Lighter standard rods - how much is too much?
 

We are having a generel discussion in our little local community about grinding standard rods.

I know many DIY enthusiasts grind their rods a little to balance them (and I do too), but some seem to take it a step further to grind off some weight as well. I have never grinded more than a few grams off to a perfect balance - but the question is: How much is too much?

Anyone with a scientific approach and actual measurements? Neil?

This particular example is from a 3.0. About 110g has been removed and the owner plan to use these for a mild 3.0 engine build with 9.8:1 CR. Max rpm will be 7500. Cross-drilled crank and standard oilpump.

http://forums.pelicanparts.com/uploa...1570517279.jpg

"why not just buy Carillo or Pauter".

I'm interested in the science and knowledge behind this. Are there ways to improve these engines, without just buying new stuff. What can be done - and perhaps more importantly : What can not be done.

Some 911 rods had surface treatments that of course would be removed by doing something like this. Might have been only the early S rods (nitriding), but not sure what other may have had. I admire the ingenuity but wonder if an an errant nick could become a stress riser.

How much does a stock 3.0 rod weigh? 670 gms, give or take? Pauter shows a 511 gm 3.0 rod, off the shelf.

When I had to run stock type rods in class racing, we'd polish the beams, then send them out for shotpeening, then replace the bolts and resize them. The cost ended up being about the same as a set of aftermarket H-beam rods. And the H-beam rods were a lot lighter.

less weight
 

I would never grind the big end but only lighten up around the small end bushing.Fred

The crank is NOT cross drilled! ... It is 11.3: 1 CR
But yes, max revolutions at 7200 ish ... I should know, cause it's mine! :)
In this case, it must NOT be shotpeened! the trick is that the surface should be as smooth and shiny as possible! (this is actually old knowledge) This "confuses" the crack formation, and distributes the load over a larger surface and thus makes it stronger. According to engineer calculations (can't find that claim right now) with up to 20%!
In addition, this must be offset by the weight removed, accelerated to a given rotational speed. The connecting rod in the picture is an org. SC forged connecting rod, that has its strength due to the direction of the structure of the metal itself, not due to a surface treatment.
They are lightened 110g, up to 10g less when finished polished. In addition, it gets new pistons that are lighter than the original, let's just say 30g. It will be 150g less per connecting rod it has to "resist".
This also goes in the right direction in terms of help on "lost" strength

This applies to an NA engine. But even in a turbo engine, you usually see that it is the arm that is bent due to too much load, and I have not removed anything from this place, only polished it.
What does it cost? sweat and evenings ....
Why do it? Cause I can! :)

If you really want to lose weight Pankl titanium GT3 rods are 425g with bolts and are the same big end dimensions and length as 3.0 rods (also avalible in 130mm length)

john

Is there a reduced lifecycle for Pankl titanium GT3 rods, compared to factory rods?

Rahl

Yep... But they cost money ... :D

I am not trying to start an argument, just trying to clear up what seems to be a misconception about shot peening...

"Shot peening is a cold working process used to produce a compressive residual stress layer and modify the mechanical properties of metals and composites. It entails striking a surface with shot (round metallic, glass, or ceramic particles) with force sufficient to create plastic deformation.[1][2]

In machining, shot peening is used to strengthen and relieve stress in components like steel automobile crankshafts and connecting rods. In architecture it provides a muted finish to metal."

https://en.wikipedia.org/wiki/Shot_peening#:~:text=Shot%20peening%20is%20a%20col d,sufficient%20to%20create%20plastic%20deformation.

