Not only that, but as the stroke increases and the rod journal gets smaller, there is less CPO (Crank Pin Overlap), which makes the crankshaft weaker. Once you get down in the 1.77" range, there is such little overlap between the connecting rod journal and the main bearing journal (on a stroked crank) that you will need a very, very expensive piece of steel to make a crank that will be stiff enough to live.

Starting with a billet piece of steel that costs $3000.00 is going to make a very expensive crankshaft, when you are done.....

 

I will sit here and bask in this knowledge. I can debug and build electrical systems blindfolded, but these types of precision requirements are a black art for me. Thanks for sharing, even a little, guys.

 

As far as the material cost is concerned, yes that is an area that needs careful consideration as to what is required for the application, that goes for many items in the engine, most will not realize how expensive some of the raw are. However there has been huge advancements in these areas as such the reliability has dramatically increased. Note the small amount of failures in F1 and NASCAR these days, it ceratinly doesn't happen by luck.

Below is a link that many may find interesting regarding, materials used in racing engines. They quote the cost of the NASCAR Cup billet at $5,000 and that would make sense as the reportedly best crank maker in the United States (Superior Chrankshaft) charges $25k for a crank although it does come down to $13k in volume.

http://www.epi-eng.com/piston_engine..._materials.htm

There is another article that is also interesting,

http://www.epi-eng.com/piston_engine...ign_issues.htm

I would have thought that (not that my knowledge is very good on these matters) that a double VIM VAR or triple vacuum remelt version of 4340 would be plenty good for the 928. Especially when you consider the design of the crank also greatly contributes to the strength, noting this is a rod thread and hopefully the opening poster is not concerned by all this chit chat know that he has his answer regarding the difference between the Porsche rods.

The design is also important when it comes to rods, there is lots of different rod designs, also lots of different ways to even mount the cap. Some use pins, said to be very good, some even use cerrations on the rod and cap that interlock, I would guess that is very stable. Other use dowels, generally it is said it is best to get the bolt as close to the inner bearing surface as possible.

Even the way the pin end is done can cause issues, I have seen some rods that don't have a radius which I would have thought that will crack if enough stress is applied, yet same material and a revised design may be fine.

One thing is for sure there is always more optimisation and interesting developments, one thing that has my interest is the new piston from JE, here's the link.

http://www.vitalmx.com/photos/featur.../bturman,20779

Greg, regarding crank pin overlap with a stroke not being greater than say 3.75" there should be no issue of weakness with a journal of 1.888" or 48 mm as the mains are large at 70 mm, the NASCAR stuff is regularly running under large loads with main journals around 50.8 mm and big ends of 47 mm. Some use the 2.25" mains which would put it at the same overlap as a stroker with 48 mm rod journals. An interesting design superior do is instead of hollowing out the rod journal with one hole, they do three, I suspect it adds greater stiffness.

 

Greg G., Don't worry about me. I'm enjoying the inforamtion you guys are sharing.

I thought this thread was going to be a dud and then the two Greg's showed up and saved it.

 

The 951 piston has different offset for the wrist pin than a 928 piston. Therefore if you use 951 pistons in a 928 block with stock rod length, the 951 piston will not reach full deck. And therefore compromise the efficiency of the compression chamber to a slight degree.

 

I think you are referring to the compression height and not the offset.

 

From what I understand, this is a difference in the piston and not the rod. Is this correct?

 

Crankshaft design and construction almost always involves compromise.

There is no question that one could build an "unaffordable" crankshaft for use in a 928 engine, but my goals of making a "brand new" design crankshaft were to make it afforable for the "normal person" building a stroker engine....or a stock stroke 928 engine.

Obviously, different materials vary in the price, per pound. A Formula One engine needs ultra expensive chunks of steel, but an 8,500 rpm 928 engine isn't stressed as highly as that Formula One engine and can use a more ecconomical chunk of steel, to start with....to a point.

I think that the important thing that the "average" reader of this thread needs to take with them is that there is a huge variance of different 4340 steels. When someone says that their crank is made from 4340....this can mean a whole bunch of different things. A "Chinese" chunk of 4340 is going to be way different than a chunk of high quality, US made, vacuum remelted 4340.

CPO (crank pin overlap) requirements have certainly changed, over the years, however CPO still effects the eventual rpm limit of a given engine. Not many people are going to "twist" a 4.500" stroke Big Block Chevy engine at 8,500 rpms...or at least, not do this for very long.....even with a really high quality 4340 crankshaft. Yet, a 3.000" Small Block Chevy engine is very comfortable doing this....with even "lesser" steels.

Taking a quick look at that 3.000" Chevy engine, which was found in the '67-'69 Z-28 vehicles, shows that the '67-'68 engines had significant crankshaft failures in racing, while the '69 engine was a much stronger engine. Chevy made a radical change to the '69 Z-28 engines when they switched the crank from 2.000" to 2.100" rod journals (note that the main bearing dimensions changed also.) Although the dimensions of the engine remained the same (302ci.), the '69 engines were certainly significantly more "robust" than the '67/'68 engines. Obviously the crankshafts were heavier in '69, but the improvement in stiffness made this "weight gain" worthwhile.

If you do the math on these "short stroke" Chevy engines, there is certainly a large CPO in both of these cranks, as compared to a lot of current day crankshafts.....and it makes one wonder why the larger diameter crank made such a significant change.

Steel, of course, comes into this equation. Chevy cranks were production pieces made from twisted 1010....a good piece of steel, but a far cry from a good piece of 4340.

Aside from the change in material, there have also been significant gains in the design of crankshafts that allow then to have smaller CPO ratios.

Back to our 928 crankshafts, CPO is also a factor, by the time these engines are stroked to 3.750". My goals, with these new crankshafts, are to increase the "power range" of the 928 engines some 800-1000 rpms. Doing this requires that I have a certain amount of stiffness. I felt that the 1.770" rod journal (or smaller) was too small for the reliability that I was looking for.

I still have no customers that are willing to "throw" their crankshafts in the trash after 500 miles....

 

Agreed on all of the above, the 1.77" journal would be too small not just in overlap but the bearing surface area would also be further reduced down from say the Honda size, in metric it would be 45 mm vs 48 mm and the smaller journals would also have slightly narrower bearings too, that adds up to a considerable loss of surface area. That loss will lead to increased wear and shorter life.

In the Australian Supercar series which use a smaller controlled version of a NASCAR style V8, probably closer to the truck series engines but fuel injected and rev limited under power not engine braking, to 7,500 rpm. Their parts last quite a long time, a crank will last for around 12,000 miles. That is pretty good considering how much strain they are under. The extra revs that NASCAR has, leads to much shorter life, as most are aware wear goes up exponentially with revs.

 

No, I'm talking about the hole for the wrist pin on the piston is offset different between the two. I had custom rods done by Pauter, that were longer than stock, so that it would bring the 951 pistons full deck in the 928 block.

 

I think you're both talking about the same thing- top of the piston to the centerline of the wrist pin bore: