How fast would a piston have to move

At 5000 RPM, the crankpin center will travel at a speed of 5000 RPM x 3" x PI = 47,100 inches per minute. With an infinitely long connecting rod, the peak piston speed will be 47,100/12 = 3925 feet per minute. A shorter rod will increase the peak piston speed.

Using a rod length consistent with a maximum 45 degree rod angle, the piston speed would be 5551 feet per minute maximum.

If you never get the RPMs up, the timing never shifts, and you might as well not have one of the VTEC engines that are available in the 2009 Acura TSX.

That was true with older “dumb” variable timing but today the engine control computer changes the shift point according to throttle opening as well as rpm. In fact, a lot of these engines are in the “high rpm” timing when the engine is idling. Why would they do that? An engine timed for low rpm torque also pumps a very high vacuum when the throttle is closed and that vacuum is a major load on the engine. By killing that vacuum with “high rpm” cam timing, the engine does not have to fight that high vacuum during the intake stroke and that results in a lower idle and part throttle fuel consumption.

An engine and AC current are very different things and the math says that the position of the piston is not sinusoidal. If you want to find the max speed, take the time derivative of the position formula and plot it.

Note that knowing the stroke and engine speed is not enough. The connecting rod length is also needed.

Builders of formula 1 race cars, Indy cars, and grand prix cars gave up on roller bearings long ago.
The biggest issue is the balls skidding instead of rolling when they can’t keep up with the cyclic speed variation of the connecting rod bearing.

Well, Ronman, I know engines have come a long way but I still don’t think anything spinning 5k to 6k RPM can last as long as one doing 2k to 3kk. The altrnator, waterpump, power steering pump, fan if it is on the engine, or belts don’t need to turn that fast.

My point exactly, common sense

It have been a lot of years since I had to do the Calculus but the formulas that Chunky Azian are of the type that are necessary to find the correct answer to the question.

Agreed.

These threads are my favorites. They make me think, challange me, and remind me that I know only a tiny bit as much as I sometimes think I know.

I like applied physics for the same reason. When the shuttle disintegrated on reentry some years ago countless brilliant engineers and physicists calculated that there was no way a ceramic tile could damage a wing. Then they ran a test. They built a wing leading edge out of shuttle parts and fired a tile at it. It blew hole in the wing leading edge the size of a basketball. Went right through it.

We know so little.

I tip my hat to you my friend, embarassed by having been the one that pointed out the derivation of the term sinesoidal as having been from the world of trig, then having been treated to a trouncing with the math from my own original point. And I had even brought in the connecting rod length as a factor.

While the term’s use has long since been expanded to include mathematical modeling of wave motion of any form, I was in fact referring to the math. I accept my grade. Guess I’d better take the oldpeople’s refresher course.

When I was a young adult, I also accepted as dogma the idea that “engines were designed to rev”. I couldn’t stand the way may grandfather drove because he always shifted way to early.
But as time went by, I started to realize that it was he who had a 1946 Chevy pickup truck that was still running, not me, and I started to question my own beliefs.
I fixed up a Honda 175 motorcycle and decided to “drive like a grownup”, ignoring the tach, in fact I even removed the tach, and used my common sense to decide when to shift instead of dogmatically keeping it at some ideal rpm.
Just how long lived and reliable a small motorcycle like a Honda 175 becomes when you drive like a grownup was one of the biggest eye openers I have ever experienced.

Today, I believe that the reason Harley Davidsons, BMW motorcycles, and Honda Goldwings have a reputation for lasting forever is because they are owned mostly by grownups and the reason 600cc sportbikes have a reputation of being worn out at 40,000 miles is that they are mostly owned by teenagers and 20-somethings.

B.L.E., I Always Wondered Why Buicks Had A Reputation For Longevity Even When Running Some Of The Same Components As Other GM Cars. Now I Know.

CSA

I should point out that the post you’re alluding to was about whether 3,000 to 3,500 rpms criusing on the highway would lead to premature engine demise. I doubt that anyone here would argue that sustained 5,000 rpms and above would wear out an engine faster than 3,000 rpms. At 3,000 rpms sustained today’s engines will generally outlast the rest of the car, barring those neglected or with design problems.

I think too that it’s safe to say that most of the respondents were pointing out that an engine treated to a life of being in a good comfort zone in its power curve will have a longer and happier life than one that lives in NYC. Constant stops and starts are the engine’s hardest tasks. It’s about the combustion during acceleration. The booms are bigger in New York City. NYC puts more stress on the engine components as well as the driver.

Chunky, you may have seen my 9:40:45 PM post. I arrived at 3925 ft/min or 44 mph with an infinitely long rod; no calculus, graphical kinematics or trig needed.

I also arrived at 5551 ft/min or 63 mph with a rod length to cause an arbitrarily chosen maximum rod angle of 45 degrees with only a little trig needed which is near but not at maximum piston speed. To cause even more piston velocity the rod would need to be shorter yet but friction generated by piston sidethrust against the cylinder wall becomes more pronounced as a rod is shortened.

Note that maximum piston velocity at any rod length other than infinite will be achieved slightly before the midway position of the piston in the bore while the piston travels downward and the 45 degree maximum rod angle will occur when the crankpin is at 90 degrees from vertical.

Feel free to check me on this.

Your comment about ignoring the tach was interesting.

I read an article once by a magazine contributor that had an opportunity to drive a formula one racecar. One comment he made was the importance of monitoring the tach. He wrote that while a regular engine could easily be felt going “over the hump” in its power curve as it revved higher (something I can attest to), an F1 engine could not. It pulled so powerfully throughout its entire curve that it would be very easy to blow right past the redline. That comment always stuck with me. I akways windered how that would feel.

Ronman, it is my understanding that “varible timing” meant that the timing changed varivly as the speed increases. I may be wrong. Maybe, as you say, it suddenly changes from low speed mode to high speed mode. Anyway I am still convinced that unnessary speed kils.

Another reason for ignoring the tach is that the peak torque and horsepower rpm applys only to full throttle operation.
As you close the throttle, both torque and horsepower peak at lower rpms.
That means an engine that is operating with the throttle only 1/4 way open wants to be shifted at a lower rpm than an engine operating with the throttle wide open.

There are a number of different variable valve systems. Some do continuously vary the valve timing with changing engine speeds and conditions. Others are of a two step design where there are only two valve timing setting, and the change between the two is sudden. My cars have the variable type. I don’t know about the Acura.

Well it seems to me that if you operate the throttle at 1/4 to 1/2 open the RPMs would automaticly be less. What do you mean by “wants to be shifted to a lower RPM”?

One thing I’ve found from both reading and experience is that excessive short-shifting is really hard on engines and trannys when driving a stick-shift. Habitually “pouring on the coals” when the engine is much below 2000 RPM ruins piston rings and is very hard on the wrist pin, crankpin, and main bearings. It is also hard on the shaft bearings in the transmission. Engine speeds around 1500-1800 RPM should ONLY be used for coasting in gear or very light throttle cruising.

I think he was referring to Luddites like me who like to do the shifting manually :smiley: