Engine Braking

[axgutt]

I have a 95 Legacy L, 2.2, AWD, 5 speed with 279,000 miles. I just started a new job at a ski/golf resort, so I'm doing a lot of up and down driving. There's one stretch where it's about a 15% grade, downhill, for about a mile. I've been leaving it in second, and it shoots up a little over 5000 RPMs before I lose my nerve and start braking near the end.

 

Am I asking to blow something up? Is there any special maintenance I should do? Should I ride the brakes a bit to keep the RPMs lower? Or, should I trust the indomitable 2.2 to take the punishment?

 

 

Thanks

 

Adam

[OB99W]

The engine will probably take that kind of treatment, for a while, anyway, but especially with the mileage you have on it that's probably not the best thing to be doing. My personal philosophy is that brake linings are a lot cheaper, and easier to replace, than engines. Unless you don't trust your brakes (in which case, they should be worked on until they can be trusted not to fade, etc., on a long/steep descent), why have the engine rev that high on a regular basis?

 

By the way, what does the speedo read just before you're hitting the brakes?

[nipper]

Engine braking is used in conjuction with the brakes.

 

The addage that pads are cheaper then an engine is true, but dangerous.

The problem with long hills and just using the brakes is that the brakes over heat and you loose the brakes. Then you find out that cost of the car and your life is much more then brake pads. Also riding the brakes will warp your rotors if its done often enough.

 

If the hill is long enough to use engine braking, use the brake pedal too, dont ride the brakes, just press it enough to bring the car speed down to where your comfortable, release, and repeat. It takes far less work for the brakes to maintain the cars speed with this method.

 

I dont know of anyone who has blown an einge during engine braking, as the engine at that point is just a giant air pump, using the work of compression to keep the speed down.

 

nipper

[axgutt]

I'll check the speed tomorrow morning on my way in to work. It's going pretty fast for second gear. What really sucks is that I basically drop down into the resort from the Blue Ridge Parkway, then climb back up to the golf course. If we could just cut over, it would save tons of mileage, not to mention brake pads. Apparently most employees replace their pads every 8 months or so.

 

Any particular model of pads that are recommended?

 

Adam

 

Engine braking is used in conjuction with the brakes.

 

The addage that pads are cheaper then an engine is true, but dangerous.

The problem with long hills and just using the brakes is that the brakes over heat and you loose the brakes. Then you find out that cost of the car and your life is much more then brake pads. Also riding the brakes will warp your rotors if its done often enough.

 

If the hill is long enough to use engine braking, use the brake pedal too, dont ride the brakes, just press it enough to bring the car speed down to where your comfortable, release, and repeat. It takes far less work for the brakes to maintain the cars speed with this method.

 

I dont know of anyone who has blown an einge during engine braking, as the engine at that point is just a giant air pump, using the work of compression to keep the speed down.

 

nipper

[nipper]

For your purpose i would recomend ceramic pads. i am sure others will chime in with a brand.

 

 

nipper

[Hotshot]

I know exactly what bit of road you are talking about. I worked there this winter, and my brother is working at the lake this summer, so I have driven that mountain my fair share of times. Nothing really to contribute, but I find it neat that someone here on the board (probably) drives right by my house every day.

[OB99W]

Engine braking is used in conjuction with the brakes.

 

The addage that pads are cheaper then an engine is true, but dangerous.

The "addage" is only dangerous if friction brakes are used to the exclusion of engine braking. Nipper, since it seems your statement was in response to mine, let me make it clear that I'm not suggesting that friction brakes be used as the sole means of slowing a vehicle on a 15%/one-mile-long grade.

 

 

The problem with long hills and just using the brakes is that the brakes over heat and you loose the brakes. Then you find out that cost of the car and your life is much more then brake pads. Also riding the brakes will warp your rotors if its done often enough.

This all depends upon the grade/length, and quality/condition of the brakes. That's why I suggested that untrustworthy brakes be worked on until they're up to proper performance.

 

 

If the hill is long enough to use engine braking, use the brake pedal too, dont ride the brakes, just press it enough to bring the car speed down to where your comfortable, release, and repeat. It takes far less work for the brakes to maintain the cars speed with this method.

