Long downhill - "engine braking"
 

I had to drive long downhills and used the "engine break" to keep the speed from increasing. I was a bit worried I may cause damage to the engine, because I was afraid that at long decelerations the motor would not get enough oil for lubing it properly. (To some extent the situation is similar to a 2stroke engine - no fuel injection no motor lubrication.)

There was a therad on "Engine braking", but it did not help to answer my question this lubing issue was not mentioned.

adding question to the question above :
i have similar situation and sometimes i just put the gear in neutral and let the car
glide down the hill until a stop sign.
is it okay to do that :confused:

Man, *nothing* compares to the smug feeling you get, gliding downhill in neutral, thinking about all the gas your saving. :) :) :) It'll take a lot to convince me to stop doing that. :)

Gliding downhill in neutral can be bad for the tranny, depending on the design. As long a fluid is flowing based on movement from the driveshaft, it is fine. Does anyone know if the 8's tranny still has fluid flow in neutral?

Engine braking supposedly is not good for the life of the engine/gas mileage. While it won't considerably cut down the engine life. But will inevitably speed up the wear and tear on the engine. But this is from experience of driving a piston engine. I for one have no experience driving rotory power car(you haven't lived until you get to drive a wankel). Also letting your car idle for long periods of time and suddenly jumping on the RPM is bad. Engines were meant to be kept running constantly at the same speed. But hey I also use engine braking all the time and cruise on neutral as much as possible. :p :rolleyes:

It is not bad to coast down hill in gear. The gas is cut off to the engine and you will inprove your MPG! If you dont have it in a gear then it is useing gas. This is how the 8 is programed.

its bad to coast in neutral... well, dangerous. if you need to make fast adjustments, being in neutral is a bad idea.

That, and doesn't the ecu go into idle fueling mode in neutral(more fuel?) vice decelleration mode when engine braking (no fuel?)

I'm sure the engineers have taken this into account. Going down hills is part of driving. Also, I imagine the oil pumps are driven by the engine, so if the engine is spinning, the pumps are pumping.

Regarding 2-stroke engines, the first 2-stroke motorcycle I ever owned (1970 Yamaha 175cc Enduro) had automatic oil injection. The oil injection did not shut off when coasting. Throttle position and RPM played a factor in how much oil was injected.

1.3L

^^^ What Old Rotor said. When you are coasting in gear, the ecu goes into Deceleration Fuel Cutoff (DFCO) mode, and actually stops injecting fuel. the motion of the car turns the wheels, which turns the tranny, which turns the engine. no fuel needed to keep the engine running.

if you are in neutral, then the motion of the car turns the wheels, whichs turns the tranny, but not the engine because you are in neutral. so to keep the engine turning, fuel has to be injected into the engine.

^^^ What Glyphon said.

Another thing to consider ... one of the tricks to driving safely is having the option to able to accelerate out of a dangerous situation. Say you're going down a hill and you have a truck behind you which loses its brakes and starts accelerating towards you. You have no shoulder and there is oncoming traffic... only way out is to go faster.

Scenario 1: You're rolling down in neutral... you plant your foot. The engine revs you go nowhere. You go to get back in gear... whack... too late.

Scenario 2: You're in gear... you plant the foot... off you go.

I know it's unlikely... but you never know.

thats what i meant, just the more graphical explanation hits home better ;)

Sorry.. re-reading your post.. yep we're on the same page. You know what they say about great minds :D ;)

It is illegal to coast in neutral in CA.

just a curiousity, how would they catch that? (if they even bother)

Born.. to be wiiiiiiild!!!....

Krankor
05 Shinka 6MT

(actually, I don't actually pull it out of gear, I just clutch it neutral, from which I can get back quickly enough)

That's gonna put excess wear on your throwout bearing...

There's no detriment to going down a hill in gear with your foot completely off the gas. It's less gas use than idling (there is *no* fuel injected when coasting down a hill in-gear, which can be verified by observing the wbo2 sensor readings) and as far as lubrication goes, the oil is injected into the rotor housings via different means than the fuel injectors, so the housings are still getting lubed.

In neutral you've got the driveshaft spinning fast whereas the idling of the engine may not be spinning the front side fast enough for proper oilflow in the gearbox (that's speculation on my part). If you push the clutch in there's that and the added wear on your throwout bearing.

You guys are full of it. Coasting in neutral does not damage the transmission. In neutral with the clutch out, the layshaft is spinning at the same speed as the engine (idle) so it is indeed spinning and getting lubricated. Same for the driveshaft assembly, which is always turning at rear-wheel speed anyway.

