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exhaust back pressure


Blue Ox
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I am thinking of putting an exhaust brake on my 2000 e7 460. I have been trying to find out what the maximum allowable exhaust back pressure is for my engine. I run 107000# and could use extra braking on hills. My other option I am considering is a telma retarder. I can find one from a mr688 and am wondering if anyone has installed one?

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Thanks for the reply. I have been studying their site. I really like the idea of the telma, wish I could see one in operation. I am just weighing the cost of each. I think in the end I would be happier with the telma.

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The telma will add about 700#. The maintenance is not bad, really just the bearings as far as I have found so far. The exhaust brake is simple enough, my concern is the back pressure. I have found one place that says the etech  is limited to 25# back pressure. I don't want to have the valves float and hit the piston. I am going to try pacbrake today and see what they say.

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I think the butterfly brakes are operated by an air cylinder. I assume you can put an air regulator on it to control the amount of back pressure the brake generates.

Find out what # springs the exhaust valves are, and what diameter they are. Run a little math, you'd have to generate enough force to counteract the valve springs to cause valves to float. You'll need to leave a safety factor to ensure the springs can still hold the valves with some force. 

Your other choice is a traditional Jake brake that opens the exhaust valves at TDC of compression stroke to dump off the combustion energy. Doesn't generate any back pressure, and certainly doesn't take 700# off your payload.

Edited by JoeH
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You have to take the 190# and convert it to psi on the surface area of the valve head. If it's a 2 inch diameter, πr^2 is 3.14159. Now take your 20 psi back pressure and you have 60+ pounds of pressure trying to open your valves, which leaves 130 pounds of spring pressure holding them shut.

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I got the 60 pounds of pressure because it's 20 pounds per square inch and a 2 inch valve has 3.14159 square inches of surface area. If it's a 3 inch valve the surface area jumps to 7 inches, which jumps to 140 pounds of back pressure acting on your valves, leaving only 50 pounds of spring pressure shutting the valves.

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That's exhaust valve diameter? That has about 1.8-1.9 square inches of surface area, so that's about 40 pounds of pressure. At higher RPMs you need more pounds of spring tension because the valves have less time to close between cycles. 40 # of pressure on the valve leaves you 150# of spring pressure. Of course the back pressure doesn't push on the valve with that 40# until the valve is shut. At fully opened the valve spring might not have much resistance, as the pressure is vented into the cylinder. But, you also have vacuum created on the intake stroke which is acting to pull the exhaust valve open, which is takes from your remaining 150# spring pressure.

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Only other thing I could think of is pushing exhaust past turbo seals? Exhaust manifold should have no problem holding that pressure. I would call pacbrake and ask. Get on the phone with a tech or engineer there and ask questions.

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1 hour ago, Blue Ox said:

Just called pacbrake. They said they don't do anything with macks???? I am thinking the chart I found on they're site isn't much value.

 I have been studying their site. I really like the idea of the telma, wish I could see one in operation. I am just weighing the cost of each. I think in the end I would be happier with the telma.

 

I will bring some experience to the table here! Take it for what it is worth!

Pre and  real early eteck there was An exhast brake Option called Stealth brake!!!! To install this unit we were  required to remove valve covers and replace or add valve seals and replace valve springs to install these! Having said that the unit helped on engine braking a fair bit ! We  however started having issues with turbos on these the exhaust gasses started over coming the shaft seals on the turbo on long downhill grades pushing the gasses into the base causing horrible blowby issues ! the fix was a special turbo with extra sealing rings on the shaft seal! if you plan on going this route.

Don't say you weren't warned!!! 

Just saying!

Telma

I have nothing bad to say about this unit , other than the extra weight added they function great!

Edited by fjh
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A traditional Jake brake is my preference. Simple, tried and true. Doesn't give the most braking, but it's adequate.  If it doesn't hold you around 1800 RPMs on the downgrade then you're in too high of a gear for the hill anyways.

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