Rod Length Discussion - Engine Masters

[Barry_R]

     "My" personal comfort zone is in the 1.200-1.400 range. Shorter pistons can be made more stable by carefully changing dimensions like the skirt shape and diameter of the ring lands. Some of the problem comes from cylinder length ( as in the 347 small block) because the piston is pulled out of the bottom of the cylinder due to the longer stroke and limited rod length. ANY time a piston comes out of the bottom of the bore , stability is compromised regardless of the piston manufacturer. This is because of the different temperatures in the piston and the undersizing of the area of highest heat , the top and ring land areas. The temperature difference is typically 400*s higher than the temperature of the skirt sometimes more. "Heat management" is another critical factor in short compression height pistons. Thermal barrier coatings "can" be employed as a band aid but the rest of the piston must be changed to adjust for the lower expansion. Short compression heights also bring the top of the rod closer to the higher heat and have been known to heat oi in the pin bores to the point where the "hydrodynamic boundry layer" ( oil film strength) is lost and piston/pin damage occurs. One of my small block engines has a 1.050 C/H but it is not a "highway" engine by any means.
    I welcome additional comments/ questions.
       Randy

You ever see where on real high end stuff where Mahle had been playing with designed in wall contact above the rings for stability (unlike the reduced diameter we all normally run in that zone)?
They also dabbled in non-round pin bores on some stuff to react to heat induced variability - kinda like ring groove up tilt but a lot harder to duplicate and impossible to service.

[FrozenMerc]

So when does a piston become too short for you guys?  1"?  Less?

Forget ring support rails and rod/stroke ratios for a minute.  Is a 1" piston as stable and seal as well as a 1.5" tall piston?

This is from the very extreme end of things - Early 2000's Cosworth Formula 1 piston and con-rod.  3.0 L V10's turning 20,000+ RPM with a life span measured in dozens of minutes.

Rod Lengths were generally around 100 mm (4 in) on strokes of about 40 mm (1 9/16")  making for approx a 2.5:1 rod/stroke ratio.

[frnkeore]

You ever see where on real high end stuff where Mahle had been playing with designed in wall contact above the rings for stability (unlike the reduced diameter we all normally run in that zone)?
They also dabbled in non-round pin bores on some stuff to react to heat induced variability - kinda like ring groove up tilt but a lot harder to duplicate and impossible to service.

I like the idea of running the top land at close to bore diameter. Does anyone have any info on that and if it's still done? And while I'm at it, what do you consider a minimum top land thickness?

Regarding the "non-round" pin bores, it's not hard to do. They would be broached, rather than bored or honed. But, the broach would be expensive.

[gt350hr]

     I found ( while doing Nascar piston development before Mahle Motorsports was created) that a second land diameter of .028 "under the bore was very good. It improved ring seal and reduced ring flutter above 8,000 rpm. An "accumulator groove HAD to be used to prevent galling of the second land. No one had gone that tight before I tried it , and a few other things like "spherical dishes". I have gone as tight as .026 under on an aluminum sprint/ dirt late model engine. CARE must be taken when doing this it doesn't apply to every piston manufacturer's parts.
   Broached pin bores are routinely done by several piston manufacturers as standard issue and optional at others.
    Randy

[frnkeore]

This is from the very extreme end of things - Early 2000's Cosworth Formula 1 piston and con-rod.  3.0 L V10's turning 20,000+ RPM with a life span measured in dozens of minutes.

Rod Lengths were generally around 100 mm (4 in) on strokes of about 40 mm (1 9/16")  making for approx a 2.5:1 rod/stroke ratio.

The last version of the NA, F1 engines, was the 2.4L (engine went from 10 to 8 cyl). Cosworth was the only one that ran 20,000+ rpm, all the others stayed closer to 19K.

I ran across this NASCAR/F1, comparison chart, about a week ago and regarding rod/stroke ratio, the two organizations, that have put much more money in research than all others combined, use a high ratio. 2.564 for F1 and 1.907 for NASCAR.

In those organizations, it may be more for reliability rather than HP but, neither would over look the HP aspect.

[gt350hr]

  The F1 engine IDLES at  7,000 because there isn't enough airflow to keep it running below that!
     Randy