FAQ 2.0! <- New Members Read First

[eaglecatcher]

BMW - Bayerische Motoren Werke

Is Bayerische bavarian in english? I have heard it stands fro bavarian motorworks, is that the english version?

[screenname]

BMW in English - Bavarian Motor Works

Yes, you'd be right.

[eaglecatcher]

yay, party all night long

[Coupe31705]

VOE- Vaccum Operated Exaust

[gage]

OHC - Overhead cam
SOHC - Single overhead cam
DOHC - Dual overhead cam
OHV - Overhead valve
HEMI - Not an acronym but means hemispherical combustion chamber (current gen HEMI's are marketing and are pentagram cylinders, not hemi)
EFI - Electronic Fuel Injection
SFI - Sequential-port Fuel Injection
DPI - Direct-port injection
CFM - Cubic feet / minute (mainly air CFM for Carberators)
LSD - Limited slip differential (positraction)

My fave:
GTO - Get Tires Often

[TomCivic]

FU - Fun Unicorns!

[six]

FSI - Audi Direct-Injection term (Fuel Stratified Injection)
DISI - Mazda Direct-Injection term (Direct Injection Spark Ignition)
GDI - Gasoline Direct Injection

[beekay]

TBE - Turbo-Back Exhaust

[jomotopia]

What RPM in X gear at Y speed?

Author- jomotopia
Submitted by- jomotopia

This gets asked a lot, and the answer is that it depends entirely on your specific car. It depends on the gear ratios, final drive ratio, and wheel/tire size. This thread and this thread discuss this topic in more depth and provide resources where you can enter your car's ratios etc to get a general idea of speed vs RPM.

[Coupe31705]

RedlinePoint at which the engine is turning at the fastest possible speed without damaging itself. Driving past the redline for an extended period of time will ruin your engine to the point at which it will not run.

The rev-limiter, mentioned earlier in this thread limits gas flow past redline in most cars to prevent engine damage.

[Johnf514]

How Do I Improve My Gas Mileage?

Author:Prodigal Son
Submitted by: Johnf514

The thought that strikes me after reading through the discussion is this: The efficiency of the engine is not the same thing as the efficiency of the car. In fact, the whole point of hybrid technology is to exploit that fact.

The efficiency of the engine, qua engine, is a measure of how much fuel it takes to produce a given amount of torque. At a certain RPM an engine produces the highest amount of torque for the fuel it consumes. What that RPM figure is will presumably vary depending on where the torque curve is in the engine, and probably other stuff as well.

When running at higher than its most-efficient speed, an engine consumes more fuel and produces less torque per liter of fuel used. But this does not mean that when the engine is running at a lower speed it also burns more fuel. At a lower speed, an engine burns less fuel, but it produces less torque per litre of fuel used, making it less efficient as an engine. At a lower speed it will still burn less fuel per hour of operation, it just won't be as efficient in terms of producing torque for the fuel consumed.

Hybrids (in my somewhat vague understanding) take advantage of this fact by trying to run the engine at its most efficient speed and using the excess torque produced to charge the battery. Since a regular car can't do that, trying to run the engine at it's most efficient RPM will sometimes involve creating torque you don't need and essentially throwing it away. The engine is at its most efficient; the car is not.

I have no idea whether the loss of torque per litre at a lower speed is ever enough to offset the lower litre per hour of a slower engine (thus making it more efficient to run in a lower gear). Frankly, I'm scared of the math. And, of course, it will be different in every car. But I don't buy the general idea that you will get lower fuel consumption overall by running in a lower gear so as to keep then engine nearer to its peak efficiency --- not unless you have some mechanism, like a battery, for storing the excess torque produced for later consumption.

How much torque a car needs at a given moment is a function of what it is doing:

Coasting: The car requires no torque. You can coast down to stall speed in fifth, since the car requires no torque from the engine. (This is why some people suggest that it is better to coast in gear -- letting the wheels drive the engine, consuming no fuel -- than to coast in neutral -- forcing the engine to idle, which consumes fuel and produces unusable torque.

Cruising: The car requires relatively little torque. A hybrid can cruise the highway just as well as a more powerful car (except at really high speeds where torque is required to overcome high wind resistance). The hybrid engine is producing just enough torque. A regular engine is producing more than is needed, and essentially wasting it. Getting the engine speed down as low as possible helps, but IC engines have a limit to how slow they will turn, so you are almost certainly going to waste torque while cruising.

Accelerating: The car requires lots of torque, and the faster you accelerate, the more torque you need. Most engines produce enough torque to provide a particular amount of acceleration and essentially waste the torque that they produce the rest of the time. (People call a car ‘fast’ based mostly on its maximum acceleration rate; not its top speed.) Hybrids use the electric motor to supply additional torque for acceleration (though still not enough to please some folks).

