Pistons for m20 Stroker

[digger]

here are some data on the Mahle alloy published by Mahle themselves

M124P is forged 4032 alloy
M142P is the "mixed" blend used on the IE pistons
M-SP25 is the 2618 alloy

chemical composition


forged


cast


typical crown temps are in the order of 250-300C
skirts much cooler maybe 100-120C

lets compare cast to forged.

"strength" forged 4032 is a step up from cast regardless, others offer slightly bigger incremental steps
"ductility" forged is better and 4032 is only the same as cast at room temperature which doesn't matter in a running engine. 4032 is actually superior at high temps.

forged vs forged

M142 is slightly stronger than 4032 and 2618 is stronger again especially at temp.
4032 is more ductile than M142 other than room temp and above 150 is superior to 2618
both 4032 and M142 are superior to 2618 by >30%

it should start to become apparent where you would use certain alloys. if the pistons going to get very hot, see very high loads / knock, and doesn't need to cover alot of miles then 2618 is the go. for everything else there might be better tradeoffs.

[ForcedFirebird]

you guys are jumping from metric to english..... adds some more confusion to this discussion. :)

As you can see from the pic I posted, one has to be on their toes ("in inches unless otherwise noted"). Even the manuals jump back and forth unfortunately.

[zaq123]

you guys are jumping from metric to english..... adds some more confusion to this discussion. :)
I think 84mm oem bore is a nominal value. Standard pistons are spec'd at 83.98mm. Standard new clearance is spec'd at 0.01 -0.04 mm. This makes bore diameter anywhere between 83.99 and 84.02mm.
FF measurement was 0.03mm (0.0012"). He didn't specify if it is a bore or piston wear. He simply stated that his high mileage engine was still tight. Looks to be still within a spec for new assembly.

If anyone cares, My IE/Mahle M142 Alloy pistons are spec'd at 0.051mm PtoB clearance.
M142 is recently designed alloy that shares benefits of 4032 and 2618, that's what they use in Z06 corvette.
4032 will have smaller clearances but less malleable, higher chance for fracture under extreme load. On the other hand, 2618 will distort much sooner, not a good choice for the long term reliability. IMHO, between those too, 4032 is the way to go unless one is building extreme duty dedicated race engine that will be in and of the car a lot.

[ForcedFirebird]

isn't nominal clearance 0.02mm ? Most Pistons say 83.98mm so with 84.00 nominal bore that is 0.02mm?

Correct, =/-.009mm allowed piston variance which almost half that but I have found the Mahle to be very stable. I used screen shots of software to make a listing of typical BMW engines and their respective piston clearances. Not only the m20 has tight jugs.

[digger]

Reread my post. Where did I say piston wear? Total piston to bore clearance is .0012" on this high mileage engine I broke down for rebuilding. I build these engines often, not just pulling numbers from thin air.

I think he means that whatever wear is there is included in the measurements. This reinforces that it came tighter from the factory though.

[digger]

isn't nominal clearance 0.02mm ? Most Pistons say 83.98mm so with 84.00 nominal bore that is 0.02mm?

[ForcedFirebird]

Just broke one down for a rebuild that has .0012" minimum total clearance with who knows how many miles.

Come on, man.
Fairy tales are not my speciality.
I give up.

Edit: If you measured 0.0012" cylinder wear, this is a totally different animal compared to piston to cylinder clearance.
0.0012" is normal for cylinder wear. But not relevant (at least not directly) to the discussion.

Reread my post. Where did I say piston wear? Total piston to bore clearance is .0012" on this high mileage engine I broke down for rebuilding. I build these engines often, not just pulling numbers from thin air.

[apostate]

Come on, man.
Fairy tales are not my speciality.
I give up.

Edit: If you measured 0.0012" cylinder wear, this is a totally different animal compared to piston to cylinder clearance.
0.0012" is normal for cylinder wear. But not relevant (at least not directly) to the discussion.

[ForcedFirebird]

Edit: By the way the clearance specs given above are from Bentley Publishers E30 manual. These are not genuine BMW specifications. I really doubt that piston to cylinder clearance can be 0.0004.
IMO 0.0016 is the bare minimum.

