I will pulling a 25' 5000lb ("dry") travel trailer this summer. I have the weight distributing hitch and new trailer brake assembly (Prodidgy). What about an exhaust brake? When is it needed? Why doesn't down shifting work with a diesel? I hear the term "air pump" when referring to a diesel but how is it anymore of an air pump than a gasser?
When the engine returns to Idle there is no fuel being apllyed to the engine until the idle set point is acheived, so a soon as you take your foot off the pedal fuel delivery stops esentualy the engine becomes just a compressor!Now plug off the exhast and stop the air from escaping you create a means of slowing the truck down by means of compression!If you don't plug off the exhast that compression being
made does very little to slow the truck down because there is no fuel being injected unlike a gaser were there is fuel all the time.
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86 F250,SC,4x4 Patched and patched and patched!335000 km and still patchin!
Fred, I was following you and it all made sense until you said
"does very little to slow the truck down because there is no fuel being injected unlike a gaser were there is fuel all the time."
I would think that no fuel would give you more compression braking. Whouldn't a bit of fuel give you a little "bang" thus reducing your braking?
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Steve
Eric -> '92, F250, 7.3l IDI, extracab, longbox, one ton spring in the rear,Borg Warner 1356 transfer case, 12500lb winch. 280k km. Since purchased in June 2003 repairs include: ball joints, tie rod ends, all u joints, water pump, radiator, brake calipers, brake pads & shoes, rear drums, front wheel bearings & seals, rear axel seals, replaced transfer case, rebuilt rear differential, replaced bent drive shaft, replaced drive shaft support bearing, K&N air filter, dual 2.5" exhaust.
No because you are firing what little fuel that is there before top dead center.On the diesel you are compressing just air and it acts as as a spring and actually pushes the piston back down after TDC!That is why on over the road truck engines The JAKE brake works !You actually let the compession go at TDC by opening the exhast valve a crack for a split second!Using the compression as a brake but Taking the spring away!!There inlies all the noise when applied on decompression!That is the noise you hear when a truck is going down hill with the jake applyed [img]/ubbthreads/images/graemlins/biggrin.gif[/img]The exhast brake is actualy doing a simular thing by giving the exhast no where to go creating back preasure outside the engine in the exhast!Not quite as effective but funtional!
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86 F250,SC,4x4 Patched and patched and patched!335000 km and still patchin!
on the diesel there is nothing restricting air flow either in or out. on the gasser they maintain an air/fuel mix and limit airflow when you take your foot off the gas this causes a vacum and slows the engine down. on the disel this is achieved with an exhaust brake which makes pressure inside the engine and this slows it down. without the brake all that is happening is the fuel is not being added.
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1994 F250 4x4 single cab long bed, 7.3L factory turbo, five speed, 165/75/16 BFG comercial ATs, open exhaust, pyro and boost gauge on A-pillar
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on the diesel there is nothing restricting air flow either in or out. on the gasser they maintain an air/fuel mix and limit airflow when you take your foot off the gas this causes a vacum and slows the engine down. on the disel this is achieved with an exhaust brake which makes pressure inside the engine and this slows it down.
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Makes me wonder if we could design a butterfly valve into the air intake that you could shut via switch and create that vacumn effect.. [img]/ubbthreads/images/graemlins/ooo.gif[/img]
Cheap mod possibilities? [img]/ubbthreads/images/graemlins/shrug.gif[/img]
Sorry to all the loyal 7.3 guys, but I traded my '91 7.3 in on a 2002 F250 Ex cab V10 4x4. SCT Xcalibrator2 with tunes by Powerplay Performance. 285/75/16 Toyo M/T's. Took a long time to get used to the ping-pop of a cooling catalytic converter...LOL.
'85 Bronco swapped up to a 302 4bbl from the 300 I6, NP435 4spd, NP208, 32x11.50x15 Radial RVT's, Soon to be 6.9 turbo powered.
86 F350 Cab&Chassis with a dump bed. 2wd 6.9 and 4 spd tranny. No mods to engine.
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I would think that no fuel would give you more compression braking.
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And you would be correct!
