It takes X amount of force to compress the air inside a cylinder as the piston travels up to TDC. The compressed gas inside will apply roughly X amount of force on the piston as it travels down to BDC. So you have 0 work done a the end of it all.

Now take X amount of force to compress that cylinder of gas again. This time, vent it when it comes to TDC. Now close that valve when the piston starts descending. The pressure will drop, and will go crank case pressure. It now takes Y force to get that piston down to BDC. So by venting the gas at TDC, you now have X + Y forces acting on it slowing it down, instead of 0 by leaving it closed.

 

If you were to block off all exhaust, and didn't have head studs the pressure could build enough on the head, givin you engage it at a high enough rpm and if the turbo was still making boost, to make you worry I would think. If you made it passed that you could have a chance of that pressure forceing the turbo in the wrong direction and that could be expansive. Might be easier to just have a small hole in you butterfly valve. Just my two cents

 

I take it your not a truck mechanic.

 

You can find them all through salvage yards on mid size trucks........I just got a 5" brake of a Cat C7 for $75.00, it was in a wrecked 06 T-300.

 

thanks i never really understood exactly how engine brakes worked but now i do

 

At a great rpm and distance I think pressure would force the butterfly open or pop part of the exhaust tubing apart before you would have engine damage. Every exhaust brake I have seen has had a small hole in it.....I think its there to somehow increase braking at low rpm, it is way too small to be any kind of fail safe.

 

Tate is talking about a compression release brake.....they crack open the exh valve right before TDC.

 

I think some are getting a exhaust brake and a engine brake mixed up, they are two completely different types of systems. The popping sound that you hear on the big trucks is the compression getting dumped before the injector fires to make a power stroke. The exhaust valves open just a little bit when the piston is almost to the top of the compression stroke, this causes no power stroke due to lack of compression there for making the engine nothing more than a big air pump. All a exhaust brake can do is slow the engine by blocking the exhaust flow and letting pressure build up in the cylinders during the exhaust stroke. Sorry if this seems like engine operation 101 to some but I hope that help's others see the difference between the two types of engine braking methods. As far as blocking the exhaust completely off, I dont think that would be a good ideal, the pressure would blow something out, be it a gasket or the piping in the exhaust system. Drilling a 1" hole in the butterfly or making it so it dosen't fully close would work fine.

 

So why don't our 5.9s get that "Big Rig Coming Down the Mountain" sound when using Exhaust or Engine (Jake) Brakes ????

 

I'm pretty sure that's because most semi's (as far as I know) use engine, or compression, brakes (the ones that control the exhaust valve opening). As mentioned earlier the engine brakes cause that cool popping sound.

I've only seen exhaust brakes available for our trucks, which essentially produce a lot of back pressure and cause more of that "whooshing, muffled slow popping sound." I think it'd have to be a Cummins factory option to get an engine break installed due to the exhaust valve control that would be required.

 

from what some one told me before, there is no room and/or technology that allows for our 5.9's to use engine(jake) brakes. only exhaust brakes are efficient enough to be used on our "smaller" engines compared to the semi-truck engines.

 

A true Jake Brake opens the exhaust valves on the compression stroke, just before the fuel is injected. that is the reason behind the cool sound that they make. It has nothing to do with blowing compression or exhaust out the intakes, although, if you can get it to do that... cool.

Some jakes ran off an extra lobe, some ran off the injector lobe. it meant that you had to sacrifice something, either injector timing or retarding power. However, you could always find a happy medium.

The newer electronic engines have it built, because you can shut the fuel off during retarding, thus allowing almost TDC travel before the valves open, makes a huge difference in overall retarding power. The ISX cummins took it one step further with the DOHC setup, where one cam runs the valves and jakes, and the other runs the injectors. NO trade offs at all. This is why the Signature 600 engine, was referred to as the first 1200hp engine. 600 to go, 600 to WHOA!!!!

Exhaust brakes are good enough for pickup trucks, and some RV's, but better to have the real deal on the big trucks

 

Oh boy. Much confusion.

There are a handful of engine brakes. Here are the four types I am aware of;

1. Jake brake.

This device opens the exhaust valves briefly as the piston passes TDC compression. Timing has NOTHING to do with this as when used properly the engine is injecting NO fuel anyway. The primary function is to relieve pressure that WOULD have been pushing the piston back down which in turn pushes the truck forward. Relieve it, and it no longer pushes the truck.

I have seen three basic types of exhaust brake.

2. Pull pin rocker exhaust brake. This is a somewhat rare system used before and around the time the more effective Jake brake came out. The exhaust valve rockers all have springloaded pins that a lever actuator pulls when the rockers are not under tension. As the engine turns over, each cylinder's exhaust rocker pins eventually are retracted leaving all the exhaust valves closed. Very similar to Honda vtec systems that leave one valve closed at low rpm.

3. Full closure exhaust brake. This is what I run. BD and Banks make them. If the pressure gets too high, it can float the exhaust valves and make them touch the pistons (!!!!!!!) This issue is addressed by BD (Banks copycatted this) by having the closure plate oval shaped and the shaft is off center. This means the pressure is imbalanced on the butterfly. So the actuator can be dialed in to only be able to hold back a set exhaust drive pressure. Once it exceeds this pressure, the pressure imbalance on the butterfly overcomes the actuator and the brake opens enough to prevent exhaust valve float.

4. Partial closure brakes. These are usually perfectly round butterfly closures on a centered shaft with a actuator that pushes a lever to flip the butterfly. Or in the case of big rig brakes, they are more like a guillotine with and inline acutator pushing a plate straight across the exhaust path.

With these types of actuators if you had full closure the pressure between the brake and the engine could overcome the exhaust valve springs and push them open allowing the piston to smash against them. So a small hole is in the closure to prevent this excessive pressure.

All types of engine brakes effectively convert kinetic energy of the truck's movement into heat from compressing air which is then dissipated by the radiator. Exhaust brakes by compressing the same air over and over. Jake brakes by preventing this compressed air from pushing back after the compression stroke.

This can be a little confusing since Jacobs exhaust brakes are not the same as "Jake brakes" which if I understand correctly simply were never made for the smaller Cummins engines.

OOPS! Make that 5!

I almost forgot! Variable displacement turbos exhaust actuator can be made to be an exhaust brake by choking off the exhaust. Either with a computer actuator, or in some cases by a cable op (which can potentially cause dangerous drive pressures-beware!)

 

Back to the original question>

A cutout would work excellent but it would be very wise to also have the electronics so the butterfly can only close at idle rpm.
Doing it manually doesn't cut it, one foot on the throttle mistake is enough to float your valves and cause major damage.
Very easy to overcome with a $2 micro switch and homemade bracket to sense throttle position.
Using a cable operated butterfly for anything other than warm up is a recipe for disaster.

 

SO the cable method being used on the HE351VGT's will float the valves? Ut oh.