meengreen 94z

Im looking for a little more input on the weight of various metals and forming methods, their pros and cons, and also some other methods used to lighten and strengthen parts. Any input is welcome.

For comparison sake heres what I know:

Cranks

Cast iron/steel- lower strength due to impurities and non uniform grain structure

Forged steel: higher strength than casting method due to more uniform grain structure. Costly large machine dies used to mold shape drive up price.

Billet: Superior ability to be custom tailored due to billet process over forging, as well as a superior heat treating process. Overall billet strength superior to forging method for crankshafts by about 25-50%. Cheaper to manufacturer than forging due to lack of costly dies, and advancements of CNC machines.


Rods

Cast Steel: Brittle formation due to lack of cohesive grain pattern.

powdered metal: Metal powder heated in a mold like a casting, but hot struck in to shaped like a forging. Pro is less weight variation than standard forging.

Forged Steel:
Superior strength due to more careful methods of machining. Fully machined forged steel rods remove surface imperfections and have increased strength with reduced mass.

Billet formed steel: Free of surface degradation found on forgings. More uniform structure resists cracking but lack of circular grain formation on the big end of the rod compromises some strength over forged rods(about 15%).


Forged Aluminum: Lighter than steel, but increased mass requires more block clearance. Rods weigh about 25% less than steel rods overall despite larger mass. Short life usually leaves them for only track cars, although "Howards rods" claims 30k-40k miles out of a street engine.

Titanium: Common metal, but requires a mixture of other metals for machining and formation methods. Has greater strength to weight ratio than steel or aluminum, but due to lengthy formation process has high consumer price tag(around $3500 for a set of sbc rods). Small surface imperfections need to be polished immediately or they can expand.

Pistons

Cast: Less cohesive design makes them more brittle. Typically run hotter than forgings.

Hypereutectic cast: Aluminum pistons strengthened by adding silicon to the mixture

Forged: Less porose than casting, greater strength, and increased ductility(metals ability to mishape rather than break off). Runs cooler than castings, yet expands and contracts at a greater rate requiring more clearances. Tends to mishape slowly allowing a period of time before complete failure

OldSStroker

I don't agree with all of your statements. You requested input. Here goes:

Cranks:

Cast nodular or ductile iron are pretty uniform throughout, but they don't really have a grain structure because they are not shaped after they solidify. Castings can be quite "clean". Forgings can easily have inclusions (impurities) and even laps or seams due to the forging process. Forging tooling isn't necessarily super expensive, and one set of dies can make a blank that can be machined into a large number of main bearing diameters and strokes. Casting patterns ain't cheap, and each crank requires the pattern be used to form the sand mold.

Forgings have grain structure, but not all forgings are equal. The grade of steel used also varies from something like 1053 carbon steel to AQ4340, aircraft quality Nickel-Chrome-Molybdenum alloy. BIG difference in cost and strength. Less expensive forgings are forged in one plane (only one set of dies required) and then the throws are twisted 90*. This doesn't really help the grain structure make the crank stronger. More expensive forgings use more than one set of dies to make a non-twisted forging. It's nearly impossible to keep all impurities out of a forging. You can do some sophisticated inspection, but that gets really costly.

A well made ductile iron cast crank can be stronger and more failure resistant than a poor or even average forging.

Powdered metal is rarely used for automotive cranks. That's mainly reserved for rods. It is difficult to get the density as high as cast or forged steel, but they are coming close.

"Billet" means the crank started out as a bar of steel, maybe 8 inches in diameter weighing nearly 400 pounds. About 350 pounds are machined off usually by a CNC lathe specifically made to do cranks. The cost is the highest of all cranks. The grain structure just goes from one end to the other along the axis of rotation because that's how the billet was shaped when hot.

Heat treatment of a billet crank or forged crank can be exactly the same, but may not be. The strength advantage, IMO, is nowhere near 25%-50% over a similar forging. Billet's main advantage is that you can make a crank from scratch in a day or so. Billet cranks run from maybe $2500 to well over $5000. A friend is having one made for a 1905 two-cylinder engine. It's gonig to be nearly $4000. Of course if forging dies had to be make, it would be more since this is a one-off. Maybe 2 or 3 of that model car still exist.

The strength advantage a billet crank (or rod) may have comes from the quality of the billet. Double vacuum melted steel of aircraft quality is about as close as you can get to perfect. Even starting with that but forging it induces impurities. There's hardly any way around that.

Rods:

Cast steel or nodular iron rods aren't really all that "brittle". Same things apply as to cranks.

Same with forgings and "billet". Fully machined 4340 forgings are probably the strongest, as you said, especially if the manufacturer x-rays the forgings for internal flaws and scraps the bad ones. You pay a lot for that.

Aluminum rods have less mass but more volume. Life is limited because they have a low endurance limit compared to steel. They also stretch and compress more.

Powdered metal (PM) should be included in rods, for the reasons stated above.

Titanium is MEGA expensive, not only to buy but to fabricate. It's not all that common a metal. It's outlawed in more racing organizations than it's allowed, mainly for cost reasons.

Pistons:

Forgings are stronger mostly because they are more dense and have grain structure/flow. Different alloys have different expansion rates so clearances vary with alloy. I'm not so sure which runs "hotter", forged or cast, but I don't think it is significant.

Most high performance pistons are forged. For custom pistons, the machining
lof either a forged blank or cast blank costs about the same, so the stronger forging is preferred. Hypereutectic cast pistons are used in OEMs a lot and can support quite a bit of power for a long time.

One lightens AND strengthens a part by 1)design of the part, or eliminating the material that doesn't carry the loads, 2) choice of material on a strength/weight basis. High strength/weight usually implies very HIGH cost. Formula 1 put limits on things like that, but of course the engine manufacturers found new materials to get around the rules. Again, cost increases. Many metals can be heat treated to increase stength and endurance. Usually processes well above room temperature are used. Some sub-zero processing can also help, but, IMO, not to the extent some folks claim. That's another tin of worms altogether.

You asked. My $.02

Stephen 87 IROC

High end racers are also experimenting with plastics and ceramics on many engine parts.

Buy what you can afford to do the job within the limits of how much abuse you're going to put the parts through. A billet crank with aluminum rods, titanium valves and forged pistons are not required for a daily driver expecting to run 100,000+ miles between rebuilds.

I'm to the point now with my engine that every winter it gets torn down for new rings and bearings and the aluminum rods get resized. I'm debating the life of my roller lifters. They also may need to be changed every winter. Many racers with very aggressive roller cams will change lifters every 50-60 runs just to be safe and that's with the high end lifters not the cheaper street replacement style. Walk through the Stock/SuperStock pits some day and ask them how many runs are on their valve springs.

Many parts will survive for a long time without having to buy exotic metal components. A 5140 steel crank is just as good for most people as a 4340 crank for a lot less money.

Unless you're building a Top Fuel, Comp Eliminator or Promod engine, invest your money somewhere else for better parts.

meengreen 94z

Thanks,this is just what I was looking for. What I wrote is stuff I've peiced together and written down from books, the internet, etc. I wouldn't have asked if I thought all my information was correct, or it included everything. Really Im just trying to learn, not teach, I just wrote what Ive read to give a basic idea of what I was asking. and yeah I was in a rush and wrote a few things out of order, I appreciate you spending the time to sit and write all that.

Thanks again.




Any more information is appreciated. Specifically about various processes used to strengthen metals, remove impurities, weight, etc. Or any other information I left out.

meengreen 94z

Thanks, lol yeah Im not really looking to build a high dollar motor, more to just get an overall understanding of various things(right now metals). Probably the best place for me to go would be in the pits or in a machine shop and get a better idea from experienced people, but I know some of those people post on here so I thought Id ask.

94bird

I would also include eutectic cast pistons in your list. They can be more appropriate than hypereutectic in some situations, and are more commonly used in OE applications now than hypereutectic. Ford is the main user of hypereutectic piston alloys. KUS and Mahle typically use eutectic aluminum alloys for pistons. Federal-Mogul almost exclusively uses hypereutectic. I believe you'll find that any GM pistons not made by F-M are eutectic. The LS6 and LS7 are 2. They're made using Mahle 142 alloy, which has a higher content of copper and nickel for greater high temperature fatigue strength (best results above 250 deg. C).

Silicon has no real advantage for most applications. It can help wear resistance in the top ring groove and is cheaper than anodizing, but it's a very poor bandaid. That is the main reason Ford switched to hypereutectic. It helped get rid of some borderline top ring groove issues and saved them 60-80 cents per piston over going to anodizing. Since then it's just stuck.

Some very high performance engines with short top land heights have gone way past where anodizing can live now and are using a copper/nickel infused weld hardened top ring groove from KUS. It's similar to a ring carrier on a diesel piston but is welded into place.

SStrokerAce

Cool info.

I have a Mahle LS1 piston as a change holder in my room.

Bret

Z28SORR

Actually, Titanium is one of the more common metals found in nature. One of the biggest deposits in the world is in Russia, which is why they make so much of there military hardware out of it.