Myth or truth? Revving in neutral

[Dave Sherman]

I've heard that it's much harder on the connecting rods and bearings than when under load, but to me this doesn't make much sense. I'm thinking they experience more load when the crankshaft is under load during the power stroke, but during the intake, exhaust, and compression strokes, the forces on them are only a function of what speed the crankshaft is rotating at. Higher rotational speed means much higher acceleration of the piston and rod as it goes between TDC and BDC.

Am I missing something? Or is it just bunk?

 

[SteveSRT8]

It's not total bunk. An older loose motor can be damaged easily.

Note that my car will not rev past 4000 rpm in neutral due to factory limiter. Must be a reason in their mind, eh?

 

[rslifkin]

I think the issue is that revving to high rpm and letting off in neutral leads to the loads and rpm changing very quickly, which is what's hard on parts.

 

[lexus114]

No load on the motor = Not good.

 

[Dave Sherman]

Originally Posted By: lexus114
No load on the motor = Not good.


Why is that?

 

[sciphi]

There's a reason lots of cars with DBW throttles are programmed to have a 3-4 second rev hang if suddenly shifted into neutral at high RPM's. The engine suddenly goes from heavy load to no load while it's decelerating rapidly otherwise. I have to think that's hard on the crank bearings. Taking a few seconds to spin down ought to reduce the load on the engine's internals.

My MT Cruze will gladly rev to the moon while in N. Our AT Fit has a limiter. I have to think some of that is AT-related, and the engineers didn't want folks neutral-dropping an AT at 6000 RPM. Bad on the solenoids and clutch packs. A manual just has a clutch to worry about.

 

[JosephHarmon]

I would say a good reason is "it's stupid"!

The point of no return comes much faster as valves float & kiss pistons or the small & big end of the con rod get a divorce

The only difference is the RATE of RPM increase. This does not increase the structural loads.

My Northstar engine won't allow over 4000rpm in park or neutral.
.

 

[Dave Sherman]

Originally Posted By: sciphi
The engine suddenly goes from heavy load to no load while it's decelerating rapidly otherwise. I have to think that's hard on the crank bearings.


Stick shifts do that all the time when changing gears, so how is taking a load OFF of a bearing hard on it?

 

[vfrkent]

It has to do with the unloading of the rod bearing against the pin when it reaches TDC and then again at BDC, with no load on the piston to keep the pin loaded to the top of the bearing it transitions that clearance at both TDC and BDC.

If you have ever heard a engine with bad rod bearings it is the nosiest with no load and quietest with a load, as the clearance increases with wear the more it pounds away at the bearing and the faster it wears.

Regards,
Kent

 

[TallPaul]

Originally Posted By: SteveSRT8
Note that my car will not rev past 4000 rpm in neutral due to factory limiter. Must be a reason in their mind, eh?
Did you read this in the owner's manual and/or did you boldly give it a try to see what would happen?

Long, long ago I had a '71 ford with a V8. Was moving to the other side to let someone drive. Engine was running and I accidently stepped on the accelerator while pushing with my foot to move over. Revved out wildly and then the alternator light came on. NO harm to the engine but it threw the alternator belt.

 

[troyb43]

After watching many NASCAR races over the years, it has been my observation that many of the engine failures seem to happen when backing off of the throttle going into a turn

 

[SteveSRT8]

Nope, I'm an old timer and I just know the car's tuning package very well.

My friend had a 68 charger 383 that his Dad started one time with the accelerator stuck to the floor!

Dead cold and held at valve float speed while we frantically scrambled to get the key turned off!

Never hurt a thing, we raced that car for years afterwards.

 

[Dave Sherman]

Originally Posted By: vfrkent
It has to do with the unloading of the rod bearing against the pin when it reaches TDC and then again at BDC, with no load on the piston to keep the pin loaded to the top of the bearing it transitions that clearance at both TDC and BDC.

If you have ever heard a engine with bad rod bearings it is the nosiest with no load and quietest with a load, as the clearance increases with wear the more it pounds away at the bearing and the faster it wears.

Regards,
Kent


I'll agree that while under load it could be less noisy under load if the bearings are worn, but at TDC and BDC there is zero torque being applied to the crank, there is only the inertia of the rod and piston, which is acting perpendicular to the crankshaft rotation. This loading and unloading would happen regardless of what load is on the crank.

So, I still have the question, how is the rod or bearing going to be MORE stressed with no load? If someone has a good understanding of physics and dynamics and knows something I'm overlooking, I'm all ears.

 

[Hokiefyd]

Originally Posted By: troyb43
After watching many NASCAR races over the years, it has been my observation that many of the engine failures seem to happen when backing off of the throttle going into a turn


I'd say that's a pretty accurate observation.

 

[IndyIan]

I read somewhere that snapping the throttle shut at max rpms causes the maximum vacuum in the cylinder as well so the connecting rod experiences the most tension at TDC in that situation. Which also might cause ring issues if the cylinder has a "lip" in the bore as well but I think that's pretty rare in "modern" higher rpm engines.
In practice, bouncing of the limiter in gear and WO, I think has got to be harder on an engine, and in autocross its very common to have that happen(often on purpose) and healthy engines seem to have no issues with it. Even my car with 270k km doesn't seem to care much?

 

[Colt45ws]

Originally Posted By: sciphi
There's a reason lots of cars with DBW throttles are programmed to have a 3-4 second rev hang if suddenly shifted into neutral at high RPM's. The engine suddenly goes from heavy load to no load while it's decelerating rapidly otherwise. I have to think that's hard on the crank bearings. Taking a few seconds to spin down ought to reduce the load on the engine's internals.

My MT Cruze will gladly rev to the moon while in N. Our AT Fit has a limiter. I have to think some of that is AT-related, and the engineers didn't want folks neutral-dropping an AT at 6000 RPM. Bad on the solenoids and clutch packs. A manual just has a clutch to worry about.

The rev hang on DBW throttle cars is because of emissions. There is a hydrocarbon burst out the exhaust when the throttle slams shut.

As far as it being harder, I dont see how.

 

[SteveSRT8]

Originally Posted By: JosephHarmon
I would say a good reason is "it's stupid"!

The point of no return comes much faster as valves float & kiss pistons or the small & big end of the con rod get a divorce

The only difference is the RATE of RPM increase. This does not increase the structural loads.

My Northstar engine won't allow over 4000rpm in park or neutral.


Agreed, I cannot see the so-called 'load' making any difference in an engine with normal clearances. Same with the piston pins, they still have to stop and change directions at the same rate per rpm.

And Sciphi, my car would not do a neutral drop even if you tried to. It's not allowed. Plus it is unlikely to damage solenoids, perhaps clutches? In my old cars the spragues were the first to go in drag racing, generally on the shift if you had good bite.

 

[itguy08]

Originally Posted By: SteveSRT8
Note that my car will not rev past 4000 rpm in neutral due to factory limiter. Must be a reason in their mind, eh?


Yup, it's to protect the transmission and it's lubrication. (http://www.marcusfitzhugh.com/CLK/DIY/revneut.html) It's also to partially protect the transmission from a neutral bomb or drop at high revs.

 

[mechtech2]

I have always heard and respected to not 'free rev' an engine.
But I can't see why, either.
Less loading on bearings, and the same loading at the 'turn around' at the unloaded TDC.

Cold revving an engine is bad - maybe that is the 99% of the truth here.

BTW, here is an interesting test:
Try de clutching while holding your gas pedal at the same position, while cruising down the highway. It is surprising how little the RPM go up. You'd think it would redline, but only goes up 1-2k.

 

[cchase]

It's important to note that revving in neutral is not a "no load" situation. There's still a fair amount of load on every part of the engine whether it's in gear or not.