TeamSwift

Home of the Suzuki mini-compacts ! Your Home for all things Suzuki Swift, Geo Metro, Holden Barina, Chevy Sprint, Pontiac Firefly, and Suzuki Cultus. TeamSwift is a technical performance oriented community!
It is currently Fri Nov 24, 2017 12:57 am

Underbody braces, turbos and more!

All times are UTC - 5 hours




Post new topic Reply to topic  [ 9 posts ] 
Author Message
 Post subject: Forged Vs. Cast Pistons
PostPosted: Thu Sep 04, 2008 11:54 am 
Offline
User avatar

Joined: Mon Jun 18, 2007 1:35 pm
Posts: 2433
Location: Regina, SK
Here is an excellent write up I found detailing, cast vs forged pistons.


Quote:
Which is better, the cast piston or the forged piston? This argument doesn't come up as often as it used to. During the 1970s it was a frequently debated topic: The answer used to be simple, because one type of piston was in fact better than the other. Today however, things are not so simple. A person can't be as dogmatic as they would like, and anyone who is insistent just doesn't know the facts. Here are those facts.
Piston Basics
The piston is under-appreciated, for sure. It may be the hardest-working part in the internal combustion engine. Following is just a brief outline of the piston's function and construction.

Three Jobs
The piston does three important jobs. It is a bearing, receiving loads from combustion and transferring them straight and true through the connecting rod to the crankshaft. The piston is also a seal, as it seals combustion's forces and compression's trapped air. Finally, the piston is a heat conductor, transferring some of the cylinder's heat to the outside. In fact, nearly 80 percent of the cylinder's excess heat is drawn away by just the piston's rings.

Piston Parts
The piston is made of essentially seven parts. The piston's top or crown takes the brunt of combustion's forces and heat. Consequently, the crown is the hottest part of the engine after the spark plug. It must therefore be quite thick so as to not collapse, though it is not always the thickest part of the piston. Moving down the piston, the next thing is the ring groove. The closely manufactured groove accepts the third part, the precisely made piston ring. In the four-stroke engine, natural harmonics cause the ring to rotate as the piston goes up and down in the cylinder. This helps the groove stay clean of carbon. The solid pieces between the grooves are called ring lands. They are similar to the lands in a gun barrel. They support the shock loads the rings receive during combustion. The next part is the piston pin hole. This hole accepts a pin that connects the piston to the connecting rod. The hole is offset from the piston's center slightly so that when the piston and rod reach TDC, they do so at slightly different times. This spreads the shock loads at high rpm, easing stresses on the connecting rod and eliminating a noise called "piston slap." Surrounding the hole inside the piston are pin bosses, thick masses of metal that support the pin when it is inserted in the hole. The pin bosses are sometimes the thickest part of the piston. In some cases, they are not as thick as the crown. In either case however, the thickness of these two parts is important, as it determines much about how the piston deals with heat. Lastly, we come to the piston skirt. The skirt is the bearing portion of the piston. It slides against the cylinder wall, bearing the force of combustion on the power stroke, and the loads of compression on the compression stroke. There are also stresses involved with rpm that the piston and cylinder are designed together to deal with. The skirt is the part of the piston most in need of lubrication. Thus most lubrication problems show up on the piston skirt first.

Piston Shapes
There are two important ways in which pistons are shaped. First, the piston is not round, but elliptical in shape. The reason is the afore-meentioned pin bosses. The bosses' mass makes them absorb a lot of heat, which makes them expand more than any part of the piston. If the piston was instead made round, it would not be when fully warmed up. That would be a problem. Therefore, the width of the piston at the area of the bosses is narrower than it is elsewhere. The resulting shape (looking downward onto the piston crown) is an ellipse (an oval). Marine pistons are sometimes called "cam ground," which refers to the same thing (however, it isn't the shape that is being referred to in that case, but rather the machine that produces it). The other (second) shape all pistons have is taper. That is, the diameter of the piston at its crown is considerably smaller than its diameter at the skirt. The reason is the same as for the piston's ellipse. Only this time it is the crown, not the pin bosses, that necessitates the shape. The crown absorbs so much heat that it must be made smaller so that when fully heated, the piston will be straight.

Piston Manufacturing Methods
Pistons are manufactured in one of two ways. Those two ways are the cast piston and the forged piston. This brings us back to our question, which is better, cast or forged? But not so quickly. The cast piston is made of molten aluminum. The alloy is flowed into a mold having the shape of the finished product, in much the same way that many other cast parts are made. However, don't imagine wooden boxes full of coarse sand, into which melted aluminun is poured. Piston molds are actually permanent dies, intricately made multiple-piece steel shapes. The molten aluminum is vacuum drawn into the mold. So accurate is the process that the resulting casting requires minimal machining. The forged piston is made very differently. The metal is not molten, but heated somewhat. A blob of this hot aluminun alloy called an ingot is placed in a female mold, and a male ram is pounded into it. The result is not a piston, but a piston blank, which must then undergo many machining operations before it resembles a piston. These two methods of making pistons continue today, and there are interesting reasons for each of them. Let's examine those reasons by looking at the history and applications of each piston type.

The Cast Piston
The cast aluminum alloy piston has perhaps the longer history. It took over for the original steel part during the internal combustion engine's early development. The cast piston is the most familiar piston type.

Casting Alloys
Early cast aluminum pistons were made with inferior alloys. The piston expanded dramatically, requiring a loose fit in the cylinder and resulting in noisy operation when cold. Harley-Davidson pistons once had steel ribs inside them to control this expansion. Since about the 1960s however, most cast pistons have been unstrutted. Their alloys have gained silicon, a material that gives the pistons natural lubricity and limits heat expansion. All modern pistons have silicon in them. However, cast pistons have historically had the most. Some of them have as much as 25 percent silicon by volume. Silicon does bring a disadvantage however. It makes the piston brittle. Dropping a modern cast piston will usually crack it, so the piston must be handled carefully.

Mass Efficiency
Probably the greatest benefit of the cast piston is the efficiency of its mass. The multiple-piece molds allow intricate contours inside and out, resulting in light weight, good expansion control, and predictable heat flow through the part. That is, the piston designer can plan in the specific thickness in each place in the part that is desired, to result in expansion at those places that is warranted. So predictable is the cast piston's heat in fact that race tuners view the undersides of the piston to gauge the combustion efficiency of the engine. In much the same way others read spark plug, they read the dark splotches under the crown.

Applications
The cast piston is however expensive to manufacture. Die casting is costly, because it requires huge machines that do very specific jobs, and can't be easily adapted to do more than one kind of job. The result is that the casting process for pistons is relegated to the large piston supplier. The downside is that the cast piston is often found only in OEM specified sizes and types. There aren't a lot of different cast pistons to chose from if you are modifying an engine. The upside of this situation is that since only large piston manufacturers can afford to make cast pistons, they are usually competently made. In fact, the cast piston generally typifies the best technology that the piston industry has to offer. However, this doesn't mean it's the best piston for every application.

The Forged Piston
The forged piston is a more recent development. It appeared first on high-powered two-stroke engines. These engines were made in low production numbers, and their performance and use resulted in frequent detonation. Both of these traits, as we'll see, made the forged piston a pretty good match for this application.

Forging Alloys
The earliest forged pistons were also made with poor alloys. In many cases however they were even worse than the alloys the cast pistons used, because when the cast piston finally got silicon, the forged piston did not. The same brittleness that makes the cast piston crack when bumped hard would have resulted in even larger defects had it been used in a forging. Consequently, during the time that the cast piston defined a piston's normal expansion rate, the forged piston was far behind the technology. The forged piston had to be fitted loose, which made it noisy and wasted power. Recently however, silicon has been introduced to the forged piston. A mixture of alloys has been found that together with silicon do not result in defective forgings. For example, nickel has been found to offset the silicon's tendency toward brittleness. However, not very much nickel can be used, as it is a heavy metal, and it affects the mixture in other ways. The result is that the modern forged piston is much more dimensionally stable than was true in the past.

Mass Efficiency
However, once again, the forged piston's mass does more to define its characteristics than does even its materials. The forged piston has historically had a crude interior shape. The forging ram is straight, which results in a rectangular rather than an inticate interior. There is too much mass there. Consequently, the forged piston has poor dimensional stability. Its expansion is not very controllable. Many engine builders overcome these two problems (too much weight, unpredictable expansion) at least partly by removing by hand the extra material inside the forged piston. This allows them to fit them tighter and rev them higher. However, many forged pistons also have overly thick skirts as well as unsophisticated interiors. This is because the forging produces a piston blank, remember, and not a finished piston. The piston wholesaler takes this blank, and from it, carves out several different sizes and shapes of pistons. If the piston being made happens to be the largest the blank supports, it ends up with the thickest skirt. While hand reworking (or CNC milling, as many do now) the forged piston can lighten it and make it behave more like an intricately made cast piston, there is still excess weight due to the thick skirt.

Applications
Unlike the cast piston, the forged piston is easy to manufacture. Smaller piston manufacturers therefore specialize in this piston type, even if some of them may not be as competent at making pistons as are the larger cast piston makers. Forged pistons have quickly become the choice of custom engine builders because they can be had very quickly, and in virtually any configuration desired. Moreover, the forged piston's added thickness is used by these builders to custom configure the piston even further. For example, flycuts on the piston's crown for high performance valve relief is an easy process with the forged piston. There's a lot of material there in which to do it, much more than there is in the cast piston. The forged piston was also the first piston type to adopt the modern ultra-thin piston ring, for the same reason. It could be done easily and immediately. There were no molds for such a piston among the cast piston manufacturers for at least a year afterward. This situation has resulted in the forged piston acquiring something of a high performance personna, even though its overall technology is less current than the cast piston's. Most of that reputation is unearned, but in at least one way it is in fact a reality. The forged piston is inherently stronger than the cast piston. Lower silicon content of course would result in this, making the forged piston less brittle. However, there is another reason as well. The forging process compresses the alloy's molecules, making the material more dense than a casting. The result is a piston that withstands the pounding of detonation better. This is why OEMs use the forged piston in two-strokes and turbocharged engines. Forged pistons are also included in many OEM high performance options kits for their street models.

Summary
To sum up, the cast piston is light and very dimensionally stable. It is found in high-rpm mass-produced engines that are not subject to modification or prone to detonation. The piston is however fairly brittle, and the cost of its manufacture has limited its availablity outside the OEM sources and applications. On the other hand, the forged piston is inherently heavy and less dimensionally stable. It is a good choice for engines in which detonation is probable, and its wide availability has made it the choice of engine modifiers. The special demands of these end users has given the forged piston its own niche in the powersports market. The next time someone tells you how superior one piston type is over another, tell them the truth. Because, as Paul Harvey likes to say, "Now you know the rest of the story." Hold the fries, please.

Source:

Mike Nixon

_________________
My cars:

J. McBean: '98 Suzuki Swift 1.3L 16v SOHC 5sp+ "Mk5" Made in Canada
The Mini Rattler: '94 Suzuki Swift .993L 6v SOHC 5sp+ "Mk3" Made in Canada *The Winter Beater*
B. Berry: '90 Chevrolet Turbo Sprint 1.0L 6v SOHC 5sp+ "Mk2" Made in Japan

I got 18MPG in a 3cyl with a 5 speed manual 4dr, '93 Metro! :yeahyeah


Top
 Profile  
 
PostPosted: Thu Sep 04, 2008 12:25 pm 
Offline
User avatar

Joined: Tue Jul 20, 2004 7:17 pm
Posts: 3566
Location: Georgetown, Guyana
An interesting write-up -although I don't think the piston and the rod can reach tdc at different times - when the top of the rod is at the top of it's travel, so is the piston. The wrist pin is offset but I don't think that is the reason.

_________________
'93 1.3 Swift GLX
'98 2.0 Grand Vitara


Top
 Profile  
 
PostPosted: Thu Sep 18, 2008 2:38 pm 
Offline
User avatar

Joined: Mon Jun 18, 2007 1:35 pm
Posts: 2433
Location: Regina, SK
fordem wrote:
An interesting write-up -although I don't think the piston and the rod can reach tdc at different times - when the top of the rod is at the top of it's travel, so is the piston. The wrist pin is offset but I don't think that is the reason.


There are a few moments, that the piston is at TDC or BDC that the con rod isn't at a perfect 90 degree angle to it. But it happens only for a couple degree's of crank rotation.

