Everything affects everything else.  When you change one aspect it dictates changes elsewhere, whether crank throw or rod length.
I don't remember what the maximum piston speed that is traditionally acceptable, my mind has lost it.  But there is a limit, and everything has to be constructed around that.  The newer materials may have extended it slightly, but if so I doubt it would be by much.

I just found this; hope it is helpful.
According to this guy the material science is critical in keeping a piston together as the forces increase.
There are some interesting numbers here.
*************

2RobHughes (Automotive)23 Mar 04 22:17
I work for a company that races and sells high performance motorcycle engine parts...here's a post I put up on one of the enthusiast boards when people started talking about wanting to raise the redline of their bikes from 5K to 6K or more. FYI the bike is Yamaha's Warrior an air cooled V-twin that displaces 102ci stock and the connecting rods are scary long and skinny...lol


From the postings:

At 5000 rpm your piston goes from a complete stop to 66mph in 2 1/4 inches in just .003 seconds.

Interesting Warrior Engine facts/figures:

Engine RPM/ Piston Velocity mph/ G's exerted on rotating mass.
1000 / 13.25 / 40.49
2000 / 26.50 / 161.98
3000 / 39.76 / 364.47
4000 / 53.01 / 647.95
5000 / 66.26 / 1012.43
6000 / 79.52 / 1457.90
7000 / 92.77 / 1984.36

For Comparison at "redline":

Chevy 350
Engine RPM/ Piston Velocity mph / G's exerted on rotating mass.
6000/ 62.20/ 1140.15

Formula 1 engine
Engine RPM/ Piston Velocity mph/ G's exerted on rotating mass.
17,000/ 83.56/ 361.27

2004 R1 Engine
Engine RPM/ Piston Velocity mph/ G's exerted on rotating mass.
12,500/ 78.58/ 250.12

2004 R6
Engine RPM/ Piston Velocity mph/ G's exerted on rotating mass.
15,500/ 79.98/ 315.69

2004 V-Rod
Engine RPM/ Piston Velocity mph/ G's exerted on rotating mass.
9000/ 76.14/ 174.50

OK guys for those who are not bored by this...here's the math.

I did my math in feet, just to keep from having to change the numbers up at the end, but trust me it's the same.

Warrior Stroke=113mm or .370735 feet.
Distance traveled by crank per revolution 1.16632 feet (remember that the circumfrence is "pie"* diameter)
Revolutions per minute=5000.
Distance crank travels per minute=5831.608202 feet
Distance crank travels per hour=349896.4921 feet or 66.2682 miles per hour.

So if the crank shaft is traveling at this rate the pistons reach this max velocity twice per revolution at 9 o'clock and 3 o'clock since they are traveling at the exact speed that the crankshaft is, remember at 12 and 6 o'clock the pistons are "stopped".

To calculate the G-Force you have to take your revolutions per second (rps) which is 83.3333 (at 5K)and figure how long it takes for one revolution or .0120 seconds (1/83.3333). The piston goes from a dead stop at 12 o'clock to full speed (or 97.1934 ft/second) in 1/4 a revolution or in .0030 seconds. So the average acceleration in a second needs to be calculated or how many times per second does this event occur 1/.0030 = 333.333 times per second, now multiply this by the speed at full acceleration (97.1934*333.333) gives the figure of 32397.82 ft/second/second as the average acceleration. G-Force is 32ft/second/second. Divide 32397.82 by 32 to get the average G's exerted on the parts...or 1012.432 G's. Whew....if you hung with my math you deserve a gold star...lol

A piston weighs in at around 400grams, so at full G's right when it is stopping it "weighs" an astonishing 892.79 lbs!!!

The g's are exponentially related to the RPM and crankshaft length relationship. The Warrior crankshaft goes from 33mph at 2500rpms with 253G's to 66mph at 5000rpms and 1012G's. At 10,000 it would be 132mph with 4050G's.

The F1 crankshafts are, I forget exactly, but like 1.65 inches. Do the math Gotta, no scare tactics here, pure physics. I was just joking about the connecting rods, but it is something to think about when you start wanting to get a few more revs out of your bike.

This calculation has nothing to do with bore or weights in the calculations demonstrated.

Rob Hughes
Orient Express Powersports

One more thing to consider is ring flutter. With 0.0463"-0.0469" (per Suzuki) rings, 6900 is about the max RPM before the rings start deforming. I can bore you with the math if you like.

I still don't believe with a 94 mm stroke that 6500 RPMs is safe. But I have no math to back that up.

The con rod is probably close to 6.5".

Didn't Cycle or Cycle World publish piston speed  with their road test figures? I seem to remember a piston speed of 2500 or 2550 being quoted as the max for reliability.

I used to operate  Pratt and Whitney R2800 engines with a 5.75" bore and a stroke of 6" The max RPM was 2700. We always observed a manifold pressure of at least the RPM selected because of piston ring flutter which could cause top ring land failure. Not a good thing!!!
Our piston speed was 4200'/ min at max RPM.

Sorry for straying off subject a little.

Kris01 wrote on 01/07/16 at 18:05:53:

This is where Art's eyes glaze over and he just nods and goes 'uh huh'

Kris01 wrote on 01/07/16 at 18:05:53:



The 4,200 ft.min is the distance the crank pin travels.....so I guess that is also the speed of the piston when the crank pin is 90 degrees to the piston travel.

The distance the crank pin travels in one rotation would be Pi x D, which is 3.14 x 6 inches.....18.8496 inches....or 1.5708 feet.
If the rotation is 2,700 rpm...then 2,700 x 1.5708 feet is 4,241 feet/min.

It appears that what Kris01 was calculating was the average piston speed per second.....and not the maximum speed.  The total distance that the piston travels in one minute (6" up and 6" down = 12" total) times 2,700 rpm = 32,400 inches...divided by 60 is 540 inches per second average.......or 2,700 feet/minute average.

Art Webb wrote on 01/08/16 at 07:34:39:


After the last few posts you're not the only one!  

Thanks for the correction guys. It's good to have someone double check your math at times.