You can decrease gas consumption by producing torque more efficiently and by not producing more torque than you need. If you produce torque efficiently but then don’t use it, your car will not be more efficient.

The chief ways you can reduce how much torque you need are by accelerating slowly and using high gears while cruising.

The chief way you can reduce the amount of torque you produce but don’t use is by buying a car with no more engine than you actually need for daily driving, or by buying a hybrid that stores the excess torque that your engine produces most of the time so that you can use it later.

Or at least, so it seems to me. Real mechanics may feel free to scoff.

[jomotopia]

What should I do when coming to a stop?

Author: jomotopia
Submitted by: jomotopia

If you are definitely going to be stopping, there is really no reason to downshift. It's a common misconception amongst new manual drivers that you have to downshift through all the gears when you are coming to a stop. That is not the case.

There is one basic thing to remember: If you come to a stop without either shifting to neutral, or disengaging the clutch, you will stall.

The easiest, most common, and most recommended (from what I have seen on these forums) method of coming to a stop is:

-Begin braking in whatever gear you are currently in.
-Leave the car in gear and continue braking until the RPM falls to around 1000.
-Continue braking while you clutch in, shift to neutral, and clutch out
-Stop.

[Johnf514]

How do Anti-Lock Brakes (ABS) work?

Authored by: GarySheehan and DW10+BE4/5L
Submitted by: Johnf514

GarySheehan says:
Here is an example of a tire's mu slip curve. The Y axis is the tires tractive capability and the X axis is the amount of slip between the tire and the road surface.



The curve relating to straight line braking is the Longitudinal Mu Slip Curve.

The far left of the curve is zero slip. So at zero slip, the tire has no traction. A tire needs to be in slip to generate traction.

The peak of the curve is the tire's maximum grip level, which occurs at a given amount of slip. A very good racecar driver is capable of modulating the brakes to keep the tire operating at or very near the peak of this curve. This is called threshold braking.

Once you exceed the peak of the mu slip curve, traction becomes unstable and the wheel rapidly slows down relative to roadspeed and locks up. This happens very quickly. It is not possible to modulate the brakes to have the tire stay in a given location past the peak of the curve. The tendency will always be towards lockup.

As you can see, as the tire approaches lock-up (towards the right of the graph) the tire's traction decreases. So a locked wheel/tire is much less efficient than a tire operating at the peak of its mu slip curve.

ABS operates in the range of slightly before the peak to slightly past the peak of the mu slip curve. When the ABS computer senses excessive slip (by comparing a given wheel speed to the wheel speed of the other tires), the ABS system releases brakeline pressure to the wheel with excessive slip to get it rolling near road speed again. The tire is now operating to the left of the peak. It then reapplies brake pressure until it senses excessive slip again and repeats the cycle. Many times per second. So ABS keeps the tire operating around the peak of the mu slip curve, but does not maintain at the very peak of the slip curve.

The more sophisticated the ABS system, the closer it is able to operate at the peak of the mu slip curve.

DW10+BE4/5L says:
The slip-mu curve, as in friction coefficient as a function of slip, depends heavily on the surface. On loose gravel and snow the greatest friction, or longitudinal force, is reached when the wheel is locked. This is because the tire deforms, i.e. digs into, the surface. IIRC, on ice the best grip is at low slip rates. Concrete and asphalt are somewhere in between.

To achieve the best braking force the ABS system should know the slip-mu curve of the surface which it of course doesn't. The ABS compromises over maneuverability and braking force.

The software of an ABS system is a closely guarded secret of its manufacturer (Bosch, Teves, Denso to name but a few). Even car manufacturers don't know what goes on in those black boxes which they just plug in. Writing such software is a tricky business because you also have to consider what happens when the friction level at the left side differs from that at right (the car might spin rapidly).

My guess is that depending on the ABS system and on how (un)lucky you happen to be, it's possible to stall the engine. In my experience (don't know whose ABS and which generation) the ABS first uses a slower cycle. It probably somehow measures the friction during the slow cycles. So if you disengage the clutch during this time the engine just might stall presuming both driven wheels are locked at the same time.

Braking with a 4WD car is more complicated because the three differentials may distribute the braking moments in many more ways than is possible with one differential.

The best way to emergency brake is IMHO to floor the brake pedal first and then worry about the clutch. If you first depress the clutch you will lose precious time that you could use to start braking. If the engine stalls you will lose power steering if it's hydraulic. I'm not sure if you will also lose brake boost during ABS braking. It depends on the system I think. In most cases there is an electrically driven hydraulic pump that restores the brake pressure during each ABS cycle, so I guess it doesn't lose the vacuum used to boost the brake pressure even if the engine is stalled and isn't generating any suction.

