[daveoxide]

This isn't really a track question, but neither is it vintage or custom. I figured folks here might have the most knowledge for the subject. Anyway, on to the question!

Maximum piston speed should be limited to how many feet/minute for a stock-ish motor? How about one with better internals?

The reason I am asking is because I am finding a lot of conflicting information. I found one page on the internet that had an exerpt from a book about engine building that had this chart:

Different "stock" Ford engines have different maximum rpms based on piston
speed:
289 - 7,317 rpm max
302/5.0 - 7,000 rpm
351 - 6,000 rpm
390 - 5,556 rpm
400 - 5,250 rpm
428 - 5,276 rpm
460 - 5,455 rpm

The page can be found here.

Now, based on other assumptions it made in the article, I should limit my piston speed to 3500 ft/min, maximum. I ran the numbers for my engine and at a mere 5000rpm my maximum piston speed is in excess of 4700 ft/min!!! For comparison I ran a 302 setup at 6000rpm (a value people say is ok for this engine to reach quite easily on stock components, from what I have read) and its maximum piston speed was just over 4900 ft/min.

In other web pages, I have also seen something called "average maximum piston speed". What the heck is that? Is it roughly half the maximum piston speed given the piston comes to a stop at TDC and BDC?

So many questions, so much confusion!!! ::

 

[bjr229]

OOOH, I think I can help on this. The theory of critical piston speed your reference goes way back, at least to the fifties.

First, the formula you are using may be wrong. The concept of crtical critical piston speed is based on the average piston speed at the maximum tolerated RPM's. It is calculated as:

Stroke x 2 x RPM= piston speed

where stroke is in feet. Using stroke in inches you would use:

Stroke/12 X 2 X RPM= piston speed

Or, if you know the maximum piston speed, you can calculate the maximum safe RPM using:

(Critical Piston Speed x 12)/(2 X Stroke)= max RPM.

For many years the critical piston speed for long life engines was considered to be 4000 feet per minute. There was a large body of knowledge supporting this. More recently, you do not see this discussed much. My take on this is that the limit of piston speed was based on lubrication properties, and the oil film on a piston would brake down if this speed was exceded. I think that oils have improved greatly, and the critical piston speed can exceed 4000 fpm today with no problems.

This piston limit was also assuming a race type engine that was carefully assembled, well balanced, and strong. It was applied to things like Manx Nortons and BSA Goldstars in the motorcycle world, and Drake Offenhausers and such in the car world. Many passenger car engine are not sufficiently strong to tolerate the forces involved with these kinds of speeds, or could not breate enough air to make power up there.

I hope this helps to clarify things. If you want some references, I think I can find some in my collection of old books on engines.

Brian

 

[gfmccann]

In other web pages, I have also seen something called "average maximum piston speed". What the heck is that? Is it roughly half the maximum piston speed given the piston comes to a stop at TDC and BDC?

Well I'm way out of my league here but I'll continue the conversation until more experts like BJR show up. As I'm sure you know piston speed is not constant. While the piston is going straight up and down it is being influenced by the rod it is either pushing or being pushed by. The rod is never actually moving straight up or down but is constantly changing the angle of its movement. A couple of years ago I started this thread on rod:stroke ratios. The conversation went on for a couple of days with a number of very knowlegeable folk climbing aboard. In short, because the piston is accelerating and slowing throughout it's stroke, I'm guessing that "average maximum piston speed" allows for the fact that at some point in its travel the piston will exceed the "maximum" numbers you've listed above (and have calculated for your engine).

I'll now step aside for the engineers to take the podium.

- Gord

 

[daveoxide]

Stroke x 2 x RPM= piston speed

This is where my confusion starts. I built a 2-D model, 3.5" stroke, 5.965" rod length (351w), and attached a piston to measure its velocity while it went thump thump. Maximum velocity measured was 4754 ft/min at about a 20º angle from horizontal. I think this is where that equation "averages" piston velocity rather than gives you an absolute maximum. But how "they" get that to be an average I can't see. If you can explain it, I am all ears.

