Forged Pistons for the S65B40(V8) engine

[M3V8SC]

Hi Bimmers,

Can someone help me find a set of forged pistons, preferably coated (92.00 mm at 12.0CR)

I need them to rebuild my engine. which will have a VF620 kit fitted.

Suggestions and contacts are much appreciated.

[Brandoch]

S85 builder knows everything about these engines. All of your questions should go to him.

[charliev68]

Stock S65 pistons are forged and coated
https://www.mahle.com/en/products-an...iston-systems/

[amrazM]

mahle powerpak

[S85 builder]

Originally Posted by charliev68

Stock S65 pistons are forged and coated
https://www.mahle.com/en/products-an...iston-systems/

The BMW documentation says the S65/85 pistons are cast, not forged with a coating on the skirt. They are made by Mahle but the are a different spec than what Mahle offers for aftermarket.

[charliev68]

Originally Posted by S85 builder

The BMW documentation says the S65/85 pistons are cast, not forged with a coating on the skirt. They are made by Mahle but the are a different spec than what Mahle offers for aftermarket.

I think you are right! I think I see casting mold lines.
That’s Weak! I think I’m going to have to get Mahle aftermarket pistons for my refresh.

[S85 builder]

Originally Posted by charliev68

I think you are right! I think I see casting mold lines.
That’s Weak! I think I’m going to have to get Mahle aftermarket pistons for my refresh.

It's cost efficient for them and has better NVH properties as a street car, which is why most OEMs don't go with forged on their street cars. If you're going to be boosting the engine you really want to go with forged. CP / Carrillo have solutions too. Mahle aftermarket is a good solution that should really be plug and play, but you really should balance the new assembly as tight as you can. Not really sure how much money you want to throw in to this tho.

[charliev68]

https://www.ecstuning.com/b-mahle-mo...197843622~mhs/
I think I’m going to go with this as soon as it becomes available. Seems like a direct plug and play

[nycplumber]

I thought the rods were the weak point and not the pistons themselves?

[Theodore2]

Originally Posted by nycplumber

I thought the rods were the weak point and not the pistons themselves?

I have the same question, are the stock cast pistons failing on the S65 when tuned or supercharged?

[S85 builder]

Originally Posted by charliev68

https://www.ecstuning.com/b-mahle-mo...197843622~mhs/
I think I’m going to go with this as soon as it becomes available. Seems like a direct plug and play

Its nice to have this ring package all matched up. Like I said earlier, you really should get the rotating assy balanced and weighed as tight as possible.

[S85 builder]

Originally Posted by Theodore2

I have the same question, are the stock cast pistons failing on the S65 when tuned or supercharged?

forced induction engines are more susceptible to detonation, the top edges of cast pistons often times are where they will get damaged, catastrophically. That's why it may be better to lower the CR if you are going to make it forced induction. It's really up to how you have it tuned, many guys reduce the built in safeguards in pursuit of more power.

[charliev68]

I feel it’s just insurance to run forged pistons. All this time I thought we had forged pistons from the get go!
Most of us run tunes and our compression ratio is 12:1. I think forged pistons are prudent in case of an abnormal ignition event. Also, the motor revs to 8600!
If you are running FI it’s stupid, not to run forged pistons.
Anyways, $1500 for a complete set of Mahle forged and coated Slugs is not bad.

[kolosy]

Originally Posted by charliev68

I feel it’s just insurance to run forged pistons. All this time I thought we had forged pistons from the get go!
Most of us run tunes and our compression ratio is 12:1. I think forged pistons are prudent in case of an abnormal ignition event. Also, the motor revs to 8600!
If you are running FI it’s stupid, not to run forged pistons.
Anyways, $1500 for a complete set of Mahle forged and coated Slugs is not bad.

it may be a good idea, but i have a feeling that most people that run FI aren't doing a full engine rebuild to go with it...

[charliev68]

Just wanted to share some new insight…
After talking to some of my colleagues within the Porsche community, I’ve come to realize that cast piston choice by BMW was purposeful and not them being cheap. I had the pistons professionally cleaned and a very reputable engine builder stated to me that our pistons are very robust and expensively cast. Meaning, it was not them cutting cost.
BMW chose to use cast pistons to ensure engine longevity and to combat bore scoring. Porsche went from cast pistons to forged pistons and the results were more bore scoring problems with their later engines. It has to do, mainly with the expansion rate of the pistons and the looser tolerance requirements of the forged piston. The result was, more rocking in the bore causing more wear and tear over time.

[drwankel]

Good video explaining the difference, and why forged isn't always better:

[baba louey]

Thanks for the video link. Very informative. Now I wonder what type of pistons top fuel dragsters would use.

Baba Louey

[djsilverjohnson]

Originally Posted by baba louey

Thanks for the video link. Very informative. Now I wonder what type of pistons top fuel dragsters would use.

Baba Louey

With their budgets it’s apples to oranges. If you are ever in the paddock for a top fuel drag event you’ll see them tearing the engine down after every pull. It’s nothing for them to blow holes in pistons and heads running on nitromethane.

[baba louey]

Originally Posted by djsilverjohnson

With their budgets it’s apples to oranges. If you are ever in the paddock for a top fuel drag event you’ll see them tearing the engine down after every pull. It’s nothing for them to blow holes in pistons and heads running on nitromethane.

Those top fuel mechanics must hold the time record for engine rebuilding. I haven't been to a top fuel meet in ages but I remember the sound and the thumping I felt on my chest.

[charliev68]

Okay. I’m contemplating between https://www.ebay.com/itm/19566871873...mis&media=COPY or https://www.ecstuning.com/b-mahle-mo...197843622~mhs/
To Forged or Cast, that is the question!
The price difference is not a big deal (~$600). I love the idea of having “stronger” internals with a bump in compression but should I mess with BMW’s recipe?
There are definite benefits to running cast pistons in a street motor!