What is less harmful: idling or 6K rpm w/o combustion?

Peterbilt · Post by **Peterbilt** » Tue Jan 03, 2006 6:21 pm

I had an interesting discussion on another forum about this. My opponent claimed that being at 6K rpm with the fuel completely cut off (just pumping air) is less harmful for the engine than idling. How would local gurus comment on this subject?

Sypher · Post by **Sypher** » Tue Jan 03, 2006 11:42 pm

not being a physicist myself, I am not quite sure, but i do know that you save a lot more gas with the injectors shut off lol...

scionkid · Post by **scionkid** » Wed Jan 04, 2006 2:00 am

Even the best engineer cannot answer that without a big supercomputer because every car is different. Disregarding the pressure inside the cylinder, at high rev, the crank is applying plenty of force accelerating and decelerating the pistons as they go through top and bottom dead center. The same can be said for the rockers and valves as they slam back at their ports and ride on cam lopes. Even though the forces are high, I don't know what kind of oil pressure keeps moving parts from touching.

Tiamat, LS1, any engineers on the forum care to comment?

LS1Leader · Post by **LS1Leader** » Wed Jan 04, 2006 2:08 am

He lost me after about ten words.

Too many factors, and I can't even think of where to start solving that problem.

Peterbilt · Post by **Peterbilt** » Wed Jan 04, 2006 3:53 am

Not including the fuel economy, how does braking from 6K rpm with the throttle shut off compare to idling. Sorry, if my original question sounded confusing.

Post by **Johnf514** » Wed Jan 04, 2006 10:54 am

Without being an engineer or a car expert, I imagine the fuel it takes to rev up to 6K (either in gear or out of gear) would negate any advantage over just idling. However, if you're driving in 3rd at 40 MPH, about to shift to 4th, it would be better to let the car slow down in gear rather than coast in neutral.

Tiamat · Post by **Tiamat** » Wed Jan 04, 2006 11:18 am

If we're not talking fuel economy, I'd say that idling is better, merely because it's less motion and less motion means less wear. On the other hand, OTHER things are happening, like combustion, which might cause wear possibly to other parts. Honestly, I don't think there's a way to say for sure. Wear is one of those things we just kinda go "Well, this is how it's worked before."

Something I almost forgot about, higher RPM means more cycles for fatigue but I'm guessing that there's a significant body of data behind any decisions car designers make, so it probably doesn' t matter.

In other words, no idea.

Post by **IMBoring25** » Wed Jan 04, 2006 1:39 pm

This question gets a whole lot easier if we assume the vehicle in question has a 5000 RPM redline.

LS1Leader · Post by **LS1Leader** » Wed Jan 04, 2006 7:26 pm

From a stress standpoint, the force of engine braking caused by slowing down from 6000rpm would be worse than idling, because you're reversing the force applied on the rotating assembly. Think paper clip; you can break a paper clip faster by bending it back and forth, left and right, rather than just bending it one way, straightening it back out, and bending it again in the same direction.

That's my limited input on this.

Charly_GK7 · Post by **Charly_GK7** » Wed Jan 04, 2006 8:17 pm

try slowing down with lower RPMs... say 4K instead. 6k's a little high and there's no need to make the car suffer, be nice with your cars...

I don't think there's any harm done when having the car running idle --but a little rate of fuel consumption- than running it at 6K slowing down w/o throttle (on a way that it wasn't meant to be done) ... think of it.

grüss

LS1Leader · Post by **LS1Leader** » Wed Jan 04, 2006 8:52 pm

I was assuming that this was a hypothetical question rather than a practical one.

Peterbilt · Post by **Peterbilt** » Wed Jan 04, 2006 9:05 pm

Of course, it was a hypothetiacal question