Whats The Cause Of The Stock 302 Block Cracking?

Like everyone said, the 302 blocks don't have very thick mains, and consider that the power the engine generates is not a one way system. Newton's 3rd law of motion- When two bodies interact by exerting force on each other, these action and reaction forces are equal in magnitude, but opposite in direction. So when you transmit 500 horsepower to the tires, suffice to say there is also a reactionary effect of 500 horsepower applied to the structural areas of the block. Not to mention stresses due to vibration, heat, friction, and loading from the cam, valvetrain, distributor, accessory drive, fastener tension, etc. The blocks take a lot of abuse, even in mild power applications. That's what is so awesome about these aftermarket blocks holding together at 1500+ horsepower... That's like a bomb continuously exploding.

The simple answer is: relatively thin material (by comparison to other blocks) and a lot of outward force.

For the more complex answer, I'm going to type as I think. So any of you physics gurus that care to chime in or correct any of my misconceptions, be my guest. I enjoy deepening my understanding of the physics involved.

The net force acting on the engine block itself is not zero. Within the block (aside from gravity) there is both a net rotational force, and a net upward force. The gross forces acting on the upper part of the block include a force perpendicular to the deck, which is to say up and out, and an opposing force pushing down and in. The critical forces to consider in regard to your question are those that oppose each other at the top and bottom of the cylinder, the piston and head. Effectively these forces are acting along the 2 axes (plural of "axis") of the engine's V. One force acts by pushing "up and out" against each head while the opposing force pushes "down and in" against each piston crown. There is also a force pushing outward against the cylinder walls. Now I'm going to break each of these three forces down into more detail (skip to #3 for the answer to your question):

1. Cylinder wall forces due to cylinder pressure are the easiest: within the cylinder, the pressure on one side of the cylinder wall is cancelled out by the pressure on the opposite side of the cylinder. If the structural integrity of the lining and the block material is strong enough for this, distortion is minimal and the elasticity of the material allows the cylinder to retain its shape (elasticity, and metal fatigue are tangent to this subject). Although cylinder wall distortion obviously does happen at extreme power levels, this is not a huge concern in our blocks.

2. You know what happens to the "down and in" force. Some of it is transferred into the "rotational force," or torque that accelerates the car, and some of it pushes the crankshaft downwards against the bearing and/or the oil coating it. Since some of the downward force converts into rotation and none of the upward force does, these forces do not completely cancel each other out within the block. That means there's an actual upward force that acts against gravity and possibly the motor mounts, too. Also, the torque output at the crankshaft is equal to the torque applied to the engine block in the opposite direction, which is also resisted at the motor mounts. It took me a bit to think through this, but the torque applied to the engine block is actually a result of the net force that the pistons apply to the cylinder side walls. The 5.0 engine produces torque in a clockwise manner as you look at the engine from the front of the car. That's why the engine spins clockwise, and the opposing torque acts counterclockwise at the engine block, which explains why the driver's side wheel and tire are always the first off of the ground in a non-reinforced chassis.

3. Ok, finally! The force "up and out" applied to the heads is resisted by the head-bolts and then through block tension. The upward force is opposed through block tension partially to the main bearings/caps where the crank is applying the previously discussed "downward" force, and partially by gravity (the weight of the block), or if the upward force overcomes the engine's weight then the motor mounts also resist it. The outward force on one side of the engine is opposed through block tension by the outward force on the other side. When the tension through the block overcomes the block's strength at its weakest point in the lifter valley, it literally pulls the block apart. This can happen in one instant, or it can happen over time as the block is pulled beyond its elastic limits and cannot retain its shape.

I'm sure that the forces applied by the rotating assembly and the counterweights contribute, but by and large these forces are resisted at the main bearings, and I do not believe they are a significant contributer to the block splitting at the lifter valley. However, at exceedingly high RPM, they are extremely damaging to the crankshaft due to the flexing they cause and also to the main bearings as the caps "walk."

Blown engines go earlier because the internal stresses on the crank are increased, and because the blown cars have to overcome the parasitic loss of the blower, which can take some serious hp just to turn the belt. You don't see this power at the wheels or even at the crank, but the block "feels" it because the cylinder pressure is still there, and thus the "outward" force is there as well. Though a turbo car is easier on the block for this reason, you're still talking about a huge amount of force that's being exerted on the block.

