonation is death to lower end bearings, if your piston crowns survive long enough.  That's a real concern in a boosted application.

[BigNate]

BigNate -

In my estimation it's pretty hard to break an FE block with boost, if you're not zinging it.  The inertia loads go up with the square of rpm, and quite often will vastly exceed the combustion loads at higher rpm.  If you're building power with boost at 5,000 rpm, you'll be able to support a lot more than naturally aspirated at 7,000 rpm.  As you mentioned, light, well balanced reciprocating components pay big dividends here. 

Yep - it is these two lines that I'm trying to mock up (inertial load @ RPM + NA combustion load vs inertial load @ lesser rpm + increased combustion loads associated with boost)...   

If you encounter detonation though, all bets are off.  Higher rpm detonation is death to lower end bearings, if your piston crowns survive long enough.  That's a real concern in a boosted application.

My detonation avoidance plans are based on running E-85, an inter-cooler, a good wide-band O2 sensor, relatively low static CR, and having a very slow and methodical hand with the waste-gates, timing and jets (or eventually fuel map)...   Mitigation will include reasonable quality rotating assembly components...  :-) 

[machoneman]

A fascinating and not esoteric topic BigNate.  The model I'm sure does exist but it's in the province of OEM engine manufacturers like Ford, Cummins, GM, etc. and something probably not available to the public. Likely the independent crank/rod makers like L.A. Billet, Crower, Lentz also have developed some pretty sophisticated models.  Bill C'.s comments on tinkering that can FEA models look silly is a testimony to hot rodders (factory based or mere mortals!) running stuff till it breaks and then making it better.

This topic begs the question though: which aftermarket FE block is the absolute strongest?

[BigNate]

I case others are chewing on this... I have found the following which appears to be course content from an ME course at Chulalongkorn University in Thailand...     Anyway - it is distinctly oriented to the student / layman to a greater degree than most of the more scholarly papers that I have found on the subject - and it appears to provide the basis for distinguishing (at least in a simple system) between the kinetic energy associated with rotating / reciprocating mass and force exerted by cylinder pressure.



Sorry for the lengthy URL - I tried to attach the PDF but no-joy....  The link below goes directly to a download of the PDF...

http://www.google.com/url?sa=t&rct=j&q=calculate%20forces%20internal%20combustion%20engine&source=web&cd=1&ved=0CCgQFjAA&url=http%3A%2F%2Fcu-ocw.eng.chula.ac.th%2Fcu%2Feng%2Fme%2F2103471%2Flecture-notes%2F14-2103471%2520Dynamic%2520Analysis%2520of%2520the%2520Internal%2520Combustion%2520Engine.pdf&ei=Xu6VT4GDNqibiALc1_GICg&usg=AFQjCNG1GyIttmjFBxmVvdfLSCfOr-fatg

[Qikbbstang]

Over five years ago I witnessed Joe's TT FORD GT go to the 1,000- 1,200+HP zone on the stock FORD GT longblock and do so time and time again. I think it's simply a byproduct of proper engineering that says "keep everything rigid" that it still does so at many times the rated limit.  When I was a Factory Rep on 10,000psi working pressure hydraulic systems I would never let on what the lab burst pressure's actually went to. There is a bunch of safety margins.....

Re:" Here's another thing we got wrong with the Ford Modular V8:  All of the FEA analysis optimized the block so that the bottom end would withstand no more than 460 HP.  (It was felt that 100 HP/Liter at the 4.6L displacement was more than the engine would ever need to put out.)
Today that same block architecture, with some development mind you, is supporting over 1,200 HP in race trim.  No one back in 1990 would have ever believed it was possible, even if you made the block out of titanium.  Even if we had been able to study today's 1,200 HP Ford GT or Shelby version of the Modular, we would have figured maybe 500 or 600 HP tops.  That's where years of building, racing, and tinkering can make the formulas and FEA models look pretty bad."

[WConley]

Yeah you're right there BB.  What we would consider "living" at 460 HP involves a 200 hr durability test on the dyno.  (Remember that thrash Ford did on the EcoBoost V6?  That's what I'm talking about.)  I bet a GT longblock would have a hard time staying together at 1200 HP for 200 hrs, but it's still super impressive that it can stay together at all!

Despite all of the torture tests we did on our engines, deep down we all knew that you could still give a car to a 16 year old and it would be toast in one afternoon 

[hotrodfeguy]

I think there are so many factors it's unreal. Stroke, Bal, is it detroit or zero, RPM. The the casting inconsistencies, not like every FE was cast perfectly the same. Just look at the bore and the core shift as a example. I think it's hard to pin down an Exact number and say and FE lives to this number for life. it's all a crap shoot when running on the edge. Kinda like driving a car on thin ICE. sometimes you can and sometimes you go through the same thickness.

[westcoastgalaxie]

So maybe someone can answer this one for me. All I know is FE's are 5.0's and most other platforms, call it the norm, that the opposing cylinders are offset? Take a look at a FE from the bottom and notice how the cylinders across from each other are offset. I was told that this is where a FE falls short. That this offset creates a twisting force in the block. This is what causes a FE to split up the mains.

[jayb]

I think all V-8s are offset like that.  It is not an FE specific characteristic.