Big Cottonwood Canyon Photoshoot and Near Disaster

[doradox]

Opening the throttle would decrease engine braking...

You did read the key off part didn't you?

At low throttle openings you aren't pumping much air. At large throttle openings you pump a lot of air. Which is harder?

On a recent trip to Colorado we climbed and descended Pikes Peak. The automatic grade braking system in the wife's Denali was most certainly opening the throttle while holding a low gear. You could hear the intake noise dramatically increase when normal compression braking wasn't doing the job. I wanted to see what was happening so I stopped and hooked my scanner up (I always carry a lot of tools when I'm on the road) and continued down. TPS reading verified what I was hearing.

You have a car. Give it a try.

Steve

[LeonV]

You did read the key off part didn't you?

At low throttle openings you aren't pumping much air. At large throttle openings you pump a lot of air. Which is harder?

On a recent trip to Colorado we climbed and descended Pikes Peak. The automatic grade braking system in the wife's Denali was most certainly opening the throttle while holding a low gear. You could hear the intake noise dramatically increase when normal compression braking wasn't doing the job. I wanted to see what was happening so I stopped and hooked my scanner up (I always carry a lot of tools when I'm on the road) and continued down. TPS reading verified what I was hearing.

You have a car. Give it a try.

Steve

I would think that an engineer would base a statement like, "maximum engine braking occurs in gear with the key off at full throttle" off something more than one data point. I also like how your responses must always be condescending, is that how you deal with people face-to-face too? Anyway...

If your wife's truck showed that it would go full-throttle under engine braking, then I think it would be safe to assume that it has some sort of variable valve timing.

The trapped air within the cylinder acts as a spring when the valves are closed. The main losses of a motored engine are frictional, with the rest being pumping losses past the valves and throttle (ignoring skin friction in runner walls). On an L-series engine, valves have a constant timing and lift profile, so the only thing you can manipulate is the throttle, thus increasing/decreasing pumping losses at the throttle.

An engine with variable valve timing can alter when the intake valve closes and exhaust valve opens, thus the exhaust valve can be opened much earlier which would then bleed off the work that was done to the air when compressing it. This would be similar to a Jake Brake in diesel truck applications.

It was YOUR recommendation so go ahead and prove it. With your Z this time and not a modern, computer-controlled truck. Personally, I'd be interesting in testing this myself but both of my Zs are disabled for the time being. My hypothesis would be that engine braking is MINIMUM at wide-open throttle.

FWIW, here's an interesting tid-bit from Cummins:

Diesel engines are 'compression ignition' engines, which means that there are no

external means to ignite the fuel and air mixture, except by compressing the

fuel/air mixture to cause enough heat to promote combustion of the mixture. The

compression ratios for Cummins diesel engines are currently around 14:1 for

larger Cummins engines and 16.5:1 or 17.2:1 for some of the B series smaller

engines and gasoline engines, for example are about 8 to 1. Since gasoline

engines are spark ignited, they have less compression ratio, where the diesel

must 'compress' the fuel/air mixture into a smaller space to create enough heat,

to initiate ignition.

A diesel, being a free-breathing engine by virtue of having no valving or

venturi restrictions (carburetor) offers inherently less braking effort than a

gasoline (spark ignition) engine even though the diesel engine has about twice

the compression ratio of the spark ignition engine (which obviously means that

it requires more effort to force up the piston on the compression stroke), that

work is given almost entirely back (less friction and heat transfer losses) when

the air is allowed to expand on the next outward stroke.

Conversly, the spark ignition engine becomes a vacuum pump when being motored

with the throttle closed.

[doradox]

I would think that an engineer would base a statement like, "maximum engine braking occurs in gear with the key off at full throttle" off something more than one data point. I also like how your responses must always be condescending, is that how you deal with people face-to-face too? Anyway...

If your wife's truck showed that it would go full-throttle under engine braking, then I think it would be safe to assume that it has some sort of variable valve timing.

The trapped air within the cylinder acts as a spring when the valves are closed. The main losses of a motored engine are frictional, with the rest being pumping losses past the valves and throttle (ignoring skin friction in runner walls). On an L-series engine, valves have a constant timing and lift profile, so the only thing you can manipulate is the throttle, thus increasing/decreasing pumping losses at the throttle.

An engine with variable valve timing can alter when the intake valve closes and exhaust valve opens, thus the exhaust valve can be opened much earlier which would then bleed off the work that was done to the air when compressing it. This would be similar to a Jake Brake in diesel truck applications.

It was YOUR recommendation so go ahead and prove it. With your Z this time and not a modern, computer-controlled truck. Personally, I'd be interesting in testing this myself but both of my Zs are disabled for the time being. My hypothesis would be that engine braking is MINIMUM at wide-open throttle.

FWIW, here's an interesting tid-bit from Cummins:

My one data point dis-proved your hypothesis. That's all it takes. Also my Denali doesn't have variable valve timing or cylinder deactivation. It's a relatively simple analysis to do if you remember your thermo. Draw a box around the engine and account for inputs and outputs. I wouldn't exactly say I recommend it either. That's a lot unburned of fuel/air mix washing the cylinders down in a carbed car.

[alternativez]

Ninja, it is well you and everyone else on that highway are safe. You were foolish but lucky. Here's another perspective: coasting down grade in neutral is not only dangerous, but also against the law in many states for just that reason. Your video has provided proof of at least one driving infraction, maybe more. Surely a driving course or the study to acquire the privilege of a dr. license would advise that it is the responsibility of the driver to be well acquainted with the laws and driving considerations of that state or any through which one drives. Youth or blame are not excuses for ignorance. Such laws are easily accessible online. The ensuing debate about downhill engine braking in a properly functioning vehicle is productive, but mute in this case when the driver should have immediately put on emergency flashers and pulled off to ensure everyone's safety. Your recorded comments demonstrated that you did recognize but ignored the fatal possibility of filming your own death. Perhaps having an in-car camera and the present day propensity to record and publicly share bizarre, exciting, risky, and even illegal behavior were motivation to discard common sense and consideration for others, including your buddy in front of you, whom I'm sure would have come back if you had put on flashers, flashed headlights, or better yet, pulled off. Sorry. I don't mean to be rude. I hope you will give serious retrospection rather than defense, and I'm glad you survived to openly and honestly present the topic for discussion and education. This thread will perhaps save lives. "Live and learn," yes, but better yet, "learn and live!"

[Kennymonster]

^All things given, filming his own death could've been the best case scenario had he lost control or under steered into oncoming traffic (there were plenty in the vid, including bikes). I also saw plenty of shoulder spaces on some stretches, that's where the vid should've stopped. Sorry to say you've risked many lives in danger, I'd hate to see a biker or a car full of family & kids in a fatal collision when the fault is 100% on the OP.

[ninjazombiemaster]

Sorry to say you've risked many lives in danger

Ninja, it is well you and everyone else on that highway are safe. You were foolish but lucky. Here's another perspective: coasting down grade in neutral is not only dangerous, but also against the law in many states for just that reason.

You pretty much did everything wrong in that scenario and were lucky that there wasn't a need for any panic stops. As soon as the engine cut, you should've pulled over to a stop as soon as it was safe to do so.

Frankly, that was just plain stupid. If your engine dies pull over as soon as you can, and don't continue hurtling down the road.

Yes, I get it. It was dumb and dangerous. If I ever find myself in that situation I will act differently, however, what has already happened cannot be changed, and was in a moment of emergency I wasn't prepared fully for at the time. What is important is I learned from the situation, and everyone came out okay.

[Captain Obvious]

they didn't really teach true manual transmission driving or technique. There were "simulators"

How many drops is this for you, Lieutenant?

I always leave the car in gear until right before I stop, no matter whether the ground is flat or I'm on an incline.

Me too. And I don't use the clutch to take it out of gear. Once you slow down to the point where the gear train is unloaded it slips out easily.

[bartsscooterservice]

The engine dying must be some other problem. There must be enough oxygen there, your buddies car keeps running..

[FastWoman]

What is important is I learned from the situation, and everyone came out okay.

Absolutely! Please don't feel beaten upon. I think we all did dumb things in our youth. I think this was just a teachable moment. I would like to think that someone would take the time to have the same sort of talk with my own kids over this sort of thing.

As for engine braking technique: It's pretty much flat where I live, and I'm of the mind that brakes are easier/cheaper to service than clutches. So I, too, leave my car in gear as I'm rolling to a stop. I don't do any downshifting unless there's really a need (e.g. I'm towing a trailer and want to keep an excessive load off the brakes). Doing this extends the life of the brakes and makes no difference to the life of the clutch.

And rolling down a long grade (your situation), I will drop a gear or two, such that I can coast down at approximately the speed I want without using the brakes. I think 3rd would have worked fine for the grade you took. If you remember seeing any caution signs that said "Grade. Use low gear," that's what that was all about, whether automatic or stick.

FAIW, I remember once driving down a grade about that long, but actually much steeper, with sharper curves, in a Dodge Ram 2500 truck, towing a rather heavy 30' camper. The entire rig was 50 ft long. My heart was pounding the entire way, and I needed about an hour at a diner to chill after I reached the bottom. I think I did it in 1st gear (automatic transmission) at approximately 35 - 40 mph, with the engine winding up within maybe 1000-500 RPM of redline. And if I could have avoided that situation, I would have avoided it. Unfortunately there was no warning before I was committed to it -- only a sign telling me I'm in deep @#$% for the next several miles, with no pull-offs available. (Thanks for the useful info, Arkansas!) Nobody was impatient with me. I think everyone on the road understood that doing it any differently could have resulted in someone's death. Mountains are different: Gravity is not your friend. Nobody will fault you for doing what you have to do to stay safe.

Edited August 23, 2013 by FastWoman

[sam280z]

The correct thing to do would have been to stop the car and get it running before descending. Barring that, descending a little at a time allowing the brakes to cool at regular intervals.

Using engine braking on a non-running carbureted engine is a good way to fill up the exhaust system with gasoline.

[Stanley]

Keeping the speed down seemed to help me once, in the old chevy truck, heading South from Oregon on the coastal route. That's a long downgrade with curves. Smelled the brakes, felt them starting to fade. the truck tops out about 75 with the 230 and a 4 point something rear end. Got it stopped on a truck run-off and saw them glowing red. After they cooled off I took it real slow - 2nd gear with frequent stops, no proble When I got back did a full brake job on it.

Another time in the truck, had a job in the Hollywood hills. Very steep narrow road with tight curves. Felt the brakes about gone at the top of a hill. Got out, tightened up the adjustment, brakes ok again.

Wonder how a z with stock brakes will do on Pike's peak for example? Is it safe to go as fast as traffic, driving ability, and the road will allow, or is it necessary to use take it slow, use engine braking, or stop every 5 minutes or so for a cool-down? What brake mods would you need to really stand on the gas on the way down? How would a rally driver in a Z take that downhill if their brakes were stock, for the best time?

[ninjazombiemaster]

The engine dying must be some other problem. There must be enough oxygen there, your buddies car keeps running..

Hes got a 280z with EFI. It should be much less effected by altitude.

Absolutely! Please don't feel beaten upon. I think we all did dumb things in our youth. I think this was just a teachable moment. I would like to think that someone would take the time to have the same sort of talk with my own kids over this sort of thing.

No worries. I live in a valley with many steep hills, and this has all been valuable information.