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Old 09-19-2014, 08:50 PM   #1
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Exhaust Brake, Compression Brake, vs Turbo Brake

As I understand it the old Caterpillar engines had an exhaust brake that closed the exhaust (choked it) to help slow down the engine. The Jack Brake or Compression Brake opened some valves to increased the back pressure on either 3 cylinders (Low) or 6 cylinders (High). I had the compression brake on my ISL 400 in my 40 foot 2007 Vectra. Loved it.
Now, I own a 44 foot Meridian with a "Turbo Brake." As I understand it the back side of the turbo puts pressure on the engine to help slow it down.
Here is my problem. It sucks! When I am going down a hill the braking power is like half for the compression brake. (I would like to see a chart that shows the difference but I cannot find anyone that has it.) But my real problem is as I go down the hill it will not hold the speed. On the Vectra with the compression brake if it peaked the hill at, let us say 45 mph, it would hold the speed between 45 and 50 all the way down the hill. Now, with the Turbo Brake on a 6% grade, it will continue to increase to 55 mph and then shift to a higher gear. That is the exact opposite of what I want. Do I have a problem with my Turbo Brake?? I cannot find anyone to help me. If I manually shift back down to a lower gear; it will, of course, hold the gear and not shift up but what good is the Turbo Brake if it just rolls up to the next shift point and then has you flying down the hill?? I really would like to shoot the engineer that decided the Turbo Brake would work on a motorhome that is 4,000 lbs more weight then my old Vectra. (Actually, I just want to put him in the coach and watch the expression on his face when it shifts from third gear, to fourth, and then fifth as he is doing 70 at the bottom of the hill.) This is hard for any shop to duplicate.
Any help is greatly appreciated.
>> Dennis
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Old 09-19-2014, 09:00 PM   #2
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Use your service brakes when your RPM's get to about 2500. Get RPM's down to about 1900, and repeat. This way your transmission wont upshift.
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Old 09-19-2014, 09:04 PM   #3
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You probably have a VGT or Variable Geometry Turbo which also acts as your exhaust brake.

It is by far not a good as any Compression Brake. There isn't an Exhaust Brake made that can compete against any Engine Brake.

Here is a good explanation of the differences.

Posted 01 September 2009 - 07:25 AM

There are three different "auxiliary" braking systems used on different diesel engines.: Exhaust Brakes, Engine Compression Brakes and Variable Geometry Turbo Brakes.

Exhaust brake: literally a "flap" which closes off exhaust flow just downstream of the turbo in the exhaust system. This causes back pressure (55 PSI on ours) which generates braking power. With the Allison transmission, it is usually tied with downshifting of the transmission to the "pre-selected" gear (usually 2nd or 4th). Think of it as a potato stuffed in the tail pipe.

Engine Compression Brake (aka: Jake brake) The exhaust valves are opened as the pistons reaches TDC (Top Dead Center) on the compression stroke after the engine has done the "work" of compressing about 18 volumes of intake air to 1 volume . If the exhaust were not let escape by the compression brake's opening the exhaust valves (i.e. coasting with brake off), the "compressed air" would mostly be returned as power to the engine forcing the piston back down. With the Jake brake on, the engine works to compress air in the cylinder, then the air is let out. This generates quite a lot more braking force than an exhaust brake. The smallest engines to offer an engine compression brake are the Caterpillar C9 and Cummins ISL.

Variable Geometry Turbo: The vanes reverse or aperture closes (depends on engine manufacturer) to create back pressure with much the same effect as an exhaust brake.

Another alternative used by Foretravel and some over the road busses us TRANSMISSION RETARDERS (see Wayne's link) on their Allison transmissions. Transmissions with retarders will have an "R" suffix such as 4060R. They generate even more braking HP than exhaust or compression brakes, but are more expensive.

Brett Wolfe


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Old 09-19-2014, 09:13 PM   #4
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96 Wideglide,
I agree with what you are saying but what benefit am I getting from the Turbo Brake? I feel I need to keep punching my service brakes and really terrified that they will get hot or I will lose a lot of air. Out West I have hit 8% or even 14% grades and I feel the Turbo Brake is like a toy. I guess I was spoiled with the compression brake but I am trying to find some justification or better understanding of the Turbo Brake.
Thanks for responding. Do you have the Turbo Brake also??
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Old 09-19-2014, 09:17 PM   #5
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It is probably upshifting because the max engine rpm has been reached. It does this in order to avoid engine damage.
The reason for it doing this is because the turbo brake is not slowing the coach down enough.
If you're used to that engine brake you won't be happy with other solutions anymore. I know I have owned both. I rarely even use my engine brake on the high (6 cylinder) setting as it slows the coach down to much usually
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Old 09-19-2014, 09:21 PM   #6
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Dr4Film,
Thank you for your response. I know of Brett but never met him. He would be a good source. Is the air coming off the back of turbo directed to the exhaust?? Or, is it directed back to the cylinders to reduce their movement? I'm just trying to better understand the theory.
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Old 09-19-2014, 09:23 PM   #7
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Yes, I do.
Never driven a heavy coach with an engine brake, so dont know what I'm missing !
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Old 09-19-2014, 09:37 PM   #8
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Originally Posted by dengraham View Post

Dr4Film,

Thank you for your response. I know of Brett but never met him. He would be a good source. Is the air coming off the back of turbo directed to the exhaust?? Or, is it directed back to the cylinders to reduce their movement? I'm just trying to better understand the theory.
>> Dennis
Here is an explanation of the VGT.

Variable-geometry turbocharger - Wikipedia, the free encyclopedia

Within the article is this explanation for the Exhaust Brake function.

Other common uses:

In trucks, VG turbochargers are also used to control the ratio of exhaust recirculated back to the engine inlet (they can be controlled to selectively increase the exhaust manifold pressure exceeds the inlet manifold pressure, which promotes exhaust gas recirculation (EGR). Although excessive engine backpressure is detrimental to overall fuel efficiency, ensuring a sufficient EGR rate even during transient events (e.g., gear changes) can be sufficient to reduce nitrogen oxide emissions down to that required by emissions legislation (e.g., Euro 5 for Europe and EPA 10 for the USA).

Another use for the sliding vane type of turbocharger is as downstream engine exhaust brake (non-decompression-type), so that an extra exhaust throttle valve is not needed (turbo brake). Also, the mechanism can be deliberately modified to reduce the turbine efficiency in a predefined position. This mode can be selected to sustain a raised exhaust temperature to promote "light-off" and "regeneration" of a diesel particulate filter (this involves heating the carbon particles stuck in the filter until they oxidize away in a semi-self-sustaining reaction - rather like the self-cleaning process some ovens offer). Actuation of a VG turbocharger for EGR flow control or to implement braking or regeneration modes in general requires hydraulic or electric servo actuation.


Your best solution is to reduce your speed and lower your gear prior to cresting the grade. Once your speed increases 5-8 mph tromp on the service brakes to reduce your speed back to a reasonable level. Continue that until reaching the bottom.

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Old 09-19-2014, 09:44 PM   #9
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Harry B,
I agree with you that the compression brake is so much better, and maybe that is my problem. I also rarely used the High setting. I will tell people it would put my head through the window if I used it. (I did use it once on a 14% grade out west at 30 mph.) But even a Low it was so much better than the Turbo Brake. Okay, I agree it shifts to protect itself but I manually put it in, let us say, 3rd gear... it will hold the speed and hold the motorhome back without shifting. I just don't know what the Turbo Brake is doing..... I guess I am not coasting but I would feel better if I felt a little resistance. Should I maybe be peaking the hill in a lower gear with the rpm's higher so that the Turbo Brake would have "more back pressure" to hold it back?? I guess I could press the service brakes just when I am peaking the hill to get it downshift if it would help the Turbo Brake to hold it back..... but my experience has been it just upshifts sooner because it almost at the peak shift point. It really feels like I am coasting with no back pressure as the engine rpm drops and I am at the bottom of the shift point in the higher "gear." I just feel like I am fighting it all the time and want to go back to the old days of just downshifting manually.
Do I dare ask for the cost of upgrading to a compression brake?? I suppose it would cost a fortune??
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Old 09-19-2014, 09:47 PM   #10
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I just returned from a trip thru western Canadian mountains.
8% grades I was starting the hill in 3rd gear, and using the method I refered to earlier, I was going about 30sec between brake applications. 3 axles of brakes, does'nt take much to get RPM's from 2500, down to 1900.
Dont think I ever came close to overheating the brakes.
Took a couple of 10% as well. 2nd gear .
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Old 09-19-2014, 10:02 PM   #11
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Richard,
Thank you for the detailed response. I am having trouble understanding one sentence: Another use for the sliding vane type of turbocharger is as downstream engine exhaust brake (non-decompression-type), so that an extra exhaust throttle valve is not needed (turbo brake).
If I understand this correctly, I have a sliding vane type turbocharger that is acting as a downstream engine exhaust brake instead of (or in lieu of) a traditional exhaust throttle valve? But, I still don't understand if air blowing out the back of the turbo is being directed to the exhaust system or to the pistons to slow down the engine? This make sense?
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Old 09-19-2014, 10:11 PM   #12
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96 Wideglide,
Hey we just returned from a trip to Banff. We really enjoyed the trip. We came up from West Glacier. All the hills were not a problem so I guess I'm getting used it. There was only two times that I manually down shifted to get it into a lower gear and held it by occasionally touching the brakes. I guess I just miss the compression brake. I didn't know they had dropped it in the Meridian and the Journey. I guess it was because they are 3,000 lbs lighter than the Tours and Ellipse. Thanks again for your comments.
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Old 09-19-2014, 10:19 PM   #13
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From what I've been told, the price up here north of the border, to have Cummins install a engine brake is about $11,000CAD.
I'll live with my VGTbrake !!
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Old 09-19-2014, 10:24 PM   #14
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Richard,
Thank you for the detailed response. I am having trouble understanding one sentence: Another use for the sliding vane type of turbocharger is as downstream engine exhaust brake (non-decompression-type), so that an extra exhaust throttle valve is not needed (turbo brake).
If I understand this correctly, I have a sliding vane type turbocharger that is acting as a downstream engine exhaust brake instead of (or in lieu of) a traditional exhaust throttle valve? But, I still don't understand if air blowing out the back of the turbo is being directed to the exhaust system or to the pistons to slow down the engine? This make sense?
>> Dennis
The air is blowing back toward the pistons similar to what happens with a PacBrake Exhaust Brake.

The only difference is that the VGT uses vanes and the PacBrake uses a valve.

I have the upgraded PacBrake model PRXB Exhaust Brake. it is far better than the normal PacBrake but only at the lower speeds like 50 mph and below.

You won't find an Exhaust Brake worth its salt for braking over 50 mph. On grades that are steep I make sure that I am in 3rd or 4th gear and doing about 40-45 mph. Again, once my speed increases 5-8 mph I tromp on the service brakes to lower the speed back to my aim point. I will never let my speed get to 54 mph doing any grade as that's where my Allison up-shifts into 5th gear and all hell with "brake" loose.

It's purely a management of speed and gears to make it safely to the bottom of the grade and under control.

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