7-14 psi turbo boost when engine compression brake engaged

Ivylog wrote:
I checked mine (ISL 400) on the way home today and depending on RPM I had 12 to 15 psi with the 15 at 2000 rpm. Sounds normal to me as there is still exhaust gas going through the turbo when you have the Jake on, but not as much if you have your foot in it.

Thanks Ivylog and everone else for their insites. I thought the turbo boost was always 0 psi with engine compression brake on, but I cant swear to it. Ivylog's confirmation that his does the same thing makes me feel better about it. All the other discussions helps to understand the system. Again, Thanks to all for responding.

Larry

I would suggest that the correct word is that an exhaust brake RESTRICTS exhaust flow. They all have a limit on how much back pressure the engine manufacturer allows. An engine is a LARGE air compressor. Even 55 PSI of back pressure (one of the standard limits) does not completely cut off exhaust flow.

wolfe10 wrote:
With an engine compression brake, there is a LOT of air flow through the exhaust side of the turbo. Remember, that is how an engine compression brake works-- it opens the exhaust valve as the piston approaches TDC on the compression stroke.

As usual Brett, you're right on. But, on an "Exhaust Brake", there is no flow or, maybe I should say, absolute minimum flow, correct? And, if there's no flow, then you can't "Build" pressure enough to show on the turbo pressure gauge, correct?
Scott

I checked mine (ISL 400) on the way home today and depending on RPM I had 12 to 15 psi with the 15 at 2000 rpm. Sounds normal to me as there is still exhaust gas going through the turbo when you have the Jake on, but not as much if you have your foot in it.

With an engine compression brake, there is a LOT of air flow through the exhaust side of the turbo. Remember, that is how an engine compression brake works-- it opens the exhaust valve as the piston approaches TDC on the compression stroke.

As most of you know, the way a turbo works and, registers PSI on the gauge is, when higher VOLUME of exhaust if flowing. So, when you step on the throttle pedal, as in accelerating, you get a ton of exhaust FLOWING, into and out of, the turbo. In lower RPMs, the turbo gauge registers lower pressure but, as you throttle up, and, you're at peak RPM during a grade climb, you're at peak turbo pressure. For mine, that's between 26-27 psi. that's on our C-7 330HP CAT, with 63K on the clock.

But, in a basic CLOSED THROTTLE operation, there is no high flow exhaust. Therefore, the turbo is close to a an idle position. I'm pretty sure that I've looked at my turbo pressure gauge when descending some grades and, I've seen nothing on there. But, I could be wrong so, I too might have to check this to be completely positive. That's if I remember to check it on the next down grade.
Scott

Since the engine is still running, there are exhaust gasses in the manifold. Also with the brake engaged, the exhaust pressure will be high, and therefore the turbo will be spinning.

However, I'm not sure what your max turbo output pressure should be. If your wastegate is set for maybe .5 bar (about 7.35 psi) perhaps your wastegate is not dumping. You might check on the correct blowoff pressure setting.

If the setting is 14 psi or so, then there should be no problem.

Dang Larry, I only see 99MPG on my Silverleaf. I gotta find out what's wrong.

I think what you are seeing for boost during break useage is normal

The turbo is still spinning so why not? Will look later today to see how much I get as I do not normally look down when using the Jake.

Double post