The best way to avoid an accident depends on the situation of course. It might be best to hit the clutch to give the best side force for steering. Or it may be best to brake as hard as you can.

Quote:
By hitting the clutch at the same time as the brakes, you will lose your engine braking (though only a slight amount anway) plus your drive wheels are no longer active and you can lose traction.

The braking torque of the engine is negligible compared to the torque of the brakes. The brakes always win even in 1st gear. And you are trying to slow down so I wouldn't worry about traction. The best thing about depressing the clutch is that the car handles better because the engine isn't braking and thus consuming precious side force of the tires or producing yawing moments. People, including me, tend to instinctively let go of the accelerator when their car starts to slip sideways. Usually this just makes the slipping worse.

I think that the ABS is at its best when a n00b like me notices too late that he should have taken that turn to the right, steers to the right, hits the brakes because he's going too fast, locks the wheels and slides to the snow bank in the corner but luckily doesn't hit any of the traffic control devices and avoids any damage at all. Been there, done that. In a van without ABS.

[jomotopia]

Heel-Toe Downshifting

Authored by: jomotopia
Submitted by: jomotopia

The purpose of heel-toe downshifting is to get into the proper gear for a turn while braking continuously.

A situation where you would use it is if you're coming up to a turn that you normally take in 2nd gear, and you are in a higher gear. So for example, if you are approaching the turn in 4th gear, you can heel-toe downshift either directly to 2nd, or sequentially to 3rd and then to 2nd. The goal is to brake smoothly and constantly while you downshift, and finish both downshifting and braking while still moving in a straight line before entering the turn.

Then you are in the proper gear for the turn, you can release the brakes and turn in, and accelerate out of the turn.

Heel-toe is not a required technique, and some consider it advanced or really only for racing. In my opinion, if you get proficient with it, it is very useful on the street.

The way you execute a heel-toe downshift is as follows:

--Begin braking
--Clutch in
--Downshift and blip the throttle, while still braking:

*This is the tricky part. You are already using your right foot to brake. In order to blip the throttle, you need to use part of your right foot to apply steady brake pressure, while using another part of your right foot to blip the throttle. The parts of the foot that you will use will depend on your car's pedals, your feet, and your preference.

It doesn't have to necesarily be your heel and your toe, although the classic way of doing it is braking with the toe, and blipping the gas with the heel.

Many people prefer a side/side method, using the left side of the right foot, or the ball of the foot, on the brake, and blipping the gas with the right edge.

I have also heard of people using their heel on the brake and toe on the gas.

The most important thing in a heel-toe maneuver is the braking. When learning make sure there is nothing around for you to hit if your foot were to slip off the brake pedal.

*You want to raise the rpm to what it will be in the next gear, however you are continually slowing down because you are braking, so the rpm you aim for keeps dropping. It will require a much smaller blip than a rev-matched downshift done at speed.

--Release the clutch.


Some cars are harder to heel-toe than others, depending on the relative height of the gas and brake pedals, and also how close together they are. The shoes you are wearing can also play a large part.

You will most likely over rev most of the time for a while, but an over rev is easier on the car than an under rev.

[Johnf514]

The Basics to Driving Stick - A Beginners Guide

Authored by: jvf1mikey
Submitted by: Johnf514

1) On launches
*give it enough gas but not too much
*do not release the clutch too fast
*if you've started the launch and feel you've given too little gas or brought up the clutch too high, clutch in partially and increase the gas a tad
*lingering at the FP is necessary on launches

2) On 1-2 shifts
*no need to rest your foot on/just above the clutch anticipating the shift; get into the habit of resting the left foot on the dead pedal
*do not shift too early; take advantage of 1st gear's quick take-off capabilities
*get your hand off the shifter between shifts
*don't release the clutch too fast; linger at the FP briefly

3) Slowing
*choose 2nd gear and use the gas pedal to negotiate speed bumps and driveways/driveway entrances
*if no-gas downshifting from 3 to 2, be sure you've slowed enough in 3rd, but not slowed too much, before clutching in, shifting, and clutching out; always clutch out slowly on these shifts
*if rev-matching (for shifting on the fly) don't wait too long to release the clutch after the blip
*try to anticipate 2nd gear corners and complete the shift before the corner, while in a straight line

4) Stopping
*no need to shift to neutral when at a stop sign; while coming to the stop, with the clutch down and brake applied, go directly from previous gear into 1st

5) General
*try to avoid shifting while negotiating significant bends in the road; concentrate on steering/control; if you need 2nd or 3rd for bends, get into the gear before the bend