Gas engines don't fire the injectors on decels, either. They haven't since the mid 80's. If you pay real close attention you can feel a bit of additional braking on a gasser during a decel. That's when the injectors shut off.
It isn't the injectors firing or not firing that creates engine braking. On a gasser it is the engine trying to suck air past the closed throttle plate. Diesels don't have a throttle plate, that's why there are exhaust brakes that are just throttle plates that are after the engine. Instead of trying to suck air past a throttle plate the diesel is trying to blow air past the throttle plate.
You could rig a throttle plate before the engine instead of the exhaust brake. It should work the same.
An intake butterfly would be good if you want to have an engine killer in case of a run away. I think it would raise the cylinder temps pretty quick.
As for the original question - You don't need one unless you are having trouble stopping. I love mine for cold moring warm ups and it works great on our NW mountain passes with our 5000# travel trailer.
I love mine. I can drive from one end of town to the other stopping at stop lights and only use the service brakes for the final stopping (below 10mph) at the stop lights. With the TT in tow (31ft 9200+ lbs) it makes towing a dream to say the least. When decending the mountain grades I just leave her in gear and with a shifter mounted switch I engage or disengage the brake (flapper in exhaust) to control my down hill speed. Makes stopping the TT a heck of a lot easier as well. I can't recommend one enough after owning one, makes for short work of bringing the truck/load to a whoa. Yes they are expensive (typically $900+ new) but they are worth EVERY PENNY.
Heck even if I was not towing I would still own one. I know that every diesel I own from here on out will have one or it won't be parked in my driveway.
As FC said makes for warming up in the morning so much easier and while idling if your worried about cylinder wash down just flip the brake on and you'll see your pyro climb up to around 300-400 degrees keeping the heat in the cylinders for ya to help complete the burn. Just can't say enough good about 'em. I should have added one to The Enterprise.
Now if you have auto then you will need some way of keeping the TC locked but that is pretty easy it's a one wire to ground thing via a switch which could be easily done with a relay off the control wiring for the EB.
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Travis
Will Work For Diesel Fuel.
To all: I am still of the opinion that it is not the lack of an intake butterfly, but the limited rev range that limits a diesel's ability to engine brake, and also the heavier loads commonly carried by a given size engine. If you could take the diesel up to 4000 RPM like a gas engine, I bet it would slow down too. I once tried this on an old truck where I could shut off the fuel from inside the cab, and when the carb went dry on the long downgrade I then played with the throttle and could detect no difference other than sound. It was a Chevy truck, 350, auto, 3.07 rear FWIW.
This is probably somewhat true but not entirely. There is still a need for intake or exhaust restriction in order for the engine to create good slowing power. If it were not this way, an exhaust brake would be of no effect (we all know that they help a ton). How much back pressure can be produced on an IDI before the exhaust valve spring's pressure is overcome?
Ok,good info guys. Now, what brand/make to consider? I have seen mention of stronger exhaust valve springs etc needed. Is there a unit out there that doesn't require this modification? Any sources of used units?
And I have to agree you likly correct here Caesy I never gave the fact of a gas fuel injected engine providing no fuel a thought I simply asumed they did!Never kept up on the gasser injection! [img]/ubbthreads/images/graemlins/rolleyes.gif[/img] [img]/ubbthreads/images/graemlins/shrug.gif[/img] [img]/ubbthreads/images/graemlins/confused.gif[/img]But Birken brings up the Rpm thing and that does have some bearing on this subject!
the fact remians the ehast brakes do work you may bave to re spring the heads with which ever brake you go with Manufactures instuctions!
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86 F250,SC,4x4 Patched and patched and patched!335000 km and still patchin!
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American by birth, Southern by the grace of God!
1991 F250 7.3l 200XXX extended cab long bed. 3.55's REDNECKED OUT!! 295/75/16 Bfg All Terrains 16 in billet rims. New front brake pads and rotors, rear shoes, glow plug controller, glow plugs, return line kit duralast batteries, mitubishi off set gear redution starter, altenator, valve cover seals, holley red pump,new injector pump, soon to be new injectors, new vacum pump. Removed soup bowl and drilled holes in air intake.
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