- As we've seen power levels climb increasingly higher in the diesel
tuning market, it becomes apparent that at some point the factory components
inside the engines aren't up to the task of putting up with all of that additional
pressure inside the combustion chamber.
At this point we need to reach to stronger aftermarket components in order
to maintain engine reliability.
I'm here with Eric from Mahle Pistons, and we're going to talk about some
of the aspects that go into the design of an aftermarket piston
for the diesel tuning market.
So Eric for a start, the factory pistons that go into a diesel engine,
there's obviously some strength considerations there.
Once we get past what those pistons can put up with,
we start reaching out to aftermarket pistons,
there's a range of different production techniques or materials that can be used.
Can we start with your cast piston?
What's the advantages of a cast material when it comes to an aftermarket piston
for a diesel application?
- Well particularly with the cast piston in a lower or entry level diesel
motorsports part, the biggest advantage of the cast piston is you retain
the cast iron insert for the top ring groove.
And that piece of the piston is the ultimate safety level for durability
that you need.
There's no way to duplicate it in any of the other types of pistons
and durability will suffer if that's not there.
So even in a performance application, starting with a cast piston
in a lot of those applications, is the preferred method to use.
- So can you just talk to us a little bit about what that insert is there for,
how is that an advantage in both OE applications as well as your entry
level cast pistons, what's it actually do?
- Yeah most of the diesel pistons have a dual keystone,
or at least a half keystone type ring that introduces radial motion in the ring
to help clean the groove from carbon build up.
But that motion of the ring in the groove also can contribute to extremely high wear
rates if you don't have a hard material there present to resist that wear.
And that's the function of the insert.
In most of the applications that we build where you're wanting that durability,
you need to keep that cleaning function, that keystone style ring,
so you've got to have that feature of the cast piston there
to go hand in hand with it.
- So if you'd move to a conventional sort of parallel style top compression
ring, you're not gonna get that cleaning effect is that correct?
- Yes and the biggest detriment to that, it'll seal up and make power,
but durability would likely suffer.
- OK so in the petrol tuning market, the cast pistons have a limit
because the material tends to be brittle and under very high power levels
particularly in a petrol application if we suffer any detonation,
they can break.
So in the diesel tuning market, where does the limit of a cast piston come in?
- We see lots of thousand horsepower type truck engines that are still using
cast pistons.
The biggest advantage that the diesel piston has is there's so much more mass
in that piston, it's well over designed, and it's very robust.
You don't have the limitations of very short compression heights
and small physical parameters in a gas engine,
or petrol engine if you will, that you have in the diesels.
So there's so much more mass there, you can design the diesel piston
with the cast material to survive at higher power levels.
- Moving up, if you're sort of beyond the capabilities of the cast piston,
you do also offer a forged piston, so obviously what we've just
talked about there with the insert for that top ring,
with a forged material it's impossible to incorporate that part,
that insert into the top ring.
So then we get into that same problem we've just talked about
with durability, so how do you deal with that?
- Well for our forged pistons we do hard anodise the groove,
which is still not as hard as the cast iron insert but it's an improvement
over the bare aluminium.
The other aspect of the forged parts is we sell those as competition
only type applications.
Naturally guys are gonna run them on the street and do different things,
but in a competition truck, whether it's a pulling truck
or a drag truck, you're not looking for 100000 mile durability,
you're looking for performance.
And there's things that we can do with added flexibility of the forge.
Starting from scratch we're not limited to stock casting parameters.
There's improvements that we can build into the forge,
it's just not gonna get that long term durability that you would see
out of a cast piston that you might want for a street truck.
- Now traditionally when we're talking about cast pistons,
because of the way the material expands as it heats up,
it allows a relatively tight piston to wall clearance to be maintained.
And one of the problems with moving to some of the forgings particularly
2618 material or the light with a low silicon content,
is that those pistons do expand more as they heat up,
requiring a higher piston to wall clearance.
So can you talk about your own forged pistons and how the piston to wall
clearance is affected?
- That's all very true and that's why one of the reasons that we build our pistons
out of a material, it's a moly material that's very similar in expansion
to the 4032 which is the lower expansion version,
it's more similar to the cast piston, is so that we can reduce that overall
expansion rate and you're still keeping relatively tight clearances for what
a diesel application would be and not having to go to the far extremes
where piston rock and things that you're trying to compensate for,
you know which will also affect additional ring land wear and movement
of the pistons, trying to keep everything sealed when you're combatting
that extra expansion, we use the lower expansion alloy in our parts
because it keeps everything fitting relatively nice and tight.
