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View Full Version : Are high RPMs/redline in Neutral ok?


CMOS
01-08-2003, 07:37 AM
Let's say I want to find out where the fuel cutoff cuts in on my car (5 sp manual).

Can I cause any damage/undue wear by revving to the cutoff while parked in neutral? Somehow, I seem to remember hearing something about this possibly causing damage.

I know I can also do this in gear, but I'm trying to keep my eyes on the road while driving like bandit at the redline!

Thanks for any input!

-CMOS

Mr. The Edge
01-08-2003, 07:38 AM
high revs with no load is not recommended

johnlew
01-08-2003, 07:49 AM
Only if you want to ruin your engine.

nate
01-08-2003, 07:55 AM
You don't have to look, you will feel the cutoff...

CMOS
01-08-2003, 08:37 AM
Originally posted by nate328Ci
You don't have to look, you will feel the cutoff...

Indeed I have felt it! Feels like someone pulling the rug out from under me.

Here's why I'm asking:
I was plotting to get the Dinan CAI+Stage II software. (I say plotting b/c its expensive enough that my wife will kill me if I come home with this.)

I'm imagining going to the dealer afterwards and the dealer unwittingly overwrites my software with the stock dealer software.

When I get my car back, how do a verify they didn't do this? I should be able to feel a different throttle response with the factory software, but that's somewhat subjective. I'm trying to think of some way I can objectively verify that I've still got Dinan's software.

nate
01-08-2003, 08:47 AM
Just see if the tach goes past 6500...

ezsce46
01-08-2003, 09:02 AM
155mph:D

Jetfire
01-08-2003, 09:25 AM
Originally posted by johnlew
Only if you want to ruin your engine.

Hmm, I've heard of this before but have never confirmed it. Why is high RPM/no load bad for your engine? And does that mean that popping into neutral (and braking) for a traffic light is bad?

StLChris
01-08-2003, 09:31 AM
So if high revs with no load are bad, then is a rev-matching throttle blip bad? At what point am I causing damage to the engine? 4000RPM? 5000? 6000?

johnlew
01-08-2003, 09:32 AM
Originally posted by Jetfire
Hmm, I've heard of this before but have never confirmed it. Why is high RPM/no load bad for your engine? And does that mean that popping into neutral (and braking) for a traffic light is bad?

I can't give you a technical answer, although technicians say it's bad and common sense tells me it's bad. Your example is not revving the engine but allowing it to drop back to idle from a high RPM. This will not hurt.

Feel free to test the question and report back the results though.

Nick325xiT 5spd
01-08-2003, 09:55 AM
The impression I got was that high RPMs with no load were a bad thing primarily over a protracted period of time... And largely because the engine would overheat/not have enough to breath.

Regardless, I would say that it's quite harmless for a couple seconds.

The HACK
01-08-2003, 10:09 AM
Originally posted by Nick325xiT 5spd
The impression I got was that high RPMs with no load were a bad thing primarily over a protracted period of time... And largely because the engine would overheat/not have enough to breath.

Regardless, I would say that it's quite harmless for a couple seconds.

I'd think this is the best rational...There's not enough air flowing through to give the engine proper breathing room.

I wonder if this is just a big, old wife's tale though.

Jetfire
01-08-2003, 10:30 AM
Originally posted by The HACK
I'd think this is the best rational...There's not enough air flowing through to give the engine proper breathing room.

I wonder if this is just a big, old wife's tale though.

Sounds plausible, but I'm with you on the wife's tale thing. Although, the sucking action of the engine's intake would probably let in more than enough air, and I would think that the coolant system is primarily responsible for managing engine temps and not air flowing around the engine on the highway.

Of course, there really isn't a need to run my engine at 6500 RPM in neutral for any length of time. Even if I were showing off my (stock) exhaust, anything more than a blip would just be dumb.

Kaz
01-08-2003, 10:51 AM
Since we don't seem to be sure about redlining at no-load, why not just do it in 2nd or something?

And the revlimiter is subtle enough on a E46 at least that you'll may not notice it for a second or two. Its a very soft bounce as opposed to the complete fuel cutoff or hard stutter that many other cars have.

·clyde·
01-08-2003, 11:30 AM
I would hit the limiter in first gear of my E30 318 regularly. The first few times I was afraid that I had just killed the car, the cutoff was that short. In the wagon, I've hit the limiter in first a couple times while autocrossing and it was subtle as Kaz mentions...much less severe than the DSC killing power.

