What does the redline on a rpm guage represent for a engine? Some say its the danger level and some say its overrevving. Ive taken my Jeep's V8 to the redline a few times and didnt see no side effects and I have a freind with a Honda that has it there at least once a day.

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99 C43
98 S420
99 C230K
01 C240

each engine has its maximum operating rpm rating. Usually, this is limited by the rpm at which the valves stop following the contours of the cam profile - valve float - which can prove catastrophic in an interference engine.

assuming the valves are not the limiting factor (e.g. desmodromic valvetrains, or Wankel engines), other factors will determine the redline, like the maximum load the reciprocating masses should impart on the bearings.

a designer would typically increase an engine's redline by:
* using high-strength, low-density materials for the reciprocating masses (valves, pistons, connecting rods); forged aluminum and titanium are typically used here
* making the valvetrain itself as direct and as simple as possible, with minimal reciprocating mass; double-overhead camshafts help in this area
* minimizing the engine stroke; the smaller the stroke, the less the linear speed of the pistons and con rods, and the lower the forces they impart on their bearings as they reciprocate

the indicated redline typically incorporates a margin below the absolute maximum for the engine. I've once read a manual for an old BMW 3series that specified a maximum SUSTAINED rpm, and a maximum ABSOLUTE rpm (what impressed me was that they were just 200rpm apart).

the new BMWs have variable redlines (implemented via LEDs on the tach), which are at their lower settings when the engine has not yet reached its operating temperature.

what is obvious is that one should not exceed the redline when pressing on the accelerator. What is less obvious is that you can easily exceed the redline when downshifting a manual gearbox (that's what those tickmarks are on a Benz's speedometer - the vehicle speed at which a given gear will have the engine at redline).

Danger and destruction lurk on the other side of the red line!
Been there done that.
I believe the manufacturer put that line there for a couple reasons. The first was to get the car out of warrenty in one piece. If you believe that then you may also want to stretch that number if you want to experience a little more power. But you really won't. All the red lines I have seen were set as the hp and torque were actually dropping. Look at the stats for your engine and you will see most redlines are there because your car will not be accelerating as well if you keep it in the lower gear. Better acceleration can be had by upshifting the transmission rather than trying to stretch the red line.
Mazda Rotary engines are very familiar to me. I have worked with the stock ones and modified a few too. Their curve was not like a piston engine but mazda had a fairly low red line because of gear rattle at high rpm's that fretted away the metal from the rotors and stationary gears. It occured if you let the engine keep a stable rpm above the red line.

You crack me up.

I blew a chevy 454, 4 bbl carb, composite prop, on an airboat a couple years ago. It was a very hot day and I was running at near the redline most of the morning. The engine stayed nearly continuously in danger of overheating. I could feel the boat getting stuck and gave it just a bit more....juice....rattle--BANG!

In think the oil had begun breaking down due to the heat. I really didn't go red any longer than I had previously so I think it was a lube problem that precipitated the catastrophic failure.

Funny thing, I heard the rattle through earplugs and shooters ear protectors. I think the rattle may have been the crankshaft giving-up. I turned my head to investigate just in time to see what looked like a large dark-amber flower bloom from the oil pan then get whipped into the props. That dark bloom was oil following a piston head toward the deck.

Its kind of funny now, but I shut off the engine and had a momentary feeling like maybe I hadn't just seen that.

There was portions of engine block, a few valve pieces, a portion of a crank shaft, a piston head and bits of journal bearing and other odd pieces scattered on the deck. The internal pieces came out of two places in teh oil pan and one big chunk of something came out of the side of the engine block. The aluminum hull got pitted by the impact of various pieces.

After saying the predictable, "Oh *****!", the next thought was how am I getting me and this boat out of this marsh?

I still have the badly scored piston head. Memento.

Botnst

Mark
Came up with this one from my MSN name did'nt ya

[slightly off topic]

My uncle in TX used to be part owner of the largest Bayliner dealership in Austin.

One day some newly hired kid decided he wanted to hear the engine on one of the biggest boats in the yard. He started it up and gave it a lot of gas.

Problem was, the boat wasn't in the water!! So much for engine coolant. That was a $20,000 mistake on his part! I think he lost his job!

OMG what a retard!

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99 C43
98 S420
99 C230K
01 C240

The red line on the tach is usually considered the "never exceed" rpm. Above this speed, severe damage will result rather rapidly. I't bright red to tell you "DON'T GO ANY FASTER".

You probably won't blow the crank the first time, but high speed causes incredible engine wear, so every time you hit redline you do some damage, and and every time you go past you do considerably more.

There is a limit to the speed at which one can yank the piston from a dead stop, down the cylinder at some fantastic velocity to another dead stop three inches away, and still keep the rod bearing cap attached. When it goes, you get parts blown out on the ground. bye-bye engine.

The rattle in the in the 454 was probably the rod bearings on the crank going loose just before one broke off. When it did, the rod was smashed through the side of the block, the piston was caught by the crank and slung, unattached, up to the head where it broke a valve off, and so forth.

The other failure mode is to get the valves out of synch and break one off when the piston hits it, bending the rod, often breaking another one, and so on.

Stay under the redline. If you want the engine to last any length of time (as in years rather than months, ) stay well away from it, preferably less that 3000 rpm.

Your money, your chioce!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Here's my question since all modern cars that I know of are computer controlled why CAN they exceed the limit in the first place? I don't know about other cars but my 360 automatically shifts at redline or stops depending what mode your in.

Other cars, when the ECM detects overspeed, either retard ignition timing, cut off fuel, or shut the motor down. Most Honda 4-cylinder engines are capable of running at redline for a few moments, since the max HP rating is so darn close to redline.

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1984 380SL-driving and service privileges.

when downshifting a manual gearbox at too high a vehicle speed for the target gear, you can exceed redline. Computer-controlled sequential gearboxes will actually delay (or prevent altogether) a requested downshift if the vehicle speed will cause redline to be exceeded.

with a manual gearbox, you can also exceed redline if you are going downhill on a steep slope in too low a gear.

though these are extreme cases, these can happen nonetheless ...

A freind of mine has the PT Cruiser (non-turbo) and for some reason all of the Chrysler engines love to rev high.

When passing a truck the other day it gently when into the redline even though we were passing the truck slowly.

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99 C43
98 S420
99 C230K
01 C240

I think that explains it for most cars, but in the 360 I cannot exceed redline no matter what I do. Yes I have tried.

I very often shift in my 16 valve around 4,000 rpm, especially when I'm out driving it fast for fun.

Are you saying that this is damaging the engine? I always figured the engine, especially on a sports car like this, would be designed for such rpms.