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Which Aluminum Forged Piston Is Right For Your Engine?
If you're rebuilding your engine you're probably upgrading to aluminum forged pistons. But which alloy should you go with? We asked Mahle to compare their 4032 and 2618 alloys to come up with some answers.

Unless you rebuilding an engine with little to no budget you're going to upgrade to aluminum forged pistons. Besides being stronger than cast pistons, the lighter aluminum cuts parasitic power loss. That translates into more power at the wheels.

Recently Mahle showed us that there are distinct differences between their 4032 and 2618 alloys. The choice to use one over the other depends on your project. Are you building a street/strip machine or an all out racer? Choosing the wrong one can be devastating. To better understand, we'll let MAHLE explain.

4032 is a high-silicon, low-expansion alloy. Pistons made from this alloy can be installed with tighter piston to bore clearance, resulting in a tighter seal with less noise. 4032 is a more stable alloy, so it will retain characteristics such as ring groove integrity, for longer life cycle applications. Relative to 2618, 4032 is a less ductile alloy, making it less forgiving when used with boosted and/or nitrous applications.

The majority of Mahle forged PowerPak kits are made with 4032 alloy and require no additional piston-bore clearance. Mahle pistons are perfectly engineered to allow for the proper clearances assuming normal operation. For example, Mahle pistons for a small block engine will provide proper .0025"-.0030" clearances--right from the box.

2618 is a low-silicon, high-expansion alloy that is used for extreme-duty racing applications such as NASCAR, ALMS, etc. Due to its high-expansion characteristic, this alloy is engineered with additional piston to bore clearance. At the start of a cold engine, the pistons expanding process can be heard and is commonly referred to as the "piston slap". Once the engine warms up the noise subsides as the piston expands to its running clearance. 2618 is a more ductile alloy and grants higher tolerances with higher resistance to detonation. The forgiving characteristics allow for the most extreme conditions, but longevity is eventually negotiated after countless heat cycles.

Mahle pistons are designed for specific applications with the alloy that is best suited for that particular application.

Piston Alloy Comparison
4032
High silicon
Low expansion
Tighter piston-to-wall clearance
Quiet Operation
Less ductile
More stable & consistent
Longer life cycle
Harder

2618
No silicon
High expansion
Greater piston-to-wall clearance required
More noise
More ductile
More resistant to detonation
Shorter life cycles
Softer

http://www.hotrod.com/techarticles/h...son/index.html

So I suppose there is another factor that hasn't been mentioned and I don't have much knowledge of, that is poor quality fuel, that may lead to detonation, this is probably just a U.S problem but I let you guys comment on that given my knowledge of that topic is second hand and I am certainly no expert.

I also read an article in Race Engine Technology that detailed what Mahle was doing in F1, they are making F1 pistons from both materials, although both are slightly altered. When MMC was banned and the pistons were increased in diameter the weight only increased around 20 grams which is outstanding given the engines also had to last 2 races instead of 1.

Porsche used Mahle 124S for our pistons and the 968 pistons which is very similar to 4032, I wonder if that is what they are now using in F1 pistons?
Greg

Jim bailey - 928 International

Daughter's Pantera rebuild it got "special" forged pistons.....the piston slap was horrible with the engine next to your right ear...it got pulled apart and different pistons installed !! NASCAR only needs to get 600 miles out of a set of pistons.

123quattro

Not that it helps in this case, but slap seems to be a lot more noticeable on V engines. I'm running 0.0055 gap on an inline engine. It doesn't rattle at all, which is surprising since 0.003 is generally considered loose.

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Hi Jim, the most likely material they used for those pistons was 2618, it is the same material that most of the strokers are using, however they should be using a smaller clearance given the different block material to the Pantera i.e iron versus aluminium. It is what saves the strokers from excessive piston noise, however I feel in parts of the country where there is a very cold climate, that may not be the case.

Greg

slate blue

I think that clearance is probably right btw and the reason that the strokers just get away with it in a moderate climate at least, my pistons which are Wiseco for the 5.35 litre and Mahle for the 6 litre are both 4032. I plan on running 0.001" but I will research this as 4032 expands less than Mahle 124S which can run 0.0008" while I would go less than that I may run it as my Mahle pistons have quiet short skirts and this will help to stabilize them.

I hope to cut down on friction in this engine so just going tighter may not be the answer, I will need time to consider this, also the coatings on pistons need to be considered, some like the ones I have on the Mahle's are great for cushioning the rocking. These are the issues that are changing the way people choose their pistons.

Greg

123quattro

Mine have teflon skirts. Maybe that helps?

slate blue

I know that teflon works on Nicasil or iron cylinders, I don't believe it can stand up to Alusil, what type of application do you have it on? I take it it is that inline 6 you mentioned. If it is I would say it does help with that slapping, that actually makes sense to me.

The reason is that teflon is a soft coating when compared to diamond like carbon DLC, in F1 where there is more coatings than anywhere else, they use a DLC coating on the piston at the ring lands. On a F1 piston the lands are not recessed like a normal piston, this area is needed for stability, so they put the hard coating here and the softer one on the skirt where they want the cushioning.

Greg

123quattro

It's an iron block Audi I5 engine with a lot of boost.

slate blue

A bit more infomation from Mahle, "Mahle motorsport currently recommends 4032 aluminium for a 24 hour gasoline engine since its relatively high silicone content alloy is more wear resistant than 2618 which is the normal choice for sprint races. Its resistance is such that it is not necessary for durability to hard anodize the ring grooves., a practice that leaves a less than perfect surface against which the surface of the ring must seal. That is highly significant for controlling blowby and oil consumption."

The temperature needs to be monitored very carefully and should not exceed 210 C as the material's properties then begin to suffer, a steady spray of oil via squirters is employed to keep the pin bore temp within the acceptable range.

Personally I will employ the squirters and have bought my DLC coated wrist pins, this should be more than enough given my engine will never be a 24 hr engine, I think it also says indirectly that 4032 is a better choice for the street as it has better durability properties.

Greg