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boost pressure on engine brake - little john - 06-02-2017
today was my first haul of the season. had work done on my engine brake over our off season . found a jake rocker not working right. rebuilt that rocker . today in hard jaking a couple time my boost went to 34 or 35 psi and held back good and hard is this ok or normal ?or is it too high for some reason?
RE: boost pressure on engine brake - Hammerhead - 06-02-2017
What motor?
What CPL?
In my understanding
My CM871 CPL 2919 has the big turbo and is approx 1.7:1 efficiency. What does that mean you ask. It always takes more pressure to drive the turbo than it actually creates, the bearings and resistance to compression in my turbo takes 1.7psi of exhaust pressure to create 1psi of boost pressure. The engine compresses the gasses in one cylinder, then the jake opens the next cylinders exhaust valve, allowing the compressed gas to be "intaken" via the exhaust, thus re-compressing the already compressed gasses which makes the Jakes so strong.
My truck holds at 28psi during hard jaking downhill at ~1700rpm. With a 1.7:1 factor, that means the equivelant of 47.6psi on the exhaust side...this is the actual boost pressure entering the cylinder. Considerably higher than the MAX recommended 38-40psi around this site. I use stock turbo tables in jake application.
Now this sounds like excessive pressure numbers that need to be lowered, but during jaking there is no power stroke. That means there is no fuel injected, so there is zero thermal combustion expansion which is why I assume this would be acceptable boost pressure by Cummins.
Video showing how the system works https://youtu.be/JF6ZeybmQ1U
So to know if you are still in the "safe zone", we would need to know your engine type and turbo (CPL) to know the efficiency rating to calculate the actual boost numbers in the exhaust tract. If the exhaust tract numbers get too high, then the valve closing can become delayed because of the excessive pressure, and you run the risk of the piston hitting the valve before it closes.
RE: boost pressure on engine brake - JMBT - 06-02-2017
Great info Hammerhead! Ha, I noticed on the video they are still saying to rotate the rocker shaft to get the proper jake detent clearence!
RE: boost pressure on engine brake - little john - 06-02-2017
hammer head, myengine is an 871 cpl 2733, i mesegged unilevers earlier today on this and he said its fine , right where it shoild be . all great inforamtion thanks
RE: boost pressure on engine brake - Hammerhead - 06-02-2017
(06-02-2017 )JMBT Wrote: Great info Hammerhead! Ha, I noticed on the video they are still saying to rotate the rocker shaft to get the proper jake detent clearence!
Yeah, I saw that too.
For anybody who doesn't know, DO NOT ROTATE YOUR ROCKER SHAFTS!!! If you have 0.030" or less clearance, either your rocker shafts detent holes are worn or your head is worn out and you are at risk of dropping a valve. Find and correct the problem properly.
RE: boost pressure on engine brake - Brock - 06-03-2017
Ya that how much my 2733 builds too....
RE: boost pressure on engine brake - little john - 06-04-2017
Brock. How much boost does your 2733engine build on jake? Mid 30's?