Now you CAN damage your transmission by towing the car with the rear wheels on the ground and the trans. in neutral. The solution is to start the engine up every 100 mi or so and let it spin the layshaft (clutch out, in neutral) for a bit to get it lubricated. A better solution is to just disconnect the driveshaft.

Finally, its always better to use the piece of your car that's replaceable, ie. the brakes to slow the car, rather than the engine. Its highly unlikely that your brakes will overheat even on the steepest of slopes, so there's no real need for engine braking.

you speak like a man who has never had his brakes go out at highway speed. The brakes themselves might not over heat, but it is possible for the brake fluid to boil. and when that happens, you essentially have no brakes. besides, engine braking going down a hill is more about controlling your speed as opposed to actually braking.

That happened to you on your RX-8? These cars have the best brakes Mazda has ever done.

I do know that most autos dont engine brake, and living in colorado there are alot of passes that are 40+ miles from top to bottom. So riding your breaks for most of 40 miles will cause any breaks to over heat and fail. I have seen hundreds of smoking breaks in the mountains. I have owned several rotaries and engine braking has never had any detrimental effect that I have ever seen.

I'm not totally against engine braking, I'm just suggesting that most of the time, its not necessary. I know some people who do it going up to stoplights on level ground thinking they are saving their brakes, which doesn't make much sense when you consider a set of pads is < $150. In any case, you won't damage the engine because fuel and oil are always being injected into the engine, even if you are totally off the throttle.

This is incorrect. Any type brake system can overheat if you drag them long enough and far enough. I've sat at the base of some long, steep grades and smelled the blistering hot brakes of some pretty fancy cars when the drivers dragged the brakes all the way down the hill. Hot brakes can boil the fluid and then complete brake failure happens. Selecting a lower gear is the solution, not dragging the brakes. Sure, it's cheaper to replace worn brake pads, but that doesn't take into account the consequences of brake failure (i.e., crashing your car) .

1.3L

nope. not on my rx-8. different car. but that doesn't mean if you ride your brakes down an extended slope that they won't fail from over heating.

i agree, that for stopping the car the brakes are the best way to go. but for controlling speed on a hill or shifting to stay in the powerband while coasting to a stop, there's nothing wrong with such behavior and it isn't detrimental to the car.

Never coast in neutral, engine braking is actually not as detrimental as you might think. It only uses your engine compression to slow down, in fact it was purposely implemented in big trucks to assist the brakes for slowing down; u know... the psssstttt sound when a bus or truck makes before stopping. In fact engine braking is one of the most important factor of proper engine break-in, I don't know if its any different in a rotary engine but in a piston engine the compression actually forces the piston ring to properly seat. Loading the engine is actually good for the engine during break-in period provided the engine is warmed up properly and it doesn't overheat if run at high rpm over long period. Furthermore the car's manual mentioned not to keep speed constant for long time is to prevent the piston ring and cylinder from glazing. Engine surface are peaks and valleys, what u try to achieve during break-in is making the peaks into plateaus so a little bit of oil still manage to lube the walls during operation. More importantly "in gear" is safer

God, I get so tired of posts like this. I have no doubt that this is technically correct. I also know that I've had 5 manual transmission cars and 2 motorcycles and coasted neutral in all of them and never wore out any throwout bearing or anything else. That includes having well over 100K miles on a couple of them. I have no doubt that if I kept the cars for a million miles or coasted to work 50 miles downhill both ways every day that I might wear out my throwout bearing or something. But you know what? I'm not gonna lose any sleep over it.

Some people know too much for their own good.

I think if you drive on some serious mountain roads you may re-assess your opinion. This is exactly the time when engine braking is most useful and contributes to safety. If the grade is long enough and steep enough it becomes a necessity. Brakes are great at slowing or stopping the car but are not the best tool to use to keep speed under control on long twisty descents. The heat involved using friction to retard acceleration from gravity on a long downhill mountain road is tremendous and can fade any braking system. The engine is a far better tool in this situation.

Spot-on Nubo.

I have just completed a 1500 mile driving holiday with my family in the RX-8. We spent a lot of time in the mountains making the most of the great driving roads up there. My wife nearly wore out the Jesus handle........but I digress :D . We did a number of steep descents where I used no bra kes at all.....and it was a hoot. Select the right gear (3rd, mostly) and rely on eng ine braking to slow us down for the corners.

End result? Saved the brakes, saved heaps on fuel (fuel is cut-off during deceleration.....idling in neutral burns heaps of gas)......and had a great time to boot. No harm done to the engine at all......in fact, probably burnt off a bit of carbon build up along the way.

