Piston Speed

What is the maximum piston speed you can safely run?

Depends on what you are talking about and the budget and parts at hand.

What is the maximum piston speed you can safely run?


You'd have to do the math to calculate the piston speed based on stroke and rod length, but I do know that at 14,000rpm the cylinder walls in a Gen1 SBC will collapse. Presumably it would be the same for a GenII. I have no idea what it is for a GenIII.

You'd have to do the math to calculate the piston speed based on stroke and rod length, but I do know that at 14,000rpm the cylinder walls in a Gen1 SBC will collapse. Presumably it would be the same for a GenII. I have no idea what it is for a GenIII.

I don't understand that at all. Care to explain? I never heard of cylinder walls collapsing.

Rich

Yeah...explain this a little bit!

How could they just collapse?

You remember back in the late 70's/early 80's when those destroked smallblocks were all the rage?

I forget what class it was that they were popular in, but I'm pretty sure they were 287ci. I do know it was one of those classes where displacement was important because of the weight breaks. Please forgive me for not remembering better. Back then I was working on my Keith Richards Merit Badge

These engine were max effort, high RPM numbers. Aluminum rods, lightweight cranks with a knife edge that could cut you, gas ported pistons, tiny rod bolts, polished lifter valleys... really ridiculous efforts to get high rpms. Like 10,000rpm plus.

I never ran into this problem personally because I never built one of those ridiculous motors. I was into big-blocks. It was cheaper and easier. More reliable too. I also wasn't one of those guys with vendor connections. I just ran gamblers.

Anyway, the cylinder wall problem...
It was I think... 1983? The story going around was that one of the cam companies (Supposedly Lunati is what I heard) was doing development work on a cam that was supposed to be good to 14,000+. Problem is, they were running into a problem with the blocks. The cylinder walls were actually collapsing. By collapsing, I don't mean something you could see with the naked eye, you needed a micrometer to tell. But what they were doing was sucking into an hourglass shape and creating a huge drag on the pistons because the clearance was going away.

There were several theories about what was going on, including a ridiculous one involving vacuum created by the fast moving piston. The one that always made the most sense to me was that the friction of the fast moving ring pack was creating friction generated heat faster than the block could dissipate it. Of course, that begs the question of; why didn't the rings give up the ghost first? I dunno. Presumably it was one of those things that had multiple factors. I don't really know more than that because, as I said... I never built one of these engines.

From what I understand, they tried using sleeves to overcome the problem but for some reason that never worked out. I never found out why.
It was about that time that those destroked engines started falling out of favor. I think it was the year before I went into the Army that I was at the Englishtown swapmeet in the spring (1984) and I saw like a dozen different guys selling off their 287s. They were all moving on to other things.

I don't know if the modern aftermarket blocks would have the same problem, partially because I could never find anyone to confirm what the specific problem was. Back then you were pretty much stuck with factory blocks for everything except the really high end stuff, like from Keith Black. Nobody really trusted new blocks anyway. Everyone wanted seasoned blocks.
Anyway, I've never heard of any problem like that occurring in other brands. Hard to say if it's because the cylinder walls in SBC's are too thin or if it's because nobody ever pushed the other brands to the RPMS that these guys were playing with. I know that there were some guys pushing the limits on the Oldsmobile DX block, which is seriously strong, and they were seeing 9-10k RPM with good reliability. But that's still a long way from 14k.

My piston speed is 4700 ft per min. Ya can do this with all forged parts.
That's about the speed of a Ford Cleveland.

EDIT:This is @7500RPM's

The destoked engines are still running strong in Comp Eliminator @ 10k+ RPM's. :)

The maximum you could run would depend on the design of the piston, pin and connecting rod. I know when engineers were planning out the Aurora IRL engine they looked at alot of other successful engine programs for a baseline. They then decided that their design would be comparable to existing competition engines with mean piston speeds of 5000 feet/min. Maximum g's, being in the 70k range @10,500 rpm, were however much higher than others but would survive with well designed components.

I think everyone remembers how successful that engine program was. :)

Either way, it's not just the piston speed... it's the acceleration.

-Mindgame

At 7000RPM's the speed in mine is 4375ft/min the "G" load at TDC is 3470G's and is do able with real good(best)parts.
The Pro boys are running in the 6000ft/min range with more G load than 3470.

Now that I'm actually thinking... I don't know how they arrived at 70k g's. That number is directly from an SAE paper on the Aurora IRL engine. Anyone have a clue?

My 457ci @ 7200rpm...
Max velocity (fpm) = 8203
Mean velocity (fpm) = 4950
Acceleration (g's) @TDC = 4060
Acceleration (g's) @BDC = 2014

-Mindgame

Got me.... maybe it was the F in N? That would make more sense. A 3" stroke at 10,500rpm is 6100g's so 1170g's hanging off the end of that crank at that RPM would be 70,000 N.

I'm guessing the 4.0L was around a 3.6" bore x 3.0" stroke. Seems to make sense with that bellhousing size they use, puts you right around 8" deck height.

Bret

Isnt there a theoretical piston speed limit around the speed of sound? and what is observed after that is sharp power decrease due to a vacuum being pulled in the cylinder regardless of flow capability?

I am building a 396 stroker with all forged parts and was just trying to figure out the maximum rpm it should see so i can pick a cam. Thanks

Bore:4.030
Stroke: 3.875
Rod: 6.000

The bottom end will support a lot more rpm than your heads and intake are likely going to be able to supply. I think you are asking the wrong question.

Rich

I am building a 396 stroker with all forged parts and was just trying to figure out the maximum rpm it should see so i can pick a cam. Thanks

Bore:4.030
Stroke: 3.875
Rod: 6.000

Yea === I agree with doc. The bottom end would stay together at 8500 (if built right-not stock block) But ya will need a set of head's and intake system that will flow 325CFM+ to make power above 6500RPM's.
Don't want to here from people saying the "are" making HP up there 'cause I haven't heard of but maybe 3 on this board with a N/A 396+ making 1.6-1.7HP per CI. The ones I have heard are running SB2.2 or 12-15* stuff so there is your intake capabality right there with the heads flowing in excess of 360CFM.

Ok...I just spent some time using derivatives to calculate the speed of a piston at a given position (degrees). I then made an equation for it, and put it in a graphing program, so that I can substitute in the stroke, rod length, degrees, and rpm (I haven't gotten to completely check it though). How does this help to build a better engine?

Ok...I just spent some time using derivatives to calculate the speed of a piston at a given position (degrees). I then made an equation for it, and put it in a graphing program, so that I can substitute in the stroke, rod length, degrees, and rpm (I haven't gotten to completely check it though). How does this help to build a better engine?

I'm not sure if it helps much. If you pick a reasonable Average Piston Speed (RPM * STROKE/6) which has worked for engines of the type and cost you plan to build, you'll come up with a max rpm.

5000 ft/min can be used if you have light pistons and strong rods/crank. That would give you 10,000 rpm for a 3 inch stroke and 7500 for a 4 inch stroke.

4500 ft/min would be my choice unless I was very wealthy. That still allows 7200 for a 383 SBC. As was mentioned, heads are probably going to be more of a limiting factor in your engine build than piston speed, unless you build Cup, PS, F1 or similar engines.

Well if you want add in how the induction system will work with the bottom end relative to piston speed.... My "good buudy" Erik and I had a discussion on the Ford Forums about this.

CSA x Ft/sec = (Bore x Bore x Stroke x .7854) x (RPM/360)

CSA x Ft/sec = (Bore x Bore) x (Piston Speed/60)

Either one of those forumlas will help you solve for the max speed you want to turn the motor with a given setup. Well as long as you have a ft/sec barrier for the average intake port velocity. I use 330 ft/sec for a really good 23° head.

Bret