forged piston question

[thedon809]

Would the pin being .011 bigger really make that big of a difference? That would mean you are also removing material from the small end of the rod potentially making it weaker, no?

[Ondonti]

I looked for a couple minutes but couldnt find this. Got a link or some helpful search terms?

http://www.turbo-mopar.com/forums/sh...=1#post1046789

That's a valid point! I haven't seen any issues with the .912 pins under 400hp, but I know several guys in the 400+ range that were trashing the pins and needed to move up on the pins to a thicker wall version. I know one guy was using the srt .866 piston pins and his were in a bad shape with only one season of racing.
We do sell the upgraded pins for people that plan to make mega hp, but they aren't cheap!

Of course pins come in different alloys, qualities, thickness, and the design of the piston can make the pin endure less stress, or more stress. A pin boss that forces the pin to do all the work (imagine a long bridge vs a short bridge) would make a huge difference but stronger bosses probably add weight or force you to run even less material in other places to keep weight down.

[acannell]

I feel like advice is just being blown off here looking for the easy way out. If you are being picky about things being perfect, there is no easy way out.

There is a thread from earlier this year with Cindy talking about the documented weakness of the stock size pin. Pin size plus the support provided by the bosses on the piston pretty much lock you into a torque/cylinder limit. Selling larger/better pins is not a gimmick. This goes across all engine platforms. Now, you only need upgraded pins if you will make enough HP to justify.

I also don't like those ring gaps, especially since Acannell likes pump gas. In FM's glory days, everybody was running race gas and still barely holding together motors or doing rebuilds in the pits.

Its a fact that the pistons determine bore size during proper machining. If your block is already completed then you can't do any better than shoot dice and see how they come out and how they match the block. If you don't want to bore farther than you are stuck. Nobody is going to modify a piston for a slightly out of round or tapered cylinder. That is not how it works. If you want things perfect, then the only option is to follow Warren's advice. I would still have the pistons ordered and then the bores can exactly match each piston's real life size.

Im not saying its a gimmick. I'm saying there is no way to separate someone making a reasonable guess about it ("bigger is better") to it being proven to being a necessary upgrade. There is a difference.

A .912 pin, has at most 5% more stiffness than a .901 pin and a negligible increase in strength unless there is a change in material. How can that possibly make any difference? Maybe the .912 wall thickness is actually thinner than a .901 and it ends up being weaker, who knows? What exactly was the failure and what were the test circumstances? What were the materials and geometry of the pins being tested? What sort of stresses were the pins subjected to?

The idea you can have perfectly circular bores is a fantasy. The bores change shape when you replace a honing plate with a head, or start the engine and add coolant pressure, and uneven heating coming in pulses dissipating through an object with all kinds of varying wall thicknesses and heat transfer mechanisms. And they change shape again when you add a few thousand miles of wear. And when power levels go up.

http://www.aera.org/engine-professio...re-distortion/

There is no fact that pistons determine bore size. I can have pistons made all day long to match whatever bore diameter I want. In fact thats exactly what I'm going to do in the next couple days. Im not asking for a piston to be modified especially for an out of round or tapered condition. And by the way, you certainly could have them modify it that way if you wanted, they will make anything you want. Although Im not sure that even makes sense since the rings are what deal with that unless you make the piston so big it seizes. Im just making sure that the bore diameter I tell them to make the pistons for ends up being a combination that works with the existing bore measurements and their variances.

There is always out of round, its just a matter of how much. The out of round I'm dealing with is within factory spec, and works fine with the rings and pistons I had in there. Theres no reason I cant swap the piston to a forged and everything wont still work just like it was.

[acannell]

Would the pin being .011 bigger really make that big of a difference? That would mean you are also removing material from the small end of the rod potentially making it weaker, no?

No it would not.

Stiffness for all steel alloys is nearly identical.

Assuming the material remains the same and the wall thickness about the same, the increase in stiffness by going from .901 to .912 would be about 5%.

Tensile strength could increase drastically depending on the material but its pretty unlikely the stock pins are made of something substantially weaker than other pins, even upgraded ones.

[acannell]

That's a valid point! I haven't seen any issues with the .912 pins under 400hp, but I know several guys in the 400+ range that were trashing the pins and needed to move up on the pins to a thicker wall version. I know one guy was using the srt .866 piston pins and his were in a bad shape with only one season of racing.
We do sell the upgraded pins for people that plan to make mega hp, but they aren't cheap!

Third-hand anecdotals with no important details. This wouldnt even make it into an uncited wikipedia article about to be deleted. And simple calculations show it doesn't make any sense that a tiny increase in pin diameter wouldnt make any difference. In fact there it makes just as much sense that you may be weakening the stucture of the piston by doing so.

There are so many things wrong with that quote I literally cant believe anyone would use it to decide anything. How do we know that guys engine didnt have oil problems? What exactly were wrong with the pins? How well were his pins matched to his rods and pistons? What sort of racing was it and how much was there? What do 0.866 pins have to do with .901 pins?

Sheesh..is that seriously the standard of fact thats acceptable around here?

[Force Fed Mopar]

People make a lot more power on smaller pins. Nascar engines run a .787 pin now. Many SRT-4 engines make 400 on .866 pins. 03-04 Cobras are making stupid power on .866 pins also. Wall thickness and metallurgy is at least as important as diameter IMO, if not more.