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Topic: Engine braking Gas Vs Diesel

Posted By: Keith Haw on 12/03/15 10:44pm

Just wondering why gas engines have better engine braking than diesel. No real reason, just wondering. With the higher compression of the diesel I would think that diesel would be better but understand it isn't. I know it was fairly good on the big trucks when I was a trucker.


Posted By: Old-Biscuit on 12/03/15 10:52pm

Simple reason

VACUUM

Gas engine controls air flow via throttle. When closed high vacuum is created in intake

Diesel engine controls fuel flow. No throttle no vacuum no engine breaking
Engine braking on diesel is done by 'Exhaust Brake' or a 'compression release brake (Jake Brake) IF they are so equipped otherwise no engine braking will occur


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Posted By: Paul Clancy on 12/04/15 12:24am

Most newer diesel pickups will have a tow haul mode. When engaged it provides a transmission braking program that downshifts the auto agressivly when you lift off fuel pedal. This provides engine braking as good as a gas truck. Many also have grade logic and cruise grade braking as well. Add turbo exhaust brake on many newer trucks and a diesel will hold speed towing down large grades better than gas.


Posted By: tatest on 12/04/15 01:45am

Engine braking for gas engines always comes from intake stroke, working against vacuum of a closed throttle. What you get is proportional to displacement and engine RPM. The 6.8 engine on my motorhome gives me a lot more braking power at 3500-5000 RPM than the 1.5 in my car at any RPM (I can safely run it up to 5000-6000), even though it has a manual transmission. But there is a lot less car to slow down.

For diesels, there are at least two approaches. Lowest cost is to throttle the exhaust, thus "exhaust brake." Yes, higher compression gives more braking on the exhaust stroke than you would get on the intake from a gasser, but there are few diesels you can crank to 5000 RPM for braking, so might be sort of a wash. And of course, displacement and vehicle mass matter.

Since most diesels today are turbocharged, there is already some back pressure in the turbine, for exhaust braking. Some recent variable vane turbo designs adjust vane angle to increase flow resistance, which is almost as effective as throttling the exhaust.

Then there is the compression release brake, which opens the exhaust valve at the top of the compression stroke (Jake brake) which can offer somewhat more abrupt braking than exhaust brake designs, but this has to be designed into the engine. As a trucker, this is probably what your were using.

Same size engine, a gas engine with throttle closed at 5000 RPM will probably provide more braking than a diesel with exhaust valve closed at 1200 RPM, but not many operators have the nerve to do that to their gas engines, more typically gear for 2000-3000 RPM where there is a lot less braking power.


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Posted By: Ivylog on 12/04/15 04:06am

Gas engine has a Butterfly valve. A diesel does not.


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Posted By: is it friday yet? on 12/04/15 04:24am

All I know, is that when I'm in tow/haul mode and have the factory exhaust brake turned on, I actually have to give it some throttle going down hills pulling my trailer or I will get run over for going too slow. I use my exhaust brake every time I drive my truck, trailer or not.


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Posted By: Sport45 on 12/04/15 04:38am

Compression really has little to do with it. The air is compressed on the compression stroke (naturally) and then pushes the piston back down on what would normally be the power stroke. So it's essentially an air spring.

Jake brakes work by releasing the compressed air at the top of the compression stroke. This means energy is used to compress the air but none is returned by the air pushing the piston back down. It's a very effective system.

Others have already mentioned how gasoline compression and what we consider normal diesel exhaust brakes work. They are effective, but not as much as the Jacobs, or "Jake" brake since they don't release energy from the system. (They're not nearly as loud either.)


Posted By: brulaz on 12/04/15 06:26am

Thanks for this. What I'm hearing is that a diesel engine without an exhaust brake has less engine braking than a same sized gas engine.

My 2011 F150 with the 3.5L EcoB has good engine/tranny braking in Tow/Haul mode, but on some steep and windy Appalachian trails I've needed to let it wind up to 4000-5000 rpm in first gear so as not to cook the truck+trailer brakes. From what I'm reading here, a similarly sized diesel without an exhaust brake would be worse.

Have read here that even the RAM 2500 with the I6 Cummins doesn't brake that well on these same trails in first gear because the exhaust brake is disabled when the auto tranny is forced into first. But in auto 2nd, the exhaust brake kicks in and works well. This makes more sense now.

It also makes sense that the RAM Ecodiesel has weak engine braking, from what I've read in the forums probably worse than my 3.5L EcoB, because it does not implement an exhaust brake.

And although nobody seems to know for sure, it's possible the Nissan Titan XD with the Cummins 5L has not implemented the turbo's exhaust brake feature, just an engine/tranny Tow/Haul mode. That would be a major disappointment for sure.

Of the smaller diesels appearing these days, the only one I know for sure has an exhaust brake is the Colorado's Duramax.


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Posted By: Me Again on 12/04/15 06:46am

I wonder how the new direct injection gas engines are doing that have no butterfly valve? Audi, BMW and Mini Cooper all have engines in production with DI.

Back to grade braking! For some reason the V8 diesels in GM and Ford trucks held back better than the Cummins I6 did in the RAMs. Maybe it was the much heavier rotating mass of the Cummins. I know from experience with my 2001.5 RAM that when I had just the smart controller on to keep the torque converter locked with the exhaust brake off, that the engine just free wheeled down grades without hardly any hold back. Chris


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Posted By: RCMAN46 on 12/04/15 06:55am

I do not believe the diesels used in the pickups have true Jake brakes.

The braking is done with the turbo and not with valve timing as in the big rigs.

Ram calls it a Jake brake but it really is not a true Jake brake.


Posted By: brulaz on 12/04/15 07:07am

Me Again wrote:

I wonder how the new direct injection gas engines are doing that have no butterfly valve? Audi, BMW and Mini Cooper all have engines in production with DI.
...


Good point. The F150's 3.5L Ecoboost is Direct Injection, so possibly no air throttle plate, but it does have twin turbos. Maybe not that different from a similar turbo diesel with no exhaust brake after all.

Can a gas DI engine with turbos and no throttle plate ever have an exhaust brake?


Posted By: rhagfo on 12/04/15 07:25am

RCMAN46 wrote:

I do not believe the diesels used in the pickups have true Jake brakes.

The braking is done with the turbo and not with valve timing as in the big rigs.

Ram calls it a Jake brake but it really is not a true Jake brake.


Well the Cummins 6.7 can be equipped with a true engine brake, PacBrake has a kit. That is not badly priced.
Pacbrake Loadleash Engine Brake


My 01 with a after market PacBrake will slow well enough in Direct drive 4th on a 5 speed that I need to either release it or apply a little throttle.

BRULAZ wrote:

My 2011 F150 with the 3.5L EcoB has good engine/tranny braking in Tow/Haul mode, but on some steep and windy Appalachian trails I've needed to let it wind up to 4000-5000 rpm in first gear so as not to cook the truck+trailer brakes. From what I'm reading here, a similarly sized diesel without an exhaust brake would be worse.


Wow, and how fast were you still moving, lowest gear I have ever needed was 3rd, on my 5 speed, traffic was down to about 40.


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Posted By: brulaz on 12/04/15 07:33am

rhagfo wrote:


...
BRULAZ wrote:

My 2011 F150 with the 3.5L EcoB has good engine/tranny braking in Tow/Haul mode, but on some steep and windy Appalachian trails I've needed to let it wind up to 4000-5000 rpm in first gear so as not to cook the truck+trailer brakes. From what I'm reading here, a similarly sized diesel without an exhaust brake would be worse.


Wow, and how fast were you still moving, lowest gear I have ever needed was 3rd, on my 5 speed, traffic was down to about 40.