- Now when you go beyond the capability of your forged material,
or the forged aluminium material, moving to the top shelf,
you've also got a steel application for some specific engines,
so can you tell us about the advantages and maybe disadvantages
with a full steel piston?
- Yeah definitely with the steel it takes the question of durability off the table.
There is no comparison between what you can do both in strength and the
ability to handle heat with the seal.
The downside of the steel is although we've never failed one from cracking
or actually physically breaking apart, you can still scuff it.
And if the clearances aren't right and it scuffs,
the other downside of the steel is, it's much harder on the cylinder walls.
The aluminium part will be more forgiving and may scuff, tear up the piston,
whereas the steel piston is, it's gonna be pretty rough on
the cylinder walls once it starts to scuff.
- So unlike an aluminium forged piston, if you do scuff up a steel piston
on the bore, you're probably gonna be over boring the engine block?
- There's a pretty good chance of it yes.
- Now in terms of the expansion though of the steel material,
it's likely, I'm assuming, to be much less than alloy so how does that affect
the optimal or ideal piston to wall clearances when you go from
a forged aluminium piston to a steel piston?
- Yeah if we put it into perspective that the average cast aluminium piston
probably runs 5000ths to 6000ths piston to wall clearance,
the forged parts with the low expansion alloy will still run seven to eight.
The high expansion alloys, you're probably running 10 plus.
The difference in the steel is you can start them out as tight as two.
And even on a very high output application, if you start getting
above 4000ths clearance, it's too much for the part just because the steel
has nowhere near that expansion that you have to compensate for
in the aluminium part.
- So I'm guessing a lot of engine builders may struggle with speccing a piston
to wall clearance that's so tight, it's sort of outside of their normal
realm of experience?
- The number one tech call we answer on the steel piston is convincing the guy
that 2000ths to 3000ths isn't a typo.
It's not missing a digit or anything else, that is the actual recommended
piston to wall clearance.
- So in terms of that material, obviously it's a denser material
than aluminium so how does that affect the overall piston weight?
- The piston itself is much heavier but through different design changes
because the structures are also different, we can take out weight in the overall
combination.
If you hold the aluminium piston to the steel piston,
the steel piston is always gonna be heavier.
But if you look at the pin design on the two,
there's a lot of weight that can be taken out of the pin.
So when you look at the combined mass that's hanging on the end of the rod,
we can get pretty close to balancing it to what a factory cast piston to pin
combination weighs.
- Now I guess with the lower rev limits used by traditional diesel engines,
the weight is maybe not quite so critical as a high RPM gasoline engine.
- No very true, but there's guys that will still spin a common six cylinder
steel piston 6000 RPM so not as critical as 10 but it's manageable.
- So the other thing I wanted to ask about there is also the design
of the bowl or dish if you like in the top of the piston.
So this really for a diesel engine becomes part of the combustion
chamber essentially.
We see with a lot of the OE design pistons that that bowl is sort of undercut
on the outside edge and there's a belief that that can lead to reliability problems
with the piston and I see that all of your Mahle pistons, forged or cast
or steel, all use a parallel side to that bowl so can you just tell us
a little bit about that design feature?
- Yeah definitely that area over the bowl is the number one region of failure.
If you imagine the piston under pressure, the gas pressure is trying to bend
that piston over the pin.
So in that axis along the pin is where the highest stress is gonna be.
The edge of that bowl is always gonna be the highest temperature on the piston,
so you're combining the highest stress area with the highest temperature area
and adding the stress concentration that occurs when you neck down
the material and have the overhang.
So we eliminate that stress concentration.
The factory designed pistons, they shape the bowl for combustion, for emissions,
and different parameters but the guys that are tuning these for the aftermarket
are using different fuel strategies, different injectors, adding more fuel,
air et cetera so all that work that they put into that bowl design to fit
under those parameters kind of goes out the window.
So we take the default approach of let's take the stress concentration out,
let the guys use the tuning and their ability to work around the smoke issues
and the emissions parts and make the part as strong as we can,
which is ultimately that straight wall type of design.
- Look that's been really interesting to get some more insight into what
goes into a performance diesel piston design.
Great to also find out the differences between those cast alloy, forged alloy,
and your steel pistons.
Now if people wanna find out more about Mahle diesel piston applications,
where can they go to?
- We actually have some more informative videos on YouTube,
Mahle Motorsport North America has a channel as well.
Or definitely go to the website, and there's direct contact information,
or they can call us at the office and we'll talk to them about any
questions they may have.
- That's great, look thanks for the chat, really been interesting finding out
more about that.
- Thank you very much.
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