how do hyper pistons work?

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Re: how do hyper pistons work?

Post by modok »

So, the question in my mind now, is have you ever seen a coated piston with four corner scuffing......that the coating wasn't worn off the middle of the skirt??
It should be possible to make a forged piston run as tight as a hyper at the bottom of the skirt, but to do that you need a anti-scuff coating or process that is a lot stronger than any I know.
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Re: how do hyper pistons work?

Post by user-23911 »

When you build an engine you measure everything........that's normal.
BUT
When you're pulling them down for a rebuild , it's good to do it all in reverse.
When you know a piston has overheated, you can confirm that by measuring the skirt and see that it's shrunk.
But if you make a point of measuring, you'll also know that if they haven't overheated, they'll come out at about the same size they went in.
I think "collapsed" might be a more common term for it?
Same as when it's been overheating, you'll often find that there's one or more headbolts down in torque.
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Re: how do hyper pistons work?

Post by modok »

yeah, although collapsed skirts are a lot more common on cast pistons, without a clear cause.
Maybe, general overheating for a long time, could just as easily see collapsed skirts. Or the skirts just got too hot, and collapsed. It's not directly a "clearance" thing.
Probably in many cases, if it had more clearance, they may still be just as collapsed.

I suppose four corner scuffing isn't seen often on smaller engines, rather you'd just see piston bits, so, to "see" the four corner scuffing you'd have to re-assemble them :lol:
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Re: how do hyper pistons work?

Post by Mark O'Neal »

You guys know that pistons aren't round, or straight?

Yes?
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Re: how do hyper pistons work?

Post by modok »

I know generally, how round most types of piston are, and I have come across quite a few that are, in fact, round.
It's what I don't know that's killing me man!
help me out
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Re: how do hyper pistons work?

Post by Mark O'Neal »

Automotive pistons are cam cut and tapered to the bottom of the oil ring. The wear you see on the corners of the skirt are from when the piston is cold. The ones on the center are from when they're warm.

The actual shape of the skirts are proprietary, and the Nascar teams skirt designs are far more secretive than that. In general the bottom of the piston is far rounder than they were in the 1980s....but still nowhere close to round.
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Re: how do hyper pistons work?

Post by modok »

no, real four corner scuffing means the skirt has grown to big for the hole, probably due to heat.
No disrespect intended, I think you know....a few things about pistons.
You are right that there would be slight wear there normally, but I mean right out of the handbooks, it's always been there.
i deal with this stuff all the time.
For instance, last week I worked on a 3408 and a 3404 cat. The pistons were made for .007 clearance but were advised to be set for .008, due to "scuffing" problems. But I have not seen any with four corner scuffing.
If what I am learning is true, the just general scuffing is MAINLY a lubrication problem in most cases, more than most I have met would expect. It's right there in the textbooks, but understanding the WHY of it, is not.
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Re: how do hyper pistons work?

Post by gruntguru »

If heat is flowing from the skirt to the cylinder the skirt is hotter than the cylinder. Heat only flows from hot to cold. So if the thrust faces of the skirt have not grown so much that they have become an interference fit (which is impossible) the structure between those faces must be at a lower temperature than the cylinder walls - reducing the expansion of the skirt.
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Re: how do hyper pistons work?

Post by Truckedup »

modok wrote: Fri Jun 15, 2018 11:56 pm yeah, although collapsed skirts are a lot more common on cast pistons, without a clear cause.
Maybe, general overheating for a long time, could just as easily see collapsed skirts. Or the skirts just got too hot, and collapsed. It's not directly a "clearance" thing.
Probably in many cases, if it had more clearance, they may still be just as collapsed.

I suppose four corner scuffing isn't seen often on smaller engines, rather you'd just see piston bits, so, to "see" the four corner scuffing you'd have to re-assemble them :lol:
One of vintage racing Triumph bike engines had a slight bit of scuffing......Forged piston in a 2.9 inch bore with .0055 clearance that was never a problem previously...I believe it was a fueling problem leading to detonation...