Using the service brakes in conjunction with engine braking is the right thing to do, but in the correct proportion. Choosing particularly low gearing that results in high engine revs, while not resorting to friction braking until the last moment, isn't the best approach.

 

 

I dont know of anyone who has blown an einge during engine braking, as the engine at that point is just a giant air pump, using the work of compression to keep the speed down.

I'm again going to assume that you're responding to my previous statement. I didn't say that this behavior would necessarily result in a "blown" engine. However, what happens under engine braking is that the forces on certain parts are not the same as those under cylinder firing. In particular, the connecting rods see high tensile forces during part of the stroke that they aren't "normally" asked to sustain. Know anyone who's had a rod fail while driving in a mountainous area?

 

One more aspect of engine braking versus friction braking has to do with driving under slippery conditions. If you begin sliding due to excessive friction braking, lifting your foot off the pedal (an almost "automatic" response for most experienced drivers) will usually help regain control of the car. If instead you've downshifted, and start sliding due to excessive engine braking, if you don't react fast enough and quickly upshift (not an "automatic" response for most of us) you may not recover from the slide.

 

I hope the above is food for thought.

[forester2002s]

My own take on this is that 5000 RPM is too high for engine braking alone.

 

I would use 3rd gear (instead of 2nd), and limit the engine to no more than about 3500 to 4000 RPM, by using occasional pedal braking.

[jamal]

You do not have anything to worry about. I'll still suggest some Hawk HPS pads. They're one of the few street performance pads for your car. I really liked them and have also heard good things (primarily from Subiegal, a real race car driver) about EBC red and yellowstuff.

 

While the motor is turning fast, it's circulating coolant and oil at a rate that will keep things cool and lubricated under full throttle, and the heat and all of the forces in the motor are much less while engine braking.

 

There is no way in hell you are going to spin a bearing because of the tensile forces in the rods. Metals are great materials because they can withstand forces in any direction. Additionally tension in the rod is going to create a much lower force on the bearing than the other side sees on a power stroke.

 

The whole reason for a redline is that you'll be pretty much fine operating the motor anywhere below that.

 

The big limitations to Subaru motors are cooling, the valve train, and lubrication to the main bearings. Cooling obviously isn't a problem for you, valves don't float until somewhere over redline, and the crank needs to be spinning at over 8krpm before the oil pressure/flow isn't high enough to lubricate the rod bearings.

[nipper]

Usually redlines are set around the point where valve float starts to happen. Valve springs can only go so fast to keep up with the cams.

 

As was stated, the problems happen when the engine is making max HP at full throttle, (or just off max HP). The oil is relativly cool, oil sheer forces low, engine coolant is relativly cool, and combustion pressures and temps are at thier lowest.

 

There is a complicated explination on how your moving the wasted energy (heat of friction) from a metal disc with limited abilty to shed the heat, to hot gasses inside the engine. We all know the engine is designed to deal with hot gasses very well. I will skip this thermodynamically leaning explanation, as it even gives me a headache (and i am an automtotive engineer).

 

Your not hurting the car.

 

I did see one cute explimantaion on a trialer towing site, saying reason #4 for using engine braking with or without a trailer going down hill. They said the odds of you showing up on youtube with your brakes on fire are much greater if you dont downshift.

 

nipper

[OB99W]

[...]There is a complicated explination on how your moving the wasted energy (heat of friction) from a metal disc with limited abilty to shed the heat, to hot gasses inside the engine. We all know the engine is designed to deal with hot gasses very well. I will skip this thermodynamically leaning explanation, as it even gives me a headache (and i am an automtotive engineer).

Nipper, based on your understanding of thermodynamics, I don't understand how you could say the following about friction braking in your post #3:

[...]If the hill is long enough to use engine braking, use the brake pedal too, dont ride the brakes, just press it enough to bring the car speed down to where your comfortable, release, and repeat. It takes far less work for the brakes to maintain the cars speed with this method.[...]

In a properly designed and maintained automotive service brake system, there should be no difference in the heat buildup whether the brakes are applied constantly (controlled braking) or stabbed/snubbed. Snub braking is a technique taught to truckers driving big rigs; it's necessary because air brakes found on some tractors/trailers don't all actuate under light braking (low air pressure), and that can cause excessive heating in the trailer brakes while leaving the tractor's relatively cool. By braking more heavily, there's a much better chance that all the brakes will actuate (due to higher air pressure), and the heat will be more evenly distributed.

