Billett mains or not?

[49-blues]

i find it hard to believe that a steel hardened crank could flex as much as a cast. is there any proof of this?

 

[Blazer406]

i find it hard to believe that a steel hardened crank could flex as much as a cast. is there any proof of this?

Ditto.

 

[Ed Valvo]

Proof? We'd need to get an authority in here on this which I am not. I did not want to mislead or appear to be an authority on this however over and over I have been told that cast cranks and steel(forged) crank do indeed have similar flex properties. It's the industry I'm in. Steel(forged) is definitely better. Steel can flex more before breakage occurs so if you're breaking cranks, you need a steel crank. If you're wiping bearing because of main cap walking, then you need to strengthen the bottom end via girdle, more main cap bolts, etc.. A steel crank will do little for stengthening the bottom end. It will just live longer under abuse before failure. Tensile, Yield, and Elongation are keys to comparing cranks. Tensile is when it breaks. Yield is when it bends. Elongation is the amount of flex before it breaks. Steel is superior in all the qualities vs. cast. Stiffness is a different measurement.

 

[49-blues]

If you have a steel crank and it flexes less then isnt it already making the lower end stronger?

 

[S2V6RACER]

Proof? We'd need to get an authority in here on this which I am not.

Thanks for clearing that up!!


You might want to do some main bearing checks on the cast crank vs. Steel cranks on higher HP applications and get back to us on what you find!!!

 

[Ed Valvo]

Ted A -- Regarding buick motors, I have. We ran into this in the pre-girdle days. I was just trying to help/contribute.

49-blues -- I never said it flexes less. It flexes about the same. I said it can take more abuse/elongation before failure.

 

[TurboP7]

Billet mains are pretty hardcore but might be the easiest to find, and the best at proving piece of mind when its all said and done

 

[John Wilde]

Ted A -- Regarding buick motors, I have. We ran into this in the pre-girdle days. I was just trying to help/contribute.

49-blues -- I never said it flexes less. It flexes about the same. I said it can take more abuse/elongation before failure.

Ed is a smart and humble man that happens to do some mechanical engineering work. We should not scare people away that are trying to contribute.

 

[S2V6RACER]

Ed is a smart and humble man that happens to do some mechanical engineering work. We should not scare people away that are trying to contribute.

Anyways, Turbobuicks post above pretty much is straight out of a experts mouth. If Eds theory holds water, we would see bearing wear patterns supporting crank flex with a steel crank after the caps are held down better (Girdled blocks and Stage II blocks). Maybe we can get some input from another engine builder. Anybody talk to Duttweiler about this?


Turbobuickman: I would go with two center steel caps. We all know at some point we step up things and you will not have to go back into the bottom end if you decide to go a little faster.

 

[ijames]

Proof? We'd need to get an authority in here on this which I am not. I did not want to mislead or appear to be an authority on this however over and over I have been told that cast cranks and steel(forged) crank do indeed have similar flex properties. It's the industry I'm in. Steel(forged) is definitely better. Steel can flex more before breakage occurs so if you're breaking cranks, you need a steel crank. If you're wiping bearing because of main cap walking, then you need to strengthen the bottom end via girdle, more main cap bolts, etc.. A steel crank will do little for stengthening the bottom end. It will just live longer under abuse before failure. Tensile, Yield, and Elongation are keys to comparing cranks. Tensile is when it breaks. Yield is when it bends. Elongation is the amount of flex before it breaks. Steel is superior in all the qualities vs. cast. Stiffness is a different measurement.

Stiffness is indeed a different measurement, and the number to go look up is called Young's modulus or the modulus of elasticity. Cast steel and forged steel do indeed have about the same modulus of elasticity, about 29,000,000 psi. However, cast iron, which is what our Buick cranks are made from, has a modulus of elasticity in the 10-15,000,000 range depending on exact composition and heat treatment. This makes a cast iron crank about half as stiff as a cast or forged steel crank, so for the same force the cast iron crank will flex twice as far (assuming you are well below the yield strength and elongation limits, which you pretty much have to be if the cap hasn't ripped out of the webbing yet). Anyone with a stock crank, a forged steel crank, a couple of V blocks, and a dial indicator can test this - set the crank up with the V blocks at the ends, put the dial indicator in the middle, and push down on the crank in the middle with a small weight. The stock crank should flex twice as far for the same push.

 

[Nick Micale]

....................Maybe we can get some input from another engine builder. Anybody talk to Duttweiler about this? .

We have done many forged cranks, and a few billet cranks, in 109 blocks and of course lots of cast cranks.

With the cast cranks, even with caps and girdle, main bearing wear shows crank flex. With a similar build in a forged crank engine, the main bearing wear pattern is even.

We installed the first forged Eagle crank a few years ago in a 109 block with 3 steel main caps. Almost 2 years ago a malfunction caused detonation, and took out a rod bearing. All the other bearing looked fine.

Since then, this low 10 sec. GN had seen lots of track time and is as solid as in the begining. Moral of this story, and others similiar which we have done, is steel caps and a forged crank [and rods], with a least 2 steel caps will be reliable in the 10's.

For low 11 sec. builds with stock cranks, we strongly recommend 2 steel caps if the car will see much track use. Many mid-11 and slower cars live long with the stock bottom end.

Of course it only takes one mis-step of something to cause detonation, and take out a rod bearing, no matter the build, or how fast the car. This is the no. 2 engine damage item behind blown head gaskets.:smile:

 

[Mike Licht]

Nick's findings are spot on with ours. Steel crank and caps are a great choice well into the 10's. The stock crank pushes on the caps when it twists. This is what breaks the caps and causes the walk, not the block moving around. My argument is this, the piston has .004 clearance commonly. .002 per side, if the block was twisting it seems to me that the piston would lock up in the bore.
Mike

 

[John Wilde]

Stiffness is indeed a different measurement, and the number to go look up is called Young's modulus or the modulus of elasticity. Cast steel and forged steel do indeed have about the same modulus of elasticity, about 29,000,000 psi. However, cast iron, which is what our Buick cranks are made from, has a modulus of elasticity in the 10-15,000,000 range depending on exact composition and heat treatment. This makes a cast iron crank about half as stiff as a cast or forged steel crank, so for the same force the cast iron crank will flex twice as far (assuming you are well below the yield strength and elongation limits, which you pretty much have to be if the cap hasn't ripped out of the webbing yet). Anyone with a stock crank, a forged steel crank, a couple of V blocks, and a dial indicator can test this - set the crank up with the V blocks at the ends, put the dial indicator in the middle, and push down on the crank in the middle with a small weight. The stock crank should flex twice as far for the same push.

Excellent post as always!

 

[Blazer406]

........ My argument is this, the piston has .004 clearance commonly. .002 per side, if the block was twisting it seems to me that the piston would lock up in the bore.
Mike

My question... is when blocks do break..... is it the crank or rod breaking taking out the block..... or did the block break first.... I have seen blocks caught early before there was any significant carnage.... and there was a definate crack in the block. This evidence would lead me to believe the block does indeed flex. I could make an argument that the cylinders in and of themselves are relatively stucturally sound and likely wouldn't deform to the point of lock-up.

If they did... could this be the catalyst that breaks the crank... or rods?

What came first... the chicken or the egg....??

 

[Mike Licht]

when we do see that problem we do not see scuffing on the piston or the bore but we do see the uneven bearing wear that comes from a twisted crank. I have never seen a broken block with even bearing wear, or where one of the caps were not broke (stock caps)
Mike

 

[mikestertwo]

Back to the original question. To run 11's, I don't think steel caps are necessary, just insurance. I have been running mid 11's on a stock long block for the last 3 years with no issues. If you have a good tune, there should be no problems.

 

[skylarkstage1]

Mike L, I plan to go with a 4340 forged crank, h-beam connecting rods and three billet main caps on my production 4.1 block. I would like to run low11"s and high 10's. So will I be ok without a girdle with this bottom end?

 

[Mike Licht]

I would be fine with it
Mike

 

[steve davis]

so mike ,your saying that the crank twisting is taking out the caps,then whats pushing ,or making the crank twist.

 

[Mike Licht]

the downforce from the pistons/rods. They push on the crank and spin it to move the car. the weight of the car, traction and areodynamics push back and it twists.
Mike