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

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

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

(OP)
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!

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

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.

 

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

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
 

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

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.

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

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.        

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

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

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/27500207+w750+st0/1005dp_02+diesel_power+2002_ford_f250_power_stroke_diesel_rebuild+cracked_rod.jpg

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

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

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

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

- Steve
 

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

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.
 

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

Why do it??

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

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

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
 

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

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

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

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.

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

" 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

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

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

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

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.

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

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
 

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

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.

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

There you go! I think Pat has nailed it.  That, I suspect, is the primary reason that it has been told/taught since as long as I can remember that "revving unloaded" is 'bad'.  It's really easy to exceed the redline...often by 'big numbers'.  Since we are, as Steve points out "off on our own" here...another little anecdote from "the old guy"...

I borrowed a '57 Ford V8 from the lot where I was working back in '58 and, like any teen I "went through the gears" several times obviously and, equally unknown to me, exceeded the "red line" by a bunch ending with a bent pushrod.  Hey, couldn't take it back like that so I stopped and removed the rocker cover, pulled the pushrod out and beat it straight with a big rock.  No one the wiser...Isn't it amazing that I have managed to live this long?

Rod winky smile  

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

The crew chief that told me that he thought that reving to an unloaded Redline was an "Automotive Urban Legend" [the real problem being the slow mechanical tach] also made the points;

1) There's no reason to do it.

2) Others will think you a fool [who needs that].

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


 A lot of people have pointed out that there is no point in actually holding an engine at high RPM for long periods.  But there is one "legitimate" excuse for doing this - and that is durability testing of camshaft and valve gear systems.  If you can't afford a Spintron machine it is a simple and cheap (even if you destroy an engine or two it still would be a lot cheaper than a Spintron) and a good real-life way to test.
 I have personally been involved in tests like this lasting half an hour or so.  One well-known cam grinder I know says that if a cam lasts even ten minutes in a test like this without any visible wear or damage it will last all day at Bathurst.
   I have heard that one well-known car maker did 24 hour continuous tests in this fashion.

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

I used to go to a lot of bike rallies some years back.  One of them always had a competition where you had to gamble on how long some old bike would last with its throttle taped open, screaming away in neutral.  It was always fun to watch it die.

- Steve
 

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

Many years ago the engineers at Chrysler published an article discussing rod loadings on their 426 Hemi.  A diagram that was included indicated that, IIRC, the max load on a rod occurred when the piston tried to follow the spent exhaust gasses out through the exhaust valve and was promptly yanked back down as the crank rotation went past TDC.  That is why it is very common for rods to break at the thinnest point at the head of the rod bolt (not likely in compression loading).

That being said, if you go to a drag race, any car that has a clutch has the rev limiter set somewhere north of 8K so that the driver can simply put the pedal to the metal after staging and be ready to drop the hammer as opposed to the old days when one had to carefully raise the RPMs to the ideal point which required looking carefully at the Tach rather than concentrating on the starting light.  I've never seen anybody lose an engine while staging.

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

Thinking a little more about this, I had a buddy back in the very early sixties (I know, showing my age) who worked for Chet Herbert Racing Cams.  He had one of the best running '55 Chevys around.  For a $100 bill, he would hold his engine at 10 grand for one minute, just to show off his cam designing prowess using the basically stock valve train.  He only made 200 bucks before everybody knew that they would lose their money.

Additionally, in the late '70s, I ran a car with a destroked small block that didn't make any power until it reached about 8K.  I had a set of (very rare) 7.14 gears in the rear that at 142 mph put me through the traps at soming around 12K.  I launched this car at 10K and never boke anything on the starting line.

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

Structurally if there was a big issue revving an engine with no load, then no OEM would undertake motoring tests on motoring dynos to ascertain an engines friction level.
Motoring engines and skip firing them will testing is normal procedure in all of the places I've worked and I've never once come across an issue in terms of catastrophic failure.
IMO it's one of those old wives tales- up there with " If you lean out your mixture too much at idle- you'll labour the engine"- you may reach lean combustion stability limits and go into misfire but you WON'T "labour the engine"!

www.auto-scape.com

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

well it isnt a good idear hence why most moddern engins have rev limmiters on them. It depends on the engine as to what ill effects can occer. as stated beffore the exhust stroke dose place a large amount of tensile stress on the rod, and as f=ma the more accelration (acc) the more force. The link between the load on the engine and the force on the conrod is that more load on the enging the lower the possible rpm and there for the lower the acc hence why you cant redline in top gear. anyawy it is unlikly that you will thow a rod in a standard road car as thay have large factors of safty but they will stretch. the engins max RPM is normaly limmited by one of two things the chocking point or the float speed. the coaking point is the place in the enging where the gas's 1st reach the speed of sound. if this is the subject engine then the most likly thing will be you will over heat and could blow a pump pipe or seal. if the float speed (the RPM where the cams are spinning so fast that the valves skip on them and dont close)is the limmiter, then you can run in to trobbel. the combination of above mentioned conrod strech and the valves always bing open plus the small clearances in the engine can result in the valve hiting the piston and breaking not good. but apart form all that it isnt good fore the trust bearings in your cluch.          

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

The op said nothing about holding at red line. Its no different with a load or without a load, reving to X rpm.  

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

I think there have been some reasonalby well-worded explanations above (correctly) explaining why unloaded reving does subject some components to higher loads.  Maybe you can tell us why you think they're wrong?

 

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