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Re: how do hyper pistons work?

Post by zums »

modok wrote: Sat Jun 16, 2018 2:44 am no, real four corner scuffing means the skirt has grown to big for the hole, probably due to heat.
No disrespect intended, I think you know....a few things about pistons.
You are right that there would be slight wear there normally, but I mean right out of the handbooks, it's always been there.
i deal with this stuff all the time.
For instance, last week I worked on a 3408 and a 3404 cat. The pistons were made for .007 clearance but were advised to be set for .008, due to "scuffing" problems. But I have not seen any with four corner scuffing.
If what I am learning is true, the just general scuffing is MAINLY a lubrication problem in most cases, more than most I have met would expect. It's right there in the textbooks, but understanding the WHY of it, is not.
How do the pin bores look in these 4 corner scuffed pistons
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Re: how do hyper pistons work?

Post by Steve.k »

IMG_2702.PNG
The basic therory on a 4 corner skid is the piston is rapidly expanding the cylinder not growing at same rate. Usually like i said before from poor warm up procedure.Very common in two stroke world but happens less with the liquid cooled engines and electronic surveillance they have now.Theres a pic of sled piston I borrowed off net. Most problems we see now are detonation.
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Re: how do hyper pistons work?

Post by clshore »

I recall reading an article on the Nissan GTP car, which used a highly modified turbocharged VG30.
The pre-starting procedure for the motor involved preheating the entire assembly.
The stated reason given was that the pistons were an interference fit in the bores below a certain temperature.

That always seemed strange to me, given that piston peak temperatures would always be higher than the block.

The blocks were modified production parts, meaning aluminum castings of whatever alloy Nissan used.
The pistons were almost certainly forgings, alloy unknown.

Is it possible that the piston alloy Tce was actually lower than the block Tce?
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Re: how do hyper pistons work?

Post by DrillDawg »

I think the old Suzuki air cooled 4 cyl big bore kits were like that.
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Re: how do hyper pistons work?

Post by DCal »

One of vintage racing Triumph bike engines had a slight bit of scuffing......Forged piston in a 2.9 inch bore with .0055 clearance that was never a problem previously...I believe it was a fueling problem leading to detonation...

I have seen this many times. When you see the ring lands scuffed like this it is lack of lubrication. And that usually is ring butting which scrapes all the oil off the cylinder walls. You can just imagine the heat that's building up in the piston, rings and cylinder walls. If the scuffing were on the skirts only it would be an indication of a clearance problem.

If there was a fueling problem causing a lean condition engine would become hotter and that could use up the available ring gap and more. If it were a flooding situation it would wash the oil off the walls and the rings would be ruined by vertical scratches which would cause loss of sealing to some degree. I would think that a properly designed piston with .0055 clearance in an aluminum case was more than adequate.
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Re: how do hyper pistons work?

Post by modok »

zums wrote: Sat Jun 16, 2018 8:05 am
modok wrote: Sat Jun 16, 2018 2:44 am no, real four corner scuffing means the skirt has grown to big for the hole, probably due to heat.
No disrespect intended, I think you know....a few things about pistons.
You are right that there would be slight wear there normally, but I mean right out of the handbooks, it's always been there.
i deal with this stuff all the time.
For instance, last week I worked on a 3408 and a 3404 cat. The pistons were made for .007 clearance but were advised to be set for .008, due to "scuffing" problems. But I have not seen any with four corner scuffing.
If what I am learning is true, the just general scuffing is MAINLY a lubrication problem in most cases, more than most I have met would expect. It's right there in the textbooks, but understanding the WHY of it, is not.
How do the pin bores look in these 4 corner scuffed pistons
Tom
I don't know, never did check. Maybe they would be oval?
Piston pin bushings were a little stressed, as you'd expect, but not enough be a problem.
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