Explaining the red line

[HawkeyeScott]

Since I began driving 15 years ago, I was told to keep the engine from revving to the red line. I would say in 175K miles or so of driving, I've only hit it a handful of times. But no one ever explained to me what exactly the redline is and how it's determined in a certain application. Why is at, say, 5200 RPM on my Jeep but can be much higher on other cars? And what is the breaking/engine injury that most often occurs if you push the tach too far past the redline? Basic stuff to most but I am curious.

 

[StevieC]

As I understand it... Red-line is the maximum amount of revolutions that an engine can sustain for a prolonged period of time during operation without mechanical failure.

 

[Colt45ws]

Redline is the max RPM of the engine it can handle before internal parts like connecting rods start becoming dis-connecting rods. The reason you can have rice burners that have redlines in the 10k zip code and your Jeep cant handle much over 5 and a quarter is they are built differently. The high horsepower per liter, low torque engine in the first example has a shorter piston stroke, shorter connecting rods, which reduces the stress on them. Your Jeep engine is built for different duty, lower HP/L higher torque with longer piston stroke, longer rods, incresing stress.. Additionally you have different materials to make the connecting rods from the really cheap and weak pressed powdered rods, the typical cast rods, strong forged rods and racing duty billet rods.
And thats just one part, you can also have lower limits due to valve springs not able to keep up with the cam (valves hang open and get smacked by the piston), or even the automagic transmission torque converter, like in my car. The engine itself in my car can take a little over 6000 safely, but its rev limited to 5500, if I recall correctly, as above that the 12" TC starts growing.
I would not worry about hitting it on a automatic car, as it will shift before the bang point, and if something happens where it does not, there is another safety, the rev limiter, built into the computer to keep you from hitting it.

 

[morris]

moe in wichita ks
i read an artical by a well known engine builder. he said that just because the engine doesnt disasimble itself when you go past red line onetime doesnt mean it isnt damage, and it may fail latter on, even at a lower rpm.

 

[HawkeyeScott]

Onmo, that's just what I'm looking for. Thanks a lot.

 

[Colt45ws]

Yup. If you go past it once, there can be latent damage which may not show up until later, maybe even after you forgot about the original incident....
It is hard to do on most cars nowadays. Although I did it once on my Mom's car. Its a Saturn with a manual transmission, with a rev limit of 6250, I think. I missed a downshift 5-4 and went 5-2 instead. Tach maxed out at 8000. Didnt blow. But, Im waiting to see what happens.

 

[StevieC]

You must let the clutch out too fast, should always ease into it to see what kind of RPM response you start to get and can catch it before it gets that high...

 

[frank83]

Can you even go past redline in newer automatic cars?

 

[Colt45ws]

I did practically drop it. I was a little steamed and wanted around this truck. Dropped it as I went WOT then realized I missed the gear I wanted. Oh well.

 

[Colt45ws]

Originally Posted By: frank83
Can you even go past redline in newer automatic cars?

Not without reprogramming...

 

[sciphi]

Heh, heh...Reprogramming...

I've taken my Buick's 3800 SII up to 5700 RPM's before with a reprogrammed computer. Probably got a little higher as the pedal was mashed still at the 1-2 shift. Redline is 6200 both stock and with this computer, IIRC.

 

[BuickGN]

Originally Posted By: sciphi
Heh, heh...Reprogramming...

I've taken my Buick's 3800 SII up to 5700 RPM's before with a reprogrammed computer. Probably got a little higher as the pedal was mashed still at the 1-2 shift. Redline is 6200 both stock and with this computer, IIRC.


You guys must have a stronger bottom end than my old 3.8L. Stock rev limiter was at 5250rpm and it's now 6,200rpm. I've now got the cam/springs/heads to make power at a higher rpm but it's still scary taking the stock bottom (except pistons) past 6,000rpm.

My TL didn't hit the rev limiter until 7,100rpm according to the dyno run. Talk about a totally different feel between the two cars.

To the original question, a thrown rod, spun rod bearing, or bent valves are the usual result of excessive rpm. Piston speed and materials used in components are the biggest factors in maximum rpm. Compare the stroke of an F1 car and it's 16,000+ redline to a typical truck's stroke and you'll see what I'm talking about.

 

[d00df00d]

Another tidbit: Different applications have different margins. For example:

- As Onmo described, truck engines usually hate playing near redline -- which is fine, because they make all their torque down low.

- Most car engines tolerate it but only for short periods of time. They prefer the midrange.

- Most BMW engines could sit at red line all day every day for years, as long as the car is always moving fast enough to keep air moving over the radiator.

- A Mazda RX-8's engine will eventually break if you don't redline it often...

 

[OVERKILL]

The red line can also be superficial.

The red line on a 302HO is 5,500RPM. The rotating assembly is good for closer to 7K. But in stock trim, it doesn't make power past 5K anyways.

On a 3rd at 5K to 2nd (went for 4th, missed it), I've hit well over 7K for a split second or two. Never did any damage.

As one poster stated: It is the "safe limit" for the stock parts. The 302's weak spot is the stock valve springs. Limiter is at 6,250 anyways though.

You will most likely hit rev limiter before you reach an RPM that would cause parts failure. Unless something is extremely worn out.... Like cooked valve springs.

 

[Colt45ws]

Yeah, youll find that type of stuff on older engines. Anymore the rotating assembly is designed with the planned powerband in mind.

 

[Gary Allan]

Inertia.

 

[ClarkB]

Red line is often displayed as being around 5200 RPMs or considerably higher. An engine that is designed for high-end HP will be designed to rev past 5200 but one that is made for low-end grunt (think trucks) will almost always have the red line around 5200.

This is due to (from what I can find) the fact that the dynamometers only measure torque, HP is calculated from torque and RPMs. Torque tends to fall off after 5200 RPMs while HP can continue to climb.

Clark

 

[Colt45ws]

HP = Tq x RPM / 5250 That equation is why HP and torque always cross at 5250RPM. Below, torque is higher, above, HP is higher.

 

[rationull]

Originally Posted By: d00df00d

- As Onmo described, truck engines usually hate playing near redline -- which is fine, because they make all their torque down low.

- Most car engines tolerate it but only for short periods of time. They prefer the midrange.


But what exactly does this mean? If truck engines "hate" to be near redline and most car engines "prefer the midrange" does that mean these engines will sustain damage if run continuously in the ranges they don't "like" or just that they subjectively don't feel or sound as good when they're out of their desired speed ranges?

Or are you just referring to where the engines are making the most of their powerbands (e.g. truck engines are tuned for low end torque and most car engines are tuned for high power in the midrange)?

 

[Colt45ws]

He is more than likely referring to powerband.