Legoland951

Using the engine and the entire drivetrain to slow the car down is THE MOST EXPENSIVE WAY as it will wear everything out faster so DON'T use your engine to slow the car down. No one can really accelerate their way out of anything in a hurry in 5th gear so that point is moot. There is a lot of misinformation out there. The manual transmission has the gears turning even when you are in neutral so oil is being circulated. When the engine is idling, auto trans fluid is also being circulated so its not a problem. The problem only comes when the auto is being towed with the engine off. Any tow truck driver will tell you this and its why they don't like to tow a car with the drive wheels on the ground without removing the driveshaft. When you are not on the gas, the computer gives the same amount of gas if not more when you are in gear since the engine is sucking in more air. There is a lot more friction in the pistons, rings, bearings, and load in the transmission when you are in gear so you will definitely get WORSE gas mileage if you leave it in gear.

I will only leave it in gear if I am not going that fast (like in 3rd) where stepping on the gas will do something.

zoltan944

Well as far as gas mileage goes-
If the porsche computer is like anything other computer
when you are costing in gear down hill you will be pulling big vaccuum, your computer will input this as coasting and more or less turn your fuel off. If you have a WB you will see it go lean off the charts.
If you go into neutral your car will turn idle on and you will go to a idle A/F mixture, somewhere in the 14's i would assume.
However since technically could coast longer in nuetral than in gear this may all cancel each other out.
As far as wear and tear goes... Frank makes sense to me.

JustinL

This is incorrect. The DME cuts fuel if the idle switch is engaged above 1800RPM. If you've ever had a sticky idle switch, you find this out the hard way when you hit a hard rev limit at 1800RPM. The other thing you can try is logging A/F ratio and watch the AFR jump to full lean when you lift from the throttle.

cegan09

I can't speak for 20 year old cars, but i'm fairly certain that at least new cars will cut fuel if you car coasting to slow down. So at least in new cars you will get better mileage (for that short time that you are coasting. Obviously the effect on overall mileage is pretty small.)


As to the question on riding the clutch, unless i'm mistaken thats bad. If you are just leaving it at the point of engaging i'm pretty sure the clutch can still slip. Slipping the clutch wears things out. I was taught for slow things like parking spaces and reversing to get the car moving a little, then disengage the clutch and repeat so you are not constantly holding the clutch there. (if i'm wrong here please correct me)

Potomac-Greg

Related question. If you flat-tow or use a dolly (front wheels up), can it hurt the transaxle to tow any distances? Obviously, the engine is off. Given that it's a rear transaxle and a closed system, I thought that there might be no harm to it.

PS: Another reason not to coast is that an engine is more succeptible to stalling, which will deprive you of power steering and brake boost. If you're in gear, you won't stall.

Legoland951

So what you are telling me is a 944 cuts fuel completely if you are coasting in gear over 1800 RPM similar to a full stall? So basically whenever you hit the gas you are "bump starting" your car all over again? If you think this is true, shut off your car at 1800 rpm in gear while your car is coasting and you will definitely see the difference (not recommended if you don't know what you are doing as you lose power steering and power brakes when the vacuum gets used up). With no fuel, you will see how fast the car really will slow down. Yes I have tried it in several cars. With some later cars with auto trans, you will get a check engine light when you restart the engine and drop it in gear while the car is coasting. The computer cuts fuel as if the car is idling in most cases, which is the same if you are in gear or not except when you are in gear, the COMPRESSION and FRICTION of all your driveline components gets you worse gas mileage and wears every part that is moving.

If you use a dolly, you can tow anything that is manual any way you want. With an auto, you will have to disconnect the driveshaft (not possible with the 944 transaxle unless you remove both half shafts). If you are towing an automatic anything with the drive wheels on the ground, you better be towing for a very short distance and very slowly as a transmission is very costly.

Anyone whos car will stall spontaneously should get it fixed before getting it on the road. All you have to do is put it in gear and let up the clutch then voila, car is running again.

Jay W

In response to Greg, only an automatic transmission car can be damaged by towing on a dolly with the rear wheels on the ground. This is because the engine turns the transmission fluid pump which is in the front of the transmission. If only the wheels are turning and the engine is off, the bearings in the transmission are not getting any fluid/lubrication because the transmission fluid pump is not pushing fluid through the transmission.

And riding your clutch at a stoplight is eating it alive. I have seen people hold their car on a hill by keeping the clutch pedal half way in. That is constant grinding of the clutch material against the flywheel. Not only do you burn up the clutch material, you heat up the flywheel and it can warp or get host spots that make it chatter. It just kills me when I see that happen because I know how much work a clutch replacement is.

Legoland951

BTW, trust me, a pair of $22 brake pads is much cheaper than an engine/tranny rebuild.

JustinL

Yes that's how the system works, but cutting fuel does not equal a stall unless you clutch in. if you did clutch in, fuel will resume at 1200 RPM and your car would idle. You also wouldn't lose vacuum as the engine is still being turned by the wheels and still pumping air. If you turn the ignition off while the car is coasting down a hill, you will not lose power steering as the pump is still spinning with the engine, and you will not lose power brakes as the cylinders are still pulling air through.

This is called decel overrun fuel cut. There are plenty of threads about it and it's also the main reason for the shudder some 944 owners report. I've data logged this with a wideband as well if you need further empirical evidence for a 944. If you want to try something, unplug your throttle position switch and see how slowly the revs come down when fuel is given during decel.

zoltan944

Correcto. I was saying this. But you aren't really bump starting your car everytime or at least feel like it. But for some reason I think that if your car doesn't start and you get going 60mph down a hill and put it in fifth with the key on and let out the clutch you really wouldn't feel much of a bump.
The difference here is clutch as many have said, but your car almost completly cuts fuel when high vaccuum is acheived on deceleration. This is how cars started getting good gas mileage. Every time you take your foot off the gas in gear it basically cuts fuel.

inactiveuser92616

It is illegal in Colorado, but mechanically fine.

pjburges

This whole thread is pretty damn silly.

Why dont you do what you want to, and if it breaks the transmission so what. Feel lucky - an N/A transaxle box is only 200 bucks, and N/A brake pads are like what 50 bucks?

Jay Gratton

I had a friend in HS who crashed his car because he used to do this. It also hurst the handling of your car as you do not have any power down to push (in this case) or pull (FWD) you through a turn. This is why he crashed because the weight of the car wanted to go straight when the driver wanted to turn. Needless to say, he went off the road. Keep it in gear and keep the power down (esp. at that speed).

LeRox

good thread......you guys are cool