rod stretch and ways to minimize it

[The Iron Icon]

So just a thought I was having after going thru some technical info is the effect of rod material and other factors on the amount of stretch you get so you can figure min. piston to head clearance.


1. Is there something better to use as a material other than steel,TI, or aluminum that will still have a signifigant life span(cost being no object)


2. how about pin offset, would that bring about more stretch into another direction even though its miniscule?

3. Tired of my off the wall topics?

[Roundybout]

The material we have today is what it is. I'm sure there are materials out there and with an insane budget could minimize it. I don't think there is any low hanging fruit there left. Ways to minimize stretch would be in the construction of the rod, the FPM the piston assembly is traveling and it's weight would be the biggest ways to reduce stretch. A properly designed, forged/billet, 300M rod is hard to beat. Each material has it's strengths and weaknesses. It's all in the design, manufacturer and quality of the material to reduce stretch.

[peejay]

If you want to fight stretch, find a way to make the piston lighter, since that is what is stretching the rod.

Some days I wonder what it would be like to have a carbon composite piston that had a stainless steel deck on top, maybe Inconel, to take combustion heat. Should be pretty cheap right? It would have to be a metal that had a very high heat resistance since it wouldn't transfer very much through the skirt.

[David Redszus]

1. Is there something better to use as a material other than steel,TI, or aluminum that will still have a signifigant life span(cost being no object)

The E Modulus of a material will determine its tensile strength. Virtually all steels have very similar E Mod numbers (30,000,000). To increase tensile it would be necessary to increase the cross sectional area of the rod. But that would also increase the rod weight and consequent loading on the rod bearing at higher rpms. A lighter piston is the obvious solution.

A short stroke engine will allow higher rpms without creating higher loads on the rod and bearings.

[PackardV8]

3. Tired of my off the wall topics?

Maybe. Can't cheat physics. It's not about rod material. It's stroke length, RPM, piston weight, rod design.

[digger]

A longer stroke engine is better at reducing stretch due to inertia effects, as it will operate at lower rpm to achieve the same breathing (piston speed) all else equal.

[The Iron Icon]

I wonder what the return would be on lightening the piston vs amount of stretching and where the diminishing returns are.

[steve cowan]

I wonder what the return would be on lightening the piston vs amount of stretching and where the diminishing returns are.

i suspect you can lighten a piston only so much before it rips the wrist pin out,i would think the piston and rod would need to compliment each other and be application specific.

[jsgarage]

As for alternate materials, titanium con-rods were often referred to as 'rubber rods' due to elongation under load. Strong and light, but stretchy. Note: hear-say only; back before I was interested in going around corners at speed, I couldn't afford to even try them. Seems that reducing piston weight and/or reducing astronomical rpms were the only sure fire ways to reduce rod stretch

[dwilliams]

The rod itself is almost always heavier than the piston and pin... and, for the same weight, I-beam rods are stronger than H-beam rods. H-beam rods were a product of the hot rod aftermarket; the H shape was easier to whittle out of bar stock with manual milling machines than an I beam.

Back in the 1980s there were dozens of SAE papers, most from VW and Polimotor, about carbon fiber rods, pistons, and pins. But that was pretty much a worst-case application for plastic resins, no matter what they were reinforced with, and after a while the shine wore off the New Hotness. "Carbon-carbon" pistons were the new thing at the turn of the century, but that's not a particularly good material for pistons either.

The strength to weight ratio between aluminum and steel is about the same at room temperature; at the temperatures pistons run at, the ratio skews heavily in favor of steel. The automotive industry once used cast iron for pistons; iron pistons went away after WWII because aluminum was lighter and cheaper to cast and machine with the processes in use at the time, and the pistons ran cooler, which was a big deal when compression ratios were being ratcheted up. But for modern engines cast iron is a better material than aluminum; some Diesels are moving back to it, and Ferrari has been doing some development with iron pistons for their F1 engines.

We're not talking about brittle "grey" cast iron here; a piston would be made of one of the "malleable" alloys. [the difference between "iron" and "steel" at the industrial level is more a matter of process than chemical composition] With selection of the proper iron, the piston could be lighter and stronger than an aluminum piston, as well as having a better bearing surface for the ring grooves and skirts. Plus the lower expansion would allow a tighter fit which would minimize rocking and help ring seal, and you wouldn't need complicated cam-ground skirt shapes to minimize slap when cold. And the lower thermal conduction would keep more heat in the chamber; with modern electronic engine management systems, we don't need or want the heat transfer characteristics of aluminum any more. And iron's melting point is double that of aluminum, giving more headroom for the occasional tuning oopsie.

Of course, I've been advocating a return to iron pistons since the 1990s. Most people just stare at me like I was talking about hot-tube ignition or wick carburetors...

[modok]

I don't really see why it's a problem. All the other parts flexing and rocking might be problems, but the rod stretching? That's more stroke man, It causes no friction.
I'd say it's a benefit

[mag2555]

Since your doing head work with DV on 289 heads which I will assume your using , then tell us how high you see yourself reving this motor and also if the stock stroke will be changed?

Ford SB Rods are so short and stiff ( in a good way) that a aftermarket Rod used in a build going to 8000 rpm would have me far less concerned then many other motors spinning to that same rpm!

Just remember that a factory block will not live too long at 500 hp and even 7000 rpm!

[strokersix]

I don't really see why it's a problem. All the other parts flexing and rocking might be problems, but the rod stretching? That's more stroke man, It causes no friction.
I'd say it's a benefit

Yup, I like the way you think.

[Warp Speed]

I don't really see why it's a problem. All the other parts flexing and rocking might be problems, but the rod stretching? That's more stroke man, It causes no friction.
I'd say it's a benefit

If that was the case, every PS engine would run aluminum rods.

[Dave Koehler]

I don't really see why it's a problem. All the other parts flexing and rocking might be problems, but the rod stretching? That's more stroke man, It causes no friction.
I'd say it's a benefit

You may want to rethink the part in bold.