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Wrist pin diameter and rpms

General engine tech -- Drag Racing to Circle Track

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Wrist pin diameter and rpms

Post by ptuomov » Wed Nov 27, 2019 6:36 am

Do high rpms put any requirements on the piston wrist pin diameter? I am thinking surface speed and oiling, assume that the piston assembly weight is given. Fully floating wrist pin.

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Re: Wrist pin diameter and rpms

Post by mag2555 » Wed Nov 27, 2019 7:36 am

What size Bore and stroke and what rpm level are we talking about here?

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Re: Wrist pin diameter and rpms

Post by ptuomov » Wed Nov 27, 2019 8:09 am

mag2555 wrote:
Wed Nov 27, 2019 7:36 am
What size Bore and stroke and what rpm level are we talking about here?
100mm bore, 78.9mm stroke, 8000rpm, 1bar boost, premium pump gas, and looking for factory reliability. Very close up Subaru EJ25 dimensions, although it’s not a Subaru.

My logic says go big with the pin diameter (short and thick wall, too), but I haven’t considered anything relating to the pin surface speed yet.

Right now, John’s drawings say 24mm OD, 13mm ID, and 5.5mm wall thickness and estimated weight of 149 grams. I’m loving that from the stiffness and pressure perspective, but is there anything related to the surface speed to worry about?

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Re: Wrist pin diameter and rpms

Post by modok » Wed Nov 27, 2019 7:48 pm

I don't understand if you consider 24m to be "BIG" or "SMALL"

IMO 24mm would be completely normal.

If took the last 50 years worth of engines as data, made a formula to predict the pin size, I bed you'd end up with 23.5-24mm

Tho applying the same "formula" to other engines, you might see they are too big or small, and...a lot of times it's true.
The surface speed thing I don't get that. Surface speeds on pins are quite low, "high" rpm should help, it may be impossible for the surface speeds to be too high even if the pin was huge.

Pin size is MAINLY tied to the piston. Because the forces the pin sees are proportional to the forces exerted on the piston, and/or caused by the piston.

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Re: Wrist pin diameter and rpms

Post by ptuomov » Thu Nov 28, 2019 8:38 am

I consider 24mm OD pin pretty large for 8000rpm engine, but I don’t have much logic backing up that view. I was just thinking out loud whether the pin surface speed is a consideration or not. The rod ratio is almost exactly 2 so it shouldn’t be too bad, no?

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Re: Wrist pin diameter and rpms

Post by ptuomov » Thu Nov 28, 2019 9:59 am

I attempted to compute the wrist pin surface speed at it's TDC maximum and got a low value. At 90-degree rod to crank angle, the velocity is temporarily zero. By my logic, high surface velocity at TDC is not a problem, the low surface velocity at 90-degree rod to crank angle is more of a problem. Since there surface speed is zero, there's no oil wedge. That's the challenge for pin lubrication, during the power stroke the pin relies on squeeze film lubrication only.

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Re: Wrist pin diameter and rpms

Post by swampbuggy » Thu Nov 28, 2019 10:33 am

24mm is bigger than you need i am sure. NASCAR cup engines have and may still run .827" diam. X .180" wall pins, but they are SHORT, 2" appx. and they are made out of something better than 9310 alloy. 9310 material with a .180" wall, shorter is better less likely to flex (and) less weight should be more than adequate for your build. Mark H.

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Re: Wrist pin diameter and rpms

Post by ptuomov » Thu Nov 28, 2019 12:12 pm

swampbuggy wrote:
Thu Nov 28, 2019 10:33 am
24mm is bigger than you need i am sure. NASCAR cup engines have and may still run .827" diam. X .180" wall pins, but they are SHORT, 2" appx. and they are made out of something better than 9310 alloy. 9310 material with a .180" wall, shorter is better less likely to flex (and) less weight should be more than adequate for your build. Mark H.
John Kuhn and Mahle run their software and it spitted out a required wrist pin wall thickness. When you round up that result to the next standard size in their catalog you get 5.5mm / 0.217” wall thickness. Length is little under 2.25”. It’s a beefy pin for sure.

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Re: Wrist pin diameter and rpms

Post by swampbuggy » Thu Nov 28, 2019 12:49 pm

Do you know the molecular of the metal ? Mark H. :)

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Re: Wrist pin diameter and rpms

Post by modok » Thu Nov 28, 2019 2:19 pm

too big.......maybe.
I'd say that might depend on how you have the boost come in.
if you had a pro-charger, yeah, maybe that's too heavy, but, I don't know what your doing with it.
for an endurance/road racing kind of thing with a turbo, it's going to have to handle a lot of cylinder pressure at lower rpms, I assume, and handle a lot of G forces shifting at 8000, then -boom- max boost in the next gear.

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Re: Wrist pin diameter and rpms

Post by 4vpc » Fri Nov 29, 2019 3:33 am

When your subtle campaign falls on deaf ears.
Or blind eyes #-o :wink:
There is no S on the end of RPM.

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Re: Wrist pin diameter and rpms

Post by ptuomov » Fri Nov 29, 2019 7:46 am

swampbuggy wrote:
Thu Nov 28, 2019 12:49 pm
Do you know the molecular of the metal ? Mark H. :)
I don't know the exact alloy. I think it's something like 9310.

I'm just trying to learn about pistons etc. as a hobby. I trust the people designing the piston to come up with the right solutions, but a big part of the fun for me is just understanding why they are doing what they are doing.

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Re: Wrist pin diameter and rpms

Post by Warp Speed » Fri Nov 29, 2019 8:02 am

Typically made from a maragin type like C300, C350 ect.

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Re: Wrist pin diameter and rpms

Post by ptuomov » Fri Nov 29, 2019 8:58 am

I found this paper interesting. It's an investigation of optimized wrist pin shape.

https://pdfs.semanticscholar.org/6351/3 ... ba8849.pdf

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Re: Wrist pin diameter and rpms

Post by modok » Fri Nov 29, 2019 10:24 pm

I have seen few of those studies on the deflection of piston pins, and every time IMO, they underestimate the flexibility of the rod and piston.
The studies show the pin deformed into a strange shape from sheer force, not a gentle arch, tho IMO MOST times it is at least closer to an arch than shown.
So in the study it appears all the forces are concentrated right near the edges, making it seem like the far ends of the piston pin do nothing and may as well be cut short and/or the ID tapered at the ends......is that true? usually no, but then again sometimes yes.


IF this were so, we'd see tapering of the edges of the pin bushing, and tapering in the inner ends of the pin bosses, on tear down.
And.....as a matter of fact SOMETIMES I have seen it, sometimes. If you do or don't see it depends on the design of the rod and piston. In fact, the worst case i ever recall was a subaru that used lightweight pins in eagle h-beams.

The rigidity of the piston pin bosses varies HUGELY between a classic old fashioned cast piston and a very compact slipper skirt...
with solid pin bosses and heavy crown, and high pin height, narrow rod width. the structural PATH, form one pin boss to the other, may be HALF the distance of a classic design, and wall thickness higher, therefore FAR more rigid, perhaps 4x
The piston may become more flexible at temperature and that should be accounted for also.
It also varies hugely between a normal I beam rod and a H-beam.

Very compact piston and a rigid H beam rod, there would be NO point using a longer skinnier pin, as your asking the rod and piston to HOLD the pin straight, which is a terrible idea. Is this subaru type piston even a good design....well, maybe yes. Not for every app, but for some yes.

You haven't yet mentioned anything about the rods, but IMO it does matter. It's all a system.
And I actually believe that for long service from plain bearings one key is matching the flexibility oi the parts to obtain more even loading.

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