Ducati Sporting Club UK - Steel Con Rods

Well you asked so here you are:

Quote:-

"The most important thing about Ti is it feels funny when you hold it. I collect surplus Ti bolts because they are cheap and hi-tech and the first thing you notice aside from the weight is the warmth. This comes about because of the lower thermal conductivity of Ti (and to answer one of your questions, Ti has a low thermal coefficient of expansion).

Anyhow, back to rods. Ti has a density about 55% that of and a tensile modulus (measure of stiffness) about 65% that of steel. What this means is if you take two identically sized bars of material, the Ti one will be about half the weight of the steel one, but if you hang a big weight at the end of each bar, the steel bar won't deflect as far. To make the Ti bar as stiff, you would have to increase the cross section slightly (but not necessarily by 50% depending on how clever you are).

So back to rods. 6Al-4V (which is one of the common alloys of Ti) has a tensile strength of 170kpsi vs. 220kpsi for 4340 steel as used in good rods when both are heat treated. That means that to make a Ti rod as strong as a steel rod (in stretching strength as in keeping the piston attached), we would have to increase the cross section of the beam by 30%. It gets better though, to prevent buckling of the beam under high combusion pressure (say that monster T04 turbo), you have to ensure that the stiffness of the beam is equivalent (to first order disregarding a whole bunch of ME stuff I'm not interested or versed in). Since Ti has a modulus 65% that of steel, we need to increase the cross section in a way that will stiffen the structure without adding too much weight. Look on too.com for a nice explanation of rod beam strength and H vs. I and consider the tradeoffs between packaging and weight by making things thicker or farther apart etc. If you got lucky, both things (strengh and stiffness) could be done at the same time and so the Ti rod is now 60 to 65% the weight of the steel rod. Pretty cool.

Ah, but it gets better. Ti likes to, on a small scale, swap spit with steel when they're rubbing against each other. That's galling. Chunks of material will transfer from one side to another or they'll just get stuck. That means you have to be real careful with your steel bolts in Ti, either by getting coatings or silver plating, or by using lots of antiseize. Also, your Ti rod is going to want to stick to the sides of your rod journal so the whole rod has to be coated with something from moly disulfide to titanium nitride to something really hi-tech whatever that might be.

Ok, but you can deal with all of that. Oh, forgot to mention, Ti is notch sensitive. That means that if you have little scratches on the surface, they are more likely to form cracks. Better hope that they treat your rods carefully when they assemble the engine and don't decide to go stamping them with a punch. (This is sort of like the time I took an ARP stud to the machine shop to see if they thought I could shorten them on the lathe with a steel tool (because they are hardened) and they whipped out a file and started filing to see!) That earlier comment about roller burnishing vs. honing, that was (I believe) due to this notch sensitivity issue. Honing is like sanding on a small scale so it leaves lot of little tears and scratches which can act like micro cracks. Roller burnishing rubs the material smooth and probably leaves a compressive stress on the surface. (squeeze a crack and it does nothing, stretch it and it grows and grows)

On top of all of that, the rods are major bucks. Very few people have forgings for import size rods [Owen, I was looking for a 146mm rod, 51mm and 20mm] and all the rest would be made from billet (back to too.com for an explanation of the advantages of forging). On top of all of that, they are DAMN expensive. $350 to $450 a rod! And on a more direct note, I asked TOO if he thought it was worth it (for what most of us want to do) and he said no. But it was an interesting little journey and I learned something along the way."

I think what he's trying to say is if ti rods were to run at high comp they would need to be 30% bigger than the same sizesd steel rod that can run that comp ratio..

Perhaps I was close...

SORRY Topitoo for going off subject!!