Piston guided Set Up Question

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ptuomov
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Re: Piston guided Set Up Question

Post by ptuomov »

Kevin Johnson wrote:Someone apparently misinformed Porsche that adding center counterweights is one approach in stiffening a stroked 928 crankshaft like in the GTS.


http://www.epi-eng.com/piston_engine_te ... issues.htm

No, I guess they knew what they were doing after all.

Do you even understand what stiffness of a crankshaft means?
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Re: Piston guided Set Up Question

Post by Kevin Johnson »

No, apparently I do not. Please explain it.
http://www.epi-eng.com/piston_engine_technology/crankshaft_design_issues.htm wrote: High stiffness is a benefit because it increases the torsional resonant frequency of the crankshaft, and because it reduces bending deflection of the bearing journals. Journal deflection can cause increased friction by disturbing the hydrodynamic film at critical points, and can cause loss of lubrication because of increased leakage through the greater radial clearances that occur when a journal's axis is not parallel to the bearing axis.

At this point, it is important to digress and emphasize the often-misunderstood difference between STIFFNESS and STRENGTH.

Metal parts are not rigid. When a load is applied to a metal part, the part deflects in response to the load. The deflection can be very small (crankshaft, conrod, etc.) or it can be quite large (valvesprings, etc). But to one degree or another, all parts behave like springs in response to a load. The ultimate strength of a material is a measure of the stress level which can be applied to a lab sample of the material before it fractures. The degree to which a given part resists deflection in response to a given loading is called stiffness. It is important to understand that the ultimate strength of a material has nothing whatever to do with stiffness. Stiffness is the result of two properties of a part: (1) the Young's Modulus of the material (sometimes called Modulus of Elasticity, but more appropriately named Modulus of Rigidity) and the cross-sectional properties of the part to which the load is applied.
...

Traditionally, many two-plane V8 crankshafts had been produced without center counterweights because of economies and difficulties forging the blanks, because the six-counterweight crank typically has a slightly lower MMOI, and because the benefits of an eight-counterweight crank in a short-stroke application were not fully appreciated. However, the bending deflection across the center main at high loadings and high speeds causes measurable losses, so many areas of racing which use two-plane V8 cranks are moving (or have already moved) to eight-counterweight cranks.
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Re: Piston guided Set Up Question

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Am I interpreting your answer correctly as a "no"?

There's the stiffness of the shaft, which is a separate concept from the forces acting on the shaft. Think about it for a little bit and you probably see where you made your logical error.

As far as 928S's crankshaft goes, it's my understanding that it will have eight counterweights, like his previous stroker crankshaft.
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Re: Piston guided Set Up Question

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ptuomov wrote:Am I interpreting your answer correctly as a "no"?
No, you are misinterpreting an annoyed sarcastic rejoinder to someone who did not bother to read the link provided.
ptuomov wrote:There's the stiffness of the shaft, which is a separate concept from the forces acting on the shaft. Think about it for a little bit and you probably see where you made your logical error.
And who further failed to understand that it addressed the issue of a piston-guided rod big-end bearing attempting to apply power-stroke force perpendicularly to a bearing journal which varies in relative surface alignment to it.
ptuomov wrote:As far as 928S's crankshaft goes, it's my understanding that it will have eight counterweights, like his previous stroker crankshaft.
That is good news. I have made patterns for stroker crankshafts in the Porsche 928 which do not use eight counterweights.
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Re: Piston guided Set Up Question

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#-o #-o #-o #-o
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Re: Piston guided Set Up Question

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Warp Speed wrote: #-o #-o #-o #-o
Yes, I know.

Warp Speed, maybe you could tie this into the discussion about design considerations between I-beam versus H-beam rods and the typically better stiffness that H-beam rods have to resist lateral bending torques when crankshaft guided rods are used.
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Re: Piston guided Set Up Question

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I'd LMFAO if it was not so sad ;(
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Re: Piston guided Set Up Question

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wyrmrider wrote:I'd LMFAO if it was not so sad ;(
Yes, I know.

Aside: reviewing Taylor V.2 again I noticed his illustration and description of H-beams being rotated 90 degrees in a redesign of an aero engine and am musing that might be around the time I-beam with respect to connecting rods entered the lexicon. Might be completely off in that, though.

What I think is sad...

When I was a teen-ager going through technical school and working in a laboratory and quizzing master technicians in their 70s and 80s about when they were teenagers and going through technical school and likewise quizzing their elders. So reaching back through about eight generations or 160 years of first-hand living history.* Then I would compare that knowledge to what was available in the library or contemporary-to-my-teen-years textbooks. Pretty scary how much is forgotten. And sad.

* Edit: that would actually be about 12 generations or 240 years. One of the terms that came up was the use of "China Clippers" -- but that only reached back about 140 years at that time.
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Re: Piston guided Set Up Question

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I think that once you (statically) place the small end load bearing surface centered under the piston and then place the big end load bearing surface centered under the small end load bearing surface, whether it's a piston guided or crankshaft guided rod setup doesn't matter in terms of any of this deflection etc. nonsense.

As far as I understand it, one needs the right side clearance at the small end, a large enough side clearance at the big end to allow for thrust bearing etc. side clearances, and enough oil to the small end.

Caveat lector, just my understanding.
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Re: Piston guided Set Up Question

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ptuomov wrote:I think that once you (statically) place the small end load bearing surface centered under the piston and then place the big end load bearing surface centered under the small end load bearing surface, whether it's a piston guided or crankshaft guided rod setup doesn't matter in terms of any of this deflection etc. nonsense.
I am glad you came to that realization. The issue is that this is not the case for the stock 928 engine nor the 944 (on the cores that I took the time to measure). I guess that just was not clear to you.
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Re: Piston guided Set Up Question

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928S wrote:To give additional piston stability, I will use a piston that uses this design, that is the upper ring lands running on the bore essentially lengthening the skirt. As seen in this picture.


Image

For piston guided operation you need pistons that have some sort skirt also under the wrist pin. With those pistons you have not side-support for rod twist and pistons will fail.

But think again what you are doing. Piston guided rods are actually not guided rods, at high rpm's there's no need for actual rod guiding. At lower revs there is sure a need to guide rod perpendicular to piston.

Volvo think at 80's that piston guided rods were good for reduced friction for low-rpm engine also. It sure wasn't but instead they got oval upper rod bearings and twisted con rods when engine pinged at low revs as rods were actually crank-guided but with massive side-clearances. But sure they got very unique sounding engines as rod clapping danced pistons.......

Nowadays everybody use with high-perf engine very short pistons without side skirts and crank guided rods for obvious reasons......
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Re: Piston guided Set Up Question

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naukkis79 wrote:
928S wrote:To give additional piston stability, I will use a piston that uses this design, that is the upper ring lands running on the bore essentially lengthening the skirt. As seen in this picture.


Image

For piston guided operation you need pistons that have some sort skirt also under the wrist pin. With those pistons you have not side-support for rod twist and pistons will fail.


Nowadays everybody use with high-perf engine very short pistons without side skirts and crank guided rods for obvious reasons......



How do NASCAR teams get away with running a piston guided design???
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Re: Piston guided Set Up Question

Post by naukkis79 »

Warp Speed wrote: How do NASCAR teams get away with running a piston guided design???
As I tried to say, at high rpm piston guiding is ok.

But I don't know whether they use it or not, as I see very short and side-skirtless pistons with crank guided rods offer less friction than piston-guided rod setups.....
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Re: Piston guided Set Up Question

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naukkis79 wrote:
Warp Speed wrote: How do NASCAR teams get away with running a piston guided design???


But I don't know whether they use it or not, as I see very short and side-skirtless pistons with crank guided rods offer less friction than piston-guided rod setups.....

Not sure I agree with that.
Just about every Cup team uses piston guided rods with a box style piston. Where do you get your friction comparison from?
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Re: Piston guided Set Up Question

Post by naukkis79 »

Warp Speed wrote: Not sure I agree with that.
Just about every Cup team uses piston guided rods with a box style piston. Where do you get your friction comparison from?
Just a quess. There ain't much friction in rod sides against crank as there is no force between them. Sure thinner rod bearing reduces
friction compared to thicker but thinner rod bearings can used with narrow size clearances:

Image

And very high revving motorcycles don't use piston guiding but very small con rod big end side clearances.

Piston with no skirts in wrist pin sides are con-rod guided in that direction, if there is too much big end side clearance piston will twist in cylinder which sure ain't good for power or piston life.

If engine makes 99,9% it's life at near max rpm it's need for rod guiding is very small as big end's rotating forces will put it perpendicular to small end, as probably is case with those nascar engines. But that won't work with street engines.
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