So what is the real redline?

[fmaxwell]

Looking at the specs of the smart fortwo reveals that it produces peak horsepower at 5800 RPM, but I could not find a documented redline.

The smart factory tach (that I just installed) shows a redline of 6000 RPM. That's referenced in some reviews as being the redline:

Edmunds Test Drive: 2008 Smart Fortwo

However, when shifting in "drive" mode under full acceleration, or when the ECU is overriding too low a gear selection in manual mode (to prevent over-revving), the transmission shifts at 6500 RPM. This is the figure given in other reviews as the redline:

It's so cute! But auto consumers can do better / Smart Fortwo has rivals for mileage, price -- and safety

Road & Track magazine has the weirdest take on it, claiming that the car has a redline of 6000 RPM and a limiter at 6500 RPM:

http://web2.roadandtrack.com/assets/...8_smart_dp.pdf

I've always taken the term "redline" to be the maximum RPM that an engine should ever see in operation -- a point to not be crossed. So if the redline is 6000 RPM, why does the transmission ever hold the gear until 6500 RPM? If the engine throws a rod at 6450 RPM under heavy acceleration in drive mode, is the fact that the car was running above redline (by design, since it chooses when to shift) grounds to deny a warranty claim?

Note: Example reviews shown were chosen solely because of the engine redlines that they specified. No endorsement of the accuracy or of the opinions expressed in the reviews was implied or intended.

[jwight]

Agree with your comments about operation in "D" - you can easily hold the rpm into the red zone on the tach (and without triggering any sort of rev limiter.) I'm not sure what the real answer is, although I try and limit those excursions past 6K rpm as much as possible.

[chieftmc]

RED LINE does NOT mean the most rpms! The red line is the point where the motor makes the most horsepower available. Spinning the engine past that point will NOT produce anymore horsepower!

[jwight]

Originally Posted by chieftmc

RED LINE does NOT mean the most rpms! The red line is the point where the motor makes the most horsepower available. Spinning the engine past that point will NOT produce anymore horsepower!

Chief, have to respectfully disagree on this one. Not that Wikipedia is always correct but:

Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine.[1] The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of components, and balance of components.

I know our Vette is not redlined at the max hp rpm, but at the "do not exceed" rpm.

[mmmeat]

Redline is the edge of the envelope where the engine can operate without damaging components or the engine.

It is not the "maximum RPM that an engine should ever see in operation" nor is it the "red line is the point where the motor makes the most horsepower available." Nobody who believes either one of those statements should be driving a car.

All modern engines with an ECU and an automatic transmission will automatically shift to the next highest gear if the motor approaches redline or, if the RPMs hit a preset number, the ECU will cut fuel to the engine until the engine is out of redline.

Your pal,
Meat.

[fmaxwell]

Originally Posted by chieftmc

RED LINE does NOT mean the most rpms! The red line is the point where the motor makes the most horsepower available. Spinning the engine past that point will NOT produce anymore horsepower!

Sorry to disagree, but that's not the case at all. Piston speed usually determines redline, though sometimes it's determined by valve float (the inability of the valve springs to cause the valve actuation mechanism to remain in contact with the cam). It's the point at which there is a danger of damage to the motor. The mass of the pistons, rods, and valve train all have an impact on this, with lighter, forged pistons usually being capable of running at higher RPMs (piston speed) safely.

Horsepower is simply (torque x RPM) / 5252. An engine which is tuned for low-end torque makes its peak horsepower well below its redline, while one tuned for maximum horsepower will have peak horsepower at redline. It might make more if one ran it beyond redline, but at a risk of engine damage). Such engines are, generally, for competition and are ill-suited to use in street-driven cars (which typically operate at about 1/2 of their redline when cruising on the road).

So, the question remains: What is the redline of the smart fortwo and, if it is at 6000 RPM as indicated on the tach, why does the transmission shift at 6500 RPM under heavy acceleration and why does the limiter not cut in until 500 RPM past redline?

[chieftmc]

I stand corrected! Thank you one and all. I love learning from this board!

[fmaxwell]

Originally Posted by mmmeat

It is not the "maximum RPM that an engine should ever see in operation"

You are wrong as evidenced from this item from Wikipedia:

Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine"

So, the engine is designed to operate up to redline without damage. Therefore, redline defines the maximum RPM that an engine should ever see in normal operation (excepting competition engines where failures are just part of the cost of the sport) If you wish to risk your engine by exceeding redline, feel free to. It's your car.

Originally Posted by mmmeat

All modern engines with an ECU and an automatic transmission will automatically shift to the next highest gear if the motor approaches redline or, if the RPMs hit a preset number, the ECU will cut fuel to the engine until the engine is out of redline.

You get, at best, partial credit for that answer.

On some drive-by-wire systems, the ECU closes the throttle butterflies as the engine approaches redline. That is because just cutting fuel can cause some engines to run "over lean" resulting in combustion that breaks piston ring lands, eventually destroying the engine (see Subara WRX STi "Service Program Campaign WVE15" for one such example).

On other systems, the ECU manipulates the spark (ignition) timing in order to implement a rev limiter, retarding it, or cutting it all together. In some cases, there is a "soft-limit" in which the engine employs a skip-fire technique where some cylinders fire and others do not. This stresses an engine and transmission less than cutting fuel, resulting loading and unloading the drivetrain as spark kicks in and out near redline.

There are other techniques employed, including limiting boost, opening wastegates, and the like. These are often done in conjunction with one or more of the above.

If you want to learn more about ECUs, I'd be happy to teach you about them. I've built comm interfaces to them, remapped the FI, changed spark timing tables, soft rev limits, hard rev limits, cooling system fan cut-on/off points, startup enrichment, acceleration enrichment, etc.

[Westfield 11]

I thought that the red line was the maximum continuous power. That is the max speed you can safely hold the engine to for continuous long term operation. On the smart 6000 rpm is the fastest the engine can turn for longterm continuous operation, but you can momentarily rev up to 6500 without doing any harm to your engine. Just don't hold it at 6500!

If you want to de-rate your engine for longer life, just use a lower redline, and you will be making less HP & torque and thus your engine will last longer in theory!!

[jwight]

Sorry pal, but I'll continue to operate my car based on this:

Main Entry:
1red•line
Pronunciation:
ˈred-ˈlīn
Function:
noun
Date:
1952
: a recommended safety limit : the fastest, farthest, or highest point or degree considered safe; also : the red line which marks this point on a gauge

Dictionary and Thesaurus - Merriam-Webster Online