Most gasoline-powered street cars run in the 8:1 to 12:1 compression range, whereas diesel-powered engines run at a much MUCH higher compression range of about 15:1 to 25:1. Compression is how the diesel fuel is ignitied..... rather than by a spark plug, like with gasoline engine. Diesel engines are usually equiped with some sort of a "glow plug" that sort-of makes the diesel engine function like a gas engine(spark plug-like) when the engine is cold and compression is lower, but once the engine is warm... it runs off of only compression to ignite the diesel fuel.

 

i just brake till i reach around 1,000 rpm and then just throw it in neutral. by that time i'm usually only going around 10-15 mph anyways. i don't see any point in downshifting unless u need the additional compression from the engine.

 

I believe if you read the car manual, it tells you to downshift for engine braking.

 

That's what I am saying... Maybe you should read what I wrote once more.

 

I'll downshift for more power or sometimes for braking. On the braking side Iwill usually only do that on a downhill or in a traffic situation where I will need to cruise along at a lower speed or be ready to accellerate again. Most times I just throw it into neutral coming to a stop without downshifting.

 

First, why would anyone put it in neutral and glide while depressing the brakes? - you're killing your pads as the car has more "mass" going down heel in neutral than if it was in gear. If anything, I would leave it in gear and brake. That way, the load is evenly divided between the engine braking and the brakes.

As to downshifting...

I usually downshift if I need to slow down fast from a high speed to a low one, like going downhill on a highway and my exist is coming up, I would rev-match and go down the gears 6-5-4 and if needed, into 3rd. I rarely go to second as there is no need, unless I'm crawling into a turn and need a quick exit.

 

Good explination.

 

As to the original question (Gas consumption): If you leave your car in gear and let it glide (foot OFF the gas pedal), there is ZERO gas consumption. The ECU shuts the injectors down. There is no need to inject gas, as at that state, the engine is being ran by the wheels. Therefore, leaving the car in neutral and letting it glide, would consume MORE gas than if you would to leave it in gear. The RPM level makes no difference - as long as your foot is off the gas pedal. As soon as you step on it, the ECU will turn the injectors back on, at which point - yes, there will be more gas consumption as you are running the car at higher RPMs while stepping on the gas. However, again, if you downshift and let off the gas you will save more gas. You can even see that if you have a NAVI. Put it on the fuel consumption screen and watch how the MPG jump up as soon as you let off the gas. The RPMs could be higher, but that's only because the wheels are running the engine at a lower gear. On the other hand, if you downshift, and step back on the gas then the engine will consume more, as now it's being ran not only by the wheels at lower gear, but also by the excessive gas injection.

 

Yes, technically it would lower your MPG. Downshifting obviously is raising the RPM's to a higher level, so yeah more gas is used (barely).

I downshift because all modern engines are engineered with engine braking in mind, and I've just got into the habit of being in the power band.

 

^ WRONG. Read my response above.

The only way the engine would consume more gas, would be if you rev-match before downshifting. That would mean you blip the throttle for a second or so, and raise the gas levels in the cylinders. As soon you let off the gas and let the engine-braking do its job, there is no gas consumption whatsoever.

 

man u guys are much less informed than the guys at 6mt.net First off, DONT engine brake too much, in the long run it messes with ur transmission and dont make me explain it cuz i cant word it right, jus know that ur are forcing revs and makin the transm. work harder

 

6Mt.net... Come on man.
Been driving MT's since 1978 and it hurts nothing at all as long as your not dropping several gears at once without knowing what the hell your doing... All automatics downshift; so are you saying that they are hurting themselves and their engines? Do you relly think manuafacturers would engineer something like this if it burned more gas or wore things out prematurelely, heck no!

Also, you can test for yourselves, there will be no noticable change in fuel consumption whether you downshift or not but you can save many brake jobs which do add up to cost as much or more than a cluch replacement especially if you don't catch it and do it in time, it could actually cost more, add up rotors, calipers, pads, labor... add up parts for replacing your Brembo's or Stoptechs and see if the numbers don't add up to making you wish you had downshifted.

 

[QUOTE=Speedoholic]First, why would anyone put it in neutral and glide while depressing the brakes? - you're killing your pads as the car has more "mass" going down heel in neutral than if it was in gear. If anything, I would leave it in gear and brake. That way, the load is evenly divided between the engine braking and the brakes.

That's an easy one to answer. If I am granny driving along in say 6th gear at a very leisurely 70km (42mph) and I see a stop light ahead i will frequently leave the car in gear for a moment and then flick the shifter into neutral so that the car will not stall as I slow down. I see two alternatives. One is to downshift through the gears wearing the clutch and engine using it to unnecessarily brake the car. The other is to leave the car in top gear and stall the engine since the rpms are already very low. In this case I prefer to use my brakes alone most times to slow the car.
For the rest of my driving at different speeds my downshifting style is pretty much how you described yours.

 

I think you mean little gas consumotion as opposed to "no" or "zero" gas consumption. The engine is still running so there is fuel being used just as there is when it is at idle when stopped.