I don't contribute here much, I normally just search the threads and gather whatever info I can. I'm currently in the process of rebuilding a motor and was considering taking it a step further and building it up for a higher compression, all motor build. One of the main things I'm having a dilemma with is what pistons to go with. Naturally I went to the search, and came up with some decent results; but I didn't find anything TOO convincing, or anything too recent (experiences may have changed over the years, regarding older threads). This isn't to say that this subject hasn't been covered, I just feel that I personally didn't find ENOUGH info.

I feel like this is a question that comes up every once in a while, but not too often; so I decided to make a thraed, not to pose a question on my behalf, but rather too get others to share info regarding their beliefs, knowledge, and experiences.

Here's what I know:
-Forged pistons and Cast pistons are quite a bit different, for the most part. Forged pistons are considered much stronger then cast pistons, therefore they can handle more abuse.

-Cast pistons however, don't expand as much as forged pistons when heated. This means a few things. Because cast pistons don't expand as much, the piston to wall clearance can be ran tighter, which can result in a slight increase in power, less oil blow by, longer ring life, and less "pison slap" noise when the motor isn't heated up yet. The opposite is true for forged; due to looser tolerances, more blow by, more piston slap, and ring life CAN be lesser.

-Forged pistons (and other parts) tend be be a bit heavier. As most of us know, adding weight to the rotating assembly can rob a bit of power, and in some cases, rev-ability. However, it has already been brought to my attention by Natural Aspirations that this isn't necessarily true, and may be a misconception that I thought to be true.
-Forged pistons are made out of several different types of materials. Now I don't know a whole lot about the different materials out there (as there seems to be a lot), but what I do know is that what the forged piston is made out of changes how it reacts to heat, and how strong it is. Some materials, such as 4032 alloy, is said not to expand as much as others. Also, the higher the silicone content in the piston, the less it expands to heat, while sacrificing some strength.

Found this link, if you're thinking about buying some pistons and have some questions you should check it out.
http://www.team-integra.net/sections...?ArticleID=183

Now I want to set the stage for discussion. Since this is the all motor thread, I'd like to keep it limited to all motor apps. Forced induction apps tend to lean on the forged side, for obvious reasons. I don't want to limit the discussion, but to set the range for now lets say, "is a forged piston more practical in a mild street build, such as a b-series ls-v, b20-v, b18c, making power in the 200-240whp range and running on 91-93 octane?"

PLEASE NOTE, I am not an experienced builder, or a manufacturer of these products. I don't claim to know a lot. If my info is wrong, or mis-guided please let me know, and I will alter it accordingly. Please share your knowledge and experiences regarding the discussion. Thanks.

 

Cast pistons are not necessarily lighter
Cast pistons tend to be more brittle
Cast pistons usually have smaller valve reliefs

Forged pistons are more expensive
Forged pistons are tougher on cylinder walls and not compatible with FRM

 

Thanks for posting. I agree that for the most part cast pistons are not always lighter, but as I stated above Forged parts are USUALLY heavier. Care to share your opinion as to when cast pistons become too brittle for an app, or when the valve reliefs are not enough? Also, in your opinion/experience, when does the price of forged pistons and the fact that they are harder on the cylinders become out-weighed by the benefits, in an NA setup.

Usually, re-sleeving greatly helps with forged pistons being harder on the cylinders, so this isn't a problem that is impossible to avoid. However, re-sleeving is expensive and not always necessary.

Something I meant to add earlier, forged pistons offer more options when building a motor. There are more compression choices available, to reach your desired compression ratio for your specific application. This is, in my opinion, one of the biggest benefits to forged pistons in a mild NA setup.

 

CTR pistons for example are known to be heavy and have close piston to valve clearances. A tank of bad gas could cause unexpected detonation that MAY break a cast piston before a forged one.