Never owned, nor consulted a Bentley manual, measured them first hand many, many, times. ;)

BMW i6 engines, and especially the m20, have tighter piston clearances than most other makes I have ever worked with. Just broke one down for a rebuild that has .0012" minimum total clearance with who knows how many miles.

[digger]

Having the piston sit without rock in the bore means the rings will have better conformity during warmup. This is one reason why OEM use low thermal expansion alloys. If the piston rocks excessively it's not good for the rings or ring lands. When you have a short compression height and skirt length it gets magnified.

When it's upto temperature they should be the same.

[apostate]

CR will be 11.5-12:1 (depending on how much the block and the head need shaving. The car barely sees 1.5-2K miles per annum and only in the hot summer months. Mainly used on winding roads and three to four track days. Not a daily driver.

@ForcedFirebird - piston to cylinder clearance in M20 for new OEM pistons is from 0.0004 to 0.0016. Both alloys exceed that specification. Cylinder wear limit is 0.0047. Pretty sure that you know very well that what seals the cylinder are the piston rings, not the piston itself. Aluminium has greater linear expansion than steel and what is important is how long it will take for the rings to close the gap rather than how long does it take for the piston to expand. In both cases (2618 or 4032) the piston rings are the same, i.e. they will close the gap for approx. the same time, may be 2618 rings will be a little bit faster due to the higher piston to cylinder clearance. At that point the rings stabilize the piston.

Edit: By the way the clearance specs given above are from Bentley Publishers E30 manual. These are not genuine BMW specifications. I really doubt that piston to cylinder clearance can be 0.0004.
IMO 0.0016 is the bare minimum.

[digger]

4032 Pistons are used on many turbo engines and high compression street engines. The metric mechanic forged Pistons for their 3.2l 11:1cr m20 engines that have been around for well over a decade use a 4032 alloy piston made by wiseco. Those engines will last over 100,000 miles. They are perfectly suitable alloy for such an application.

[ForcedFirebird]

There are always advantages and disadvantages when you start comparing materials. 4032 is a better choice for a mixed street/track engine that is expected to last several miles/years, and 2618 is more suited for an engine that will be torn down periodically (every race or weekend for drag cars, seasonal for road racing), as they will definitely wear much faster.

...and while saying 2618 "only" needs .0008" more clearance on an 84mm bore, that is actually more than the stock clearance. Forgings are already over, with the 2618 sounds like marbles in a can until they warm up (and that was in a v6 with a much better 1.7:1 rod/stroke ratio). You add that to a high compression engine like an m20 with a very short rod/stroke ratio (R:S ratios get worse on stroked m20's).

I was surprised I saw no mention in your questions of the intended purpose of the engine, as that should be the true determining factor in piston material choice. That's the first thing I as every engine we build or consult on.

[apostate]

I don't need TopEnd to make up my mind, it was just a quotation of what they recommend also. The price is the same for both alloys so it doesn't make any difference for them which one I shall choose.

The decision of which alloy to be used depends on the application and should be based on the contents of the two alloys. As stated above, the lack of titan and the high silicon content of 4032 are huge disadvantages for a high CR engine piston.

[digger]

Here is a part of an e-mail exchange with TopEnd with regard to the piston material:

top end use JE, here is what JE say, no need to ask steve as you get the printout of the relevant pdf below in the box lol

https://www.jepistons.com/PDFs/TechC...instrc2618.pdf

https://www.jepistons.com/PDFs/TechC...instrc4032.pdf

0.0020-0.0025" compared with 0.0030-0.0035"

be careful when you trust people who are trying to sell you something, its always better to seek the advice of an impartial person if you don't know enough about the subject to form your own opinions ;)

if you cant tune then 2618 is better, but you will soon realize that even that isn't enough and detonation will still kill them, and the guy down the road who can tune is probably still running on stock cast pistons making more power. for what most people here need the stronger forging doesn't offer any additional performance they actually need and they infact will wear out quicker.