Effects of free-reving an engine
Originally Posted by PontiacFan
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!
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!
I always was taught that free-revving as you guys call it is bad on rod bearings and rods due to the change in both rotational and axial loading on the metals and the bearing surfaces. You have a change in directional loading when the rod is no-loaded after the rev and also in the load on the thrust surface as well since the thrust tends to go forward on the rev, and towards the rear on the decrease in rpm. It's like watching a guy scatter a motor in the burnout box because he wants to wop the throttle eight or ten times thinking the noise will scare his opponent(usually he just gets laughed at) About the sixth whack of the gas the rods come out the bottom because of the negative loading on the bearings, turns a bearing, builds some heat and the rod bolt breaks........... the vicious cycle is repeated countless times in burnouts all around the world!! 
Teching In
Joined: Apr 2006
Posts: 37
Likes: 0
From: Princeton, MA
One of the guys at the shop where I work revs the snot out of cars all the time. It is definitely a bad idea. He still does it even after he's windowed an engine block. He spun a rod bearing and the rod bolts broke. Aparently my boss has more tolerance for "free revving" than an engine does because he still has a job. 
Originally Posted by WS-6Will
Aparently my boss has more tolerance for "free revving" than an engine does because he still has a job.
Originally Posted by bygblok
it's OK, there were loads of idiots working at the GM shop I worked at for quite a few years......... it's almost as bad as working on aircraft!!!! Scary isn't it?
I destroyed 700 hp 350 in my brothers Nova while out pounding it.... i heard/felt "something" break at 6000 RPM but I knew as soon as I let off the fast pedal/hit the clutch that engine was gone... lol Sure enough, as soon as I let out the awfull meatal/explosion/oil coated windshield/ etc. occured... that was the first time I had ever seen a crank actually break... I've had the same happen with U-joints... seems like when you let off or hit the clutch after a heavy load thats when u-joints come apart or twist out
On The Tree
Joined: Aug 2005
Posts: 133
Likes: 0
From: TX
Ok, so as long as you're giving the motor gas and it's moving/burning air/fuel charge, there should be a combustion pressure to put that compressive force out to counter the inertial force. OldSSStroker, am i wrong?
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by Adnectere
Ok, so as long as you're giving the motor gas and it's moving/burning air/fuel charge, there should be a combustion pressure to put that compressive force out to counter the inertial force. OldSStroker, am i wrong?
A free reving engine is only producing enough power to ovecome internal friction and pumping losses which is maybe 12-15% of the max power it can produce when loaded. So you have 12-15% of the combustion forces "helping" rather than the 100% you'd have with the engine under load.
FWIW you are not "giving the motor gas" with your right foot. You are controlling how much the engine is "throttled" or how much air is let in. If you flat-foot the accelerator pedal, aka the "throttle pedal", the engine will hit the rev limiter with the computer limiting spark and fuel to produce just enough power to maintain the engine speed. Even if you adjust the "throttle pedal" to free rev just below rev limit, there is only a little fuel being injected (or metered by the carb). The engine produces power "on demand" with that demand being the load placed on it by the vehicle or a dyno.
Make sense?
On The Tree
Joined: Aug 2005
Posts: 133
Likes: 0
From: TX
Ok, I'm better with that explanation, probably the last sentence about on demand. Never really thought of it that way. I just figured the 2 things in a torque equation would be % throttled (well in my thinking, unthrottled) and rpm and thus would give you power regardless of load. Very interesting...brings me back to a previous post you had to explain. I'll chew on it for a few days. As always, a pleasure.
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I think most of the wear would be on the valvetrain, like the timing chain and springs. Since I do my own work, I rarely free rev the motor. Dont want to make extra work for myself for no reason.
Launching!
Joined: Mar 2004
Posts: 231
Likes: 0
From: Exton, Pennsylvania
Originally Posted by PontiacFan
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!
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!
Staging Lane
Joined: Mar 2006
Posts: 50
Likes: 0
Load reversal is probably why driveshafts snap on a decel, especially if there's slack in the gears. That load reversal also gets into the crankshaft, where before the reversal, the crank was twisted slightly in the torque-making direction, and then when the vacuum is applied to the manifold, via the closed throttle, the crank twists slightly in the torque-recieving direction, via inertia of the drivetrain that's still connected to the crank. Reversing stresses will break components faster than just constant forces.
The rods (and bolts) probably see a lot of stress when vacuum is applied. Stress in the stretching direction. During compression and exhaust with load, there's plenty of pressure to help avoid too much stress in the stretching direction, and the intake stroke is only pulling on near atmospheric resistance. But, when sucking on a closed throttle, it'll apply quite a bit more force to the piston (think surface area and pressure = force), in the stretch direction, compounded with the stress of high RPM.
Other people float a valve, and smack pistons.
Probably just repeated what half the posts say, sorry.
The rods (and bolts) probably see a lot of stress when vacuum is applied. Stress in the stretching direction. During compression and exhaust with load, there's plenty of pressure to help avoid too much stress in the stretching direction, and the intake stroke is only pulling on near atmospheric resistance. But, when sucking on a closed throttle, it'll apply quite a bit more force to the piston (think surface area and pressure = force), in the stretch direction, compounded with the stress of high RPM.
Other people float a valve, and smack pistons.
Probably just repeated what half the posts say, sorry.
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by dug
youre right about the vacuum part during high rpms with closed throttle. However that happens everytime you shift when racing.
Actually a slight partial lift is probably preferred for drivetrain life, but all you are really doing is a slight torque management to unload the gears. A .1 second or less spark retard would probably be effective.
high oil pressure, pushrod will either bend or fly through the hood... trust me i've seen both and did one- the one was by missed shift though and it resulted in free revving.