Down west slope of Newfound Gap in Smoky Mountains National Park. Speed limit was 35mph, many hairpin turns with 20mph speed limit.
The Dragon's Tail just south of the park is similar but not as steep.
Not talking Interstate Highway travel here!
Think I was up to 30mph at 5000 rpm in first gear.


Posted By: Terryallan on 12/04/15 07:43am

Thing is. Diesels need add ons to have the same braking as gas engines do naturally. Mostly because of back pressure. Diesels have tons of compression on the front side. But have a HUGE hole on the back side. Result, no back pressure, and no engine braking. Which is exactly why Jake brakes were invented. To give diesels engine braking.

IMOP. No diesels should be sold with out some kind of add on braking system. Be it exhaust, or, trany brakes.


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Posted By: kyle86 on 12/04/15 07:50am

Me Again wrote:

I wonder how the new direct injection gas engines are doing that have no butterfly valve? Audi, BMW and Mini Cooper all have engines in production with DI.


I was under the impression that DI just places the fuel injector in the combustion chamber instead of the intake runner. They should still have a throttle body to let air into the engine. I could be wrong.






Posted By: ChooChooMan74 on 12/04/15 07:51am

brulaz wrote:


It also makes sense that the RAM Ecodiesel has weak engine braking, from what I've read in the forums probably worse than my 3.5L EcoB, because it does not implement an exhaust brake.


The EcoD does not come with any exhaust brake from the factory. It does have some transmission braking. If you get the Green Diesel Tune, they have an option of Exhaust braking about 2700 RPMs, Transmission in 6th of less (Doesn't work in 7th or 8th).


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Posted By: mowermech on 12/04/15 08:05am

RCMAN46 wrote:

I do not believe the diesels used in the pickups have true Jake brakes.

The braking is done with the turbo and not with valve timing as in the big rigs.

Ram calls it a Jake brake but it really is not a true Jake brake.


If you go to the Jacobs website you will see that they say that ANY engine braking system that is manufactured by Jacobs IS a "Jake Brake".
If the manufacturer says the Jacobs Rambrake used on Dodge/Ram pickups is a Jake Brake, that is good enough for me!


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Posted By: mkirsch on 12/04/15 08:17am

I still don't buy into this whole idea that gas engines are so much better at engine braking.

On the farm, I spent many years hauling loaded wagons from the field to the barn that weighed upwards of 10 tons. It was generally downhill back to the barn so much of the trip involved "engine braking."

For many years in the '80s and early '90s, I hauled with a gasoline powered tractor, and "engine braking" was only good for a couple of seconds at the top of the hill. From there on you rode the brakes the rest of the way down the hill to keep the engine from overspeeding.

In the mid '90s we got a diesel tractor about the same HP and weight as the gas tractor we had been using. That diesel tractor handles the load SO MUCH BETTER. It holds the same loads almost all the way down the same hills at the same speeds with almost no braking needed.

Same thing can be shown with an empty tractor. Going down the road, you pull back the throttle of the gasoline tractor, and it just coasts, driving the engine. Without brakes, you'd sail right on by the driveway. Pull back the throttle on the diesel, and it's like you threw out the anchor.


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Posted By: coolbreeze01 on 12/04/15 08:22am

Jacobs builds engine brakes as well as exhaust brakes.

I bet Jacobs, BD, and or PacBrake will build exhaust brakes for the new diesels if they don't start showing up OE soon.


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Posted By: goducks10 on 12/04/15 08:22am

is it friday yet? wrote:

All I know, is that when I'm in tow/haul mode and have the factory exhaust brake turned on, I actually have to give it some throttle going down hills pulling my trailer or I will get run over for going too slow. I use my exhaust brake every time I drive my truck, trailer or not.


x2. I hate using TH mode when descending. It always wants to go down an extra gear and I slow down too much. I always take it out of TH mode when descending. I suppose if I was maybe towing real heavy, like 12-15,000lbs then maybe it would work better. But I'm only towing 9000lbs.
Towed the same TT with a gasser and no comparison.