It is your motor - do what makes you happy. SmileWavy

I would really like to see documentation of the 20% increase in fatigue life. I admire the amount of effort you have expended. I don't believe polished is better that shot peen. The industry and engineering data I know of support a robust compressive stress to increase fatigue life. Curtiss-Wright Surface Technology uses a massive Laser to create a steam hammer effect to "shock peen" critical metal parts to improve fatigue life. To do better, for this contest, you need to bring a big hammer 😃

I worked on life extension programs for Sikorsky helicopters and submitted technical reports to the FAA for approval of Time-Life Limited parts on helicopters. One of our main processes for for extending part life and getting acceptance thru the FAA was our shot peening process. I do not think the polishing method will help strengthen anything (except reduce the smallest amount of air resistance). If you think about the surface on a microscopic level you really have not changed or reduced any imperfections or possible crack initiation areas by polishing.

Rahl

Engineering and science prevail... yet again. GO figure... ;-)

Cracks (dislocations) are failures resultant of tensile stress.

Polishing will improve fatigue life by reducing the probability of a surface defect where the critical crack length will result in formation of a dislocation.

Shot peening creates a compressive surface layer that must be exceeded in addition to the minimum stress for a given defect critical crack length. Additionally the plastic deformation of the surface probably improves things further. Similarly nitriding also creates a compressive surface layer through a diffusion process.

Nevertheless solutions cannot be applied without thought to the failure mode, at best these are best practises without specifying a desired fatigue life and applied stress.

Doesn’t seem like the reward to risk ratio is high enough to warrant lightening stock rods. The Pauter rods are so much lighter and actually decrease risk of failure that it seems a much better option.

Removing material from the rod cap ridge on a 911 rod is never a good idea. The "ridge" is structural and designed to provide rigidity. Some people grind this boss (ridge) to balance the rod but even this small amount compromises the rigidity and may allow excessive cap flex at higher rpm.

Henry's caution is also what came to my mind. A shop I used send my stock rods off for balancing. These guys were used to Chevy type rods, where the big boss on the bottom was there specifically for balancing. They cut down the ridge quite a lot. Very embarrassing for the shop, which found another set of good used stock rods for me and had them balanced. Typically very little is removed in order to get a set balanced. There are several spots which apparently don't affect strength, but this ridge isn't one of them, and you don't have to be an engineer to appreciate its function.

I am dubious that very much weight can be removed without altering where the highest stress levels are, or without increasing those stresses. There are programs and techniques for determining what and where these stresses are. FEA? I've got a saved picture of a rod showing just that, but it certainly is not really adequate for home brew weight redistribution.

http://forums.pelicanparts.com/uploa...1606525375.jpg

This appears to be a Carillo type H beam rod. I wonder if the analysis was done only for compression? The bottom of the rod is what takes up tension on that end as the piston hits TDC on the exhaust stroke.

Here is a similar view of stresses on a rod bolt.

http://forums.pelicanparts.com/uploa...1606525813.jpg

Not sure just why the stesses are so concentrated in the threaded end, though the press fit nature of the rest of the bolt may spread the tensile forces out more than at the nut end? The bolt I had fail on me broke about in the yellow area - you could see how both pieces were necked down there on each side of the break.

When Porsche finally did FEA with the redesign in the 993 the rod changed significantly. Most of the reduction was in the beam. These same rods (and crankshaft) were used in the 996/997 turbos as well. Picture of 993 rod.

http://forums.pelicanparts.com/uploa...1606595571.jpg

The 964 rod which was also used in the 3.2 Carreras and originated in the 3.3 turbo from 78. It looks much like a SC rod. Picture of 964 rod.


http://forums.pelicanparts.com/uploa...1606595691.jpg

My guess is that you might get away with a rod with material removed like in the original post (or not) because the rod is way over engineered for a 180HP motor with a 6300 redline.

John

If the goal is 180hp and 6300 reline why bother lightening them at all?

