Why forged pistons when everybody says......

Jebus!

The comment "Do forged pistons not contain explosives?" is full of awesome.

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Kirk Coughlin
Woodbury, MN and River Falls, WI

Quemadmodum gladius neminem occidit, occidentis telum est.

I've run hypereutectics for a good number of events (like maybe 15 on the same engine) without incidents. But the compression was kept low, the boost was kept to below 13 psi, and this was in an engine that does not rev high (turbo), and I still would prefer forged. The one disadvantage for forged tends to be the clearances are looser when cold and so I don't expect them to last as long before getting a bit loose in the cylinders and needing a re-bore and new pistons.

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starion887
The one disadvantage for forged tends to be the clearances are looser when cold and so I don't expect them to last as long before getting a bit loose in the cylinders and needing a re-bore and new pistons.

The difference in clearance is at most THREE ONE THOUSANDTHs of an inch.
Or more graphically, the difference is about the thickness of one sheet of ordinary paper

That's at amient temp.

I assure you that THAT difference is gone in a matter of seconds one the motor is making heat...

The difference is one of those "Yes its true, but the significance is an itty bitty non-important thing" dealies we read all the time one the web..

I'll betcha a nickle few people have ever held a hunk-o-aluminum and micked it at 65F then warmed it up to 265F and measured

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

True, but still makes them looser for some time. I'm one of those geeks who has done slow temp runs in an oven and measured pistons diameters; interesting how the overall shape changes with the built in design....and how they go so quickly way out of spec when they go over normal operating temps and run out of the compesating methods to keep the skirt changes to a minimum.

The forged pistons HAVE worn out faster (i.e., lose some compression) for me in the past, but I accept that for racing and high performance. It's just something to be aware of, IMO.

Quote
starion887
True, but still makes them looser for some time. I'm one of those geeks who has done slow temp runs in an oven and measured pistons diameters; interesting how the overall shape changes with the built in design....and how they go so quickly way out of spec when they go over normal operating temps and run out of the compesating methods to keep the skirt changes to a minimum.

The forged pistons HAVE worn out faster (i.e., lose some compression) for me in the past, but I accept that for racing and high performance. It's just something to be aware of, IMO.

Kinda busy so short:
AH! But what alloy were those?
Do you remember when and what brand?

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Why forged? And hbeams? Cuz DNF doth sucketh.
Mr. Scray you have my sympathies. I strongly dislike turning an engine inside out. Hoping that ARP and Wiseco and Eagle etc etc etc will make it so this new one holds together a bit longer...

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Jay Woodward
Snohomish, WA
'90 Mazdog Frankenprotege
Chronologically, 46...



Edited 1 time(s). Last edit at 09/04/2012 12:36AM by Jay.

"there’s a common misconception that forged pistons always require greater skirt-to-wall clearances. This is a notion that isn’t necessarily true because clearances depend on the type of alloy that’s used in a forged piston, the design of the piston itself and the application in which the piston will be used. Some forged alloys actually have a lower coefficient of thermal expansion than the alloys commonly used in conventional cast pistons!" www.enginebuildermag.com

Coefficient of thermal expansion (microns/*C)

Cast alloys:
201.0-T4, Sand cast: 19.3
204.0-T4, Sand cast: 19.3
208.0-T4, Permanent Mold Cast: 22.3
222.0-T61, Sand cast: 22.1
242.0-T61, Permanent Mold Cast: 22.7
413.0-F, Die-Casting Alloy: 20.4
443.0-F, Permanent Mold Cast: 22.1

Forged Alloys:
2618-T6: 22.3
2011-T6: 22.9
2014-T6: 23.0
2024-T6: 23.2
4032-T6: 19.4
4043-H18: 22.1
4045-__: 21.1
4032-T651: 19.4

As you see not too much difference from both but there is substancial more growth with temperature than paper sheets.

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john vanlandingham

Quote
starion887
The one disadvantage for forged tends to be the clearances are looser when cold and so I don't expect them to last as long before getting a bit loose in the cylinders and needing a re-bore and new pistons.

The difference in clearance is at most THREE ONE THOUSANDTHs of an inch.
Or more graphically, the difference is about the thickness of one sheet of ordinary paper

That's at amient temp.

I assure you that THAT difference is gone in a matter of seconds one the motor is making heat...

The difference is one of those "Yes its true, but the significance is an itty bitty non-important thing" dealies we read all the time one the web..

I'll betcha a nickle few people have ever held a hunk-o-aluminum and micked it at 65F then warmed it up to 265F and measured

Edited 1 time(s). Last edit at 09/04/2012 05:47AM by Cosworth.

Quote
Cosworth
Coefficient of thermal expansion (microns/*C)

Cast alloys:
201.0-T4, Sand cast: 19.3
204.0-T4, Sand cast: 19.3
208.0-T4, Permanent Mold Cast: 22.3
222.0-T61, Sand cast: 22.1
242.0-T61, Permanent Mold Cast: 22.7
413.0-F, Die-Casting Alloy: 20.4
443.0-F, Permanent Mold Cast: 22.1

Forged Alloys:
2618-T6: 22.3
2011-T6: 22.9
2014-T6: 23.0
2024-T6: 23.2
4032-T6: 19.4
4043-H18: 22.1
4045-__: 21.1
4032-T651: 19.4

As you see not too much difference from both but there is substancial more growth with temperature than paper sheets.

So, you wouldn't happen to have the equation that lets me use these constants to tell how much the metal is going to expand and contract? Preferably without partial derivatives and is this for volume, or area expansion?

Is it like volumetric where its just coeff * temp change?

---

Kirk Coughlin
Woodbury, MN and River Falls, WI

Quemadmodum gladius neminem occidit, occidentis telum est.

All were TRW designs from the 70's, both quench-dome types and dish top low compression. No clue here on the alloys......I also just tend to run on the wide end of clearance for everything; I'd rather have some extra blow-by that any chance of scuffing.

Quote
darkknight9
So, you wouldn't happen to have the equation that lets me use these constants to tell how much the metal is going to expand and contract? Preferably without partial derivatives and is this for volume, or area expansion?

Is it like volumetric where its just coeff * temp change?

I kinda doubt it; the expansion is a function of a lot of things like how the skirt thickness changes around the diameter from face towards pin area, tapers in thickness vertically, cuts here and there, and sometimes things like steel strip inserts to compensate and control the piston diameter changes over the regular operating temp ranges. From my measurements, the diameters do funny things within a small range up to about 250 degrees or more but stay closely controlled and then go waaaay out of spec reeeealy fast once you get over the maximum design operating temp and the expansion compensating design features run out of compensating ability. 'Splains why pistons seize up so badly once you run out of cooling.

Quote
starion887
All were TRW designs from the 70's, both quench-dome types and dish top low compression. No clue here on the alloys......I also just tend to run on the wide end of clearance for everything; I'd rather have some extra blow-by that any chance of scuffing.

You know why I asked.. The traditional US forged pistons from Jahn's, Arias, Venolia etc were specced at enormous clearances so for example these ancient Opel slugs that the Old Crusty Dave Clark got from Rick Delamere in the late 80s/early 90s that Rick had laying around forever that he'd gotten from his dad who had gotten them in presumably late 70s, those things were 92mm and wanted 010-012" cylinder to wall clearance. And that was for normal aspirated.
Bigger bore like 'Merikanski stuff had to be even bigger.....

Contrast that with forged Mahle pistons for OEM applications like Xratty, Cossie, or Volvo B23FT and there you see about .0015" to .0017, and maybe a thou more for hard use like rally.

And yeah I've seen worn to hell bores in short order in motors using those old school Venolia or Arias things, like set up at 010 and after 8 months and 5-6k miles it was at 020+".

Eeeeech!
So for me 1970 thru 1997 I used forged mahle in everything I built for hard use. And they seemed to last for years.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Slightly off-topic but I'm curious...

Does gravity have an effect on piston or ring wear? ie does a Boxer engine have different wear charateristics than an upright inline? or even a V?

If so, are they designed in a way to anticipate that?

Am I just an ignorant, keyboard engineer?

Simon
USUK Racing

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Simon Wright
Owner/Driver USUK Racing - Home of the USUK Racing LED Light Bars!

North America Rally Map

Quote
simoniac
Slightly off-topic but I'm curious...

Does gravity have an effect on piston or ring wear? ie does a Boxer engine have different wear charateristics than an upright inline? or even a V?

If so, are they designed in a way to anticipate that?

Am I just an ignorant, keyboard engineer?

Simon
USUK Racing

No.

The main side load on the piston are from the connecting rod being at an angle relative to the bore centerline. Gravity is negligible.

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john vanlandingham

Quote
starion887
All were TRW designs from the 70's, both quench-dome types and dish top low compression. No clue here on the alloys......I also just tend to run on the wide end of clearance for everything; I'd rather have some extra blow-by that any chance of scuffing.

You know why I asked.. The traditional US forged pistons from Jahn's, Arias, Venolia etc were specced at enormous clearances so for example these ancient Opel slugs that the Old Crusty Dave Clark got from Rick Delamere in the late 80s/early 90s that Rick had laying around forever that he'd gotten from his dad who had gotten them in presumably late 70s, those things were 92mm and wanted 010-012" cylinder to wall clearance. And that was for normal aspirated.
Bigger bore like 'Merikanski stuff had to be even bigger.....

Contrast that with forged Mahle pistons for OEM applications like Xratty, Cossie, or Volvo B23FT and there you see about .0015" to .0017, and maybe a thou more for hard use like rally.

And yeah I've seen worn to hell bores in short order in motors using those old school Venolia or Arias things, like set up at 010 and after 8 months and 5-6k miles it was at 020+".

Eeeeech!
So for me 1970 thru 1997 I used forged mahle in everything I built for hard use. And they seemed to last for years.

The TRW's for low comp dish pistons for Opel 1.9's had a recc clearance of .0024 and I used .003 to .004 to account for imperfect boring. TRW L2407 hi comp for the 351C had a clearance spec of around .0035. Never had any pistons with .010 or .012" clearance spec'd!