Posted By: brulaz on 12/04/15 08:23am

kyle86 wrote:

Me Again wrote:

I wonder how the new direct injection gas engines are doing that have no butterfly valve? Audi, BMW and Mini Cooper all have engines in production with DI.


I was under the impression that DI just places the fuel injector in the combustion chamber instead of the intake runner. They should still have a throttle body to let air into the engine. I could be wrong.


Apparently some eliminate the air throttle plate according to Wikipedi anyway:
Wikipedia on GDI wrote:

In addition some engines operate on full air intake. That is, there is no air throttle plate eliminating air throttling losses in some GDI engines, when compared to a conventional fuel-injected or carbureted engine, which greatly improves efficiency, and reduces piston 'pumping losses'. Engine speed is controlled by the engine control unit/engine management system (EMS), which regulates fuel injection function and ignition timing, instead of having a throttle plate that restricts the incoming air supply.


I'm seeing aftermarket replacements for the 3.5L Ecoboost throttle body, complete with butterfly valve, so I guess that engine does have an air throttle plate. http://www.stage3motorsports.com/1822-1-........013-F150-37L-BBK-73MM-Throttle-Body.html, so that should help with engine braking.


Posted By: ShinerBock on 12/04/15 08:42am

brulaz wrote:

Thanks for this. What I'm hearing is that a diesel engine without an exhaust brake has less engine braking than a same sized gas engine.

My 2011 F150 with the 3.5L EcoB has good engine/tranny braking in Tow/Haul mode, but on some steep and windy Appalachian trails I've needed to let it wind up to 4000-5000 rpm in first gear so as not to cook the truck+trailer brakes. From what I'm reading here, a similarly sized diesel without an exhaust brake would be worse.

Have read here that even the RAM 2500 with the I6 Cummins doesn't brake that well on these same trails in first gear because the exhaust brake is disabled when the auto tranny is forced into first. But in auto 2nd, the exhaust brake kicks in and works well. This makes more sense now.

It also makes sense that the RAM Ecodiesel has weak engine braking, from what I've read in the forums probably worse than my 3.5L EcoB, because it does not implement an exhaust brake.

And although nobody seems to know for sure, it's possible the Nissan Titan XD with the Cummins 5L has not implemented the turbo's exhaust brake feature, just an engine/tranny Tow/Haul mode. That would be a major disappointment for sure.

Of the smaller diesels appearing these days, the only one I know for sure has an exhaust brake is the Colorado's Duramax.


1) In order for a diesel turbo exhaust brake to work in an auto transmission the torque converter has to be locked. If I am not mistaken, none of the current truck makes have TC lock up in first gear.

2) The pickup version of the ISV 5.0L that will be in the Nissan and Toyota trucks use a compound turbo setup while the non pickup ISV 5.0L found in medium duty trucks use a VGT. One of the draw backs to a compound turbo set up is the fact that you cannot utilize an exhaust brake like a variable geometry turbo. This is why Ford switched to a VG turbo on the 2015 and up Powerstrokes. The 2011 to 2014 Powerstroke have a compound turbo.

3) The 2.8L Duramax in the Colorado/Canyon does have a VG turbo and does have an turbo exhaust brake.

4) Although the 3.0L Ecodiesel uses a VG turbo, it was not designed with an exhaust brake. I suspect this is due to the fact that the engine was initially designed to go in a Cadillac sedan which would not need an exhaust brake since it was not meant to go into a vehicle that tows.

* This post was edited 12/04/15 08:59am by ShinerBock *


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Posted By: N-Trouble on 12/04/15 08:59am

Modern diesel exhaust brakes don't work well in 1st gear because the trans torque converter does not lock in 1st. Simple answer...

Modern diesel with exhaust brake will slow a heavy load better than any gas motor. No comparison.


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Posted By: Terryallan on 12/04/15 09:06am

N-Trouble wrote:

Modern diesel exhaust brakes don't work well in 1st gear because the trans torque converter does not lock in 1st. Simple answer...

Modern diesel with exhaust brake will slow a heavy load better than any gas motor. No comparison.


True. BUT it needed an add on to do it. The gas engine does not. I think that is the point. I firmly believe that every diesel sold should be required to have an exhauset brake.


Posted By: brulaz on 12/04/15 09:15am

Thanks Shiner, good info.

So do the early 2011-2014 Powerstrokes just use a separate exhaust valve as an exhaust brake?

There is Holset Turbo Brochure that talks about the Holset M2 with its rotary valve used in the Titan XD. And it says this:

Holset Brochure wrote:

it may be necessary briefly to make the engine work harder in order to raise the exhaust temperature to regenerate a diesel particulate filter in the exhaust aftertreatment system. Positioning the rotary valve to restrict the exhaust passage produces the necessary increase in exhaust back pressure to make the engine work harder, and therefore hotter. If the rotary valve is closed completely, the exhaust passages are blocked providing an engine brake without needing any extra hardware.


Perhaps the rotary valve doesn't provide as good/fine control as VGTs, I dunno. But as Nissan never mentions Exhaust Brake, just tranny braking in everything I've seen, my guess is it's not being used for whatever reason. Too bad, as it would be a real selling point for me.


Posted By: N-Trouble on 12/04/15 09:16am

Terryallan wrote:

N-Trouble wrote:

Modern diesel exhaust brakes don't work well in 1st gear because the trans torque converter does not lock in 1st. Simple answer...

Modern diesel with exhaust brake will slow a heavy load better than any gas motor. No comparison.


True. BUT it needed an add on to do it. The gas engine does not. I think that is the point. I firmly believe that every diesel sold should be required to have an exhauset brake.


That's what I said... Modern diesel WITH exhaust brake. And it's not really an "add on" as many are calling it here. As long as the truck has a VVT which all have had for a long time now it's all accomplished through tuning.

Without an exhaust brake advantage goes to gas.


Posted By: ktmrfs on 12/04/15 10:33am

Me Again wrote:

I wonder how the new direct injection gas engines are doing that have no butterfly valve? Audi, BMW and Mini Cooper all have engines in production with DI.

Back to grade braking! For some reason the V8 diesels in GM and Ford trucks held back better than the Cummins I6 did in the RAMs. Maybe it was the much heavier rotating mass of the Cummins. I know from experience with my 2001.5 RAM that when I had just the smart controller on to keep the torque converter locked with the exhaust brake off, that the engine just free wheeled down grades without hardly any hold back. Chris


Direct injection gas engines still have a throttle butterfly to control air intake.


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Posted By: wilber1 on 12/04/15 10:37am

When it comes to engine braking, I've never had a gas engine that was as good as a diesel with an exhaust brake.


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Posted By: ktmrfs on 12/04/15 10:37am

brulaz wrote:

kyle86 wrote:

Me Again wrote:

I wonder how the new direct injection gas engines are doing that have no butterfly valve? Audi, BMW and Mini Cooper all have engines in production with DI.


I was under the impression that DI just places the fuel injector in the combustion chamber instead of the intake runner. They should still have a throttle body to let air into the engine. I could be wrong.


Apparently some eliminate the air throttle plate according to Wikipedi anyway:
Wikipedia on GDI wrote:

In addition some engines operate on full air intake. That is, there is no air throttle plate eliminating air throttling losses in some GDI engines, when compared to a conventional fuel-injected or carbureted engine, which greatly improves efficiency, and reduces piston 'pumping losses'. Engine speed is controlled by the engine control unit/engine management system (EMS), which regulates fuel injection function and ignition timing, instead of having a throttle plate that restricts the incoming air supply.


I'm seeing aftermarket replacements for the 3.5L Ecoboost throttle body, complete with butterfly valve, so I guess that engine does have an air throttle plate. http://www.stage3motorsports.com/1822-1-........013-F150-37L-BBK-73MM-Throttle-Body.html, so that should help with engine braking.


I'm suspicious of the wiki article claim. Gas engines need to operate over a vary narrow air/fuel ratio. So there must be some way to control the air volume entering the cylinder. Throttle plate or something. It may be located downstream of where one might expect it, but still there must be a way to control air volume in the form of some kind of variable restriction on the airflow.

Diesels will operate over an extremely wide air fuel ratio, so the throttle plate is not needed to control the amount of incoming air.

Hence less pumping losses but also less restriction under no throttle operation and hence less braking.


Posted By: Bedlam on 12/04/15 11:10am

I wonder if variable valve timing could accomplish the same control as the throttle plate.


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Posted By: ktmrfs on 12/04/15 11:52am

Bedlam wrote:

I wonder if variable valve timing could accomplish the same control as the throttle plate.


By varying intake valve duration, lift and timing It could, but the effect would be the same, a restriction at low loads. In either case it would end up "sucking through a straw" under low loads.

And many vehicles do vary intake valve duration timing or lift to to give a broad torque curve and high horsepower. In fact, I'd say the majority of gas engines are doing this.



In fact, even back as far as 1991 this was done on some cars. I have two 199y7 mercedes that vary the intake valve timing over about 30 degrees to give good torque and good idle at low rpm and good breathing at high rpm for high peak power. The result is a very broad flat torque curve. This was done starting in 1990 on the engines I have. Back then it was pretty amazing in that they were getting above 1HP/cubic inch on a normally aspirated gas engine. Course today that is ho/hum, but then it was state of the art.

The other thing that can be done is using the atkins or miller cycle for improve efficiency. That entails adjusting the valve timing and duration such that most of the intake air goes through the exhaust at low rpm giving very little air in the combustion chamber during the compression stroke. The downside is very poor low end torque. so it has been either combined with turbocharging or used with hybrid vehicles. Of course to do this, it requires direct injection so the intake air isn't also full of fuel.

Thinking about it, this may be the engines wiki was referring to that don't have a throttle plate.

And I would expect these gas engines wouldn't have much engine braking either.


Posted By: ShinerBock on 12/04/15 11:57am

Bedlam wrote:

I wonder if variable valve timing could accomplish the same control as the throttle plate.



Variable Valve Timing only controls when the valves open, and not how much it opens which is what is needed to control the air/fuel ratio. VVT basically rotates the cam to control the moment when the cam lobes push the valve down. The amount the lobe pushes the valve down is not controlled. You can in theory have a slant lobed cam that moves front to back to control how far the valve opens, but that would be adding more cost and more complexit to something as low cost and simple as a throttle valve. Or you can have some sort of motor controlling tha valves, but again it will add cost and complexity with not much of an added benefit over a throttle valve.

* This post was edited 12/04/15 02:07pm by ShinerBock *


Posted By: lenr on 12/04/15 12:14pm

2008 - 2010 were the only years that Ford used a compound turbo. 2011 - 2014 used a single turbo with a double compressor wheel. The desired effect was quicker spool which it does. Two downsides are reduced engine braking through turbo vane control and running out of air at higher altitudes. So, in 2015 Ford went to a larger single wheel turbo to get more torque, more HP, more go at high altitudes, and more slow coming down the mountain. Repotedly it does spool slightly longer. I don't believe that Ford ever had an exhaust valve style brake; just vane control starting in 2011. I find that my 2012 works just fine in tow haul mode with cruse within 10 mph of the cruse setting. So just set it 10 mph less than what you want to max at as you crest the mountain. You do need to be able to watch the rpms run up toward 4000, but that is where red line is.


Posted By: ShinerBock on 12/04/15 12:58pm

lenr wrote:

2008 - 2010 were the only years that Ford used a compound turbo. 2011 - 2014 used a single turbo with a double compressor wheel. The desired effect was quicker spool which it does. Two downsides are reduced engine braking through turbo vane control and running out of air at higher altitudes. So, in 2015 Ford went to a larger single wheel turbo to get more torque, more HP, more go at high altitudes, and more slow coming down the mountain. Repotedly it does spool slightly longer. I don't believe that Ford ever had an exhaust valve style brake; just vane control starting in 2011. I find that my 2012 works just fine in tow haul mode with cruse within 10 mph of the cruse setting. So just set it 10 mph less than what you want to max at as you crest the mountain. You do need to be able to watch the rpms run up toward 4000, but that is where red line is.


You are correct, the 2011 to 2014 did use am SST
Sorry, I am more up to date on Cummins engines than the new Ford diesels.


Posted By: lenr on 12/04/15 01:43pm

It is confusing because Ford used the play on words "Single Sequential Turbo" which makes it sound like 2 turbos. No harm.


Posted By: Bedlam on 12/04/15 02:12pm

lenr wrote:

...I don't believe that Ford ever had an exhaust valve style brake; just vane control starting in 2011. I find that my 2012 works just fine in tow haul mode with cruse within 10 mph of the cruse setting. So just set it 10 mph less than what you want to max at as you crest the mountain. You do need to be able to watch the rpms run up toward 4000, but that is where red line is.

The 6.0 PSD used a VGT that was vacuum driven and did provide exhaust braking. It was not as effective as my new 6.7 CTD, but it worked well on my F250.


Posted By: jus2shy on 12/05/15 01:57pm

ktmrfs wrote:

brulaz wrote:



Apparently some eliminate the air throttle plate according to Wikipedi anyway:
Wikipedia on GDI wrote:

In addition some engines operate on full air intake. That is, there is no air throttle plate eliminating air throttling losses in some GDI engines, when compared to a conventional fuel-injected or carbureted engine, which greatly improves efficiency, and reduces piston 'pumping losses'. Engine speed is controlled by the engine control unit/engine management system (EMS), which regulates fuel injection function and ignition timing, instead of having a throttle plate that restricts the incoming air supply.


I'm seeing aftermarket replacements for the 3.5L Ecoboost throttle body, complete with butterfly valve, so I guess that engine does have an air throttle plate. http://www.stage3motorsports.com/1822-1-........013-F150-37L-BBK-73MM-Throttle-Body.html, so that should help with engine braking.


I'm suspicious of the wiki article claim. Gas engines need to operate over a vary narrow air/fuel ratio. So there must be some way to control the air volume entering the cylinder. Throttle plate or something. It may be located downstream of where one might expect it, but still there must be a way to control air volume in the form of some kind of variable restriction on the airflow.

Diesels will operate over an extremely wide air fuel ratio, so the throttle plate is not needed to control the amount of incoming air.

Hence less pumping losses but also less restriction under no throttle operation and hence less braking.


There is only 1 engine maker I'm aware of running without a need (but it still has one) for a throttle plate. That's BMW's valvetronic motors. They only use the throttle plate for start-up and warm-up (as in a choke), but once warm, all throttling is handled by valve lift.

I believe Audi/vwag still uses a throttle plate. I owned a 2007 Saturn Sky Redline that had direct injection, but it also had the customary throttle body as well.


E'Aho L'ua
2013 RAM 3500 Crew Cab 4x4 SRW |Cummins @ 370/800| 68RFE| 3.42 gears
Currently Rig-less (still shopping and biding my time)


Posted By: Keith Haw on 12/06/15 09:10pm

Thanks for the replies, I was just wondering.
Yes my big truck had a jake brake but unloaded truck and trailer( 30k lbs) it wasn't needed.


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