_________________
My cars:

J. McBean: '98 Suzuki Swift 1.3L 16v SOHC 5sp+ "Mk5" Made in Canada
The Mini Rattler: '94 Suzuki Swift .993L 6v SOHC 5sp+ "Mk3" Made in Canada *The Winter Beater*
B. Berry: '90 Chevrolet Turbo Sprint 1.0L 6v SOHC 5sp+ "Mk2" Made in Japan

I got 18MPG in a 3cyl with a 5 speed manual 4dr, '93 Metro! :yeahyeah


Top
 Profile  
 
PostPosted: Thu Sep 18, 2008 3:34 pm 
Offline
User avatar

Joined: Tue Jul 20, 2004 7:17 pm
Posts: 3566
Location: Georgetown, Guyana
What's that got to do with anything?

TDC or top dead center is defined as the point in the crankshaft's rotation when the piston is at the top of it's travel and furthest away from the crankshaft - by necessity, the rod HAS to be in a dead straight line, and the crank at the top of it's throw.

And even if this was not so defined - the piston is at the top of it's travel at the same time the top of the rod is at the top of it's travel - regardless of the angle the rod is at - it is impossible for it to be any other way, assuming the piston is attached to the rod.

_________________
'93 1.3 Swift GLX
'98 2.0 Grand Vitara


Top
 Profile  
 
PostPosted: Thu Sep 18, 2008 4:35 pm 
Offline
User avatar

Joined: Mon Jun 18, 2007 1:35 pm
Posts: 2433
Location: Regina, SK
Ahh... I got confuzzled, dead centre is the time when the cylinder can't apply a turning force to the crank, so everything is inline.

But the simple fact is, just like a piston doesn't travel at a constant speed while on it's up/down stroke, or even the same distance, there is play in the system. The rotation of the crank is a horizontal movement briefly.

Had a teacher explain it to me once, and it made sense, because to time certain older sled engines, you put a dial indicator on the head, extended down through the plug hole. You could rock the crank very(very) slightly without having the indicator move, and these things are alot tighter than any car, roller bearings on the crank and wrist pin.

I may be wrong, but this is part of learning no?

_________________
My cars:

J. McBean: '98 Suzuki Swift 1.3L 16v SOHC 5sp+ "Mk5" Made in Canada
The Mini Rattler: '94 Suzuki Swift .993L 6v SOHC 5sp+ "Mk3" Made in Canada *The Winter Beater*
B. Berry: '90 Chevrolet Turbo Sprint 1.0L 6v SOHC 5sp+ "Mk2" Made in Japan

I got 18MPG in a 3cyl with a 5 speed manual 4dr, '93 Metro! :yeahyeah


Top
 Profile  
 
PostPosted: Thu Sep 18, 2008 5:34 pm 
Offline
User avatar

Joined: Tue Jul 20, 2004 7:17 pm
Posts: 3566
Location: Georgetown, Guyana
Look - there is no point in discussing how to detect top dead center - chances are we're not going to disagree.

This is not about how to detect when the piston is at top dead center, but whether or not, the piston, attached to the rod, can reach the top of it's travel at any time when the rod is not at top of it's travel - or vice versa - and I think we can ignore any play in the bearings.

Sure the piston will remain virtually stationary at the top of it's travel over several degrees of crankshaft movement as the crankshaft throw approaches top dead center, passes through it, and then travels away from it - but the top of the rod, is also stationary in that the movement is angular around the piston pin and not up/down.

The rod is what is pushing the piston up - when the top of the rod stops moving upward, so does the piston - when the top of the rod starts to move downward, so does the piston, this time pulled by the rod - they are physically attached to one another, the one cannot move without the other.

Once we agree on the paragraph directly preceeding this, then we must also agree, that whatever the reason for the piston pin offset - it can have nothing to do with the rod and the piston reaching top dead center at different times, because they move together.

_________________
'93 1.3 Swift GLX
'98 2.0 Grand Vitara


Top
 Profile  
 
PostPosted: Fri Sep 19, 2008 1:10 am 
Offline
User avatar

Joined: Mon Jun 18, 2007 1:35 pm
Posts: 2433
Location: Regina, SK
There is more at work than the simple idea of well when that moves, this moves.

When you rotate the crank 50% of the way from TDC to BDC the piston has moved 60% of it's travel.

http://www.epi-eng.com/piston_engine_technology/piston_motion_basics.htm wrote:
The reason is that as the crank rotates toward BDC, the crankpin also moves horizontally back toward the center of the cylinder and "restores" the effective length of the rod. That horizontal motion of the crankpin opposes the downward movement of the piston, subtracting from the half-stroke of vertical motion produced from 90° to BDC.


Bolded on my part to highlight signifigance.

In theory, you are right, but in everyday practice, theory caves out to wear, time, and metal being wierd under heat/pressure. There is a certain amount of play in all systems, givin that the two points of connection are round, rod does and can move before the piston goes downward, just the same as the piston will hit the peak of it's travel shortly before the crankpin hits the center line, and it becomes TDC.

The graphs on the link in the qoute show, that for a few degrees of crank spin, the movement of the piston stops dead before and after TDC/BDC.

_________________
My cars:

J. McBean: '98 Suzuki Swift 1.3L 16v SOHC 5sp+ "Mk5" Made in Canada
The Mini Rattler: '94 Suzuki Swift .993L 6v SOHC 5sp+ "Mk3" Made in Canada *The Winter Beater*
B. Berry: '90 Chevrolet Turbo Sprint 1.0L 6v SOHC 5sp+ "Mk2" Made in Japan

I got 18MPG in a 3cyl with a 5 speed manual 4dr, '93 Metro! :yeahyeah


Top
 Profile  
 
PostPosted: Fri Sep 19, 2008 9:02 am 
Offline
User avatar

Joined: Tue Jul 20, 2004 7:17 pm
Posts: 3566
Location: Georgetown, Guyana
gamefoo21 wrote:
There is more at work than the simple idea of well when that moves, this moves.

When you rotate the crank 50% of the way from TDC to BDC the piston has moved 60% of it's travel.

http://www.epi-eng.com/piston_engine_technology/piston_motion_basics.htm wrote:
The reason is that as the crank rotates toward BDC, the crankpin also moves horizontally back toward the center of the cylinder and "restores" the effective length of the rod. That horizontal motion of the crankpin opposes the downward movement of the piston, subtracting from the half-stroke of vertical motion produced from 90° to BDC.


Bolded on my part to highlight signifigance.

In theory, you are right, but in everyday practice, theory caves out to wear, time, and metal being wierd under heat/pressure. There is a certain amount of play in all systems, givin that the two points of connection are round, rod does and can move before the piston goes downward, just the same as the piston will hit the peak of it's travel shortly before the crankpin hits the center line, and it becomes TDC.

The graphs on the link in the qoute show, that for a few degrees of crank spin, the movement of the piston stops dead before and after TDC/BDC.


Like I said - chances are we will not disagree - once we take the endplay into consideration, the crank, rod and the piston move simultaneously - by the way - you don't appear to notice that I am not discussing the crankshaft, but the rod.

Read my last post - you'll see I already mentioned the crank turning without the piston moving up and down - however the issue there is one of detection, it is not that the piston is not moving, but rather the movement is so small that it is lost in the end play - if the crank is not marked, that is best done with a degree wheel and some sort of depth gauge - find the highest point of the piston travel, turn the crank one way until the piston moves down by a given amount, mark the degree wheel, turn the crank the other way so the piston rises and then falls by the same amount, mark the degree wheel a second time, and then split the difference on the degree wheel

When the crank is at TDC, the piston is also at TDC.

_________________
'93 1.3 Swift GLX
'98 2.0 Grand Vitara


Top
 Profile  
 
PostPosted: Fri Sep 19, 2008 10:13 am 
Offline
User avatar

Joined: Mon Jun 18, 2007 1:35 pm
Posts: 2433
Location: Regina, SK
fordem wrote:
When the crank is at TDC, the piston is also at TDC.


Agreed.

8)

My input comprehesion has been a little off, while I deal with a nasty case of strep throat. :|

_________________
My cars:

J. McBean: '98 Suzuki Swift 1.3L 16v SOHC 5sp+ "Mk5" Made in Canada
The Mini Rattler: '94 Suzuki Swift .993L 6v SOHC 5sp+ "Mk3" Made in Canada *The Winter Beater*
B. Berry: '90 Chevrolet Turbo Sprint 1.0L 6v SOHC 5sp+ "Mk2" Made in Japan

I got 18MPG in a 3cyl with a 5 speed manual 4dr, '93 Metro! :yeahyeah


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC - 5 hours


Who is online

Users browsing this forum: No registered users and 3 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group