Why use Al for pistons?
03-20-2014, 02:08 AM
#21
Titanium valves have issues with guides, rods like to Gaul rubbing eachother too.
Can't imagine rubbing against a cylinder wall would it would last.
But I have no experience with it.
I have worked with inconel stuff before during machining.
It's rough to work with, kills bits and is rough on the equipment.
03-24-2014, 03:43 PM
#23
I work with inconel (which is a trade name, btw. It goes by alloy 600/800/840/etc) on a daily basis for temperature sensors and electric heaters that go up to 900C, and James had it right. The stuff is a huge PITA to machine, especially in volume. It obviously conducts pretty well, but not as well as aluminum...plus, that much more rotating mass is a bear on a crankshaft.
It's nice that it doesn't creep under temperature and maintains its strength, but it's more suited to very high temperature applications...like in a turbocharger.
From what I remember in my materials classes and random things I've picked up, titanium would be good for short-term applications. Eg, open the tolerances 10% so it glides easier and run it for a few races or a single season. I don't think I'd want it on a car that will hit 30k miles or more.
It's nice that it doesn't creep under temperature and maintains its strength, but it's more suited to very high temperature applications...like in a turbocharger.
From what I remember in my materials classes and random things I've picked up, titanium would be good for short-term applications. Eg, open the tolerances 10% so it glides easier and run it for a few races or a single season. I don't think I'd want it on a car that will hit 30k miles or more.
03-24-2014, 10:55 PM
#24
Wow, so many people greatly over thinking this.
Why is aluminum used for pistons? Because its not the weak point. Stock Hypereutectic pistons are capable of supporting close to 150 HP EACH. In 90% of OEM engine failures where high HP is involved, it tends to be the rod bolts that let go. In 90% of all OEM rotating assemblies, the rod bolts are the weak point. They give out because of the forces exerted on them during an intake stroke. At high RPM, even with a lightweight aluminum piston, they let go between the top of the exhaust stroke and the bottom intake stroke. Somewhere between the piston coming to a dead stop with no downward force being exerted, or the piston being accelerated back downwards with no downward force coming from above. All of the force in these 2 instances is on the rod bolts.
In aftermarket applications, stronger rod bolts can handle the load better. They can handle a 4" bore piston at 8k RPM all day long. The thing is, even the aluminum piston can handle this. What would happen here if you tripled the weight of the piston? Even that nice expensive ARP rod bolt is going to let go at 6000 RPM.
Changing the piston to a heaver, stronger material is not removing the bottleneck. The lighter the piston the better, in all applications. If aluminum is strong enough to handle whatever abuse is being thrown at it, there is no need for an upgrade. Top fuel dragsters produce 1000 HP per cylinder, and not only do the aluminum pistons hold up just fine, but so do the ALUMINUM connecting rods. The aluminum rods are strong enough to handle the abuse, and soak up extra vibration. This keeps the rod bolts in place.
You might as well discuss why engine blocks themselves are often made of aluminum in 3000+ HP builds and instead aren't made of solid Inconel or some 210,000 + psi tensile strength alloy. The fact of the matter is, the point is still moot because you aren't discussing any real weakness.
Why is aluminum used for pistons? Because its not the weak point. Stock Hypereutectic pistons are capable of supporting close to 150 HP EACH. In 90% of OEM engine failures where high HP is involved, it tends to be the rod bolts that let go. In 90% of all OEM rotating assemblies, the rod bolts are the weak point. They give out because of the forces exerted on them during an intake stroke. At high RPM, even with a lightweight aluminum piston, they let go between the top of the exhaust stroke and the bottom intake stroke. Somewhere between the piston coming to a dead stop with no downward force being exerted, or the piston being accelerated back downwards with no downward force coming from above. All of the force in these 2 instances is on the rod bolts.
In aftermarket applications, stronger rod bolts can handle the load better. They can handle a 4" bore piston at 8k RPM all day long. The thing is, even the aluminum piston can handle this. What would happen here if you tripled the weight of the piston? Even that nice expensive ARP rod bolt is going to let go at 6000 RPM.
Changing the piston to a heaver, stronger material is not removing the bottleneck. The lighter the piston the better, in all applications. If aluminum is strong enough to handle whatever abuse is being thrown at it, there is no need for an upgrade. Top fuel dragsters produce 1000 HP per cylinder, and not only do the aluminum pistons hold up just fine, but so do the ALUMINUM connecting rods. The aluminum rods are strong enough to handle the abuse, and soak up extra vibration. This keeps the rod bolts in place.
You might as well discuss why engine blocks themselves are often made of aluminum in 3000+ HP builds and instead aren't made of solid Inconel or some 210,000 + psi tensile strength alloy. The fact of the matter is, the point is still moot because you aren't discussing any real weakness.
03-24-2014, 11:12 PM
#25
I'm a machinist (actually a tool & die maker) and I make and sell aluminum strut brace mounting brackets for the w-body cars in my garage. I use high strength aircraft aluminum because it's strong and lightweight plus it doesn't rust. I have sold many thousands of these parts and have not had one single failure.
A lot of people don't realize how strong aluminum really is, even though it's softer and lighter than steel. In fact pound for pound it's actually stronger than steel, which makes it ideal for pistons.
A lot of people don't realize how strong aluminum really is, even though it's softer and lighter than steel. In fact pound for pound it's actually stronger than steel, which makes it ideal for pistons.
