Opinions on RPM's

[CJ_Boxster]

Originally Posted by saaber

How wrong you are. I came off the highway and leasurely cruised into town and flowed with traffic dyadreaming and never shifted down... and it did accelerate just fine and was not about to stall at all.

I wonder if a person could start in 6th and go to 162mph in one gear? Now that would be something. It probably can't be done but maybe in 5th? Not willing to burn my clutch to try but it would be interesting. These engines are amazing.

Well since you admited to driving slow.. the torque you felt at 35mph in 6th gear felt fast enough to you while it wouldnt to some of us that routinely hit the 4k rpm range to feel that torque against our backs and asses.

Thats probably why it seems good enough for yah.

[saaber]

Originally Posted by CJ_Boxster

Well since you admited to driving slow.. the torque you felt at 35mph in 6th gear felt fast enough to you while it wouldnt to some of us that routinely hit the 4k rpm range to feel that torque against our backs and asses.

Thats probably why it seems good enough for yah.

Don't know what you are talking about "driving slow". I rev over 4k often if for no other reason to hear that sweet sound above 5k or so and I shift at the redline when driving fast. I was talking about how the engine has amazing torque even at ridiculously low rpms.

[986chris]

TOP GEAR BBC did a demo starting in 5th but ran out of road before redline. speaking of revs @ track days i'm having a hard time shifting to 2nd from 4th whats a safe # in 3rd. or if your in 3rd and slowing it's always safe to pull 2nd. 2002 S stock.

[Brucelee]

Well-designed conventional reciprocating engines operating at conservative powers and speeds, irrespective of size or type, generally last about 5×108 revolutions before wear-out of the piston rings and the cylinder bore. Note that the engine life expectancy is a function of the number of revolutions rather than the total piston travel distance. Such a fact implies that wear-out is primarily due to the ring reversal event, i.e., the reversal in direction of ring travel that occurs twice each revolution. It is well known that maximum bore and ring wear occurs at ring reversal due to the "squeeze-out" of the oil film between the ring and bore when the ring is not moving.

Also, higher piston speed increases wear rates because of the increasing difficulty in maintaining an adequate oil film. It is generally accepted that average piston speed should not exceed about 2000 feet per minute, and low speed engines enjoy a life advantage over high speed engines.


As can be seen from FIG. 3, as engine speed increases, the frictional horsepower losses become quite significant in the conventional engine

[Brucelee]

Piston Speed


Vehicle speed and engine speed are both obvious quantifications we consider every time we turn the ignition over and step on the gas. But piston speed is something just as important, and is more often than not forgotten about.

It's usually measured in feet per minute and is the rate at which the piston travels up and down the bore. But the rate changes, as you might expect, alongside rpm changes.

At high rpm, the piston can reach speeds over 100 mph as it nears the center of the cylinder and then slow down to a near stop at the top or bottom of its stroke. It's not hard to see the abuse both the rods and pistons undergo from this extreme acceleration/deceleration process.

The strains imposed by these excessive speeds can snap connecting rods and crack piston skirts. Another not so serious side effect is that power can be lost due to the piston outrunning the flame front. Piston speed is related to crankshaft stroke, so being able to calculate it can potentially help you steer clear of a dangerous situation before swapping in that stroker crank on your next buildup. (fig. 4

[gmboxster]

Thanks Bruce Lee for the info, but it doesn't do anything to answer my original question.

[FTD]

Originally Posted by Brucelee

Piston Speed
At high rpm, the piston can reach speeds over 100 mph as it nears the center of the cylinder and then slow down to a near stop at the top or bottom of its stroke. It's not hard to see the abuse both the rods and pistons undergo from this extreme acceleration/deceleration process.

The strains imposed by these excessive speeds can snap connecting rods and crack piston skirts. Another not so serious side effect is that power can be lost due to the piston outrunning the flame front. Piston speed is related to crankshaft stroke, so being able to calculate it can potentially help you steer clear of a dangerous situation before swapping in that stroker crank on your next buildup. (fig. 4

Now I'm not going to relax and enjoy my track days. bummer.

[Brucelee]

A point of clarification on my posts. In the past, we have had folks state that driving your Porsche up near the red line was "good for the car."

I have stated that while it may be good for you, for your soul, so to speak, it was not a formula for longevity. Simple physics, as the small posts that I made indicate.

That in no way suggests that you should not drive your car safely at whatever RPM you want. You can ride around all day at 5000 RPM or more and that is perfectly fine with me (maybe not with your local PD).

The point was that as any engineer will tell you, on balance, slow moving engines last longer than high RPM motors. That is one of the reasons why diesels run for so many miles.


Now, to be fair, it is NOT good for your engine to be "lugged" either, so there is usually a happy medium. I have found that for normal street driving, 2000- to 4000 is a nice range for good gas mileage and being easy on a Box. Larer engines can normally be driven at lower RPM than smaller. Another reasons why large V8s can be run for so many miles.

That is not to say I don't go higher, just not all the time.

Hope that helps.