How To Determine Safe Piston Speed?

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?

 

Ummm? If the Venolia engineer doesn't worry about it should you? Looks like you can do the math why dont you figure some big inch motor combinations? I seriously doubt piston speed will be your downfall. Clem? You been readin them fancy books again?

 

Get goatropers opinion on this.

 

Just because they don't worry about it, doesn't mean I shouldn't! Remember, they probably sell 1000 sets of pistons or more, for every set they sell where piston speed is an issue.

Most high revving engines have a very short stroke, so piston speed isn't an issue.

Lets take a SBC with a 3.48" stoke. You would have to spin that 8000 RPM's to get the mean piston speed I have at 6000 RPM's. SBC's that are turning higher RPM's often have a shorter stroke to compensate.

Tommyd, I'm going to see Tony next week, and will be chatting with hiom about a lot of things!

 

Those long favored piston speed calculations were pretty general, and really included broad concerns for other engine components, not piston breakage specifically.

The rod design and preparation >may< result in operating stresses at full rpm being safely below the material's endurance/fatigue limit, but if the crank deflects enough to edge load the bearings, bad things may still happen.


One substantial "speed limit" is when max TDC acceleration for a given ring width that will overpower cylinder pressure and flutter sets in.

Both are discussed understandably here, in a "two stroke" book from the 70s -
http://www.vintagesleds.com/library/manuals/misc/Two-stroke Tuner's Handbook.pdf

pages 5 thru 10

 

Been building mc motors for years,cars not as long. In your nitro thread I told you I worked on a HD TF bike for a few years. We used a 3 13/16 piston with a 5.250 stroke. Trap speed was 7000 rpm. We played with gearing and tried to keep piston speed in said good range.bottom line,it usally wanted to spin harder. For street bikes my favorite stroke is 4 3/4 so my theory is suck squish pop phooey.motor is a motor,if you cannot get the wind through it throw arm at it to make torque.I know some engine builders would scream how can you!! but think outside the norm and look how much power some small displacement big stoke motors make. Piston speed we were aware of but didn't pay much mind to.Thrust sides of the piston took a beating,eventually put nylatron buttons in the skirts helped a lot.With nitro it's make it live just long enough to win.if you play it too safe the guy who beats you pushed it a little harder.

 

Well Goddamnit Clem! You figured it out. A 350 Chevy going 8000 are pee em's is the fastest dam engine speed out there..... Dude! Seriously. Piston speed with high quality forged pistons is the least of your F@&king worries!

 

If you are really that worried about it, venture on over to www.speedtalk.com and do a search.

 

Dan and Gary, thank you! That's a good book, and Gary, your experience is helpful!

I agree that piston speed probably isn't my biggest worry, but knowledge is a good thing! So Slick, what are my ' F@&king worries', pray tell?

 

I would say your worries are...don't run it lean. I would trade a tad bit of piston weight for some good domes. Lippy

 

It used to be that 3800- 4000 ft /min was a safe maximum speed for longer duration runs than drag racing. Used to be said by many top builders according to old books... Better materials now have likely raised the limit.

 

You can always run it till the rods fly out, then back it down a bit.

 

I agree lean is bad! I think my biggest concerns are crank flex with only three mains, and how much cylinder pressure this 73 year old block will take!

We have popups, but not domes. We run a Harley KR design chamber, but not the dome chamber design. We'd have to design new chambers to run domes.

 

I meant piston top thickness.

 

Lippy, they are THICK!

 

I would for sure be monitoring cyl temps during a pass. Your gonna have to sneak up on this deal.

 

How thick.

 

I don't have them here, but about 1/2" if I remember correctly.

Of course, if the nitro and aluminum combine to make fuel, they'll be thinner! Lol!

 

I don't think piston speed is of much consequence. Max stress at high speed will be on the wrist pin and connecting rod small end as the piston is yanked down from TDC at the beginning of exhaust stroke. Wrist pins must be well oiled for max longevity for high speed runs at full throttle.

Use the best stuff you can afford, take utmost care in assembly, and don't worry about it.

 

Lets hope that does not happen with all the work and time you've invested. It seems to me jmo here, the fuel system is gonna be a big player here. As well as timing. Being able to control timing and fuel on a pass. Lippy

 

there are lots of sbc race engines with 4.125 stroke turning in excess of 8k, i'm not seeing piston speed as a problem....time at that speed might be a problem though

 

With the switch to fuel, I'm also changing classes. I'll have a computerized ignition, MSD box, computer engine management, atc., so we can control things better than we could in the vintage class

 

I'm sure Venola is right there piston won't fail before something else does that they have no control over. I'm not a flathead man but won't the power curve fall way off before 6000 rpm?

 

As far as i know, the only time 'piston speed' is a consideration is when you can compare different connecting rod lengths for the same stroke engine.
For instance, a 400" sbc with a 3.75" stroke can have a 5.50" up thru 6.125" connecting rod (probably more if you offset the pin in the piston).
For comparison, an engine spinning at 600rpm - these numbers are illustration numbers only to show the comcept - with a 3.75" stroke will travel about 37 1/2" per second. Now, here is the difference: with a 5.5" rod the piston lingers at top and bottom of the stroke for 10degrees (i am just making a presumption here, i don't have actual numbers) so at 10 degress top and bottom means that the acceleration occur only in 340degrees of crank rotation. If we put a degree wheel on an 6" rod motor with the same 3.75 stroke we find the piston lingers much longer at top and bottom - again i am just guessing, but i do have a degree wheel and can pull the head off an engine for actual numbers - but i believe the piston lingers close to 30degrees so that now the accelation occurs in 300degrees of crank rotation - the piston speed is much greater while the actual engine rpm and travel remains the same.

 

I will toss in some possibly relevant info..
I just wish I could remember the source and the formula !
Back in 1990 I put together a IMCA modified with a 500 CID caddy , stock internals, except for the cam... Mixed & matched ( mismatched ?? ) heads / pistons to get almost 12 to 1 compression..for methanol fuel.
TONS of torque....broke a few pg trannys & 9" rear ends before we got it dialed in..

Someone who knew a lot more than me told me to limit the rpm to 5600 MAX.
He was sure that anywhere above that rpm it would yank the pin right out the bottom of the piston..
His term was " terminal velocity". Keyword terminal !
This was for the stock cast pistons.
He was a retired GM engineer ...
For those who know the formula the 500 has bore 4.300 x stroke 4.300

Dave

 

think about this....piston speed vs fuel combustion speed ...you will need a faster burning fuel with a high piston speed ....just think about it ..... Check out MIT's publication The Internal Combustion Engine.. The section on fuels ...

 

No, I don't think so. On the dyno, we're still climbing at 570 HP @5500 RPM. with fuel, I think we'll still be building HP.

Thanks OJ and Dave!!

 

How many gallon pump? Lippy

 

Lippy, it's 7.1 gal/min

 

38, My $0.02: I don't think you are getting into piston speed areas where it will be a real problem if you keep the engine below 6000 RPM or so. As a comparison, at 6000 RPM your average piston speed is equivalent to a Ford 427FE at about 6700 RPM, And I know of guys who often spin those 427's up to 7500. However, I don't know who the dumba** engineer at Venolia was, but that is an absolutely terrible answer and they should know what the failure force (speed) on that piston is.

I think you have 2 things to worry about:

1: Will the acceleration forces on the piston and rod cause either one to fail at high speed? Force = mass * acceleration. You know the mass of the rod and piston, and you know the acceleration (change in velocity over time, 0 to 125 ft/sec in your case, and about 70 deg's of crank rotation). So you should be able to figure out the acceleration force on the piston. That info might help you pry some more details out of the guys at Venolia.

2: That rod is going to stretch at those kinds of speeds, especially with a long rod. The good news is you don't have to worry about touching valves, but will the dome of the piston touch the head??? I don't know what kind of clearances you are running, but I assume they will be fairly tight to keep compression up. But I wouldn't be surprised to see as much as 0.030" to 0.040" of rod stretch.

Good Luck and Keep up the good work.