7-14 psi turbo boost when engine compression brake engaged

A compression brake forces the engine to do work. The air exhausting the engine is hotter than the air entering it by the amount of work done in compressing the air. Adiabetic heating. The turbine continues to extract work from the exhaust gases which produces boost.

Fuel, after all, is just a way to heat air so it expands.

"A H" you might want to look at my post on 08/27/15 at 01:24pm... 7th from the top.
I do not put my Allison in N while moving and I did not miss your question nor the edits to your post.

You aren't really reading my posts as I posted what engine you have hours before you responded, about 8 hours actually. I said you have an ISL and I told you about the Cummins C brake that is a combo of compression release and exhaust braking and that is why you get boost. Cummins uses it on the ISL's to increase retarding hp.

A compression release brake without aid from a VGT won't build boost, or at least it shouldn't.

If you have enough turbine restriction to build boost (and a lot of it) freewheeling at rated rpm there is too much turbine restriction and your drive pressure while actually burning fuel would be epic. This boost wouldn't be a function of the compression release brake thou, and rather an undersized turbine for emissions reasons and the boost would be present brake on or off.

You also missed my question of how much boost are you building at 2000 rpms and the brake off? Both in N and decelerating.

So if you truly took a nice pull maybe you should have taken a reading pill too.

Like I said above, you need to read my posts as I posted which engine I have and that it has a true Jake compression brake with Hi/Low (6 or 3 cylinders). I like it when someone doubles down... even when they are wrong.:W I thought the loud sound out the stacks would have helped you figure this one out.
PS: I was tempted to used the smile two below this :W but I took my nice pill this morning.:B

Ivylog wrote:
AH64ID, guess you did not read my post above as during the use of my true Jake (compression release) brake yesterday, I had 12 to 15 psi of boost depending on the rpms, not how long it was on. Guess that loud sound of a truck without mufflers while using a Jake is not lots of air coming out the stacks by way of the turbo.

It is very odd for a compression release brake to build boost. There isn't any additional flow out the exhaust with the brake on or off, therefore there should be no change to the turbo speed.

What model engine? Does it have a VGT that may be closing the vanes at the same time as the compression release brake?

Are you building boost decelerating at higher rpms with the brake off?

How much boost do you build in N at 2,000 rpms? This will be more boost than you can build without fuel at the same rpm.

How would the air go any other way but by the turbo?


EDIT: I see you said you have a ISL 400. That motor uses a VG Turbo which tells me that the turbo vanes are closed to provide an exhaust braking feature. That feature increases the boost which then increases the compression release retarding power. So it is a hybrid system of sorts.

An engine with only a compression release brake and a non-VGT turbo shouldn't build boost while braking.

Looks like it is called a C brake by Cummins and they claim an additional 40hp of retarding power by using the VGT in conjunction with the compression release brake.

AH64ID, guess you did not read my post above as during the use of my true Jake (compression release) brake yesterday, I had 12 to 15 psi of boost depending on the rpms, not how long it was on. Guess that loud sound of a truck without mufflers while using a Jake is not lots of air coming out the stacks by way of the turbo.

I notice the same on my Powerstroke with no exhaust brake.
But on long grades, when I am not touching gas pedal for several minutes, the boost keeps going down.
It is at 10 psi on top of the hill, but after couple of minutes it drops to 4 psi.
I figured out it is the heat that air pumps from the engine into exhaust.
Since there is no burning fuel we don't have increase in coming gases to spin the turbine, but they do expand due to warming up.
Would be interesting if somebody can make other theory.

If the exhaust brake is a function of a VGT turbo then it is normal. A VGT acheives exhaust back pressure by closing the variable vanes all the way creating a back pressure between the pistons and the turbine wheel. The pressure is released over the turbine wheel and that spins the turbine wheel which in turn spins the compressor wheel and you get intake airflow and thus boost.

On an exhaust brake that is post turbo you will not build boost as the pressure is the same on both sides of the turbine wheel. One of the reason for the activation delay is to allow the turbo to slow down before the airflow decreases.

A compression release brake shouldn't build boost either as the exhaust flow shouldn't be sufficient enough to spin the turbo fast enough to build boost. The exhaust flow brake on is the same as brake off. If you turn the brake on and gain boost something is wrong.

So more info is needed. What engine/brake configuration.

I have a true Jake brake... compression brake, not a exhaust brake so yes 10-15 psi air going in will increase the braking because the compressed air is released before it can push the piston back down.

A exhaust brake (variable turbo or not) is much like the butterfly in a carb on a gas engine... it restricts the air flow making it harder for the engine to turn over. Sorry 06Fargo, but higher pressure going in is not going to help as the compressed air is not let out before it can push the piston down.

Doesn't the variable geometry turbocharger on the ISL increase compressor exposure during retarder operation to raise intake manifold and cylinder pressures to increase retarder performance by increasing the power requirement needed to turn the engine?

Think of an air compressor "normally aspirated" by piston vacuum on the intake stroke vs one that is supplied with air compressed at 10 psi to start with - the second one takes more power to turn...