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!

 

[evelrod]

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 ;-)

 

[sreid]

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].

 

[HRHtex]

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.

 

[SomptingGuy]

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

 

[madferraristi]

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.

 

[madferraristi]

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.

 

[Marquis]

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"!

http://www.auto-scape.com

 

[kk72]

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.

 

[dicer]

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

 

[ivymike]

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?