cacmf

I'm a newbie troubled with the subject of forged pistons. My apologies in advance if it's been discussed.

Looking to order a MY14 C63 to catch one of the last NA engine, and now it comes down to the decision if I should add the 507 edition option.

Everything I read says that forged pistons are more likely to expand with heat; therefore, requires higher piston-to-cylinder wall clearance. As a result, the pistons may slightly 'rock' in the cylinder prior to warming up and cause cylinder wear. This sound like a trade-off between power and longevity. Any expert opinion would be greatly appreciated. Many thanks!

I'd like to keep the car long, so longevity is critical, but the extra power and the BS hood are so nice.

CarHopper

You could always get a non PP/507 and just get a BS hood and a tune and be pretty much at the same place.

Seeing as you're intentions are longevity I assume you won't be tracking the car or doing any crazy modifications such as supercharger so there really isn't an advantage to having the internals forged from factory.

Forged internals can have something called piston slap, but it isn't going to be catastrophic to an engine nor is it likely to come up in an otherwise unmodified car. You get that kind of piston slap out of say something like a subaru sti or evo with a fully built bottom end meant to handle a massive amount of power.

Bottom line, if you have the money, I'd go for the PP/507 if I were you and just leave it alone as far as performance is concerned and get the longevity you want out of it.

austinauflick

I'm going to say if you have the funds, opt for the forged internals. I'm a hot rod guy and I'll tell ya when performance and longevity are required, and the option exists, forged internals will be used every time. Now that's not to say that a standard M156 equipped with hyper eutectic pistons is worse; a standard M156 will run its *** off for many hard miles before the standard rotating assembly lets go.

So, what it boils down to in your case - forged always trumps hyper eutectic in terms of overall strength and durability. But if you're keeping the car free from super high RPMs, super advanced timing and/or power adders (e.g. boost or n2o) I'd say you can probably get away with the cast stuff. But if you have the funds and like the 507, by all means know that it comes equipped with what us hotrod guys like to call a "built motor" from the factory and it's not going to fail you without some serious pushing from modifications.

ilovemaui1

Almost all strokers use forged internals. Cast pistons tend to shatter when they fail and takes a lot of other stuff with it. The forging process does a better job of forcing the grain to follow the shape. Forged pistons can take higher RPM. When I built my 408 stroker motor for my Bronco I went with all forged internals, pistons, rods and crank because I built it for torque. If you can afford forged then go for it.

PTE

http://www.enginelabs.com/engine-tec...piston-secrets

Cheers _PTEngineering

Richard_AMG

Thanks for posting this, the whole forged M156 forged piston thing has puzzled me ever since I was deciding to buy my C63?

Why do Mercedes offer this option? If the “base” M156 is designed to be a “race” engine and forged pistons are so much better, then why did the base engine not have forged pistons? In the case of performance pack that only gives an extra 30BHP they include forged pistons, does it really matter or is it purely a marketing tactic?
if they were so critical to handle the additional performance why are there so many “tuned base” C63’s with no pistons problems, i.e Eurocharged etc

Obviously the standard cast pistons are more that capable of 500BHP+ and a decent longevity.

The reason this bothers me is because I have my car tuned and don’t have forged pistons, does it really make a difference at the power levels released with a tune.

Rsquad911

It's not just forged pistons... It's also forged connecting rods and crankshaft.
Straight from the SLS M159 motor.

Richard_AMG

But is it really necessary for the extra 40 odd BHP? Surely the base engine can’t be that stressed/on the limit ?

austinauflick

Absolutely not. The forged rotating assembly just happens to be 6lbs lighter than the standard rotating assembly. Lighter inherently has more power potential due to less reciprocating mass. Thats why AMG felt they could "allow" another 30hp to be unlocked from the tune. Now, most likely the weight savings come from the forged crank. A nodular iron crank that most likely comes in a standard M156 weighs a good deal more than a forged crank. Now, typically a set of forged pistons are heavier than the hypereutectic pistons. Usually when someone just throws in forged "slugs" they'll lose just a tick of power due to the added reciprocating mass. Again, the forged pistons can take a good deal more walloping than cast.

Now, hypereutectic pistons have been seen to handle 800+ hp it's just a matter of how long it'll endure that kind of power output. If you're not going to be spinning your motor up to 7500+ RPMs, or plan on doing some serious modding with power adders, the standard rotating assembly will still handle ~700bhp for a good while under spirited driving (if tuned properly).

austinauflick

...another benefit is that a forged will be slightly more forgiving when a less than optimal condition presents itself. Namely, fuel pump going out under load, a slightly off tune, fuel quality etc.

noodleman

i'm fairly certain that the P30 did NOT have forged pistons.

crazy1eye

Correct, started with the P31 pp

cacmf

Thank you all for the insighful information. I guess the takeaway is that forged interanl is always better and stronger, and I shouldn't be overly concerned about the piston slap without pushing the engine with mods.

It's correct that I wouldn't modify the engine with any mean of forced-induction. The stock power (pp or not) is probably enough for me to hurt myself if not careful.

I guess my case will probably come down to if my wife can stand that two holes on the BS hood

ZephyrAMG

It's splitting hairs....forged internals...something that in reality won't make the slightest difference or "feel" any better or stronger. Dads C63 proved what a beast the M156 is and what power it can deliver and what a beating it can take....

austinauflick

Exactly my point. The cast internal'd M156 can be tuned to probably ~700rwhp safely and take a good bit of abuse.

Diabolis

The forged internals are in this case really much more of a "safety margin" feature than an actual necessity for the resulting power gain, but given the option I would always pick the stronger internals. It's like buying forged vs. cast wheels. Both are round and work fine under most circumstances, but if you do hit a pothole, the stronger (forged) wheel is less likely to get damaged.

If Mercedes decides to use forged internals in a production engine, you can be pretty sure they have carefully weighed all the pros and cons and determined that it is the right option. I wouldn't worry about piston slap or anything of the sort unless you're building a custom competition engine from scratch.

adv4nced53

The base M156 has forged rods, forged crankshaft, and cast pistons. The P31 has the M159 forged rods and crankshaft, as well as forged pistons.

Casting creates small voids in the material (due to the manufacturing process) and therefore the material is less dense. This means it's slightly weaker, more brittle, but also lighter. In high horsepower applications where you're making 200hp per liter, the typical failure mode is the rods, not the pistons because the forces are distributed better over the face of the piston. Oh and regarding the OP's original question, forged pistons expand more with heat due to their density. Difference isn't big but you shouldn't beat on the car before it's fully warmed up, which is also true for the cast pistons.

What I'm getting at here is the following:
- Base M156 does not need forged pistons and it can be of benefit to have the lighter, cast pistons.
- I don't know what the exact difference is between the rods but even if M156 has H-beam rods and M159 has I-beam rods, either is perfectly sufficient to make a lot more power than what we get stock. That's why you don't hear of people snapping rods...or breaking anything else as a matter of fact. If the M159 rods were specifically redesigned to take weight out of them, then it is actually likely that they may be weaker than the M156 rods.

ZephyrAMG

Great additional assessment....

Diabolis

Completely agree - you'd need to be in the 200 hp/l specific output bracket where you really need to start taking these things into account, and the usual failure is a broken conrod as opposed to the piston itself.

Which sort of makes me think that it would be a fun project to try and squeeze as much HP out of the M156 as possible. With its 6.2L displacement, you could probably push the P31 motor to 800-900 crank HP without too much difficulty and still have decent reliability. I am not talking about boring or stroking it - just getting more juice out of the existing block and internals. Modified heads, intake and exhaust, fuel delivery and injectors, ignition, high-lift cams (possibly valve train?) and an M159 dry sump ought to do it. Has anyone attempted anything of the sort?

I am new to the board and the C63 in general, but I think I read something about a member here (DadC63?) that has a modified motor. Anyone have more info about this?

cacmf

thank you, Stoyan!