 

Snub braking a car isn't wrong, especially if the road is dry and free of loose material, but under slippery conditions it can lead to a slide. Controlled braking is less likely to cause that. Of course, if a car is in a slide, pumping the brakes can help in regaining steering control while still slowing the car. (In case anyone is wondering why I keep mentioning the possibility of a slide, it's due to "I just started a new job at a ski/golf resort" in the original post. It may not be ski season yet, but developing good habits before then is better than trying to break bads ones when it counts.)

 

Assuming good initial design, brake balance can be retained by proper maintenance. That means pads/shoes have to be decent, the rotors/drums free of significant rust and not cut beyond spec, nothing is hanging up due to corrosion or lack of lubrication, etc.

 

Get the brakes in shape (if they're not at this time), then combine correct gear selection with prudent brake pedal usage, and there shouldn't be a problem descending mile-long 15% grades.

[nipper]

Downshifting is much safer then braking going down hills in slick conditions.

 

The key to maintaining control on a car is to keep the wheels spinning, so that they do not loose traction with the road. Brakes can loose traction easily by locking up. It is nearly impossible to loose traction by downshifting a car (especially a subaru) , ice being the exception.

 

Thats why one should downshift in snowy conditions, on a slick road surface. Unless your driving like a fool, the mass of the car will slow you down without skidding on any surface. Brakes Can apply more braking force then is needed, and lock up the wheels. Thats an instant loss of traction.

 

nipper

[Virrdog]

You'll be fine as long as your engine sees those kind of revs at other times.

 

But they definitely don't have to be exclusive (engine braking and, umm... brake pad braking) use the brakes every once in a while to keep your revs in check if it worries you.

 

I love my Hawk HPs because of how they handle high heat (with increased grip) but supposedly they are rough on the rotor. So by time you replace the pad, you will probably be getting new rotors. But that is not that bad. Having good fluid is key since the boiling point of the fluid will drop as it ages. Putting SuperBlue or some other synthetic fluid in will also help with its improved ability to handle high heat.

[OB99W]

[...]There is no way in hell you are going to spin a bearing because of the tensile forces in the rods.

Who said anything about spinning a bearing? Con-rod failure is often due to stress fractures, and the rod breaks at the beam near either the big or small end. An engine with 279k miles on it may have had lots of stress on the rods already.

 

 

Metals are great materials because they can withstand forces in any direction. Additionally tension in the rod is going to create a much lower force on the bearing than the other side sees on a power stroke.[...]

Metals, like many materials, are typically much stronger in compression than in tension. There are reasons why mountain driving sometimes leads to a rod through the block; the additional stress caused by uphill power requirements isn't the only one.

 

Conditions under high revs with closed throttle are not the same as under identical high revs but with the throttle open. The increased tension in the rod isn't great, but it isn't insignificant either. Most people think that the reason engine braking works is due to losses during the compression stroke; it seems obvious that forcing those air molecules together would expend lots of energy, but you get most of it back when the "air spring" pushes the piston back down again as soon as TDC (top dead center) is passed and the air reexpands (darn thermodynamics ). Actually, the energy loss (and therefore braking effect) is mostly due to the effort required on the intake stroke, pulling a vacuum against the closed throttle. Whatever energy that is, the connecting rods see the increased tension caused by it.

 

Back to the original focus of the thread -- proper pedal braking in conjunction with a suitable gear selection for engine braking is the right approach.

[OB99W]

Downshifting is much safer then braking going down hills in slick conditions.

 

The key to maintaining control on a car is to keep the wheels spinning, so that they do not loose traction with the road. Brakes can loose traction easily by locking up. It is nearly impossible to loose traction by downshifting a car (especially a subaru) , ice being the exception.

The key to maintaining control is to have minimal difference in the tangential speed of the tires relative to the road surface. Anything done to induce a significant change can cause loss of traction. The tires and road surface don't care whether a speed difference is induced by pedal braking or engine braking.

 

Pedal braking can certainly induce the speed difference, but if a loss of traction happens, getting off the pedal normally results in the wheels coming back up to speed in a reasonable time (except, perhaps, on glare ice). Unfortunately, if downshifting results in loss of traction, getting the drive train speed back up so that the tires and road are "in sync" again requires either a lot more time to accelerate the drive train against the engine braking, upshifting, or actually getting on the gas (totally non-intuitive for most people, particularly if they're going downhill).

 

 

Thats why one should downshift in snowy conditions, on a slick road surface. Unless your driving like a fool, the mass of the car will slow you down without skidding on any surface. Brakes Can apply more braking force then is needed, and lock up the wheels. Thats an instant loss of traction.

Sure, downshifting with caution, not picking too low a gear, and the engine braking developed can slow the car without too much loss of traction. Downshifting unnecessarily on a slick surface can lead to more likelihood of traction loss, AWD or not, since the wheel torque is higher the lower the gear.

 

Pedal braking (like many other things in life ) can be done to excess, especially by inexperienced drivers. Learning how to drive under all kinds of weather/road conditions is always a good idea.

[jamal]

Who said anything about spinning a bearing? Con-rod failure is often due to stress fractures, and the rod breaks at the beam near either the big or small end. An engine with 279k miles on it may have had lots of stress on the rods already.

 

So, to get this straight, you're saying that engine braking can pull a rod apart in tension? Specifically in an EJ? When has this ever happened?

 

 

Metals, like many materials, are typically much stronger in compression than in tension. There are reasons why mountain driving sometimes leads to a rod through the block; the additional stress caused by uphill power requirements isn't the only one.

 

Uh, no, actually, most metals have nearly the same yield strength in both tension and compression.

 

I can all but guarantee that a rod through the block in a Subaru is caused by one of two things:

 

1) oil starvation to the rod bearings due to sustained high revs.

2) detonation due to overheating after sustained high load.

 

These are not things that happen on a stock car in good working order, and sometimes on cars are modified for power and tracked continuously. Rods are not just going to snap because you have 250k miles and are driving in the mountains.

[MDW25gt]

My own take on this is that 5000 RPM is too high for engine braking alone.

 

I would use 3rd gear (instead of 2nd), and limit the engine to no more than about 3500 to 4000 RPM, by using occasional pedal braking.

 

I agree!

[OB99W]

So, to get this straight, you're saying that engine braking can pull a rod apart in tension?

No, I'm saying that it can contribute to fatigue that may lead to eventual failure.

 

 

Uh, no, actually, most metals have nearly the same yield strength in both tension and compression.

That's theoretically true, but mostly borne out only in lab testing of samples that have no irregular surfaces. In the real world, metals often have rough finishes and develop surface fissures/cracks as they are manufactured/used/abused. Those irregularities tend to close up during compression and so aren't much of a factor for strength in that mode; however, under tension they can open and grow, possibly becoming severe enough to cause complete fracture.

 

 

I can all but guarantee that a rod through the block in a Subaru is caused by one of two things:

 

1) oil starvation to the rod bearings due to sustained high revs.

2) detonation due to overheating after sustained high load.

 

These are not things that happen on a stock car in good working order, and sometimes on cars are modified for power and tracked continuously. Rods are not just going to snap because you have 250k miles and are driving in the mountains.

So barring 1) and 2) (and ignoring bearing surfaces), are you saying that a rod with 279k miles of use on it is in the same condition as when it was manufactured? Are you also saying that mountain driving with high-RPM engine braking doesn't stress engine components such as connecting rods more than level driving, or perhaps that it only stresses them in compression but not tension?

 

As to cars that are modified for racing, a typical approach is to use parts such as connecting rods that are carefully radiused and polished in order to minimize the development of stress cracks that lead to failure.

[subaruplatt]

Just wanting to add something.....

Last month I returned home coming south from the east side of algonquin park. There where long steep hills and I found that fourth gear in the auto didn't provide the engine breaking necessary for the cruise control to work effectively down steep hills. I was reaching speeds of 140+ KPH .

I discovered that it was quite convent to downshift the auto into third.

To my amassment the cruise control didn't kick out and with the increased engine breaking the speed was really well controlled without touching my breaks, or the gas. I didn't even think that revs were really that excessive.

 

I believe the cool circulation of air does adequately offset the increased engine RPMs.

Not talking about dropping the shifter and flooring it or revving it upwards throughout the gears.