Jspeed
01-08-2003, 12:27 PM
The analogy I read somewhere was that this:
It's safer for a person to run upstairs full speed, but if he tries to do the same going downstairs he's going to trip.

My interpretation is that you need a certain level of resistance to balance the forces.

Pinecone
01-08-2003, 01:10 PM
It is bad enough that BMW SPECIFICALLY warns against doing this with E46 M3s.

It loads the bearings differently and may cause premature bearing failure.

A quick blip for rev matching is not a big problem. But consistantly running the engine to the upper part of the rev band in neutral is NOT good.

As too cooling that is not a problem. Since the engine is under no load, very little pwoer is being produced, therefore very little heat. The fan will move enough air to prevent overheating for any reasonable amount of time (less than several minutes).

As to testing your car, much easier, run it to redline in 1st or 2nd. Most cars do about 60 - 65 MPH in 2nd at redline.

Nick325xiT 5spd
01-08-2003, 01:13 PM
With all due respect, M3s are not exactly known for resilient bearings. :p

CD-55
01-10-2003, 09:40 AM
High RPMs are obviously stressful for any engine, but I have to believe that an engine under load is MORE stressed than an unloaded engine.

Someone mentioned that a free wheeling engine in Neutral may not get enough air to cool, but it is also not burning as much fuel… I guess because of added fuel and pressure a loaded engine would be much hotter.

It is probably a bad idea to spin any engine at high RPM’s for a long time, but it is perfectly balanced and well lubricated, IT IS NOT GOING TO RIP-APART OR GRIND INTO PIECES! What could possibly go wrong that would not have failed if the engine was loaded?

Every way I have thought about it, a loaded engine has many parts that would be under a lot more stress.

I say rev them like you drive them!

Jspeed
01-10-2003, 11:18 AM
Originally posted by Pinecone
It is bad enough that BMW SPECIFICALLY warns against doing this with E46 M3s...
Some cars also have additional rev limiters when the transmission is in neutral. From my personal experience I noticed the Mercedes ML can't rev to redline in neutral, as well as the previous generation Accord V6.

CD-55
01-10-2003, 12:09 PM
Originally posted by Jspeed
Some cars also have additional rev limiters when the transmission is in neutral. From my personal experience I noticed the Mercedes ML can't rev to redline in neutral, as well as the previous generation Accord V6.

Yeah, I know that is a fact that these are in place... I just cannot understand why.

About 10-years ago I knew this guy who use to rev-up his T-Bird, then drop it into "DRIVE". Maybe this is why?

rwg
01-10-2003, 12:13 PM
Originally posted by CD-55
High RPMs are obviously stressful for any engine, but I have to believe that an engine under load is MORE stressed than an unloaded engine.

Someone mentioned that a free wheeling engine in Neutral may not get enough air to cool, but it is also not burning as much fuel… I guess because of added fuel and pressure a loaded engine would be much hotter.

It is probably a bad idea to spin any engine at high RPM’s for a long time, but it is perfectly balanced and well lubricated, IT IS NOT GOING TO RIP-APART OR GRIND INTO PIECES! What could possibly go wrong that would not have failed if the engine was loaded?

Every way I have thought about it, a loaded engine has many parts that would be under a lot more stress.

I say rev them like you drive them!

Cars overheat at idle. It's b/c there isn't enough air moving through the radiator at idle speeds (even with a properly working fan, sometimes - but the overheating from the bad fans didn't occur on the highway).

As for an engine under load having more stress - well I think that's the point and the reason why idle red lines are a bad idea. That stress CHANGES things. It changes how parts work together, it affects alignment of things, it has a huge impact on how stuff works together. The engine was designed to be under load. I have no doubt that an engine could be designed to redline with no load, but ours are not.

CD-55
01-10-2003, 01:05 PM
Originally posted by rwg
snip

As for an engine under load having more stress - well I think that's the point and the reason why idle red lines are a bad idea. That stress CHANGES things. It changes how parts work together, it affects alignment of things, it has a huge impact on how stuff works together. The engine was designed to be under load. I have no doubt that an engine could be designed to redline with no load, but ours are not.

Which engine parts in particular are more likely to fail in neutral than at full throttle?

If neutral at high RPMs is worse than full throttle at high RPMs, if you were to close the throttle at high RPM… would that be even worse than just neutral, since the load forces would be in the opposite direction for many engine parts

Jspeed
01-10-2003, 01:46 PM
Originally posted by CD-55
Yeah, I know that is a fact that these are in place... I just cannot understand why.