Factory B16 pistons weigh between 295(P30) and 325(PCT) grams where for example wiseco offers pistons in the same compression that weigh between 268-275 grams in std sizes. I am not sure of many Forged pistons that weigh more, so in reality forged pistons weigh LESS

 

Wow, this is actually entirely new info to me. Thanks for sharing. So you're saying that most known forged piston brands (wesico, as well as others) actually weigh less then oem cast pistons, for the most part? I knew that concerning oem cast pistons, ctr's are pretty hefty; but never would've guessed they are heavier then forged. I'm not doubting that you are right in any way, but is there a way we can verify this? I viewed wiseco's catalog and the heaviest weight for a b18c was 284, which is pretty light. I'll have to see if I can something that states how much oem pistons weigh to compare this too. In the mean time, I will add this to the top. Thanks man.

 

Everything you said sounds right to me too. Including forged pistons generally being lighter than cast ones...
This is a forged wiseco 85mm piston...
I THINK it actually weighs less than a 81mm oem piston.

 

Good info guys, exactly the sort of thing I was looking for. So do you think it's a common misconception that forged pistons are generally heavier? Because like I said, I've been doing my research, and when I searched most of the threads that had good info were a bit older, but it seemed the consensus was that forged was heavier then cast. Thanks for clearing it up.

I'm still searching for something that shows how much oem pistons weigh but most forged piston catalog says how much they weigh, 270-280 grams seems to be about right for most of them.

 

forge!! FTY

 

[QUOTE=Shimakid12;41462045]So do you think it's a common misconception that forged pistons are generally heavier? QUOTE]

Yes. I think people associate the word forged with being tough, which automatically kinda makes you think of heavy.
Since the material is stronger,they can redesign the whole structure and reduce the amount of material used.
Aftermarket forged connecting rods on the other hand are usually heavier than the OEM ones, at least from what ive seen...

 

I don't know exactly how much my pistons weigh. I do know that my current piston and rod combination weighs more than 100 grams less than factory parts (per piston/rod combination). I want to say something like 170 grams, I can't remember though.

 

Okay but why??? Within what app?

This is really good info regarding the topic, and I appreciate you contributing. I've also read that one of the reasons why forged rods in a mild NA setup is not necessary is because they weigh more and it is not needed; stock rods are strong enough. Could the same logic sort of be applied to forged pistons; aside from the fact that I've just learned that forged pistons are lighter. Are forged pistons necessary in a mild NA build?


This is also good info, thanks for sharing. I wouldn't imagine saving that much weight with forged internals.

 

Which is 'best' depends on the application and budget. Cast pistons are usually cheaper and (IMHO) are better for daily driver type builds, even fairly aggressive ones.

Forged pistons are usually lighter and stronger, but cost more and (almost always) need more room in the bore to allow for expansion. I have seen a lot of engines with forged pistons that will smoke slightly until they warm up.

[edit - posted this in a hurry and originally had 'cast' where I should have had 'forged'. Corrected now.]

 

I've got piston slap in my d16z6, run this engine till it goes? or a restore/build?
engine seems in very strong shape but sounds like piston slap is happening. 120kms and ran from 80kms to 120 kms on 7psi greddy turbo sorry i dont know the specifics, working on it... first post on honda tech, alot of smart posts around, hello everyone

 

The casting is lighter because of the density of the metal. The problem is that you can build a custom forged piston in a lightweight design that you can not do with a casting. It is easy to call a piston manufacturer and have them move the pin, use a smaller pin, and make the piston much lighter and then have it sitting in your hands in a few weeks. You can't do that with a casting, and you couldn't run that same design in a casting anyway as it would fail.

As for piston to wall, not all forgings are low silicon metals. Some Mahles and Supertechs are higher silicon and they do allow you to run a tighter piston to wall. Honestly, I see some CRAZY loose piston to wall because people are misreading the wear on their pistons. Some of that wear is from their loose p2w. There are a couple things that can be changed in the design of the shelf forged pistons that allow tighter p2w even with the low silicon, less brittle aluminum.

 

I was wondering when someone was gonna mention silicon content and expansion.

