Author Topic: How To Determine Safe Piston Speed?  (Read 16442 times)

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Offline 38flattie

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How To Determine Safe Piston Speed?
« on: January 01, 2013, 08:05:41 AM »
Let's talk piston speed.

I have Venolia forged pistons. I have a long stoke at 4.625", so I'm concerned about the piston speed. No problem, I'll call Venolia, and they should be able to give me what they consider a safe speed.

Well, the salesperson didn't even know what I was talking about, and informed me I should talk to the engineer, Ron. Ron said they didn't worry about piston speed, as their "pistons don't fail."

Um, okay!?

I plan on pushing this thing hard this year, so I'm looking at a lot of things we didn't look at last year.

At 5500 RPM's, the average piston speed is 4239.59 ft/min, and max speed is 6893.2 ft/min

At 6000 RPM's the average piston speed is 4625 ft/min, and max speed is 7519.8 ft/min

So, with 3 mains, billet crank, steel rods, forged pistons, etc., how do I arrive at a safe piston speed?
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Offline Koncretekid

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Re: How To Determine Safe Piston Speed?
« Reply #1 on: January 01, 2013, 08:20:42 AM »
Just keep increasing RPMs until it breaks, then slow it down!
We get too soon oldt, and too late schmart!
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Offline 38flattie

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Re: How To Determine Safe Piston Speed?
« Reply #2 on: January 01, 2013, 08:22:34 AM »
Just keep increasing RPMs until it breaks, then slow it down!

Somehow, I just knew that was coming!  :cheers: Happy New Year! :cheers:
With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925

You can't make a race horse out of a pig. But if you work hard enough at it you can make a mighty fast pig. - Bob Akin

http://www.flatcadracing.org/
http://youtu.be/89rVb497_4c

Offline Koncretekid

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Re: How To Determine Safe Piston Speed?
« Reply #3 on: January 01, 2013, 08:29:05 AM »
On a more serious note, I'd check with your connecting rod manufacturer, and crankshaft maker as well.  Other than that, unless someone else has more experience with 3 main bearing cranks, you're probably on your own.  I have broken pistons (pin pulled out of the bottom, probably not Venolia, and connecting rod (old Honda CB350), and the crankshaft on an old Bridgestone MC, so not much help to you.
Tom
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Offline kiwi belly tank

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Re: How To Determine Safe Piston Speed?
« Reply #4 on: January 01, 2013, 11:35:40 AM »
Thats funny stuff Buddy, according to them you should be good to go!!
But coming back to real-land you will probably find that the engine will nose over when the pistons start to out run the flame. Whether the three mains can hang on to all that might be a different story.
How about just running a bigger gear & not swinging the engine so high & start changing blower pullies?
  Sid.

Offline tortoise

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Re: How To Determine Safe Piston Speed?
« Reply #5 on: January 01, 2013, 11:57:26 AM »
Actually, piston speed is not the problem. There are no inertial forces generated by moving at a constant speed. Piston acceleration makes inertial stress. Fortunately for flatheads, long rods lower the acceleration. Google "piston acceleration calculator".

Offline Dean Los Angeles

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Re: How To Determine Safe Piston Speed?
« Reply #6 on: January 01, 2013, 11:58:33 AM »
Everything fails at some point. The piston engineer has to know what that point is.
Lighter pistons are better to reduce the rotating mass. Lighter pistons break easier.
Heavier pistons just the opposite. The pistons would be made out of titanium, steel or something stronger if strength was the overriding issue.

The amount of aluminum has to be engineered so that when everything comes to a screeching halt from 85 mph to zero in 2.3 inches, handles a massive pressure increase and really hot temperatures, above the melting point briefly and repeat every few milliseconds . . .  :-o

Having said that, the piston speed has less to do with failure than huge cylinder pressures, ignition knock, lean conditions that melt the piston, lean conditions that seize the piston . . . let me see, how else have I destroyed pistons?
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Offline Tman

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Re: How To Determine Safe Piston Speed?
« Reply #7 on: January 01, 2013, 12:01:09 PM »
Liek Dean said, I would be worried about the start/stop of the piston. What does your coating guy say about the switch to Nitro? I read your posts here and on the HAMB about this and think you are right for asking the questions.

Offline JasonS

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Re: How To Determine Safe Piston Speed?
« Reply #8 on: January 01, 2013, 02:26:03 PM »
Actually, piston speed is not the problem. There are no inertial forces generated by moving at a constant speed. Piston acceleration makes inertial stress. Fortunately for flatheads, long rods lower the acceleration. Google "piston acceleration calculator".

This is just my thought, but piston speed is still important because while it's true there are no inertial forces when at a constant speed, the faster the piston is travelling the longer it is under stress during the acceleration and deceleration periods to get to that constant speed.

