A-man930

iTrader: (2)

What are the consequences of reving the snot out of an engine while not under load? For instance, sitting at a stoplight and bouncing off the rev limiter... What happens to the rings? bearings? etc... I personally try not to do this because I always figured it was bad for things. I think of pedaling a bike as hard as you can while going down a hill, you're not actually pushing anything, but you're still banging the pedals like hell. The motor has no resistance to speak of, but you're banging the crank like hell... make any sense at all?

*98Formula*

iTrader: (2)

Don't do it is the best thing I can tell you. Since there is no load on the motor free reving the motor doesn't load the piston's and rings. Short term you will be fine but if you do it alot you will have bad rings and you burn up bearings quicker because the oil pump can't always keep up with the speed of the engine. In other words your revs are coming up faster than the oil can pump this is bad for rings. I enjoy watching ricer turds do this to me at stoplights and then the next thing you see is smoking coming from under the hood because they just spun bearings or something. I know of 3 times when I have seen this.

Old SStroker

Any time the engine is run to the rev limiter it is applying maximum inertia loads to the rotating/reciprocating parts whether there is a power load on it or not. Actually when there is a maximum power load, the combustion pressure helps counteract the inertia loads trying to fling the piston out of the top of the block on one half of the revolutions, so free reving is the WORST case, and is about the quckest way to runing over your own crank.

Unless you are drag racing and use a rev-limit clutch dump to start, free reving the engine for sh**s and giggles is a very bad idea. Other than some of the "coffee-can muffler" ricers, do folks actually do this? (I wish there was a smilie for "shakes head").

Some OEM engines with automatics drop the rev-in-neutral limit to around 3500 to protect the driveline from a high rev "drop". It also helps the engine of course.

"Mother Nature must love stupid people...She allows so many of them to exist."

gun5l1ng3r

So without a load on the engine, can't parts "bounce" forwards and backwards in their rotation because there is no interial load to keep things spinning in the same direction?

I never understood why a load is better for the engine, but I have always accepted that fact. Also when you rev an engine in neutral, it doesn't sound the same as WOT in gear. It almost sounds like it rattles a little but versus a smooth scream when you are in gear.

If this doesn't make sense let me know!

BREAKPARTS

i rev my engine evry once in a while...mostly when a much slower car wants to run em and i would rather not due to a pointless reason to be arrested. so would it be better to take off hard and quick to scare them away. usually a solid rev gets em to leave me alone. so mabey im bass ackwards

Old SStroker

Inertial loads on the rod and crank are the loads resulting from trying to stop the piston at top dead center. From basic physics, F=Ma or the Force on the con rod = the Mass of the piston, rings, pin and most of the rod x the acceleration (or "gs") the piston sees. This acceleraton is determined mostly by rpm and stroke length (with rod length being a minor factor). It can be 4000 gs or more for higher rpm engines. F1 engines which rev past 19,000 but have tiny (1.56 in) strokes still see 10,000+ gs!

If you swing a bowling ball tied to a rope around your head like a lasso, the force you feel in the rope is "inertia force". It's because the ball wants to fly away from you and you are pulling it back in. It's the same force you feel on your body when you go around a turn. It's called lateral g force in vehicle dynamics talk. 1 g is huge. Most people rarely see 1/2 g even on a scary trip around a feeway off ramp. Now imagine 4gs or 400gs or 4000 gs. It makes me feel sorry for the poor connecting rod!

gun5l1ng3r

Isn't the bowling ball's acceleration called centripetal acceleration, the force of the acceleration tangential to the radius in which it is being swung?
And how would a load on a motor change the forces inside the motor? Isn't 4000RPM always 4000RPM in a motor(disregarding the charge density)?

00transamnh

iTrader: (1)

thats what i was thinking

Old SStroker

I believe it's centripetal force, the result of accelerating the ball toward the center of rotation away from the tangential path it wants to take, or will take if the rope breaks. The amount of force depends on the mass of the ball and the magnitude of the acceleration which depends on the length of the rope and the rpm or velocity of the ball. It's constant with a fixed rpm and radius, unlike the reciprocating piston.

The inertial loads that do the most harm are those from decelerating the piston as it nears TDC and then accelerating it again after TDC . This puts the rod in tension.

Yes, at 4000 rpm the inertia forces don't change but the compression/combustion forces are pushing down on the piston crown and acting against the inertial forces. Of course this happens only every other stroke, but that reduces the cumulative total load the the rod endures. While damage may not occur immediately, metal (like a woman) remembers every stress that was ever placed on it (her), and when the fatigue limit is reached, things can go boom. Ironically, the higher the power being produced, the more the inertial loads are reduced.

Did you ever see a short track car experience a blow up just after the driver lifted at max rpm entring a corner? It didn't blow under max power, but at max rpm with no load.

jimmyblue

iTrader: (11)

It grows hair on your palms and makes you go
blind.

irnbru83

To give an example from the electrical side... If you have a short circuit, then you get tons of current (Force) through your components (Rings and Bearings) with basically no resistive load and they will burn out very fast (Fire) if you didn't design a current limiter.

Richiec77

iTrader: (3)

So the force acting against the roatational force isn't helping. It's the actual air and fuel that is the cusion to the inertia. So the air is acting like a pillow and helps contain the inertia in some respect.

I see the bad part of free-reving the engine is that there isn't much of a load acting against the rotation and in turn makes the engine linger at the high RPM longer and doesn't help control the amount of acceleration and deceleration.

TheRealDeadApe

free reving will not cause your motor to blow-up, nor will it cause noticable damage. It will cause faster cylinder wear, as will any sort of high rpm driving. it will cause increased oil consumption as will any sort of high rpm driving. Moral of the story its only going to turn around so many millions of times, but your not likely to notice any ill effects in the lifetime of the car.

Richiec77

iTrader: (3)

Ok. Some mistakes here. Electromotive Force is Voltage. Load is Current Amperage.

A short circuit places is the path of least resistance. If something shorts out, there is nothing to contain the Force or load.

That's the same as no air/fuel mixture in the chamber to ease the piston as it nears TDC.

RevGTO

iTrader: (12)

LMAO ...

2002_Z28_Six_Speed

iTrader: (19)

No offense intended but can someone please lock this thread?

RussStang

Why?

PontiacFan

iTrader: (104)

Because it's phucking retarded to even do something like this, 'free-revving'?!?
What a stupid thing to do, ...
Like all those stupid motorcyclist who have to sit there & keep revving the engine while at a stop light. For Christ's sake man, let the damn thing idle!

ArcticZ28

iTrader: (14)

I actually always wanted to know the physics behind why it was so horrible rather than just "knowing" it's bad for the engine. I got a lot of tangible info from this.

2002_Z28_Six_Speed

iTrader: (19)

Acceleration of the parts in a engine causes wear. An engine with no load has large changes in acceleration compared to a loaded engine. The loaded engine stays at the same operating speeds and temps for the least wear.


I do not question the intelligence of anyone here. I do question free-reving. It's not like it will be the cause of an engine failure, however, it is malpractice. I think of it like not wearing your seatbelt on the highway.