On one descent we caught up with a Ford wagon at the bottom. My kids commented on a smell as we followed this car into town. The smell was burnt brake linings. Obviously a low performance driver.

Gomez.

I just got back from a road trip to the Adirondacks. The descent from Whiteface Mountain was about 5 miles and I kept the tranny in 3rd gear to utilize engine braking. This is somewhat off topic but I noticed a really odd thing. The coolant temperature kept steadily dropping and eventually dropped all the way to the left. When the road leveled off and I went back to normal cruising, the coolant temperature dial immediately went back to the middle. I'm pretty sure that there wasn't any kind of damage but I'm wondering if anyone else had ever have this happen.

Heh, i notice this as well on my FD (with linearized temp gauge):)

Edgardo

I think before you start criticizing other people for their knowledge.. you should get some of your own... he said that if you coast down a hill with the clutch "IN" as in pressed down.. you will put excessive wear on your throwout bearing... and this is true.. it's even written in the rx8 manual.. "don't keep the clutch depressed at stop lights" and such this and that..

Anyway..

With other cars I've owned (all piston engine), Engine braking wasn't a problem to use, but to save the life of the engine a little more, I would always just use 5th gear and rely on the handling of the car to bring it around the curves using only light braking to slow down.

In the rx8, I've found that coasting down in 5th or 6th for most of the mountains around here allows me to go around 45-65mph all the way down.. and the amazing handling of the rx8 makes it super easy to do. Theres not much load on the engine at the high gears as I try to hold the throttle at a little bit of a "neutral" position.

And if you read what I had said, I said that that's what I do, doing the coast with the clutch.

Now, you tell me, are you really claiming that the 8 has a radically different clutch then other cars? I'm open to the possibility that this may be so, but I'm telling you that on *every other manual car I've ever had this has never been a problem*. This is the knowledge I have acquired, this is the knowledge I have passed on, and it is absolutely true. All you have done is repeat what he said and I'm still not buying it. If there's something special about the 8's transmission I will revise my opinion, but so far nobody has demonstrated such.

[QUOTE=SoSaRX]I had to drive long downhills and used the "engine break" to keep the speed from increasing. I was a bit worried I may cause damage to the engine, because I was afraid that at long decelerations the motor would not get enough oil for lubing it properly. (To some extent the situation is similar to a 2stroke engine - no fuel injection no motor lubrication.)

Your owners manual says to downshift when going downhill. The engine will still get oil - the fuel injection has nothing to do with lubrication. If you don't use engine braking on a long downhill, you WILL have to use your brakes. And if they get too hot, they may fade, and then you would be in trouble!

I din't know, but every book on auto mechanics I've read has advised against riding the clutch, as well as my stepdad, and he's an ASE certified tech w/ about 13 years or so under him. Not everybody who does the neutral drop destroys their transmission, but that doesn't mean it's a good idea. And you made no mention of laying on the clutch, you only said coasting in neutral.

That's the "Ive been smoking for 20 years and I didn't get cancer yet" argument.

The tranny in the rx8 isn't any different to any other tranny and the concerns regarding accelerated wear on the throwout bearing apply to the other cars you've been driving like that also. That you havent experienced failure or seen any detremental effect from that habit doesn't mean that it's a good one to have.

Please feel free to coast around with your foot on the clutch as much as you want. I know that it's not good for the tranny so I don't do it, if you want to then that's your choice.

Multisync, my BMW has the problem as well, basically the engine is not generating any heat as it is not burning anything, or very little, and coupled with the cooler temps of higher elevations and your coolant system working it cools the car off. It really sucks in the winter sometimes here in Colorado cause the heater will suck then!

Engine breaking down long hills is not bad for the engine, any worse than driving it.

For REALLY long hills its recommended (like when you drive up and down Mt Washington or Pikes Peak) because if you relied soley on your brakes to slow you down, they wouldn't last. Halfway down your brakes would be so hot you'd have no way to stop. :eek:

Hold the gas down and upshift when its "a little past" necessary.

I used engine braking CONSTANTLY in my rx-7.
Flat ground or hills. Every stop I EVER made in the car.
I always down shifted.
Sold the car at 130,000+ miles after 12 years.
Still ran like the day I bought it!!!!!! NO PROBLEMS FOR A ROTARY FROM MY VIEW.
Same goes for all the piston vehicles I have owned.

I remember years ago, my father told me not to engine brake too much when doing city driving. he tells me engine braking just eats up too much fuel. I guess he's used to driving the older cars where the fuel doesn't get cut off when still in gear. I love to engine brake, since my brakes sucks anyways.

on a somewhat related subject, I have heard that in an automatic tranny it's not a good idea to coast down a hill in neutral, can anyone shine some light on this?

