Author Topic: How To Determine Safe Piston Speed?  (Read 16443 times)

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Offline 38flattie

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Re: How To Determine Safe Piston Speed?
« Reply #30 on: January 07, 2013, 04:57:41 PM »
Thanks everyone!

I guess it's a case of 'run what you brung'!

We have what we have, so we'll build for 6000 RPM, and see if we need kitty litter or not! :-o
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Offline fordboy628

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Re: How To Determine Safe Piston Speed?
« Reply #31 on: January 08, 2013, 06:41:19 PM »
I guess it's a case of 'run what you brung'!

We have what we have, so we'll build for 6000 RPM, and see if we need kitty litter or not! :-o

Buddy,

Based on my experience, useable mean piston speeds have increased in the last 20/30 years, primarily due to better materials, heat-treating & coatings.   Mean piston speeds of 5000 ft/min are now being used regularly in endurance (500 miles) racing.   From an engineering standpoint, lower is always better, but often you are locked into a build geometry.  Higher mean piston speeds are being used for brief periods, ie: drag racing.   Your requirements are somewhere in between, based on inspection/service interval.

For your engine geometry:  5000 ft/min mean piston speed ~ 6500 rpm;  this is 7870 ft/min maximum piston speed.
                                         This results in 119,023 maximum piston acceleration in ft/sec/sec.  This is lightweight, narrow ring territory
                                         & it also requires very tight ring to groove clearance to prevent the rings from "hammering" the grooves @
                                         high rpm.

Your BMEP in psi is lower than I expected with your combo & dyno test results, which is good.  Until you give me the info I need to calculate the load in "g's", that remains my primary concern.

Expanding the performance envelope always requires a couple of things.  Awareness of the engineering numbers is the first step.  Large or scary numbers can prevent a "guess" that is actually a poor choice.   Developing the engine in incremental stages, with plenty of inspection is the second step.  Some intestinal fortitude helps, but unfortunately, a fat wallet is better.  Even the best engineering cannot forsee or prevent every failure........

You ran the engine briefly to 6100 out on the Salt; that's 4702 ft/min mean P/S; 7386 ft/min max P/S and 104,825 ft/sec/sec max P/A.  Upon inspection @ teardown, there was no evidence of cap walk/fretting/scuffing/etc.  You can check the entrails of a chicken if you want, but I'd say the signs are good to push the rpm up a bit, from 5500/5700 to a maximum of 6100/6200, as long as the load in "g's" is sane and the inspection intervals are short.   You also need to do a dyno run to the max bhp rpm or the max rpm that calculates out to sane loads..........
 :cheers:
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