How To Determine Safe Piston Speed?

38,

Have you tried contacting Rick Schnell, he built and owns the Slider FED which runs a 930+ Hp blown Flathead on nitro. His shop is in Anoka, MN. He may have some good info for you.

 

And its not speed that kills, its the sudden stop at the end. I know I have mentioned this before, but its peak piston acceleration you need to look at, not piston speed.

 

That's not really true. Piston speed is a concern because it begins to encroach on the ability of known materials (aluminum, steel, iron, oil) to handle the friction and resulting heat. In other words, you begin to 'friction weld' the pieces together as they are unable to shed heat fast enough to stay within their comfort zone.

 

Simple solution. Just run the longest rod possible. Of course that will require more pistons, but rod length has a direct effect on piston speed. Other than that, it's pretty much out of your hands. I don't know that I would use the word "worry", but I concern myself with everything when building an engine and when deciding on a particular purpose for it..

 

No effect on piston speed. It DOES affect peak piston acceleration.

 

If the pistons do begin to exceed the maximum recommended speed limit... Couldn't you just drive slower? I've been vindicated Goatroper weighed in as well and confirmed piston speed accounted for only one variable. Hey it's all in good fun here. Good luck on the salt!

 

so many rpm's in a flathead,what holds the bottom together?

in late model dirt cars for this year they are asking for 9000 rpm limiter to make fair competition,
like who was running these rpm's when everyone was saying [yea i really held off] ,like i was running 1/3 throttle and could not get enough off to keep rubber locked to the dirt.
and then someone else asked for the limiter's and they are in.
why not just run 2 cycle engines?? ha

 

That's funny. Incorrect as well.

 

show me the math...

 

Piston Speed (ft/min.)
a = RPM
b = Stroke of Engine (ft)
Formula: 2 x a x b
Example: 2 x 8000 x .333 = 5328 ft/min

Rod length is not a factor in calculating piston speed. falcongeorge has it right.

 

Hell, that's only 60 mph... What's the problem?

 

I was gonna give the guy 24 hrs to respond, then drop this on him. You beat me to it.

 

I thought I would come back today and shed some light for you folks and hope to solve some issues and discrepancies here for the discussion.

Let's refer to piston speed for this discussion, as " mean " piston speed.
" Mean " piston speed remains the same for any given rod length. The word " mean " refers to the " average " speed the piston is going when traveling from TDC to BDC and back again to TDC.
This distance is " linear " and there for ONLY affected by crankshaft stroke and engine speed or RPM.

Another point is that the mechanical stresses within the reciprocating and rotating masses are inherent traits of increasing or decreasing the rods lengths. A longer rod allows the piston to stop at TDC and accelerate more SLOWLY away. This in fact LOWERS the velocity and stresses imposed on the piston.

A longer rod will have less " bore center line angle " given the same crank angle than the shorter rod and exert lower side loads as well on the cylinder wall.

I'm jumping around a bit here and this should answer a few questions, and more than likely open several cans of worms as well. Volumes and volumes are and have been written on this subject and would take some serious band width to discuss fully.

I'll really throw a wrench into the pot here and let you in on the fact that cylinder head flow is a MAJOR concern as well in choosing and optimizing connecting rod length. Just touching on this subject is much more than I want to one finger type at this time, sorry guy's.

TR

 

Further to TR's comments, basic kinda rough generalization, a "short" rod engines optimum port volume for a given combination will tend to be larger relative to the optimum for an otherwise similar combo with a longer rod. The "short" rod combo yanks harder on the intake charge early in the pistons motion, whereas the longer rod combo starts out accelerating the piston more slowly, but maintains a higher rate of acceleration farther down the bore.

 

This ^^^^ is what I was referring to. I stand corrected. If the rest of you want to busy yourselves counting fucking rivets, go right ahead.

 

I coulda sworn a shorter rod also allows the piston to stop at TDC.....

 

Well, looks like its time for all the barking dogs to start marking thier territory. Hell of a shame, every time a SERIOUS tech thread breaks out on the HAMB, they come out of the woodwork, and the thread goes for a shit. Thats why I usually dont even post on engine tech threads on the HAMB, I save that stuff for speedtalk, where people are actually interested in contributing and sharing KNOWLEDGE, rather than pissing matches, bullshit fests and chest pounding. What TR is trying to say is that a long rod rod combination increases piston dwell at TDC, relative to a short rod combo. He should expand his comment on lowering stress on the piston to include the rod, crank, bearings and block, as the forces generated as the piston changes direction ultimately end up in the main caps/webs, but I think he is assuming that this is obvious. He's over-estimating the crowd hes talking to. Clearly, its time to pull the plug on this. Knock yourselfs out guys, I'm done...

 

George, I didn't start this thread for pissing matches- I started it so that I could learn, and hopefully keep my engine together.

You have a lot of knowledge, and it would be a shame if you didn't share it with those of us sincere in learning, IMHO.

 

Honestly, The best advice (per Mr. Waters) on this thread is to take it over to Speedtalk. These threads are the kiss of death here. Guys over there are serious, and almost all the guys on here that DO have a clue about this stuff are over there as well, and the atmosphere is much more suited to a serious exchange of information, rather than the kind of crap we see on the last page of this thread. The problem with arguing with idiots on the internet, is that its hard to tell whos who.

 

Oh....so our tech talk is meant to include everyone knowing what the other guy meant to say, not what he really said? That's not engineering talk, that's hair salon talk. So far you're batting about 50% - that would be great in baseball but not so great when it comes to engine talk.

 

I feel compassion for those that feel that " speed talk " forum is the end all to engine building discussion's.

Is the " Miller Welds ", which I am on by the way, the end all to welding discussions? I think that comment would tick off quite a few VERY knowledgeable HAMB members. There are MANY brilliant minds on that forum, but we have many as well when it comes to welding.

We ask machinist, metal shaping, engine building questions here on our beloved HAMB, and can generally expect concise and accurate answers. There will always be members who don't really know but mean well, and those who don't even read the O/P's post and just blurt out non sense, it goes with the territory.

To say that " speed talk " is the end all of engine building is quite insulting, illiterate, and childish. The discussions there are no different than ours- some educated and some pure bull shit. I went to that site once for all of 5 minutes and picked a rod length discussion. 10 guy's all good engine builders, and 10 CORRECT and 10 DIFFERENT answers and theory's. You guy's might want to read that last sentence again- SLOWLY!.

Just because you are in the business, or have a dyno in your shop doesn't make you the end all to engine building discussions. There are engine builders on this forum that have as many titles and championship wins as any of the members of " speed talk ".

For the record, this post was NOT to create drama, but to help, like I always try to attempt here on our forum, thank you.

Take it away Alex .

 

Speed Talk is just another source of information and opinions. It is no more the "end all" of engine building than The Hamb is the end all of traditinal hot rod building. Some people on both forums are very good sources of info. and many are not.

 

I have cleaned some of the Drama off this Thread.

This is a Technical discussion, y'all should be able to get technical facts across without puffing up your chests and personal attacks.

Next Drama post will more than likely result in Infraction Points and/or a Ban.

 

I'll take one more kick at this thing, and I just know its gonna cause a shitstorm. I scoured the thread, couldn't find a rod length so I could calculate peak piston acceleration, but like several other guys, I REALLY dont think thats gonna be your main issue, and it sure as hell wont be piston speed.
I'll preface this by saying I have no first-hand nitro experience.The main issue I see with this thing on nitro is main web/cap integrity and crank flex. And once the webs and caps start moving around, the shit is going to hit the fan. If it were me (TR, Gregcon and Cutaway Al are gonna freak ) I would look very hard look at switching to aluminum rods. They will go a long ways to absorbing some of the violence that is otherwise going to end up being absorbed by the main webs, and that crank that is only supported by three main caps. Even on a light nitro load, you are in a whole different world as far as bottom end loads are concerned. I have seen clear evidence of main web/cap movement at around 650 hp in naturally aspirated 2 bolt main blocks, I hate to think of whats gonna happen with a load of nitro and three main webs. I have had good experience with BME rods.
I'm outta here, Anybody standing in front of the fan should probably move now...

 

We have redesigned the main caps and girdle. The mains are now doweled, and are wider going across the block. The center is a wedge shape, and far stronger than before. We are currently looking at cross-bolting it. Still, it remains to be seen if it will be enough!

Rods are 8.75", with a 4.625" stroke. I looked at aluminum rods, but with the narrow bore and long stroke, no one could/would make any that fit!

I guess I can let you know what happens on the dyno, in March!

 

That center cap looks really good. I didn't realize that the Cad's had such a deep skirt on the block, that will really help your bottom end strength. Cross bolting that thing will also help stabilize it. One more shot of the Slider for inspiration. If they can build that kind of power out of a Ford, I know it can be done with a Cadillac. Keep up the Good Work.

 

8.75???!!!

 

Yep- Getting HP and RPM's out of this engine is no easy feat! That's one of the reasons I research what others take for granted!

 

I dont like the idea of cross-bolting that center main, isn't pulling inwards at that point going to tend to wedge the cap away from the block? This would be complicated to make, but how about a girdle with integral main caps that bolts to the pan rail and original main cap bolt locations? Just a thought...