What, exactly, is bad about revving under no load? Is it the fact that you have no load, or is it that the revs climb too quickly? Is it not bad as long as you only rev up to half of your red line?

 

the piston rings expand with combustion chamber pressure (part of the design). Revving under no load (with a computer controlled engine) results in lower combustion chamber pressures... which in turn doesn't expand the rings as tightly against the cylinder wall as they would be under heavily loaded acceleration... which in turn will allow more oil to leak past the rings.

At least that's the way I understand it.

 

Wouldn't that just cause you to burn oil?

 

yes; and I thought that was the direction of your question, relating to the reported oil consumption/ring problems.

 

Actually, it's the lower than atmospheric pressure created by the piston moving down inside the crankcase the causes the rings to seat against the cylinder wall, if you can explain why that changes due to load against reciprocating assembly I'm all ears.

 

Just a guess, but I think by keeping the engine "loaded" you may avoid posssible engine destroying harmonics.

Mark

 

No, I meant the thing I commonly hear about engines being damaged or broken by repeated revving (but not above redline, which I know will cause damage.)

 

There is another effect:

If you reach redline with an unloaded engine, the partial vacuum in the induction tract will still be pulling up on the piston as it passes TDC with both valves open, increasing the tension in the conrod at this heavily stressed point. But the LS1 is redlined due to valves not conrods, so this case probably doesn't hold for us, even though it does for racing engines.

 

the rings need maximum pressure on the downstroke, after ignition. The cylinder pressure is greatest just after combustion. The combustion chamber pressure seeps around the piston top, travels downward along the gap between the piston and cylinder wall, and gets into the piston grooves where the rings are situated. The rings and the piston grooves are designed to direct this pressuer inbetween the piston grove and the back side of the ring... pushing/expanding the ring to increase its force against the cylinder wall.

On a computer controlled engine, the combustion pressure is controlled by varying the A/F ratio. When the PCM sees a demand for high RPM (but also detects no/low load) the AF ratio is leaned; which in turn causes lower combustion pressures within the combustion chamber.

It is this reason that GM said the oil burining problem was due to drivers with manual tranny's that kept the engine at a constant RPM with no load (sustained high rpm with low loading). The rings were not being pressed hard enough against the cylinder walls to keep oil from passing the ring barrier.. due to low compustion chamber pressures from a leaner AF ratio. But the oil pressure was higher solely due to the RPM. So you had high oil pressure along with low combustion chamber pressures during the sustained high rpm with low/no load on the engine.


[Modified by Mike Mercury, 1:36 PM 11/14/2002]

 

Our cars issues from revving to redline in neutral (no load) are in the top end (heads/valvetrain), not in the bottom end. Sure, you'll burn some oil revving, but that's nothing bad.

Revving to redline in neutral means there's no load to slow down the motor's movement. By the time fuel cuts at the redline, the engine's moving parts have enough momentum and are still accelerating to carry the engine's actual RPM past the redline, to the point of valvetrain damage, whether from interference or harmonics.

If you want to rev, just keep it below 5k, you'll burn fuel and oil, get nice snap/crackle/popping, and do no damage.

Sam

 

Thanks Sam. That is just what I wanted to hear, plain and simple. :rolleyes:

 

by saying this, you are implying that rpm's above the stock limiter are dangerous to our motors, therefore, setting the limiter higher (LS1Edit, other programming) is dangerous.
Bouncing off the rev limiter is not the best thing to do too often, under load or not. This is a separate issue from this post.

 

Yes indeed, I do imply, and believe that - I personally will NEVER raise the factory rev limiter on a stock valvetrain engine.

Sam

 

Thanks, Sam, your explanation sounds like a very likely source of the "revving is bad" school of thought. So it's not the revving under no load that's bad, it's the going to red line while doing it that's bad.

 

I have to differ with sam:

[QOUTE]By the time fuel cuts at the redline, the engine's moving parts have enough momentum and are still accelerating to carry the engine's actual RPM past the redline, to the point of valvetrain damage, [/QUOTE]

Once the fuel or spark is cut, the engine stops accelerating--period.
The engine may already be over redline when the cut happens, but once you quit making power--no acceleration
F=MA, F=0 implies A=0 even if it is in the rotational sense.
Momemtum does not cause acceleration, momemtum is velocity times mass, momentum (actually the mass) resists acceleration!

Time to go back to science class--sam!

 

Mitch, you're right, I apologize if my post mislead anyone - reading it now it doesn't quite jive with Physics.

What I mean is when you rev in neutral, electronics reaction time becomes a very important factor, though fuel cut is set for a certain RPM, the tach trigger may not read that RPM and trip the fuel cut until the actual engine is hundreds or thousands of RPM higher. As a result, the fuel system doesn't cut until it's too late - remember, fuel cut time/placement is based on the engine being loaded.

Sam

 

So what you're saying is that when an engine revs that quickly, the tachometer and the fuel cut-off RPM meter can't keep up?

 

I posted a while back about rev limiters....I was told that you can not do damge unless you are downshifting and wind up with RPMS above redline.......and that if you are just spinning your wheels or revving that you can not blow your engine or damage it with a manual 6 spd due to the limiter.........damn....who is right???????? Not that I spin my wheels..or over rev......but made me feel better letting someone rev or drive my car!

 

The thing about having to shift into too low a gear to do damage refers to redlining under load. This discussion is about revving with no load. In other words, in neutral or with the clutch disengaged.

 

Scissors, exactly - the sensor's read rate falls behind actual rpm value when it changes that quickly - an unloaded engine revs almost instantly.

skidmarc, revving itself won't hurt the engine, over-revving will. Downshifting and missing the proper gear will overrev the engine, revving in neutral to redline will also overrev the engine. Spinning the tires into redline puts more load on the engine and as a result slows down the rpm changes, so it is less likely to result in damage.

rwj383, can you elaborate on what would be damaged from extended bumping of the rev limiter?

Sam