For many years the critical piston speed for long life engines was considered to be 4000 feet per minute.

I am guessing critical piston speed to be this so called "average". Looking at the velocity graphical output of my model seems to show that the true average to be much less than the calculated average for my engine at 5000rpm.

I don't mean to attack you here, I just want to know why this equation is the way it is and why average piston speed is more important than the actual maximum speed. Does piston acceleration play a part in this? Shouldn't that be more important than the actual piston speed?

 

[daveoxide]

Gord,

I read that entire thread, very good discussion there.

I just did more tinkering with the 2-D simulation. It seems if I crank the rpm up to 6300 rpm for my motor, I will hit an "average" piston speed of 3000 ft/min. 6300 rpm seems like a lot for a stock 351w, doesn't it?

 

[daveoxide]

AH HA!! I just figured out where they got that goofy equation from! Let me see if I can explain it...

stroke * 2 * rpm = "piston speed"

Ok, so first multiplying the stroke by 2. That will give you the total distance the piston will travel in one revolution of the crank, in my case, 7 inches or .583 feet. Multiply that by the rpm, 5000, and that gives you how far the piston has traveled in one minute, 2915 feet. :: This does not accurately represent the actual speed the piston is moving at any given time, nor is it a true "average" of the piston speed.

This "critical piston speed" must have something to do with oil and its abilities to lubricate effectively. As to who came up with it and determined that 3000 ft/min is a good number to stay below I can't answer, but maybe someone here can.

I have seemed to come full circle with part of my original question. Nothing like determination and spare time to answer your own question! But now, what determines this critical piston speed to be what it is? Oil manufacturers? Engine manufacturers?

 

[bjr229]

This whole thing is empirical in nature. It came from the observations of many engien builders and engineers over time, as they observed that many different engines, with different strokes actually shared a common limit to the rpm's they would survive. You AH HA moment is correct. The formula is based on distance travelled in one revolution.

More later, back to work.

Brian

 

[roadracer]

I think we're all missing a critical point here....

It's not the speed that matters.... it's the acceleration.

Force = Mass * Acceleration,.... acceleration is the rate of change of velocity (speed) expressed as dv/dt = A

In one revolution, a piston and it's connecting rod go through 2 accelerations and 2 decelerations and they pass through Zero velocity and acceleration twice with each revolution...it's the forces on the connecting rod during the exhaust stroke as it passes through TDC towards intake that is usually the limiting factor.

Acceleration is a function of crankshaft throw and rod length and is a more complicated calculation because it varies with position.

So... using piston speed is an approximation (ie: "rule of thumb") that can be used to infer the acceleration effects and thus the forces the rod/piston/wristpin combination see.

In short... don't worry about it for a SBF.

Just buy good rods and pistons and spin that baby as high as your valvetrain will allow!

We do! :biggrin:

 

[LMan]

Right off you can see that a chart that puts a 351 redline at 6K is wrong. It may be done pulling by then, but it will certainly go higher (valvetrain permitting). Limiting a 460 to 5400 RPMs is laughable.

 

[JSHarvey]

Total distance traveled (given by the equation) divided by the time (one minute in this case since the total distance traveled is measured over one minute) gives an average by definition.

John Harvey

 

[GRR456]

One item no one has brought up, but is very critical in determining max RPM is piston/rod weight. That is why all of the race engines try to have the internals as light as possible. Our race engines with a 4.125 bore can still run at 8500 RPM+. That is because we use titanium rods and ultra lightweight pistons and pins. Our 4.03 bore engines can go 9,000 RPM. We've also found it to be beneficial to run the longest rod possible. It is better to have the pin take away some of the oil ring groove so that you can run a longer rod.

Remember, this is for racing engines that see 8500 RPM lap after lap and they'll stay together.....

Gary