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Red line on the tach

I was always curious about the red markings on the tach. Let’s say it is red at 6600 rpm. I assume the engine will not blow up at 6601, or even at 6610. So what will happen if it is tuning on the red line for a while? Is there a correlation with temp and rpm in that range? Just theoretically will it run at 6800 for a minute? Or 7000 for 40 sec? I am just curious. For obvious reasons I do not have an intention to try any of thus out on my car…

It’s hard to say at what RPM it will blow. The engineers set the redline with some margin of safety, and on modern cars, there is usually a fuel cut off slightly above the redline. At 6,000 RPM, the crankshaft is spinning 100 times a second, and it is yanking the pistons back and forth accordingly. There is considerable strain on the connecting rods, which have to endure tensile, compression, and bending strain that rises with RPM. Also strains the wrist pins.

Red Line on the tach is telling you that your motor is a risk if you rev it beyond the red line. In lower gears, 1st and 2nd, you can go well beyond the red line quickly unless the motor is protected by a rev limiter. In 4th, 5th, or 6th you would likely get up to the red line only if you are racing as you would be way over most speed limits. In higher gears you would likely not risk the motor as much, but if you sustained those high revs for a long time (10 or 15 min.) you could make some part of the motor break and at those revs the motor would then just fly apart. Most often a piston rod gives out and the piston crashes into the head and locks up, or shots out the bottom of the motor through the oil pan and into the road.

Most street cars now are computer controlled and programmed to not allow the motor to rev much beyond the red line. Most motors that have good oil and are in good condition car run right up to the red line without damage. The power “sweet spot” is often 3000 rpm up to about 5,500 rpm. From 5,500 up to the red line is where there is a drop off of power so running beyond the red line isn’t necessary anyway.

Every vehicle I’ve owned since 2000 had redline limiters. The engine would start shutting fuel off to engine when got near the readline.

With older cars I’ve gone past redline…but not for long. No immediate engine damage…not sure if there was long term damage though. It’s NOT something you want to do. I don’t even come close to redlining anymore. Not worth it in my opinion.

This might not be the best analogy. Think of a paper clip. When you slip it over a couple sheets of paper the wire is stressed. Remove the clip. It returns to its original shape. If you stick to clipping only a few sheets the paper clip will last almost forever. Start increasing the number of paper sheets. At some point the clip won’t break but it won’t return completely to its original shape either. You have exceeded the paper clip’s proportional limit. Most people know that if you twist a paper clip enough enough times it will break. This is a fatigue failure. If a paper clip had a red line, it would be the number of sheets of paper the clip’s designer determined could be used, still function correctly and not fail in a reasonable lifetime. A car’s red line is a sort of grey area where the engine won’t break but may begin to experience abnormal wear and a decreased design life. I’ve had a couple of cars that had a yellow line in addition to the red line. Running the engine at red line was OK for short periods. The yellow line, set about 600 rpm lower, was the limit for sustainedoperation. In the days before electronic rev limiting was common the red line was usually set lower to provide a larger safety factor for the exuberant driver. Now that rev limiting is more common the designers can place the red line closer to the point where the engine will begin to experience abnormal wear making the engine lighter and cheaper.

That’s a very good analogy. It explains fatigue really well.
I’ll have to remember that one.

Redline is there for a reason. There’s probably some margin but you don’t want to be too close to it.

Redlines are under ideal conditions. I doubt that a motor that hasn’t had it’s oil changed frequently or isn’t completely warm or…has the same red line indicated as it does practically. Safety margins disappear with age a neglect…just like our own bodies.

If you ever blew a rod in a snow blower and had it come through the block, you wouldn’t run an engine anywhere near redline out of respect for mechanical devices.

Running an automobile engine at its redline for any length of time is likely to cause some damage. Ford, GM and Mopar engines installed in trucks are red lined at <4,000 rpm when the same basic engine installed in a car is red lined at 4,800 to 5,600. The truck rating is apparently for continuous operation. I just can’t imagine a 454 holding itself together long at 5,600 rpm.

And those snow blowers can ice the throttle and prevent the governor from controlling the rpm with catastrophic results.

That paper clip comparison is quite good, MTraveler.

Usually, what happens first, the VALVES start to float, their springs unable to control their motion…This can make a lot of noise but most engines survive a moderate over-rev event… If the driver persists, parts are likely to start flying in all directions…

There are a lot of reasons that red lines are chosen where they are:
-Some engines can take it mechanically, but there is a huge power drop off because the engine can’t “breathe” at that point.
-Some may not have enough lubrication beyond that point to survive for long.
-Some may be destroyed by the inertia from the pistons/rods.
-Some may be able to go higher, but the valve train can’t keep up (valve float like Caddyman said)

I think you’re safe hitting redline if you have a healthy engine, but you’re not only putting stress on the engine, but on everything under the hood that’s spinning fast, such as your water pump, alternator, power steering, air conditioning, flywheel or flex plate, input shaft of your transmission, etc.

You can bet that it’s a combination of factors. When manufacturers develop engines, they run the engines screaming at redline on a dynamometer to see how long they can take it and what problems develop. This is part of how the maximum safe RPM is chosen.

I rode with a guy in a junkyard truck once. (a Chevy I think) He had some unknown problem with his fast idle cam on the carburetor. (was a mid 70s truck with a 6-cylinder) To unstick the fast idle, he repeatedly put the vehicle in neutral and revved the thing as high as it would go. I don’t know how he thought this would solve the problem. It had no tach, but he had to be hitting 7,000 or higher easily by the sound of this ungoverned engine. I’m guessing redline was probably about 4,500. I kept cringing, waiting for something to blow up, but it held together. He probably did this every day too.

Thanks for all the educative comments… Of course I do not plan to try this on my car and that is why I started this discussion in the first place.

All he had to do was unscrew the fast idle cam screw a little. This was kind of a problem back then.