My goal is 250 + hp and red line at 7000 - 7200

but ... if I go for 100mm cyl and pistons (= 3.3) I probably can't keep it near 250, with the ideas I have for the engine in general. And my wepcam 20/21 will probably be too small too.... and my SSI exhaust :rolleyes::rolleyes:

well, I also have these in reserve ... But they are not as light as "mine" and I do not want to use mine with anything other than lighter pistons than the original.
I still think they are seriously over-engineered. Speaking of which, weren't the original connecting rods also the ones they used in the turbo variant of the 3l block ?!
the engine was designed to run with up to 800hp in their race cars.? Was that with other connecting rods? I can't find any info on this...

http://forums.pelicanparts.com/uploa...1606676485.jpghttp://forums.pelicanparts.com/uploa...1606676507.jpg

They are more finished now, just the final touch... and lighter still! :)
http://forums.pelicanparts.com/uploa...1606684225.jpg
http://forums.pelicanparts.com/uploa...1606684225.jpg

But I'm actually considering putting them on the shelf now, because I're leaning towards bigger and stronger ... (when do I learn) :rolleyes:

One needs to be careful when considering what Porsche did with its race engines. One approach was to make them just strong enough to last one race, which by and large is not something which will work for most of us.

Here is a Gen 4 LS Chevy rod that people run over 1000hp and 7000rpm with a 105mm (495g + 145g pin) and 92mm stroke. All the LS family run a 6.1in rod for a 1.7 rod/stroke ratio on the 6.0/6.2. Energy the rod has to take from RPM is E=mv^2 so the Chevy rods are taking MUCH more stress even at 400hp then any 7000rpm 911 motor.

Needless to say, stock 911 rods are damn near indestructible. The much thinner 993/996Turbo rods don’t bend until 600+hp.

http://forums.pelicanparts.com/uploa...1606799227.jpg

You are right, the rod is showing stresses in compression which is usually not a problem for a rod to handle unless its very high boost and high cylinder pressures. Whats critical for a NA engine are stresses in pulling direction which where it will come apart at high revs.

Regarding the rod bolt, the plot is showing deflection under tension.. so not really saying anything. The stresses are of course highest in the neck down area

With this discussion I think an intuitive explanation of why shot peening works would be a big help.

A crack requires a place to start. If it can't start it won't crack. Cracks will always start at the surface. ("The surface" includes the surface of voids and other internal defects but those are exceptions, especially on forged parts.)
So anything you can do to make it harder for that first hint of a crack to start on the surface you can significantly reduce cracking. Most treatments to increase the durability of parts come down simply to making the surface compressed. Nitriding and other hardening treatments change the surface chemistry by forcing atoms into the alloy's crystalline matrix that make the molecules want to be slightly bigger. This makes them "chemically compressed". Shot peening compresses the surface by striking it to plastically deform the surface so it is physically compressed.

And here we get to the simple reason these methods work. When you start to stretch the metal the compressed surface isn't stretching, it is just becoming uncompressed. You can put a lot of stress into the part and the surface is just becoming 'relaxed'. You have to stress the part a lot more to get it to the point where the surface is starting to come under tension and until you get to the point where the surface exceeds the limits of elastic deformation where you will start fatiguing the part because the crack must start on the surface.

Polishing will increase the fatigue resistance of a part by removing surface variations that provide a starting point for cracks but it is not anywhere near as effective as shot peening or a surface treatment like nitriding. That is why there is a milspec for shot peening but not for just polishing a part. If you polish a part and then shot peen (or nitride it) it you probably will increase its fatigue resistance beyond a simple surface treatment but it will get expensive in a hurry.

I had a set of 2.7 rods shot-peened in 1985. Also balanced and crack tested.

Built it as a 2.8.

Those rods go to 8000 RPM on every shift from 1990 up to today!

I'll admit that luck might be part of the equation.

Brookfield Engine Builders baby!

2.7 rods are quite capable of 8,000 rpm as they are. The main reason to use something else on motors of this era is to lose weight. Maybe they get marginal at 9K, but my 2.7 needed to be shifted below 8K to optimize accelleration, but would easily stretch to 8 (I'd been shifting at about 8,200 before I dynoed the motor and realized I was better off at 7,600) depending on the track.