I have always used engine braking on piston-engined cars, keeps you in control and keeps you from going too fast. I also knew for a fact around half that time that it uses no gas when doing it.

How does the rotary rate in deceleration when compared to a regular piston engine? I found this thread because I read somewhere that the rotary decelerates less.

Riding the clutch isn't the same thing as coasting. Riding the clutch is keeping your foot on the clutch - applying some pressure whether you intend to or not - and therefore resulting in slight slip and premature clutch death.

Right - downhill, engine braking:

Manual - leave it in a lower gear. Engine compression will provide engine braking like the exhaust/"Jake"/brake on a truck but without the ability to hold valves shut. Because the car is in gear it is using NO fuel, compared to idle fuel. You've surely felt moden fuel injected cars 'shunt' slightly when coasting in gear on the freeway; that's the injection shutting off/coming back (Audi/VW are really good for demonstrating this).

Automatic - choose a lower gear. Depending on the speed you want to maintain, a good example is a very steep downhill near Whitby I have to use sometimes - my little Sera automatic could be left in D, and it would happily scare the crap out of people by running off down the hill. Stick in in 2 and it was fine. Acted just like leaving a normal car in 2nd or 3rd. Automatics DO NOT RUN AWAY if you know how to use them.

THERE IS NO REASON TO COAST IN NEUTRAL. EVER.

This is all "Driver's Ed 101", so I really hope that people are not being as stupid as they seem to be here whilst in charge of a decent car like the RX8.

It's a bad idea to coast in neutral, period.

However, this is like saying "Don't stick your finger in a wallsocket" and smart people think too much. What if the wallsocket hasn't got a fuse on, is it safe then? Never mind that wanting to do it in the first place is a bad idea, let's explain why:

Automatic transmissions rely on oil pressure fed from the engine (speed/rotation of the input shaft or similar arrangement. Not the engine oil. Should clarify that). If you are coasting, you are providing idle-speed oil pressure to the gearbox and road-speed input from the prop. This can, in some cases, be a bad thing. However, what WILL be a bad thing is when you go to engage drive. Your car is going to react very slowly, and it is going to be very very unhappy when it is asked to go straight into 4th because you tried to engage D at 50mph.

You are also relying entirely on your brakes to control speed. Brakes are not some miracle device, and when most motoring lore was created, they were basically a slightly better alternative to opening the door and trying to slow down with the heel of your shoe.

Finally, you might not even be able to engage D when you need it. You might be about to slam into an elk, twirling the wheel, and then discover you have no drive and therefore, no control over the car.

Modern gearboxes work in a variety of ways, so the oil pump argument is a bit old hat; even torque convertors are going the way of the dinosaur as people create things like DSG. However, the driver control and wear & tear/expected behaviour arguments ring true. Feel free to stick fingers in wallsockets, but don't complain when you get a shock.

BS. The car (at least the RX-8) uses some amount of fuel regardless of load.

Listen, my Dad's best friend's cousin's grandpa was in dubbya dubbya two and he said that coasting in nuetral causes cancer. So, go ahead and get cancer.

Seriously though, why would anyone who has an RX-8 really care about fuel economy? Maybe it uses a little bit of gas, maybe it doesn't. Maybe it'll mess up your throwout bearing or your flux capacitor, and maybe it won't. In the end it's illegal to coast in nuetral in most states because of safety concerns. Follow what the owner's manual and common sense dictate. As long as you're enjoying what you're doing and you're not impinging on other people's ability to do the same, do whatever you think is right.

Fair enough. I'll take the word of the people who can provide a technical explanation of why it doesn't use fuel when coasting in gear and using engine braking vs. why it does use fuel when idling because you are coasting in neutral, over the word of someone who claims bullshit without any technical explanation and with an erroneous statement ;)

My father told me years ago when I was learning to drive that I should never let the car coast in neutral...whether on a decline or on a flat surface because when the car is not in gear you do not have the control you do when in gear. I have coasted down a hill in neutral and then did the same hill in gear...there was no doubt to me which felt like I had control...which felt the safest. I am sure some will drive as I do and others will not, but at least everyone is being pretty civil in this discourse as the thread has not been closed!

this has been explained a few times in this thread (see my post on the first page), but if you don't believe me (backed-up by sCO immediately afterward), here is Rasputin explaining it...

Well I guess we learn new things every day. :dunce:

and that's why we all keep coming back...that and hemanrulez avatar :mdrmed: