Revving in neutral is bad, but... ??

[rationull]

Disclaimer: I have absolutely no interest in revving my car in neutral, this is purely a curiosity question.


Whenever anybody asks whether it's OK to rev their car while in neutral (here or elsewhere) the responses tend to indicate that it's not good, mainly because it stresses the rods. Responses vary in *how bad* unloaded revving supposedly is for the engine but there seems to be consensus that it's not good.

However, when drivers who are new to manual transmissions start asking about downshifting, more experienced drivers inevitably suggest rev matching techniques.

Unless I'm grossly misunderstanding something (and I don't believe I am -- I know how to drive stick) rev matching involves a fair bit of unloaded revving. The only difference being that much of the time (unless you're driving hard) you won't rev close to redline while rev matching (but then you're not necessarily revving close to redline when you give it gas in neutral, either).

Seems to me there's a conflict here. Anybody have any thoughts on it?

 

[rpn453]

I didn''t think it mattered. Can anyone explain why it would? I do a lot of rev-matching, so I guess my engine is doomed if I'm wrong!

 

[Camu Mahubah]

Remember when you were a little kid going all out on your bike and the chain came off or broke... you'd be busting your knees on the handlebars or worse. It was painful and stressful on your joints spinning the pedals without the chain attached. Same thing for your rods and crankshaft in your motor.

 

[Camu Mahubah]

Rev matching prevents lugging the engine which is also hard on components. I think rev matching okay, reving in nuetral any other time bad.

 

[Camu Mahubah]

I think I got that last part wrong because in rev matching you are shifting into a lower gear correct... so one blips the throttle to match engine speed more closely with driveline speed...this makes for an easier transition for clutch, engine, u-joints, etc.

 

[Brogy]

Cars were meant to be worked, as long as you're not driving in too high of a gear, or bouncing off the rev limiter, you'll be fine.

 

[Shannow]

I was always told that free revving in neutral was bad, but that was heaps of revs (like near redline).

Explained to me that the types of cars that I was driving (old), had likely been driven by more gentle fol, and would have a ridge of either metal or carbon at the top of the bore.

Revving unloaded may allow the ring to contact that ridge, upsetting the rings, or braking them.

 

[rationull]

A few responses..

Camu: I don't think the bicycle analogy is a valid one. It would hurt your legs to be spinning the pedals that fast with or without a load on them, but continued operation at high engine speeds isn't necessarily bad for an engine (depending on your definition of "high").

And yes, it makes perfect sense that rev matching makes downshifts easier on the rest of the driveline. The issue here is whether rev matching is actually bad for the rods. I'm not trying to make an argument for or against rev matching, just trying to clarify what seems to me is a conflict between pieces of advice given at different times.

Shannow: Interesting.. but wouldn't the piston probably contact such a ring at higher RPMs anyway, whether loaded or unloaded?

 

[rpn453]

I don't agree with the bicycle analogy either. Clipped in and without a chain, you can spin as fast as your legs will move; the load on bones, joints, muscles, and the bicycle cranks is small. Without clips, the only risk is that your foot is likely to slip off the pedal if you go too fast or if it is unexpectedly unloaded and throws you off balance (like a chain break).

 

[Camu Mahubah]

I always thought it was painful to spin the crank on a bike with no load...a lot less painful underload. I still think it is a bad idea to rev an engine in neutral... Rev matching like I said earlier okay if done correctly, in fact F1 style paddle trannies have throttle blipping programmed in...

 

[98LSC32V]

It depends on how high your rev limit is. I rev my 98 LSC in nuetral to 3000-3500 once in a while to hear the exhaust, as long as you don't take it to 4000-5000rpm often you should be fine. My limiter is at 7000rpm btw... Now if you have a big block with a 4500rpm rev limiter it's a different story.

 

[mechtech2]

Keep it under the red line, and you'll be fine.

 

[Shannow]

Quote:

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Shannow: Interesting.. but wouldn't the piston probably contact such a ring at higher RPMs anyway, whether loaded or unloaded?

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I think you are probably right, me, my brother and our mates all revved our older cars much much harder than their previous owners...only saw one problem on a Ford 302 that went from tradesman's ute to burnout king.

 

[rationull]

I'm not hearing any convincing arguments that rev matching is somehow different from revving unloaded in neutral (which .. err.. is what rev matching actually *is* from the engine's perspective).

I guess the only conclusion to come to is that unloaded revving, multiple times a day, is just fine for an engine (provided you're not bouncing off the rev limiter or whatever).

I'd just like to state again for the record that I was not asking about this because I had any particular interest in revving at stoplights like a tool, or because I'm worried about engine speeds.. it was just an interesting observation I had about conflicting advice you see on lots of forums.

 

[Black Bart]

Rev matching is the lessor of two evils.
While reving the engine with no load is not recommended down shifting without rev matching will soon wear out the synchronizers.
As long as you are not over reving it the engine will suffer little and the tranny will thank you for it.

 

[cmhj]

If reving in neutral is bad, then many engines are doomed. Many emission tests are done with the engine run, in neutral, at 2,500 RPM for a minimum of 30 sec.

 

[Black Bart]

Quote:

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If reving in neutral is bad, then many engines are doomed. Many emission tests are done with the engine run, in neutral, at 2,500 RPM for a minimum of 30 sec.

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I would consider 2500 as a high idle speed more than reving the engine.
Especially on the little engine in most of the cars now days 2500 is nothing you need to rev it nearly that high before engaging the clutch

 

[CaspianM]

If natural frequency/resonance frequency rpm of an engine is reached without load can do some damage over time. Having said that n.f is unsually near the idel speed or near the top by design. Also beaing oil film formation needs some load and rpm to reach its safe optimum thikness to avoid metal contact. As long as reving the engin is not too high (like over 2500 rpm) and is very brief (do not keep it reving without load) it should not be a problem.

 

[mechtech2]

I have always heard that free revving an engine was bad, and accepted it.
But I don't see why there should be any problem, if the normal rev limit is not exeeded.

 

[Matt_S]

There's a big difference between the blip on the throttle one would do when rev-matching and the extended vrooooom-vrooooom when revving in neutral. What I would call revving would last for a much longer duration than what I would think would be necessary for rev-matching.

This reminds me of the old guy at church who starts his engine, hits the gas and lets it bounce off the rev limiter for 10 - 15 seconds. It makes me cringe every time I hear him do it.