I'm on a local forum and "Geek" is flaming the hell out of some guy and comes up with this. Can somebody verify that free revving is really this bad? I never do it anyway unless I'm under my hood but yeesh...

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">Running a motor at redline in 3rd gear is *far* easier on the components than running it to redline with no load. Do you understand why that is?

If the motor is loaded, there's always force pushing down on the piston tops - both during exhaust and compression strokes (your cylinders fire every OTHER time the pistons are at the top of the cylinder, since this is a 4-stroke engine).

The best analogy I can find right now is with a boxer. If he throws a punch and lands it, the person or object he HIT is absorbing the force required to stop his fist. However if a boxer throws that same punch and misses entirely (i.e., there's no force to slow down his fist) then HIS BODY (arm, wrist, etc.) must abosorb the exact same force that was required to accellerate the fist. It's widely known in boxing, if you throw a punch and miss, it's almost the exact same effect on you as being punched by another guy. I.e., it's real bad to throw a punch and miss because it wears you out faster.

Same with the piston. As it reaches the top of the combustion chamber, if the motor isn't loaded, there's less combustion pressure (on the compression stroke) to stop the piston, and if it's an exhaust stroke (especially on boosted cars) there's little or no backpressure to stop the piston moving upwards. The rod bearing must take all this force!!


Top fuel dragsters wear out the BOTTOM of their rod and main bearings because they're under load at all times.</TD></TR></TABLE>

 

I think load is realative in the engine. Yes, free reving there isn't load on the engine, but that doesn't mean that a combustion hasn't occured. There has to be for the rpm's to climb. I would think that a 3rd gear to redline pull would put more strain on an engine that free reving to redline anyday!

 

Well, in the exhaust stroke the piston is pushing out exhaust gases, and at high speed the exhaust gases should develop some sort of pressure being expelled out of the cylinder. So, it may be bad, but I dont think its as bad as he makes it out to be.

 

I would think that a 3rd gear to redline pull would put more strain on an engine that free reving to redline anyday! [/QUOTE]


i would have to agree with this also!

 

When there is no load, you can get torsional oscillations in the bottom end that do not occur under load. Depending on the engine design, this could be worse than running it under load. The boxing analogy is also valid.