 

Would you go as far as to say that oem cast pistons are superior, in most cases, in common dual purpose, DD all motor builds? I'd agree that there is the smoking during Initial start up, and is one of the things I listed at the top as a con for forged pistons; do you think this is something that is possible to avoid, or is it an inevitable compromise that must be made with forged pistons?

Really good info here, thanks for posting. So is it safe to say that it is possible to have a forged piston made, with higher silicon, that can run tolerances as tight as oem cast pistons to get rid of some of the main cons, such as blow by (smoking) and piston slap? If this is possible, then it pretty much means that forged pistons are not inferior in any way, other then price, for the most part. If the tolerances can be ran just as tight as oem, then just as much power can be made, there wouldn't be 'more' stress on the rings and cyl. walls with forged, and there wouldn't be any "slap" issues associated with forged.

Basically what I'm poking at is, if the tolerances can be ran just as tight with certain types of forged pistons, then there really are no cons when compared to cast. Is it safe to say that this is possible? Is the longevity of a motor compromised at all with forged pistons in a scenario where the tolerances are just as tight as oem?

I also agree that one of my main problems with cast is that the design availability is so limited compared to forged. Forged pistons can be made in so many different shapes, strengths, designs, and sizes.


I did, in my very first post. I didn't get into it too much though because there's a lot I don't know about the make up of forged pistons.

 

Forged pistons dissipate heat better from what I've read (in books, not just on the internet :p ), so you can runner higher compression or wilder timing as there is not such a concentrated heat buildup at the top of the piston promoting detonation. I have a diagram in an engine blueprinting book; if I remember I'll post it up.

 

and i totally fail @ reading, sorry

 

This is a really good point and something that I didn't think about really. By dissipating heat better, forged pistons are better for avoiding detonation. If you find that diagram post it up. Thanks for sharing.

Haha, it's fine man, it happens.

 

I personally feel that in a daily driver (without very aggressive compression, nitrous, or some other specific reason you need the properties of a forged piston) cast pistons are the way to go. They are less likely to smoke on start-up and are cheaper (leaving money for other things). This is subjective, and there may be people who completely disagree with me.

It is possible to get forged pistons with a high silicon content that does not result in as much expansion. This allows for tighter piston to wall clearances (as you do not have to allow as much room for expansion) and will reduce smoking at start-up, as well as piston slap. However, it is a compromise, as changing the silicon content also changes the metallurgical characteristics. Silicon makes the piston expand less, makes it harder and less susceptible to scuffing but also makes it more brittle. It also absorbs less heat.

 

So from what I'm gathering, a forged piston made up of more silicon doesn't necessarily have any con's, except that it is more brittle? In your opinion, at what point would a high silicon forged piston become 'too' brittle for an NA app?

I tend to agree with you, that for the most part in a daily driver (say a 180-240ish whp range) doesn't really need a forged piston. But is there any actual downsides to using a forged piston in this sort of app, do you think? Will a motor with cast pistons last longer?

I hope I'm not beating a dead horse, but I wanted to create this thread for opinion and fact based discussion, and also hear experiences.

 

yes forged pistons also dont need as much lubrication to run because they can withstand heat alot better than oem cast

 

and if u builf everything right with the right gaps and tolerences and thne compliment that with a good tune than u can run forged, cast, anyhting and have a safe reliable engine

 

The point where high silicon content becomes a factor is when you approach the hypereutectic range, meaning it is overloaded (for lack of a better term) with silicon. These pistons are used in some oem engines for precisely the reasons mentioned above- they will expand less, meaning closer p2w clearance, they are extremely light, and -now im pretty sure but i might be wrong- the modulus of elasticity is not affected by the high silicon content. the downside, however, is with hypereutectic pistons any sign of detonation causes a catostrophic engine failure, b/c the pistons basically shatter. like glass. (high temp+Si+O2=glass)

 

**** i just realized that usually hypereutectic pistons are cast anyways. well the basic characteristics are the same.