Forged or Cast pistons ?
Discussion
They would hold up more rpm, more heat and stress and boost pressure..
Are you planning street or race engine?
If fast road/street why not go to hypereutectic piston? I will say they are +50% better than std cast piston and -50% worse than forged.
Forged piston setpack is piston rattle when engine is cold + it might cause slightly oil consumption because piston to bore clearance is large vs STD cast piston.
Are you planning street or race engine?
If fast road/street why not go to hypereutectic piston? I will say they are +50% better than std cast piston and -50% worse than forged.
Forged piston setpack is piston rattle when engine is cold + it might cause slightly oil consumption because piston to bore clearance is large vs STD cast piston.
steve-V8s said:
That sort of confirms what I have been told.
The people that built the bottom end of my Rover V8 say that the higher oil consumption and sump compression is normal because of the forged pistons.
Afraid I'd say not true.The people that built the bottom end of my Rover V8 say that the higher oil consumption and sump compression is normal because of the forged pistons.
Unless the pistons are pretty poor, bore clearances should not be huge.
Even a good quality turbo piston on most setups can run around 3-4thou these days.
So for a n/a setup they should neither be noisy or use much oil. Even a forced induction engine I wouldnt be happy with one that used a lot of oil.
It's either to speccing bad parts, building to sloppy tolerances or just doing something wrong.
I spoke to V8 developments couple of year go. Not sure is it Rob or Roy. I ask my 5.2litre oil consumption problem. They say it is quite normal that they like to eat oli approx 1L/1000km or like similar amount. They say don´t worry about it.. 
My engine eat approx 0.8-1L/1000km no smoke or smell at all.
I also ask this same question for ROSS racing piston tehchnical help service. Get 2 answer of 2 different guy. They told me it is normal when engine is especially cold and also rod/crank ratio has got small effect for this.
My engine has got ROSS forged piston´s total seal ring and piston to bore clearance +0.1mm. Bored and honed (fine sock honed to race specks)to recommend tolerances.
I also have hear this same oil sonsumption problem gouple of guy´s who use forged piston´s.
My engine eat approx 0.8-1L/1000km no smoke or smell at all.
I also ask this same question for ROSS racing piston tehchnical help service. Get 2 answer of 2 different guy. They told me it is normal when engine is especially cold and also rod/crank ratio has got small effect for this.
My engine has got ROSS forged piston´s total seal ring and piston to bore clearance +0.1mm. Bored and honed (fine sock honed to race specks)to recommend tolerances.
I also have hear this same oil sonsumption problem gouple of guy´s who use forged piston´s.
http://www.strokerengine.com/RodStroke.html
The shorter piston then tends to rock in the cylinder as it moves up and down, because there is less skirt surface area to support the piston.
Further complicating matters is that we typically use a forged piston have 0.004" or more clearance allowing even more rocking, than a cast piston with 0.002" clearance would experience.
This rocking makes it more difficult to keep the rings straight and sealed.
Even though the rocking is not very much it does reduce the effectiveness of the ring seal leading to some increase in oil consumption.
increasing stroke lengths can reduce piston and ring life due to the less than ideal rod to stroke ratio and can cause some oil consumption.
These issue are not desirable, but it is the compromise we must make to have the extra power this combination produces.
To minimize problems I focus on the details, such as paying close attention to proper cylinder honing and cleaning.
I will believe V8 developments word´s
If you use STD size piston, normal lenght skirt etc and STD stroke, i will belive oil consumption is less than long stroke monster engine.
I have been build couple of STD rover V8 engine with Original piston/ring´s and no oil consumption. My first build forged piston motor use oil.
The shorter piston then tends to rock in the cylinder as it moves up and down, because there is less skirt surface area to support the piston.
Further complicating matters is that we typically use a forged piston have 0.004" or more clearance allowing even more rocking, than a cast piston with 0.002" clearance would experience.
This rocking makes it more difficult to keep the rings straight and sealed.
Even though the rocking is not very much it does reduce the effectiveness of the ring seal leading to some increase in oil consumption.
increasing stroke lengths can reduce piston and ring life due to the less than ideal rod to stroke ratio and can cause some oil consumption.
These issue are not desirable, but it is the compromise we must make to have the extra power this combination produces.
To minimize problems I focus on the details, such as paying close attention to proper cylinder honing and cleaning.
I will believe V8 developments word´s
If you use STD size piston, normal lenght skirt etc and STD stroke, i will belive oil consumption is less than long stroke monster engine.
I have been build couple of STD rover V8 engine with Original piston/ring´s and no oil consumption. My first build forged piston motor use oil.
On the (forged) pistons I had made recently, on a 4.5" bore, they recommended a 6 thou clearance for marine, NA, closed cooled, use
stevieturbo said:
Afraid I'd say not true.
Unless the pistons are pretty poor, bore clearances should not be huge.
Even a good quality turbo piston on most setups can run around 3-4thou these days.
So for a n/a setup they should neither be noisy or use much oil. Even a forced induction engine I wouldnt be happy with one that used a lot of oil.
It's either to speccing bad parts, building to sloppy tolerances or just doing something wrong.
Unless the pistons are pretty poor, bore clearances should not be huge.
Even a good quality turbo piston on most setups can run around 3-4thou these days.
So for a n/a setup they should neither be noisy or use much oil. Even a forced induction engine I wouldnt be happy with one that used a lot of oil.
It's either to speccing bad parts, building to sloppy tolerances or just doing something wrong.
Boosted LS1 said:
You may have high oil consumption if you're running an open breather circuit or a pcv circuit without a restriction in it. If you have pressure in the crank case the rings may be worn or have huge gaps.
When i´m running plenum, I have got 1/2" pipe right hand side rockercover to plenum. + I´m also trying to resist it, no effect to oil consumption if is it straight or resisted. I also fit oil catch can, no effect.Now i´m running ITB´s with open breather, oil consumption stay same.
I replace head gasket after engine has been drived in approx 2000km. No classing in bores and look like new.
Compression pressure is same all 8 cylinder. Approx 200psi(rally cam)
My opinion:
As for the link. If your piston slap´s, then oil can pass other side of the piston skirt to the ring´s. And because piston to bore clearance is big it can pass to consumption champer backside of the piston ring croove and ring.. Every time when piston start to go up and down skirt moves another side of bore and can´t keep the ring´s straight and sealed then it would pass slightly oil to consumption champer..
My understanding STD cast piston skirt/tight bore clearance is "designed" to scrape extra oil away from cylinder wall? And when using forged piston + extra clearance oil is also lubricate piston skirt and ring´s..
If my ring´s are bad, i will estimate that my compression pressure is say 140psi..
When i build my engine i douple check ring end cap + etc..
+ When i tune 2 stroke engine´s i usually round(shape ina curve)piston skirt to get extra oil to lubricate piston/cylinder wall.. Because sharp edge skirt scrape oil away..
Hope some one understand my English..
I recently bought a set of 4" forged CP pistons for a n/a engine. They recommend 3thou clearance.
Wiseco suggest similar clearances. 6 thou just sounds huge unless it's going to run extreme duties for extended periods.
I would also agree on the breathers. Any performance engine needs effective breathing systems in place. Crappy egay catch cans dont count.
And the dipstick should definitely not be popping put !
And Ive had an engine that did have 7 thou bore clearance apart. The pistons were very sloppy and bore seal was very poor. It is wayyyy too much.
Even it didnt use a litre of oil every 1000km or indeed 1000 miles. 1/2 litre maybe and I thought that was excessive.
Wiseco suggest similar clearances. 6 thou just sounds huge unless it's going to run extreme duties for extended periods.
I would also agree on the breathers. Any performance engine needs effective breathing systems in place. Crappy egay catch cans dont count.
And the dipstick should definitely not be popping put !
And Ive had an engine that did have 7 thou bore clearance apart. The pistons were very sloppy and bore seal was very poor. It is wayyyy too much.
Even it didnt use a litre of oil every 1000km or indeed 1000 miles. 1/2 litre maybe and I thought that was excessive.
Piston alloys need different clearances. Some expand more then others. I use one brand which recommends .007" on a 93.5mm bore for turbo use. They run nice and quiet, don't rattle and don't burn oil. Could just be down to rod angle.
Slotted skirt cast pistons have cooler skirts and hotter crowns so run with tight skirt clearances, maybe as tight as .0005". Solid skirt forged pistons transmit a lot of heat from the crown into the skirts which is one reason for the bigger clearance.
