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I pulled up at a light the other day next to a kid driving (what I assume to be) his dads 5.0 Mustang. As he proceeded to bounce off the redline and roast away half the clutch when the light turned, I was reminded of an old rumor I came across on a Civic forum years ago.

The rumor was that revving your engine in neutral, when the engine wasn't under load, could cause damage. I believe the reasoning was akin to throwing out your arm because you weren't holding a ball. EDIT: Thanks to Jason C for the info, the prevailing argument seems to be that oil pressure cannot keep up with a free-revving engine.

I feel like the rumor was BS, but for the life of me I can't think of a reason. So my question is, on a normal street car with a rev limiter and wet sump, can any excess wear occur from revving your engine to redline at full throttle in neutral that wouldn't also happen if you took it to redline in 1st - 6th gear?

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    I would love to see a convincing, physics based answer to this. I've spent the last 20 minutes reading endless forum arguments about this, and there are convincing but baseless theories both ways. I hope any answers posted go into some more rigorous analysis. Theories for doing damage seem to focus on revving past the limit (but afaik certain or even all types of limiters make this not actually possible) or oil pressure not keeping up with a fast rate of rpm increase (but gut feeling is once rpms are stable this is moot). Theories against include less physical stress on linkages. – Jason C Aug 4 '16 at 18:02
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    @JasonC thanks for the insight, I edited it into the question. As soon as I read your comment about oil starvation i was hoping I could come up with an answer using the Toyota/Yamaha 1LR-GUE V10 from the Lexus LFA as an example, since it can rev from idle to 9500 in 0.6 seconds, but it also uses a dry sump system, which would more or less eliminate the problem. – MooseLucifer Aug 4 '16 at 18:45
  • I do get the feeling that even though people seek a general answer to this question, that it's actually highly specific to the vehicle. I hope any answers can go into detail about how various differences in engine design affect the outcome, if there truly is an effect. The dry sump system is a good example. – Jason C Aug 4 '16 at 19:11
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It is indeed mostly a rumor, but with some background.

Temperature

The main problem is with the cooling. If you rev the car while standing still, the radiator will receive less air, thus it will cool the coolant less efficiently. Usually the fan will force some air to the radiator if the car is not moving fast enough, (like if you are stuck in traffic). This means that the engine will operate in the higher threshold of temperature. The problem will be worsened if the car already has cooling problems.

Unnecessary wear

The secondary is that you are turning the engine and naturally wearing it without moving the car. The engine wears for a combination of factors and one of the most strong of them is the number of revolutions it will do over its lifetime. It is like driving the car for more miles that it actually has in the odometer.

Either way, both factors only offset the engine wear for very little, unless if you keep revving the car for 30 minutes then driving it for 5 miles a day. The car will seem to have a low mileage, but the engine will wear out much earlier than other comparable engines.

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    I'm not doubting your answer, but I personally would like to see something a bit above and beyond what's already out there. To that end: Can you provide at least some basic heat transfer / air flow calculations to back up the statement that the fans are inadequate for cooling at certain RPMs (you can pick an example vehicle to illustrate), and if so, how long you'd have to rev for it to create enough heat to do damage? – Jason C Aug 4 '16 at 21:49
  • Continued: The "unnecessary wear" part seems reasonable, though, although that's kind of a default assured consequence of any of these types of actions so I'm not sure if it's worth pointing out unless you do some calculations that show % expected lifetime decrease. E.g. if you hold 5000 RPMs instead of idling at 800 for 10 seconds that's 700 extra rotations in an engine with an expected lifetime in the hundreds of millions, which is pretty inconsequential. – Jason C Aug 4 '16 at 21:52
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    (Also fwiw, anecdotally, I was diagnosing a cooling issue on my car recently, a 2001 Honda Prelude, 190k miles. After I fixed it I did a number of tests. After letting the car warm up to a stable 195F coolant temperature at idle, I held it at 6500 RPM for about 20 seconds and the coolant stabilized around 210F, well within the usual operating specs and comparable to temperatures reached during normal driving, suggesting no additional heat vs. normal driving conditions. I'd be happy to run other abusive tests too if you think of any, I'm getting a new head gasket next week anyways.) – Jason C Aug 4 '16 at 21:58
  • @JasonC Maybe I did not read through the question the same way you did, but I understood that the OP meant about driving at a higher than usual rev, like using a lower gear or keep revving the car stopped. As I mentioned in the beginning of the question, those problems are very small and usually other habits will reduce the lifetime of the engine more than this. – Gabriel Diego Aug 4 '16 at 23:37
  • And about your Integra, you heated the car all the way up to 210 in 20 seconds means that the engine heated much quicker than if you where driving normally at lower revs. You have two (very small) problems there: quick heating while operating (will all the rings, pistons, gaskets expand at same rate?) and operating the engine at the higher temperature threshold. – Gabriel Diego Aug 4 '16 at 23:45
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Yes, this causes engine wear.

