Mike Simard

I have a crank that Moldex just finished, It's at Moldex and ready to ship.
It's a basic 6cw 3.75" for a 6" rod and Chevy 2.1 journals.
mike@simardracing.com

Mike Simard

OK, I'm going to have it balanced for the titanium rods I use and put this thing in my nice 87 along with a few other bits. I hope this doesn't snowball too badly ;-)

IcemanG17

Mike
BE CAREFUL.....you know how quickly this slope gets slippery!!!!

However a "street" 7.0 does have a nice ring to it!!!

123quattro

GM Ti rods are sweet. Just don't run them in a race engine for more than 24 hours.

IcemanG17

I thought forged Ti rods are stronger than just about anything else available?

GregBBRD

With the offset being completely wrong for the 928 engine, I'm amazed they ran that long.

Mike Simard

123', I think this Ti rods that fail in 24 hours claim is a myth.
Can you give me an example of a GM rod failing at all, ever? Not where the piston or something else broke but the rod itself. This sounds like someone once said something, it got passed down through a few thousand people over the internets who have no direct experience. In other words, I disagree

Greg, are you assuming I'm running these rods out of the box with no changes to put the pin end centered with the piston?

Here's what I do:

The stock pin bushing is removed.
A new one is put in that is made from material that I've tested and know to have high strength and little friction. I tested this with simulated lab type stuff and by running them (over 24 hours) in a race engine and comparing. This bushing gets oil channels CNC machined and the whole rod and bushing is cut on a Wire EDM to final shape. This is when it is effectively moved .02" to be centered with the piston. While I'm at it the pin bore isn't just honed to size. No, it gets Wire EDMed to an oval shape to account for changes to the rod and pin under loading. Rods come out looking excellent after running that way. The material I use really is noticeably better than stock, they come looking nice with no lines on the pin.

Also noteworthy in this pic is that the pistons don't extend out of the cylinders at all. Another benefit of the sleeving that is surely subject to the same passed-on internet wisdom as Ti rods, 928s having flimsy timing belts and Chevy motors being 500 pounds lighter.

Anyone care to guess how much those pistons weigh?

ptuomov

Thinking about geometry, what actually determines the relevant side loading or rather absence of it? Is it the centering of the small end on the pin such that small end side clearance is the same on both side? Or is it that the beam is exactly centered relative to the piston? Or is it that the big end is centered relative to the piston?

For example, one could have the small end and the big end exactly centered relative to the piston but the beam to the side:

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Wouldn't this still cause a side loading in the rod? And if the beam is not completely rigid, wouldn't it also lead to a side loading between the piston and the cylinder wall.

Mike Simard

Hi Tuomo.

I owe you a thank you for lower case oiling mods that I've done. I'll have to tell you about that some time.

About the rods, it's common to have the rod offset. The stock 928 has its offset by an amount greater than the .020 required to use a .940 wide Chevy rod. It looks like the stock 928 rod is offset on the big end. Possibly to keep it centered over the bearing itself?

The offset is determined by the big and width and cylinder bank offset.
The 928's cylinder banks are offset at .985" so anything other that a rod that was .985 wide would need to be offset. If they added the extra rod width to the area with the fillet than that makes sense.

I actually didn't care that the Chevy rods weren't centered at first. I felt it was too small to matter and I ran them that way for a while with no noticeable issues.
The reason I made that mod was to be able to use a tight, box piston design and get away from those generic piston forgings with far apart pin bosses, ugh. These are my dream pistons. Greg Brown saying that centered is important was the final prompting;-)
Now all 928s rods will need to be centered which is fine with me although one could contemplate just how the centering is done.

123quattro

My comment comes directly from experience on C6 Z06 cars. Several development cars have kicked the rods out right near the 24 hour (of hard tracking) mark. It's repeatable.

blau928

Hmmmmmm, 3 words here....

piston

guided

rods....

Looking good Mike..!

Mike Simard

Please tell me more about these cars. I'm going to be skeptical until I get details and I would like to learn more. It's not that I don't trustyour advice, it's just the nature of the internets.

ptuomov

Credit for any actually useful mods should go to the person who's been able to sort out the useful bits from incoherent internet ramblings such as mine.

I think it is probably the case that sensible designs have the small end centered relative to the piston. Among the reasons the ability to use the lightest and strongest pistons available as you have.

What I don't really understand is whether it is important to have the beam is centered relative to the piston. I think that if the rod is perfectly rigid and strong enough that it doesn't shatter then it doesn't matter where the beam is relative to the piston center. But what happens if the rod flexes? And how does not centering the beam relative to the piston impact the strength of the rod? Are there a lot of production engines with the beam offset relative to the piston?

BC

I'd like to understand this better:

You are moving the pin bushing and putting it back in a different offset, and this suffices for the rod offset on the 928?

ptuomov

Mike -- Out of curiosity, what's the compression height and weight of those pistons? Best, Tuomo