Stuck in V-8 mode?

Dunc · July 10, 2015

Engine braking was not on. I have been driving those roads all the time and it has dropped to V-4 all the other times.

Engine braking isn't a feature, it's a characteristic of all car engines so it's essentially always "on." I meant it as though maybe there is logic in software that trys to regulate speed increases (before grade brake activates.)

ember1205 · July 10, 2015

Engine braking isn't a feature, it's a characteristic of all car engines so it's essentially always "on." I meant it as though maybe there is logic in software that trys to regulate speed increases (before grade brake activates.)

Yes, engines will naturally slow a vehicle down when you take your foot off of the accelerator, working essentially like a brake. Many of these trucks have Grade Braking in them, though, where the truck adjusts the gear (and the engine speed) to increase the -amount- of braking that the engine provides. This feature, I believe, is what was actually being discussed.

Debra in Cali · July 10, 2015

Engine braking isn't a feature, it's a characteristic of all car engines so it's essentially always "on." I meant it as though maybe there is logic in software that trys to regulate speed increases (before grade brake activates.)

Yes, engines will naturally slow a vehicle down when you take your foot off of the accelerator, working essentially like a brake. Many of these trucks have Grade Braking in them, though, where the truck adjusts the gear (and the engine speed) to increase the -amount- of braking that the engine provides. This feature, I believe, is what was actually being discussed.

Ember nailed it. Sure saves on the brakes when you are towing. Very cool feature.

ember1205 · July 10, 2015

Ember nailed it. Sure saves on the brakes when you are towing. Very cool feature.

While I'm all for saving the brakes at that exact moment in time so that they aren't overheated if I really need them (like towing or hauling heavy, downhill, and steep), brakes are maintenance items. "Over" heating the transmission fluid (making hotter than it needs to be, not necessarily overheating it), overworking the driveline, and running the engine at higher RPMs and wasting gas are what that features "costs" you. I do like having it there as it helps keep the vehicle traveling at a reasonably constant speed and keeping me from having to keep correcting it. But, it comes at a cost - one that most of us really don't think about.

Jon A · July 11, 2015

Will leaving all 8 firing increase engine breaking? I would think so, but don't know. Perhaps the truck leaves all 8 on to try to satisfy some software logic about engine braking.

Yes-- sort of. Technically the cylinders aren't firing but they are pumping air against a resriction. The effectiveness of engine braking is directly related to the displacement of the engine--V4 mode cuts the displacement in half. This is exactly why GM goes through the trouble of collapsing lifters and sealing off four cylinders in V4 mode--to prevent them from pumping air. You could get the engine to run on four cylinders by simply shutting off four injectors, but if those cylinders are still pumping air they are in effect "engine braking" when you're trying to push the vehicle forward with the other four cylinders--not exactly good for efficiency.

While I'm all for saving the brakes at that exact moment in time so that they aren't overheated if I really need them (like towing or hauling heavy, downhill, and steep), brakes are maintenance items. "Over" heating the transmission fluid (making hotter than it needs to be, not necessarily overheating it), overworking the driveline, and running the engine at higher RPMs and wasting gas are what that features "costs" you. I do like having it there as it helps keep the vehicle traveling at a reasonably constant speed and keeping me from having to keep correcting it. But, it comes at a cost - one that most of us really don't think about.

1) It doesn't cost any gas. The injectors are shut off. Your aren't using any gas at all.

2) It puts very little heat into the transmission--dramatically less that going up the hill does. These trucks keep the converter locked so the transmission is running very efficiently when grade braking. The transmission on these trucks while towing will cool down fairly quickly while going down hill if they had heated up noticeably going up whether they are engine braking or not.

3) While the engine does run at higher RPM, it is burning no gas so cylinder pressures are low and the engine cannot produce anywhere near as much negative torque as the positive torque it produced to get up the hill, so again, relatively speaking it's not a worry. If it was cheaper in the long run to not use the engine and simply replace the brakes more often, that's what commercial trucks would be doing.

ember1205 · July 11, 2015

Yes-- sort of. Technically the cylinders aren't firing but they are pumping air against a resriction. The effectiveness of engine braking is directly related to the displacement of the engine--V4 mode cuts the displacement in half. This is exactly why GM goes through the trouble of collapsing lifters and sealing off four cylinders in V4 mode--to prevent them from pumping air. You could get the engine to run on four cylinders by simply shutting off four injectors, but if those cylinders are still pumping air they are in effect "engine braking" when you're trying to push the vehicle forward with the other four cylinders--not exactly good for efficiency.

1) It doesn't cost any gas. The injectors are shut off. Your aren't using any gas at all.

2) It puts very little heat into the transmission--dramatically less that going up the hill does. These trucks keep the converter locked so the transmission is running very efficiently when grade braking. The transmission on these trucks while towing will cool down fairly quickly while going down hill if they had heated up noticeably going up whether they are engine braking or not.

3) While the engine does run at higher RPM, it is burning no gas so cylinder pressures are low and the engine cannot produce anywhere near as much negative torque as the positive torque it produced to get up the hill, so again, relatively speaking it's not a worry. If it was cheaper in the long run to not use the engine and simply replace the brakes more often, that's what commercial trucks would be doing.

if the engine is firing, it's using fuel. Period. There isn't a motor on the planet that burns "zero fuel" because the vehicle is coasting downhill. This also negates your argument in point three.

Jon A · July 11, 2015

The engine is running, it's spinning, just not "firing." You might want to google DFCO (Deceleration Fuel Cut Off) and do some reading. I have a data log of me towing down a mountain pass where my injectors basically didn't fire at all for about 5 minutes. And my transmission cooled down by 20 degrees during that time. 14,200 GCW and I didn't have to touch the brake at all the whole way down. These new trucks rock....

I did think the same things you did years ago on the subject and there may have been some truth to them at some point as engines, clutches, transmissions, etc weren't nearly as durable back then, and computer controls not as sophisticated, but times have changed. The data speaks for itself.

j-ten-ner · July 11, 2015

if the engine is firing, it's using fuel. Period. There isn't a motor on the planet that burns "zero fuel" because the vehicle is coasting downhill. This also negates your argument in point three.

Go to "instant fuel economy" on your DIC and you can actually see when the fuel shut off happens.

And you can actually feel it as well.

so long

j-ten-ner

redwngr · July 12, 2015

if the engine is firing, it's using fuel. Period. There isn't a motor on the planet that burns "zero fuel" because the vehicle is coasting downhill. This also negates your argument in point three.

With electronic fuel injection, it is possible for there to be 'zero' fuel injected.