Slotted skirt cast pistons have cooler skirts and hotter crowns so run with tight skirt clearances, maybe as tight as .0005". Solid skirt forged pistons transmit a lot of heat from the crown into the skirts which is one reason for the bigger clearance.
Stevie or Boosted.
Can you made me new top hat Rover 5.2 or 5.4litre bottom end with nice set of forged pistons and manley rod´s and give me a 1 year/+5000km warranty? If it eat oil i can send it back and you give my money back?
I have always use Valvoline VR1 20w50mineral racing oil. And i notice when oil is new, it dosen´t eat as mutch as when it is old/drived approx +1500km. (almoust 0 oil consumption when oil is new/drived below 1000km).
My local race engine builder say that this oil is loosing some particles after +1000km. It is designed to race/short use only, not "long life" oil like mobil 1 or castrol 10w60 TWS. Haven´t try any other oil, but i think it is good idea in next summer..
Or can you tell me why so many guy´s ask oil consumption problem with forged piston´s for quick google search?
There is always 2 opinion with oil consumption and forged pistons.
Can you made me new top hat Rover 5.2 or 5.4litre bottom end with nice set of forged pistons and manley rod´s and give me a 1 year/+5000km warranty? If it eat oil i can send it back and you give my money back?
I have always use Valvoline VR1 20w50mineral racing oil. And i notice when oil is new, it dosen´t eat as mutch as when it is old/drived approx +1500km. (almoust 0 oil consumption when oil is new/drived below 1000km).
My local race engine builder say that this oil is loosing some particles after +1000km. It is designed to race/short use only, not "long life" oil like mobil 1 or castrol 10w60 TWS. Haven´t try any other oil, but i think it is good idea in next summer..
Or can you tell me why so many guy´s ask oil consumption problem with forged piston´s for quick google search?
There is always 2 opinion with oil consumption and forged pistons.
I think the issues I'm seeing raised here of oil consumption with forged pistons are missing the real point and abjectly failing to correctly identify cause and effect. OE engine designers perform extended testing on their pistons and bore finish to optimise all aspects of service life and performance. Even then a few engines slip through the R&D net and have known problems which necessitate a redesign.
The real issue is not with "forged pistons" but with "aftermarket pistons". Pistons made from generic blanks with generic skirt profiles and clearances specified to try and make sure they at least don't seize up in unknown engines with wildly different specifications and operating conditions and generate a fault claim. No wonder so many of them can't possibly be optimised for every individual engine. Because nearly all aftermarket pistons for uprated duty are forged because it's not cost effective to make expensive casting dies for low volume production it should be obvious that when problems occur it's going to be with forged pistons. Most aftermarket cast pistons are OE replacement so they are just copies of the stock piston. Hardly surprising these generally work fine.
Many OE high performance engines also have forged pistons but these are developed for their specific application with the normal extensive R&D and run perfectly happily which rather proves the point that it's not forged pistons that have a problem but incorrectly designed or installed pistons in general.
As with the other shiny "must have" bits that people insist on fitting to their engines like billet rods, uprated rod bolts and stainless valves I have little doubt that most engine specs have not the slightest need for forged pistons. Well designed cast pistons can cope with enormous abuse but any engine that's not been correctly set up will fail if it detonates or bore clearance is too tight. Fitting forged pistons might mask the underlying problem but certainly won't cure it.
Another issue is there are two wildly different materials used for forged pistons. 2618 is a high thermal expansion, low (almost zero) silicon alloy originally developed by Rolls Royce for the V12 Merlin engine during WW2. It has very good high temperature strength and cured the original problem of supercharged aircraft engine pistons failing under high boost pressure. However it has rapid ring groove wear because silicon contributes much of the wear resistance in aluminium alloys.
The other main forging alloy is 4032 which has exactly the same silicon content of 10% to 13% as the main casting alloys such as LM13. It therefore also has the same low thermal expansion coefficient as LM13 because the silicon content is what primarily determines aluminium alloy expansion rate. In fact it's a straight line curve with a simple equation. In millionths per degree C the thermal expansion coefficient (TEC) of aluminium alloys is about 22.3 - (0.25 x silicon content %).
So 2618 which has almost zero silicon has a TEC of 22.3. 4032 and LM13 with say 11% silicon content have a TEC of 22.3 - (0.25 x 11) = 19.5
Hypereutectic alloys (greater than approx 13% silicon) have even better ring groove wear resistance than eutectic casting alloys but are more brittle. With 16% to 19% silicon content their TEC can be as low as 18. This means they can run with a little less bore clearance than LM13 (maybe 3 or 4 tenths of a thou) and the top ring groove can be closer to the piston crown which reduces crevice volume exhaust emissions.
But it's not the alloy expansion rate that primarily determines the required bore clearance. Time for more maths. Lets assume a 90mm bore engine, 95 degrees C for the bore wall and piston skirt and 15 degrees C for ambient. Cast iron has a TEC of 12. At 80 degrees C above ambient it can be easily calculated that the bore expands by 3.4 thou and a cast piston skirt with a TEC of 19.5 by 5.5 thou. Therefore it would appear the required piston/bore clearance must be at least 2.1 thou. However stock pistons routinely run 1 thou clearance or even less. Why don't they seize? Because the skirts are not round but oval - smaller across the pin boss axis by about 20 thou than perpendicular to the pin. The skirts distort when hot to accommodate the extra expansion sideways towards the pin bosses.
If we specify 4032 alloy for a forging the skirt expansion should be exactly the same as with LM13 and with 2618 at a TEC of 22.3 it should be 6.3 thou leading to a theoretical requirement for only an additional 0.8 thou clearance over a cast piston. If a stock cast piston can manage on 1 thou clearance why can't a 4032 forged one manage on the same and a 2618 forged piston on only 1.8 thou instead of the usual 4 or more?
The reason is that it's piston shape not alloy type that mainly determines the required bore clearance. Cast pistons have flexible skirts and usually have a groove behind the oil ring that thermally isolates the crown from the skirt except for heat transmission down the pin bosses. Forgings can't be made with undercuts and are designed for strength not flexibility. So more heat transmits from the crown to the skirt, the crown runs cooler which helps reliability in extreme service, the skirts flex less so they accomodate distortion less well and they run hotter into the bargain which generates even more expansion.
So what piston to choose? For most applications stock should be just fine IMO. For very high performance road engines and most race ones 4032 will have a longer service life and nicer operating characteristics than 2618 which I'd reserve for only very high boost engines and ultra high rpm race and drag ones that only spend their life at full chat. Putting 2618 into a road motor that spends most of its life with the piston barely up to temperature and not fully expanded to bore size is a recipe for piston slap, high oil consumption and other woes.
I'd also suggest that most road engines with aftermarket forged pistons are running far too much clearance if the manufacturer's specs are followed. Their concern is that you don't seize one. They are a damn sight less worried about a bit of oil consumption or piston noise. The trouble is you don't have the budget to experiment with smaller than recommended clearances until you stop seizing pistons. The OE engine manufacturer does!
One thing is for damn sure. You can tune a stock engine a hell of a lot, even stick a turbo onto an engine that doesn't have one as stock and never give a thought to the OE piston clearance which could be as little as half a thou. I've never seen a stock piston seize just because the engine was modified or because it was used in a limited modification race series unless there were other engine faults. I'd therefore suggest that when you look at the recommended clearances for aftermarket forged pistons which increase dramatically with each additional category of usage or turbo boost over and above basic road use that most of this extra is to give them peace of mind rather than you an optimised engine.
The real issue is not with "forged pistons" but with "aftermarket pistons". Pistons made from generic blanks with generic skirt profiles and clearances specified to try and make sure they at least don't seize up in unknown engines with wildly different specifications and operating conditions and generate a fault claim. No wonder so many of them can't possibly be optimised for every individual engine. Because nearly all aftermarket pistons for uprated duty are forged because it's not cost effective to make expensive casting dies for low volume production it should be obvious that when problems occur it's going to be with forged pistons. Most aftermarket cast pistons are OE replacement so they are just copies of the stock piston. Hardly surprising these generally work fine.
Many OE high performance engines also have forged pistons but these are developed for their specific application with the normal extensive R&D and run perfectly happily which rather proves the point that it's not forged pistons that have a problem but incorrectly designed or installed pistons in general.
As with the other shiny "must have" bits that people insist on fitting to their engines like billet rods, uprated rod bolts and stainless valves I have little doubt that most engine specs have not the slightest need for forged pistons. Well designed cast pistons can cope with enormous abuse but any engine that's not been correctly set up will fail if it detonates or bore clearance is too tight. Fitting forged pistons might mask the underlying problem but certainly won't cure it.