Just talking about wisely anticipating the decline and picking your gears accordingly.

I believe that truckers state that you should always go down a hill in the same gear as you went up the hill. We did feel the car down shift on some of the inclines, especially if we were trying to gain momentum. http://www.ultimatesubaru.org/forum/images/icons/icon14.gif

[DerFahrer]

Can you guys explain to me how the rods are ever in tension?

 

When the engine is working to turn the wheels, the air is compressing against the rods to turn the crank. When the wheels are working to turn the engine, the crank is pushing against the rods to compress the air.

 

Unless there's something that I'm not understanding, the rods are always under compression as long as the engine is turning.

[zyewdall]

 

I dont know of anyone who has blown an einge during engine braking, as the engine at that point is just a giant air pump, using the work of compression to keep the speed down.

 

nipper

 

A friend of mine threw a rod on her '91 EJ22 while engine braking around 5500rpms on a long hill in 2nd gear. It still ran afterwards, but on three cylinders, and oil was splashing out of the hole in the top of the block. I'm not sure it had anything to do with the high rpms though -- because i'm convinced that she has a poltergiest in her car. It was the second EJ22 that her car ate in a year. And she regularly (like every two weeks) gets flats on her driveway when no one else does. Scoff if you will, but I've never seen a subaru have so many problems. And when she borrowed two other vehicals while getting the engine in the suby replaced, they both stopped running too....

[jamal]

So barring 1) and 2) (and ignoring bearing surfaces), are you saying that a rod with 279k miles of use on it is in the same condition as when it was manufactured? Are you also saying that mountain driving with high-RPM engine braking doesn't stress engine components such as connecting rods more than level driving, or perhaps that it only stresses them in compression but not tension?

No, no, yes. Show me a rod that failed in tension due to engine braking. Actually, just show me a rod that failed in tension. It doesn't happen. Either the rod bearing stops working and things go crunch, or detonation occurs, causing a melted piston and/or rod bending.

 

Can you guys explain to me how the rods are ever in tension?

 

When the engine is working to turn the wheels, the air is compressing against the rods to turn the crank. When the wheels are working to turn the engine, the crank is pushing against the rods to compress the air.

 

Unless there's something that I'm not understanding, the rods are always under compression as long as the engine is turning.

 

The throttle plate is closed on the intake stroke.

 

I was going to do the math and calculate the tension in a rod assuming atmospheric pressure in the cylinder at tdc but don't really feel like doing that right now.

 

 

A truck's compression brake will actually have sort of a reverse v-tec thing that actuates valves differently when it is turned on to create more engine braking. They even have engine braking horsepower ratings.

[OB99W]

No, no, yes. Show me a rod that failed in tension due to engine braking. Actually, just show me a rod that failed in tension. It doesn't happen.

As I've been saying, and you've been ignoring, tensile forces can cause fatigue which leads to failure. The final, complete fracture might well occur at any point, including a time of compression. That doesn't mean tensile forces didn't lead to or compound the problem.

 

 

Either the rod bearing stops working and things go crunch, or detonation occurs, causing a melted piston and/or rod bending.

Certainly those are two possible causes, but not the only ones.

 

 

I was going to do the math and calculate the tension in a rod assuming atmospheric pressure in the cylinder at tdc but don't really feel like doing that right now.

Please do the math, and let us know the results. Once you do, and think you have "proof" that the forces couldn't be sufficient to be a problem, I'll have further comment. You may be surprised at what's involved that may not be immediately obvious.

 

Oh, and to avoid confusion, I'm OB99W, not obw99.

[DerFahrer]

The throttle plate is closed on the intake stroke.

 

But doesn't the IAC valve open whenever the throttle plate is shut?

[hankosolder2]

But doesn't the IAC valve open whenever the throttle plate is shut?[/quote

 

When the engine is doing 5000+ RPM due to engine braking and the TPS reads that the throttle is fully closed, I believe that a common engine management strategy is that the ECM cuts the fuel to the injectors and the IAC would be closed. The fuel is restored once the engine speed drops below the 1500 rpm mark and the IAC will begin to open once the engine speed nears normal idle speed. (Please note: I don't know for a fact that Subaru's engine management strategy works this way, but I do know that many cars use a similar strategy.)

 

Nathan