When the transmission is in neutral and the engine is “revved” without any load, the spinning engine internals will accelerate, gathering rotational and lateral forces at a faster rate than designed by the manufacturer.

Why will the engine wear? (not an exhaustive list):

Piston compression ring expansion

Rapidly revving an engine will heat up the piston rings much faster. Because they have a much smaller thermal mass than the cylinder liners they expand at different rates. If the compression rings expand too much, they will generate increased friction on the cylinder liners, causing the cylinder liners to wear out (reducing compression).

In the worst case scenario, the compression ring ends touch, pinch the cylinder liner and will most likely cause the piston to crack. Then you’ll have metal fragments flying around destroying the head and cylinder.

On a very cold engine or while lean (not enough fuel), it is easier to get a larger thermal differentiation between the cylinder liner and piston rings.

Some early rev limiters implemented used a fuel only cut which could cause a cylinder to lean out "slowly" and develop a piston hot spot from detonation.

Contrary to other comments & answers given, the cooling system is NOT able to help, as piston failure happens within 4-10 milliseconds after a hotspot develops on a piston or compression ring. (See video for references to fuel injection millisecond times below)

Oil system starvation in wet sumps

More common in continued over revving - the cylinder head & block doesn’t drain the oil at the same rate that oil is pumped out of the sump, leaving the sump empty, common in the Rover V8 engines. This is very common in engines that don't have the minimum oil required. Engines that are not serviced have dirty oil systems and are prone to blockages at high rpm.

This can also be caused if the block uses the same pipes to drain the oil that is also used to ventilate the sump causing oil vaporisation. However the engine would most likely already be overheating for this to occur.

Some poorly designed oil pumps (and water pumps too) can aerate and are unable to pump oil when increased in rpm too rapidly.

The next two points are much more common in performance engines where the manufacturer has tweaked output leaving no room for improvement. You’ll find plenty of videos on YouTube of motorcycles and Italian supercars revving without any load and destroying themselves.

Engines that can handle repeatedly being held on the rev limiter without any failure generally have a conservative rev limit set or have lots of performance parts available.

Connecting rod (conrod) warping

Some conrods will stretch/bend during excessive rpm acceleration. Worst case scenario is it will put greater unbalanced forces on the crankshaft and bearings (higher compression ratio only needs 10 thou). If the engine has tight tolerances it will also bend valves.

Crankshaft warping

The crankshaft or engine block isn’t designed to withstand such a sudden increase in internal force (similar to harmonic imbalances). It only takes a thou or two (0.0254 - 0.0508 millimetres) for the crankshaft to gouge a main bearing. This is common in performance & motorsport engines so they have increased strength built into the block by adding more ribs and webbing to the cast or billet.

If you’re curious what a “catastrophic” harmonic engine failure looks like at 11,000 rpm with load on a dyno, watch https://www.youtube.com/watch?v=1LkxGx5WJzA and skip to 14:20 where Cosworth gently explore the limit of their turbocharged 4 cylinder during F1 engine research & development.

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In my humble opinion, to take an engine to high RPM under load (where the piston is cushioned at the top of the stroke by a large charge of fuel/air) is far better than to reach the same RPM unloaded. An unloaded engine, with a relatively small fuel/air charge, relies entirely on conrod bearings to stop the piston at the top of its stroke. in a "loaded" engine, the large charge and therefore high compression pressure, cushions the piston action.

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Depending on the year/model, the rev limiter will not allow you to redline in neutral.It will cut off the engine before the rpms can do damage. Usually cuts it off at 4000 rpm - depending on year/make/model.

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Don't listen to any BS people say here... If the engine is in the operating temprature, it is okay to rev it getting not too close to redline. And not revving it for long periods of time... I treat my cars like that and they are still golden.

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