Another issue is there are two wildly different materials used for forged pistons. 2618 is a high thermal expansion, low (almost zero) silicon alloy originally developed by Rolls Royce for the V12 Merlin engine during WW2. It has very good high temperature strength and cured the original problem of supercharged aircraft engine pistons failing under high boost pressure. However it has rapid ring groove wear because silicon contributes much of the wear resistance in aluminium alloys.
The other main forging alloy is 4032 which has exactly the same silicon content of 10% to 13% as the main casting alloys such as LM13. It therefore also has the same low thermal expansion coefficient as LM13 because the silicon content is what primarily determines aluminium alloy expansion rate. In fact it's a straight line curve with a simple equation. In millionths per degree C the thermal expansion coefficient (TEC) of aluminium alloys is about 22.3 - (0.25 x silicon content %).
So 2618 which has almost zero silicon has a TEC of 22.3. 4032 and LM13 with say 11% silicon content have a TEC of 22.3 - (0.25 x 11) = 19.5
Hypereutectic alloys (greater than approx 13% silicon) have even better ring groove wear resistance than eutectic casting alloys but are more brittle. With 16% to 19% silicon content their TEC can be as low as 18. This means they can run with a little less bore clearance than LM13 (maybe 3 or 4 tenths of a thou) and the top ring groove can be closer to the piston crown which reduces crevice volume exhaust emissions.
But it's not the alloy expansion rate that primarily determines the required bore clearance. Time for more maths. Lets assume a 90mm bore engine, 95 degrees C for the bore wall and piston skirt and 15 degrees C for ambient. Cast iron has a TEC of 12. At 80 degrees C above ambient it can be easily calculated that the bore expands by 3.4 thou and a cast piston skirt with a TEC of 19.5 by 5.5 thou. Therefore it would appear the required piston/bore clearance must be at least 2.1 thou. However stock pistons routinely run 1 thou clearance or even less. Why don't they seize? Because the skirts are not round but oval - smaller across the pin boss axis by about 20 thou than perpendicular to the pin. The skirts distort when hot to accommodate the extra expansion sideways towards the pin bosses.
If we specify 4032 alloy for a forging the skirt expansion should be exactly the same as with LM13 and with 2618 at a TEC of 22.3 it should be 6.3 thou leading to a theoretical requirement for only an additional 0.8 thou clearance over a cast piston. If a stock cast piston can manage on 1 thou clearance why can't a 4032 forged one manage on the same and a 2618 forged piston on only 1.8 thou instead of the usual 4 or more?
The reason is that it's piston shape not alloy type that mainly determines the required bore clearance. Cast pistons have flexible skirts and usually have a groove behind the oil ring that thermally isolates the crown from the skirt except for heat transmission down the pin bosses. Forgings can't be made with undercuts and are designed for strength not flexibility. So more heat transmits from the crown to the skirt, the crown runs cooler which helps reliability in extreme service, the skirts flex less so they accomodate distortion less well and they run hotter into the bargain which generates even more expansion.
So what piston to choose? For most applications stock should be just fine IMO. For very high performance road engines and most race ones 4032 will have a longer service life and nicer operating characteristics than 2618 which I'd reserve for only very high boost engines and ultra high rpm race and drag ones that only spend their life at full chat. Putting 2618 into a road motor that spends most of its life with the piston barely up to temperature and not fully expanded to bore size is a recipe for piston slap, high oil consumption and other woes.
I'd also suggest that most road engines with aftermarket forged pistons are running far too much clearance if the manufacturer's specs are followed. Their concern is that you don't seize one. They are a damn sight less worried about a bit of oil consumption or piston noise. The trouble is you don't have the budget to experiment with smaller than recommended clearances until you stop seizing pistons. The OE engine manufacturer does!
One thing is for damn sure. You can tune a stock engine a hell of a lot, even stick a turbo onto an engine that doesn't have one as stock and never give a thought to the OE piston clearance which could be as little as half a thou. I've never seen a stock piston seize just because the engine was modified or because it was used in a limited modification race series unless there were other engine faults. I'd therefore suggest that when you look at the recommended clearances for aftermarket forged pistons which increase dramatically with each additional category of usage or turbo boost over and above basic road use that most of this extra is to give them peace of mind rather than you an optimised engine.
Pumaracing said:
I think the issues I'm seeing raised here of oil consumption with forged pistons are missing the real point and abjectly failing to correctly identify cause and effect. OE engine designers perform extended testing on their pistons and bore finish to optimise all aspects of service life and performance. Even then a few engines slip through the R&D net and have known problems which necessitate a redesign.
The real issue is not with "forged pistons" but with "aftermarket pistons". Pistons made from generic blanks with generic skirt profiles and clearances specified to try and make sure they at least don't seize up in unknown engines with wildly different specifications and operating conditions and generate a fault claim. No wonder so many of them can't possibly be optimised for every individual engine. Because nearly all aftermarket pistons for uprated duty are forged because it's not cost effective to make expensive casting dies for low volume production it should be obvious that when problems occur it's going to be with forged pistons. Most aftermarket cast pistons are OE replacement so they are just copies of the stock piston. Hardly surprising these generally work fine.
Many OE high performance engines also have forged pistons but these are developed for their specific application with the normal extensive R&D and run perfectly happily which rather proves the point that it's not forged pistons that have a problem but incorrectly designed or installed pistons in general.
As with the other shiny "must have" bits that people insist on fitting to their engines like billet rods, uprated rod bolts and stainless valves I have little doubt that most engine specs have not the slightest need for forged pistons. Well designed cast pistons can cope with enormous abuse but any engine that's not been correctly set up will fail if it detonates or bore clearance is too tight. Fitting forged pistons might mask the underlying problem but certainly won't cure it.
Another issue is there are two wildly different materials used for forged pistons. 2618 is a high thermal expansion, low (almost zero) silicon alloy originally developed by Rolls Royce for the V12 Merlin engine during WW2. It has very good high temperature strength and cured the original problem of supercharged aircraft engine pistons failing under high boost pressure. However it has rapid ring groove wear because silicon contributes much of the wear resistance in aluminium alloys.
The other main forging alloy is 4032 which has exactly the same silicon content of 10% to 13% as the main casting alloys such as LM13. It therefore also has the same low thermal expansion coefficient as LM13 because the silicon content is what primarily determines aluminium alloy expansion rate. In fact it's a straight line curve with a simple equation. In millionths per degree C the thermal expansion coefficient (TEC) of aluminium alloys is about 22.3 - (0.25 x silicon content %).
So 2618 which has almost zero silicon has a TEC of 22.3. 4032 and LM13 with say 11% silicon content have a TEC of 22.3 - (0.25 x 11) = 19.5
Hypereutectic alloys (greater than approx 13% silicon) have even better ring groove wear resistance than eutectic casting alloys but are more brittle. With 16% to 19% silicon content their TEC can be as low as 18. This means they can run with a little less bore clearance than LM13 (maybe 3 or 4 tenths of a thou) and the top ring groove can be closer to the piston crown which reduces crevice volume exhaust emissions.
But it's not the alloy expansion rate that primarily determines the required bore clearance. Time for more maths. Lets assume a 90mm bore engine, 95 degrees C for the bore wall and piston skirt and 15 degrees C for ambient. Cast iron has a TEC of 12. At 80 degrees C above ambient it can be easily calculated that the bore expands by 3.4 thou and a cast piston skirt with a TEC of 19.5 by 5.5 thou. Therefore it would appear the required piston/bore clearance must be at least 2.1 thou. However stock pistons routinely run 1 thou clearance or even less. Why don't they seize? Because the skirts are not round but oval - smaller across the pin boss axis by about 20 thou than perpendicular to the pin. The skirts distort when hot to accommodate the extra expansion sideways towards the pin bosses.
If we specify 4032 alloy for a forging the skirt expansion should be exactly the same as with LM13 and with 2618 at a TEC of 22.3 it should be 6.3 thou leading to a theoretical requirement for only an additional 0.8 thou clearance over a cast piston. If a stock cast piston can manage on 1 thou clearance why can't a 4032 forged one manage on the same and a 2618 forged piston on only 1.8 thou instead of the usual 4 or more?
The reason is that it's piston shape not alloy type that mainly determines the required bore clearance. Cast pistons have flexible skirts and usually have a groove behind the oil ring that thermally isolates the crown from the skirt except for heat transmission down the pin bosses. Forgings can't be made with undercuts and are designed for strength not flexibility. So more heat transmits from the crown to the skirt, the crown runs cooler which helps reliability in extreme service, the skirts flex less so they accomodate distortion less well and they run hotter into the bargain which generates even more expansion.
So what piston to choose? For most applications stock should be just fine IMO. For very high performance road engines and most race ones 4032 will have a longer service life and nicer operating characteristics than 2618 which I'd reserve for only very high boost engines and ultra high rpm race and drag ones that only spend their life at full chat. Putting 2618 into a road motor that spends most of its life with the piston barely up to temperature and not fully expanded to bore size is a recipe for piston slap, high oil consumption and other woes.
I'd also suggest that most road engines with aftermarket forged pistons are running far too much clearance if the manufacturer's specs are followed. Their concern is that you don't seize one. They are a damn sight less worried about a bit of oil consumption or piston noise. The trouble is you don't have the budget to experiment with smaller than recommended clearances until you stop seizing pistons. The OE engine manufacturer does!
One thing is for damn sure. You can tune a stock engine a hell of a lot, even stick a turbo onto an engine that doesn't have one as stock and never give a thought to the OE piston clearance which could be as little as half a thou. I've never seen a stock piston seize just because the engine was modified or because it was used in a limited modification race series unless there were other engine faults. I'd therefore suggest that when you look at the recommended clearances for aftermarket forged pistons which increase dramatically with each additional category of usage or turbo boost over and above basic road use that most of this extra is to give them peace of mind rather than you an optimised engine.
The real issue is not with "forged pistons" but with "aftermarket pistons". Pistons made from generic blanks with generic skirt profiles and clearances specified to try and make sure they at least don't seize up in unknown engines with wildly different specifications and operating conditions and generate a fault claim. No wonder so many of them can't possibly be optimised for every individual engine. Because nearly all aftermarket pistons for uprated duty are forged because it's not cost effective to make expensive casting dies for low volume production it should be obvious that when problems occur it's going to be with forged pistons. Most aftermarket cast pistons are OE replacement so they are just copies of the stock piston. Hardly surprising these generally work fine.
Many OE high performance engines also have forged pistons but these are developed for their specific application with the normal extensive R&D and run perfectly happily which rather proves the point that it's not forged pistons that have a problem but incorrectly designed or installed pistons in general.
As with the other shiny "must have" bits that people insist on fitting to their engines like billet rods, uprated rod bolts and stainless valves I have little doubt that most engine specs have not the slightest need for forged pistons. Well designed cast pistons can cope with enormous abuse but any engine that's not been correctly set up will fail if it detonates or bore clearance is too tight. Fitting forged pistons might mask the underlying problem but certainly won't cure it.
Another issue is there are two wildly different materials used for forged pistons. 2618 is a high thermal expansion, low (almost zero) silicon alloy originally developed by Rolls Royce for the V12 Merlin engine during WW2. It has very good high temperature strength and cured the original problem of supercharged aircraft engine pistons failing under high boost pressure. However it has rapid ring groove wear because silicon contributes much of the wear resistance in aluminium alloys.
The other main forging alloy is 4032 which has exactly the same silicon content of 10% to 13% as the main casting alloys such as LM13. It therefore also has the same low thermal expansion coefficient as LM13 because the silicon content is what primarily determines aluminium alloy expansion rate. In fact it's a straight line curve with a simple equation. In millionths per degree C the thermal expansion coefficient (TEC) of aluminium alloys is about 22.3 - (0.25 x silicon content %).
So 2618 which has almost zero silicon has a TEC of 22.3. 4032 and LM13 with say 11% silicon content have a TEC of 22.3 - (0.25 x 11) = 19.5
Hypereutectic alloys (greater than approx 13% silicon) have even better ring groove wear resistance than eutectic casting alloys but are more brittle. With 16% to 19% silicon content their TEC can be as low as 18. This means they can run with a little less bore clearance than LM13 (maybe 3 or 4 tenths of a thou) and the top ring groove can be closer to the piston crown which reduces crevice volume exhaust emissions.
But it's not the alloy expansion rate that primarily determines the required bore clearance. Time for more maths. Lets assume a 90mm bore engine, 95 degrees C for the bore wall and piston skirt and 15 degrees C for ambient. Cast iron has a TEC of 12. At 80 degrees C above ambient it can be easily calculated that the bore expands by 3.4 thou and a cast piston skirt with a TEC of 19.5 by 5.5 thou. Therefore it would appear the required piston/bore clearance must be at least 2.1 thou. However stock pistons routinely run 1 thou clearance or even less. Why don't they seize? Because the skirts are not round but oval - smaller across the pin boss axis by about 20 thou than perpendicular to the pin. The skirts distort when hot to accommodate the extra expansion sideways towards the pin bosses.
If we specify 4032 alloy for a forging the skirt expansion should be exactly the same as with LM13 and with 2618 at a TEC of 22.3 it should be 6.3 thou leading to a theoretical requirement for only an additional 0.8 thou clearance over a cast piston. If a stock cast piston can manage on 1 thou clearance why can't a 4032 forged one manage on the same and a 2618 forged piston on only 1.8 thou instead of the usual 4 or more?
The reason is that it's piston shape not alloy type that mainly determines the required bore clearance. Cast pistons have flexible skirts and usually have a groove behind the oil ring that thermally isolates the crown from the skirt except for heat transmission down the pin bosses. Forgings can't be made with undercuts and are designed for strength not flexibility. So more heat transmits from the crown to the skirt, the crown runs cooler which helps reliability in extreme service, the skirts flex less so they accomodate distortion less well and they run hotter into the bargain which generates even more expansion.
So what piston to choose? For most applications stock should be just fine IMO. For very high performance road engines and most race ones 4032 will have a longer service life and nicer operating characteristics than 2618 which I'd reserve for only very high boost engines and ultra high rpm race and drag ones that only spend their life at full chat. Putting 2618 into a road motor that spends most of its life with the piston barely up to temperature and not fully expanded to bore size is a recipe for piston slap, high oil consumption and other woes.
I'd also suggest that most road engines with aftermarket forged pistons are running far too much clearance if the manufacturer's specs are followed. Their concern is that you don't seize one. They are a damn sight less worried about a bit of oil consumption or piston noise. The trouble is you don't have the budget to experiment with smaller than recommended clearances until you stop seizing pistons. The OE engine manufacturer does!
One thing is for damn sure. You can tune a stock engine a hell of a lot, even stick a turbo onto an engine that doesn't have one as stock and never give a thought to the OE piston clearance which could be as little as half a thou. I've never seen a stock piston seize just because the engine was modified or because it was used in a limited modification race series unless there were other engine faults. I'd therefore suggest that when you look at the recommended clearances for aftermarket forged pistons which increase dramatically with each additional category of usage or turbo boost over and above basic road use that most of this extra is to give them peace of mind rather than you an optimised engine.
Well explained,you aren't my old mechanical application teacher are you? Must repeat to myself power stroke combustion is an even burn not an explosion. Off to get some painkillers.......
. On a side note our Focus ST170 has OEM forged pistons and don't use any oil (yet).lemonslap said:
On a side note our Focus ST170 has OEM forged pistons...
So did the venerable 150ish bhp Vauxhall C20XE Astra GTE 16v engine introduced over 20 years ago and that is one of the most reliable and frequently modded engines ever made. Pistons were made by Kolbenschmidt and bore clearance was only 0.03mm. When the C20LET turbo version was introduced power went up to 200 bhp and the Mahle pistons that used had a minimum specified clearance of 0.05mm, no doubt to cope with the extra heat.Those figures for an 86mm bore engine might give some idea of how tight forged pistons can be run in different applications and I have little doubt that aftermarket 4032 ones could be used very similarly.
For comparison Wossner do in fact use 4032 for their XE pistons and specify 0.07mm clearance for the XE ones and 0.08mm for the LET turbo ones. That seems to indicate that a good chunk of those clearances are for arse covering purposes or for coping with the ultimate modifications you might potentially throw at the engine rather than what you actually do.
http://www.rosspistons.com/information/automotive_...
My piston setup: Bore is 3.779"/96mm
INSTRUCTIONS ‘B
Bore size 3.476 – 4.499......................................... .004
Huge?
My piston setup: Bore is 3.779"/96mm
INSTRUCTIONS ‘B
Bore size 3.476 – 4.499......................................... .004
Huge?
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