About 10-years ago I knew this guy who use to rev-up his T-Bird, then drop it into "DRIVE". Maybe this is why?
That's certainly plausible. Another possible reason is that the engine revs so quickly in neutral that the standard redline limiter might not catch it in time.

bls
01-10-2003, 04:14 PM
The only rationale I can think of is that if the engine is running with a load, then the force between the crankshaft and piston rod bearing will all be on one side. If there's no load, then the crank will bang back and forth between the top and bottom bearings, to the extent that the tolerance allows it.

Think of it like being on a bicycle where there's no ratchet gear on the wheel, so you can't coast: the wheel and pedals always turn together. If you're pedaling hard, the pressure is on the bottom of your foot. You can even "engine brake" on such a bike, by pulling your feet up against the pedal clips. But if the clips aren't tight, and you just relax your legs while cruising at high speed, your feet will bang around in the clips.

To make the analogy complete, you should imagine that your feet wrap around the pedal, instead of the pedal wrapping around your feet. Also, you should imagine yourself as an insect, so you're pedaling with six feet instead of two.

mwette
01-10-2003, 05:56 PM
Why would revving the engine at high RPM under no load cause damage? Here is my guess.

The engine has a flywheel to smooth the high frequency dynamics caused by the fact that the engine is getting power intermittently (e.g., six pisons generating power). A load on the engine may perform a similar effect to adding mass to the flywheel. If you rev the engine at high RPM you are generating lots of high frequency vibrations throughout the engine. If the engine is under load I would guess that this dampens, or even changes the frequency of the induced vibrations. Again, just a guess.

Matt

AF
01-10-2003, 08:01 PM
Originally posted by CD-55
High RPMs are obviously stressful for any engine, but I have to believe that an engine under load is MORE stressed than an unloaded engine.



I am no mechanic but if high rpm's are stressful on an engine, then the people in Germany driving at high speeds whom are at a pretty high rpm on a regular basis would be doing a lot of damage to their cars.

I could be wrong but another example of engines running at high rpm's for a pretty long period of time would be a boat engine and as you might know, some of the I/O (inboard/outboard) engine's are based on car engines.


Let me just say again, I know very little about mechanics so I am just speculating here and could be totally wrong.

AF
01-10-2003, 08:03 PM
Originally posted by CMOS
Indeed I have felt it! Feels like someone pulling the rug out from under me.

Here's why I'm asking:
I was plotting to get the Dinan CAI+Stage II software. (I say plotting b/c its expensive enough that my wife will kill me if I come home with this.)

I'm imagining going to the dealer afterwards and the dealer unwittingly overwrites my software with the stock dealer software.

When I get my car back, how do a verify they didn't do this? I should be able to feel a different throttle response with the factory software, but that's somewhat subjective. I'm trying to think of some way I can objectively verify that I've still got Dinan's software.


Why not have a passenger sit in the backseat directly behind you and rev your car to redline while driving and let them tell you when the cut-off happened.

I say they should sit behind you because if they sit in the passengers seat, the angle they see your dashboard might lead them to give you an incorrect reading.

mullhaupt
01-10-2003, 09:51 PM
I have read and heard that the E36 had problems with people miss shifting and the cars were over revved and the valves hit the pistons. I have not seen this personally but heard it was happening

Jspeed
01-11-2003, 12:44 AM
Originally posted by bls
...Think of it like being on a bicycle...
You just gave me another idea. If flooring the throttle in gear is like pushing down on the pedals really hard forward, then doing the same in neutral would be like pedaling really hard backwards. With this analogy I can clearly imagine how I would hurt myself pedaling really hard w/o resistance.

visor
01-11-2003, 01:00 AM
Another analogy: for the sports players out there. Imagine swinging a racket/bat at a ball in tennis/squash/baseball/etc with the maximum force that you can (ie. "redline"). It's great if you connect and transfer the force of your swing into the ball's kinetic energy, but what if you miss (ie. redline with no load)? You would end up not transferring that momentum/energy and end up with a pretty sore arm wrapped around your face!

The same goes for the engine at redline with no load. Maximum mechanical energy is being produced from gas combustion, but there is no place for that energy to be transferred (because the driveshaft is offline), hence some fragile moving parts are going to bear the brunt of this excess energy. Like valves/lifters/bearings etc...