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Piston wrist pin length

General engine tech -- Drag Racing to Circle Track

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ptuomov
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Piston wrist pin length

Post by ptuomov »

I’m looking at Mahle 100mm Subaru pistons with 23mm diameter piston wrist pins. The bare piston weighs 406g. The wrist pin is about 57mm long. The gap between pin bosses (for the rod small end) is about 29.5mm. This leaves about 13.75mm (0.54”) of pin inside the piston per side when the pin is perfectly centered. Seems low, no? Must be thoroughly optimized. Welcome to the modern era.
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Re: Piston wrist pin length

Post by Walter R. Malik »

ptuomov wrote: Fri Jun 29, 2018 7:17 am I’m looking at Mahle 100mm Subaru pistons with 23mm diameter piston wrist pins. The bare piston weighs 406g. The wrist pin is about 57mm long. The gap between pin bosses (for the rod small end) is about 29.5mm. This leaves about 13.75mm (0.54”) of pin inside the piston per side when the pin is perfectly centered. Seems low, no? Must be thoroughly optimized. Welcome to the modern era.
If the pin is of good quality with enough strength and the piston pin boss is designed to not flex much; that is plenty.
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Re: Piston wrist pin length

Post by DCal »

If the rod is no narrower than 26.5 mm its probably OK but I certainly wouldn't call .540 "plenty" particularly if the pin bore has a radius or chamfer. If, for instance, the chamfer is.040 then the effective pinbore length is really only .460, that's pretty sketchy IMO.
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Re: Piston wrist pin length

Post by ptuomov »

DCal wrote: Fri Jun 29, 2018 12:32 pm If the rod is no narrower than 26.5 mm its probably OK but I certainly wouldn't call .540 "plenty" particularly if the pin bore has a radius or chamfer. If, for instance, the chamfer is.040 then the effective pinbore length is really only .460, that's pretty sketchy IMO.
These have 13.75% of the bore diameter in pin-to-boss overlap. BMW race pistons that a friend measured have 20%. My old porsche pistons have something like 17%. Even other Subaru aftermarket pistons have much more than these Mahle pistons, but then again these Mahle pistons are pretty light for turbo pistons. I am not saying it doesn't work, this design/manufacturing is from 2012 or before that. I am just trying to wrap my head around it how it works. Oh well.
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Re: Piston wrist pin length

Post by modok »

Subaru stuff is on the edge right from the factory.
It's pretty cool IMO.
But they guys trying to hotrod them are mostly delusional.
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Re: Piston wrist pin length

Post by digger »

Post up an image of you can I'm interested in seeing what they look like
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Re: Piston wrist pin length

Post by ptuomov »

digger wrote: Fri Jun 29, 2018 9:09 pm Post up an image of you can I'm interested in seeing what they look like
I will, when they fix the site to accept iPhone photos without conversion.
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Re: Piston wrist pin length

Post by ptuomov »

Here after recycling them:
5A12E77D-7024-4D59-87A7-F4DDD63F48F6.jpeg
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Re: Piston wrist pin length

Post by ptuomov »

modok wrote: Fri Jun 29, 2018 8:24 pm Subaru stuff is on the edge right from the factory.
It's pretty cool IMO.
But they guys trying to hotrod them are mostly delusional.
From the masses of deluded, some rise above it and run those things at 800 hp from that 2.5L boxer four.
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Re: Piston wrist pin length

Post by modok »

I believe it, and it's all the more impressive.
Those are.....not the ones I am dealing with.
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Re: Piston wrist pin length

Post by digger »

Have a set of mahle here it's about 10-11mm from radiused edge to the start of the lock groove. 20mm pin 48mm pin length . Bosses are 26mm apart. No turbo though
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Re: Piston wrist pin length

Post by peejay »

ptuomov wrote: Fri Jun 29, 2018 10:08 pm
modok wrote: Fri Jun 29, 2018 8:24 pm Subaru stuff is on the edge right from the factory.
It's pretty cool IMO.
But they guys trying to hotrod them are mostly delusional.
From the masses of deluded, some rise above it and run those things at 800 hp from that 2.5L boxer four.
There's a lot of power locked away in the heads, people usually just go for the easy button and slap a big turbo on it, then joke about ring lands or blowing the cylinders out or beating the case halves apart.

https://www.youtube.com/watch?v=wqnuj58lFQI

This is 352hp at the wheels, on an AWD dyno, from a 3.3, which is the same geometry as the 2.2l. If this was a 2.2 then it would have about the same power as a STI engine, with less displacement and no turbo.

I personally don't see why people bother with the 2.5l. Naturally aspirated, the 2.5 technically makes more power than the 2.2 but in the real world the 2.2 feels a lot better because it actually accelerates, the 2.5 feels like a turd.
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Re: Piston wrist pin length

Post by ptuomov »

peejay wrote: Sat Jun 30, 2018 8:18 am
ptuomov wrote: Fri Jun 29, 2018 10:08 pm
modok wrote: Fri Jun 29, 2018 8:24 pm Subaru stuff is on the edge right from the factory.
It's pretty cool IMO.
But they guys trying to hotrod them are mostly delusional.
From the masses of deluded, some rise above it and run those things at 800 hp from that 2.5L boxer four.
There's a lot of power locked away in the heads, people usually just go for the easy button and slap a big turbo on it, then joke about ring lands or blowing the cylinders out or beating the case halves apart.

https://www.youtube.com/watch?v=wqnuj58lFQI

This is 352hp at the wheels, on an AWD dyno, from a 3.3, which is the same geometry as the 2.2l. If this was a 2.2 then it would have about the same power as a STI engine, with less displacement and no turbo.

I personally don't see why people bother with the 2.5l. Naturally aspirated, the 2.5 technically makes more power than the 2.2 but in the real world the 2.2 feels a lot better because it actually accelerates, the 2.5 feels like a turd.
Not turbocharging a Subaru is like kickboxing with your hands tied behind your back. Sure, some can do it, but it’s generally not the winning strategy.
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Re: Piston wrist pin length

Post by modok »

yeah I totally agree. It's a great engine it's just the owners I think have a strange attitude about what to do with it.
If it was NA power I'd de-stroke a 2.5, and REV it
For big boost, I'd use a 2.0 or 2.2 because they have more metal holding them together.
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Re: Piston wrist pin length

Post by Mark O'Neal »

digger wrote: Fri Jun 29, 2018 11:14 pm Have a set of mahle here it's about 10-11mm from radiused edge to the start of the lock groove. 20mm pin 48mm pin length . Bosses are 26mm apart. No turbo though
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