Is reving an engine to redline without a load (in neutral) bad? 2

[foscoe944]

Hi,

I'm new to the forum. I'm not in the engineering field, but I race and track BMW's as a hobby. I'm wondering if anyone here can clear up something for me. I've heard that it is bad to rev an engine to red line if there is no load on the engine (i.e. in neutral). Is this true and if so, why?

Thanks!

 

[heulum]

I guess it's not bad but it's not good either. It doesn't hurt engine unless it is overheating due air not cooling the radiator and overheated oil.

 

[SomptingGuy]

It does mess with the piston/crank load calcs somewhat if they've not been done at this condition. I've revved the nads off typical engines at no load for minutes on-end (zero load transients) before (as part of a test) with no real evidence of damage.

- Steve

 

[ivymike]

Not the engines you're thinking of, but generators will commonly run at their rated speed with no load and no harm done if it's not for hours at a time.

 

[HRHtex]

There certainly was once a view that holding an engine at peak RPM in an unloaded state was potentially dangerous.
The idea being that there was little compressed gas in the combustion chamber to "cushion" the piston at the top of its compression stroke.
Popular legend has that the very long and partly downhill Conrod Straight on the Bathurst circuit (in Oz) was so named because drivers would worry about holding the engine at peak revs for so long and lift off the throttle to give the engine a rest - resulting in a conrod immediately breaking.
Just how dangerous it is to the engine running unloaded at peak revs is I don't know - but is probably wise to avoid doing this if possible.

 

[patprimmer]

Load may influence a:f and ignition timing vs same rpm at load. It depends on the tune as to whether or not it is a problem.

Also high manifold vacuum at high rpm does reduce the pressure over the piston as it passes over TDC is reduced so the stretch on the rods is higher at least on the power strokes and probably also on the exhaust strokes as there is little exhaust gas to displace.

If the engine is tuned so there is no high speed lean out and increased spark advance, then the engine should be safe enough, but the question is why rev to red line for no real reason as any engine will have increased stress and wear and will use up a portion of the engine life for no benefit.

Regards
Pat
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[Tmoose]

As pointed out by others, redline with a 4 stroke at exhaust TDC subjects the rod to ~ max tensile loads, but running no load would likely make the former firing TDC load similar, with a slight discount of at least compression pressure X piston area. The end result would ~ double the rate at which those max loads are accumulated.

I don't think the valve train loads are much different under power, except maybe the force required to crack open the exhaust valve.

A lot, if not most rod breakage (and metallic part failure in general) is the final stage of progressive cracking from fatigue. In the days when lots of passenger cars were still using forged steel rods I used to magnaflux rods at a hot rod engine shop. It was common to find small "indications" in the area brutalized by factory machining to accept the rod bolt head. That was true of abused engines as well as used passenger engines. It was much less common to find indications along the I-beam or around the wrist pin eye.

One of the differences between specialty race rods and poor pitiful passenger rods was the bolt seat detail.
Studebaker sixes were actually FSB'd shortly after they adopted a sharp corner in the bolt seat and bod breakage began.
For years Chevy V8s had a gruesome torn finish on a fairly generous radius.

http://image.hotrod.com/f/32887725/hrdp_9903_07_+chevy_connecting_rods+arbor_press.jpg

High performance Small block Fords had a 3D Curve to accept football shaped bolt heads. Generelly speaking having the rod material wrap around the bolt is "better" (lower stress concentration) than a straight across notch.

http://partsnetworkonline.com/images/gn_Ford/DSC07018_02.jpg

In addition to gorgeous materials and superb preparation Carrillo rods used a cap screw, and the area around the tapped hole was sculptured.

https://www.theboombopshop.com/v/vspfiles/photos/carrillo/carrillo-rod-1.jpg

Powdered metal con rods are popular today, and often include low stress details in bolt area of the rod.
(Factory PM rod is the upper rod in this picture.)

http://image.hotrod.com/f/9278752+w750+st0/113_0708_13_z+426_hemi+rods.jpg

Even good details are not always enough.

http://image.dieselpowermag.com/f/2...0_power_stroke_diesel_rebuild+cracked_rod.jpg

 

[evelrod]

Does revving to the red line overstress an engine? I dunno. From an early age I was taught not to do it. As an engine builder I find it often necessary but still uncomfortable. Every (well, almost) engine I've blown and, I have scattered more than a few, failure occurred as I lifted for a gear change or at the end of a high speed run...Some were 'more' than spectacular I must say. Most modern street engines are rpm limited in neutral under no load. Every engine manual I'm familiar with recommends that revving under no load conditions be avoided.

From a practical point I do not do it on my engines. However, I have a couple friends, non engineers/non engine builders that rev their race engines (Fiats) to red line and hold them their while "adjusting" whatever. Scares me just to watch.

Rod

 

[SomptingGuy]

Lifting off at high revs (negative load) is a bit different from revving under no load.

- Steve

 

[ivymike]

forced overspeed is indeed a different situation than HSNL operation. Any engine should be able to tolerate a little bit of forced overspeed (certain ag. ones that I'm familiar with are designed to withstand more than 50% forced overspeed - so a 2400rpm engine can be forced to 3600rpm indefinitely with no permanant damage (although I imagine you'd pump a lot of oil out the exhaust if you really tested my "indefinitely"). I expect that 50% overspeed capability is the exception in the vehicle world, but I'd expect at least some amount on all engines... how much depends on just how close you want to go on size/weight/cost.

 

[MiketheEngineer]

Why do it??

Can't be good - might be bad..... Why push the envelope if there is no good reason??

 

[SomptingGuy]

Standing-start races perhaps? Although they would usually not be at the red-line, but at the speed of peak torque, often held for several seconds before the lights go out.

- Steve

 

[ivymike]

probably a good place to start or end a sweep when collecting torsional vibration data

 

[sreid]

What I was told was that in the era of mechanical tachometers there was a big delay in the RPM reading. So if you bliped the throttle to rev the engine to redline you would actually exceed the max RPM by lot.

 

[Tmoose]

" in the era of mechanical tachometers there was a big delay in the RPM reading. "

The 1966 movie "Grand Prix" comes to mind .

Dan T

 

[evelrod]

Steve, I only meant to point out the fact that I have never lost an engine by simply revving it to red line in a no load scenario. No matter. It still makes me uncomfortable to do so on any engine. As I said, most new engine have multi staged rev limiters and revving it to redline unloaded is not normally possible.

Still my two Fiat/Abarth racers, both old dudes like me, have peaked their engines since, forever with no failures. I don't have a dime invested in their race cars but I still don't like to watch them do it.

Just one more little tid bit. I was "tuning" the carb on my sons old 318 Dodge pickup leaning over the RF fender when with just a "slight" rev (???) one of the connecting rod poked it's little end out the block just a few inches below my CHIN!!!!! Gets your attention.

Rod

 

[dicer]

I say its okay. And much easier on stuff than going down hill and throttle closed redline. I'd say the important thing is not to close the throttle quickly.

 

[patprimmer]

Engine rpm can increase very quickly under no load, and a quick throttle off might be necessary to avoid going well over red line

Regards
Pat
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[SomptingGuy]

We are well beyond the OP here and having our own debate, which may be a good thing. It is interesting though.

One phrase that some of our petrol-heads here use is "bore polishing". Don't really know enough about the trib of pistons/rings/liners to comment, but they think it's an issue with unloaded running at high speeds.



- Steve

 

[ivymike]

bore polishing is a problem with unloaded running regardless of speed (driven by piston carbon buildup) but could be worse in some engines at higher speeds due to increased propensity for carbon-bore contact as a result of greater piston energy/tilt.