Chris

This is one reason that 347s scare me.
Don't freak out, this is just my though process and opinion, I am not bagging on anyone's choice to run a 347...

The short rod ratio in a 347 puts heavy side load on the block, which is the exact force we are talking about when we discuss the block splitting in half from being pushed apart. Combine that with oil control issues from the piston pin height, and it rules out a 347 for me.

But then I am too conservative to use nitrous, and many of you use it with great success.

My response to the original question would be that the roller blocks were thinned greatly in an attempt to save weight, and the pressures/side loads put on the block in power adder situations is just too much for the stock roller block. Pretty simple.

ratio411 said:

This is one reason that 347s scare me.
Don't freak out, this is just my though process and opinion, I am not bagging on anyone's choice to run a 347...

The short rod ratio in a 347 puts heavy side load on the block, which is the exact force we are talking about when we discuss the block splitting in half from being pushed apart. Combine that with oil control issues from the piston pin height, and it rules out a 347 for me.

But then I am too conservative to use nitrous, and many of you use it with great success.

My response to the original question would be that the roller blocks were thinned greatly in an attempt to save weight, and the pressures/side loads put on the block in power adder situations is just too much for the stock roller block. Pretty simple.

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Couple of things here, and please don't take this as a flame:

1. the short rod ratio doesn't have any significant impact on the life of the piston, rings, or cylinder walls. Typically something else fails. Besides, what's the big deal in replacing the rings when it's time for a rebuild anyway? bearings and rings are pretty standard stuff, even when a motor just needs to be "refreshed."

2. The only side load that I mentioned is what causes the motor to torque over on the motor mounts in the opposite direction that it propels the drivetrain. This side load does not oppose the side load from the other bank. So, it has nothing to do with the "outward" opposing force. That is simply due to the force placed on the heads due to cylinder pressure.

3. There are no oil control issues in well built 347s. There are even short rods made specifically to pull the pin completely out of the oil ringland, but even the ones whose pins do intersect the oil ringland can be controlled without issue.

I'm saying all this having built a 331, but my reasoning for doing so had nothing to do with your 347 concerns.

ratio411 said:

This is one reason that 347s scare me.
Don't freak out, this is just my though process and opinion, I am not bagging on anyone's choice to run a 347...

The short rod ratio in a 347 puts heavy side load on the block, which is the exact force we are talking about when we discuss the block splitting in half from being pushed apart. Combine that with oil control issues from the piston pin height, and it rules out a 347 for me.

But then I am too conservative to use nitrous, and many of you use it with great success.

My response to the original question would be that the roller blocks were thinned greatly in an attempt to save weight, and the pressures/side loads put on the block in power adder situations is just too much for the stock roller block. Pretty simple.

Click to expand...

331, 347, or even 302, there are a million documented examples of them breaking around the 500 rwhp mark. While arguably there may be differences in the stresses the block sees between the different stroke variations, it seems to be a moot point at extreme power levels.

i would say the biggest reason they break there is because the valley floor is only about 3/16" thick, if that. add in all the oil drain holes, and the spider hold down holes, and its the most flimsy part of block. all the aftermarket blocks are considerably thicker there, as well as on the main supports.

FastDriver said:

The short rod ratio doesn't have any significant impact on the life of the piston, rings, or cylinder walls...

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No flame taken... It's just my thought process on it.
Like I said, I don't like nitrous, but a huge number of folks have great success with it.
I am just conservative that way.

As for rod angularity, a short ratio allows the rod to push sideways against the cylinder walls as the piston travels.
Longer ratios keep the rod more vertical, keeping stress off the cylinder walls.

I remember reading about John Kaase's builds for engine masters. In one of them, I remember him talking about the advantages in detonation resistance, because a short rod results in a faster piston with less dwell time at the top of the combustion chamber. So many other builders I've talked to about this always say they get the rest of the specs figured out and then put the rod that fits in there. So, different strokes (almost literally) for different folks...

The pro of longer piston dwell is that the piston absorbs more of the energy produced by combustion and pushes it down on the crank harder and more vertically, giving more power in a stroke. I am sure that more energy transferred to the piston equals more heat, so I can see the detonation resistance angle he was referring to.

A shorter rod also equals higher piston speed, which puts higher stresses on the pin and piston integrity below the rings. Kinda like G-forces of having to come to a stop quicker and then return down the cylinder faster. It goes super fast up, then in an instance all that mass comes to a stop and reverses direction just as quickly. Not a bad thing, but requires either lighter parts and/or stronger parts.

ratio411 said:

The pro of longer piston dwell is that the piston absorbs more of the energy produced by combustion and pushes it down on the crank harder and more vertically, giving more power in a stroke. I am sure that more energy transferred to the piston equals more heat, so I can see the detonation resistance angle he was referring to.

A shorter rod also equals higher piston speed, which puts higher stresses on the pin and piston integrity below the rings. Kinda like G-forces of having to come to a stop quicker and then return down the cylinder faster. It goes super fast up, then in an instance all that mass comes to a stop and reverses direction just as quickly. Not a bad thing, but requires either lighter parts and/or stronger parts.

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Food for thought:

Rod length is irrelevant to mean piston speed, which is dictated by stroke and RPM only. Rod length affects acceleration of the piston at different points in the stroke, and while shorter rods increase acceleration near TDC, they also reduce acceleration near BDC, where a long rod increases it. Arguably, the rod is under the most stress while pulling the piston away from TDC, but in any sort of stock 302 block with a halfway decent rotating assembly, the block will split before the rods give up.

Out of curiosity, I've crunched the numbers of a 331 using a 5.315" rod and one using a 5.4" rod. At maximum rod angularity, the 5.315" engine has less than .3 degrees more rod angularity than the 5.4" engine. I'd be curious to see the tolerances allowed by Ford on those blocks and how it compares to that.

A 347 with a 5.4" rod has ~.837 degrees more angularity than the 331 with a 5.4" rod. I also did a free body diagram on the geometry, and the resultant horizontal force, the piston side loading, is about 5.4% higher in the 347. (If my math is right. I'm drinking beer and switching between Fox news and the Olympics as I type this, haha.)

So is increasing just one of MANY inefficiencies within the engine assembly by 5.4% worth giving up the extra 16 cubic inches, or is it just pis sing in the wind? My opinion is to stick with the tried and true- there's no replacement for displacement. Any stroker kit is putting more stress on the stock block than it was originally designed for. If you're silly enough to put more than 500 hp through a stock block, you're flirting with disaster regardless of what rod and crank is in it. If I was going to do a stock block NA stroker, it'd be a 347 no question. If I was going to do a stroker for boost, it wouldn't be a stock 302 block period. The only time I'd even consider a 3.25" stroke would be in Chris' situation, when a lot of dish and compression height is needed in the piston.

And you're sure you're going to run 5.4" rods, I think. I could have gotten away with a longer stroke if I had found a great deal on some shorter billet rods. I stole the oliver billet 5.4s with bolts worth more than the rods themselves for $600 new. Most builders do it the other way around when starting from scratch. I had a custom piston made to fit the crank and rod, rather than finding a rod to fit a crank and piston.

Back to the argument at hand, nice work on the calculations, Nik. Pythagorean theorum for the angularity and just a statics problem to determine the side load when the crank is perpendicular to the axis of the cylinder's centerline?

I think there's more than enough material in this thread for someone to come to their own conclusion about how they should go. So, it was a good discussion. But also, a realtively boring and common one.

FastDriver said:

And you're sure you're going to run 5.4" rods, I think. I could have gotten away with a longer stroke if I had found a great deal on some shorter billet rods. I stole the oliver billet 5.4s with bolts worth more than the rods themselves for $600 new. Most builders do it the other way around when starting from scratch. I had a custom piston made to fit the crank and rod, rather than finding a rod to fit a crank and piston.

Back to the argument at hand, nice work on the calculations, Nik. Pythagorean theorum for the angularity and just a statics problem to determine the side load when the crank is perpendicular to the axis of the cylinder's centerline?

I think there's more than enough material in this thread for someone to come to their own conclusion about how they should go. So, it was a good discussion. But also, a realtively boring and common one.

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Haha, yep. The 331 VS 347 debate is

By now, everybody should know that you don't build either- you build a Windsor.

BTW, I used trig, not the Pythagorean- but yea, all of that was based at 90 degrees ATDC (or 90 BTDC, however you want to look at it).

Great discussion folks!

Love it.

I have always wanted to do a Chevy 400 rod 302 build, just for giggles, but I rarely have enough dough to build one engine that isn't experimental, so the ideas just float around in my head. It's good mental exercise to get them out in a discussion once in a while.
I learn more everytime this sort of thing comes up.