Offline jdincau

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Re: How To Determine Safe Piston Speed?
« Reply #9 on: January 01, 2013, 02:42:04 PM »
I thought piston speed was related to piston ring sealing ability?
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Offline Jon

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Re: How To Determine Safe Piston Speed?
« Reply #10 on: January 01, 2013, 03:48:29 PM »
Hardest time for a piston and rod on a non turbo motor that is rpm related is intake/exhaust crossover, as the piston is decelerated and accelerated under tension.
How harsh this happens is effected by the stroke length, rpm and the rod/stroke ratio.

I'd be more worried about you 3 main crank than the ability of a modern forged piston to keep its top on from the intake/exhaust crossover.

Probably teaching you to suck eggs I know.
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Offline JustaRacer

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Re: How To Determine Safe Piston Speed?
« Reply #11 on: January 01, 2013, 04:05:22 PM »
I thought piston speed was related to piston ring sealing ability?

AFAIK, until you tear it in half, the limit is point of ring flutter (ring tension pulls ring off land) which causes land damage and blowby erosion.  Used to be ~4000ft/sec average long ago.  Much higher today.
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Offline fastman614

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Re: How To Determine Safe Piston Speed?
« Reply #12 on: January 01, 2013, 04:55:06 PM »
At first glance, the Venolia engineer's comment regarding their pistons NOT failing may seem a little flippant. But it probably is not.....

Going through records and recollections of our engine failures, we came up with several "piston failures" - however, the pistons themselves were NOT the cause of the failures.... oiling to the wristpins was compromised on more than one occasion, resulting in the seizure of a wristpin - the evidence was there on the "unfailed" pistons..... seizing a rod bearing which resulted in side forces so great that it tore the bottom of the piston out sideways (as well as a piece of the cylinder)....etc..... some of the problems were "cured" by getting rid of our drag racing engine building ideas as well as upgrading to a new "priority oiling style" block and fitting our oil pressure relief system to a remote location at the opposite end of the engine from the pump (in hydraulicspeak, it is technically a sequence valve when used that way).

The problem that you have, though, is that there are no upgraded priority oiled, high nickel alloy, flathead blocks that come with "CNC" pre-porting available (are there?).... and when you take out a flathead block, it is pretty much the same as, to us overhead valve racers, also losing both heads.... so you are right in being extremely concerned about any and all safety aspects.

When we were having problems with our short stroke big block, we had a fairly lengthy discussion with a NASCAR engine builder..... his words, at the time, were that, with piston technology as it was at that time, pistons would live well at a speed of not more than 4800' per minute.... implying an engine rpm of 8800 or so, if equipped with a 3.25" stroke....I know now that they have exceeded 4800 feet per minute in NASCAR (like up to probably 5000 feet per minute).... keep in mind that this is for engine longevity in the 500 mile range. He alluded to drag racers pushing that number up some....  he did not commit himself on the idea that Bonneville racing could also push that number up some....

What I will say is that there are several good tips so far about keeping your engine together..... and, long (as you can get) rods is a way of not overly side loading the wristpins and pinbores (as well as the skirts and cylinder bores) which usually rely on "drip and/or splash" lubrication that can be spotty....

As an aside here, there used to be a guy up our way who was a top alcohol drag racer....He had no big dollar sponsors and he was runing iron '92s.... his ongoing lament was that he could not afford the cost of any of the Donovan 417 blocks that he had come across for sale at the time.... and that good iron '92 blocks were getting nearly impossible to find for any price.... He fabricated several 392 style blocks out of plate steel and had them furnace normalized prior to machining them...The were NOT super light or anything but they were bullet proof, so to speak.... and he leaned hard on those engines for a few years..... He eventually got some sponsoring and bought a couple of K-Bs though....

So. it got me thinking...... if a guy up in Canada can fabricate a 392 hemi engine block, what is stopping people elsewhere from fabricating flathead blocks...... and putting in priority oiling, 5 bearings, KickA** porting, rerouted and/or better cooled exhaust ports etc....?

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Offline tortoise

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Re: How To Determine Safe Piston Speed?
« Reply #13 on: January 01, 2013, 05:30:01 PM »
So. it got me thinking...... if a guy up in Canada can fabricate a 392 hemi engine block, what is stopping people elsewhere from fabricating flathead blocks...... and putting in priority oiling, 5 bearings, KickA** porting, rerouted and/or better cooled exhaust ports etc....?



Just that stupid rulebook.

Offline stay`tee

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Re: How To Determine Safe Piston Speed?
« Reply #14 on: January 01, 2013, 05:42:22 PM »
i agree with what fastman614 has posted above, in particular the oiling to the wristpin, "forget dragracing ideas", were(LSR) doing approx 20 quartermile passes each time out there,, have to get plenty of oil to the pin and under the crown to help elievate the heat build up issues that occur,, also, keep an eye on wrist pin/rod clerances,, 8-)
detonation loves causing havoc in this area too, and can sometimes mask the oiling issues,, :roll:,,
« Last Edit: January 01, 2013, 06:37:09 PM by stay`tee »
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