Downshift or exhaust brake?

Me Again wrote:

4x4ord wrote:
I took my Ford out this morning and conducted a test that I think demonstrates the effectiveness of an exhaust brake.

On a level stretch of highway I revved the truck to 3500 rpm in 3rd gear before backing off on the accelerator. I watched the speedometer as the truck started to slow down and with the exhaust brake off I timed how long it took for the truck to slow from 100 kph to 80 kph. I then followed the same procedure with the exhaust brake on. Then I timed how long it took for the truck to slow from 100 to 80 kph with the transmission in neutral. After recording the results I found an online calculator that allowed me to calculate the negative horsepower acting on the truck in each of the three cases. Here are the results:

Exhaust brake off ............. 5.25 seconds ...... negative 142 horsepower
Exhaust brake on ............. 3.98 seconds ...... negative 187 horsepower
Transmission in neutral .... 16.5 seconds ...... negative 45 horsepower

So the results demonstrate that on my Ford turning the exhaust brake on gives about 45 additional braking HP over transmission braking alone with the engine revved in the range of 3200 to 2600 rpm. I agree with Me Again in that you can feel the difference. To use the full potential of the exhaust brake my truck would have to be revved to 4000 rpm.

Here is a link to the online calculator: click

You need to repeat your little test with the trailer behind you!

Why is that?


Edit: The reason I ask is because doing my little test with the trailer behind should not make much difference to the behavior of the exhaust brake. Due mainly to added wind resistance and a little more rolling resistance I would expect the data to look something like:

Exhaust brake off ............... 9.7 seconds ....... 192 negative horsepower
Exhaust brake on ............... 7.9 seconds ....... 236 negative horsepower
Transmission in neutral .... 19.5 seconds ........ 95 negative horsepower

What do you you expect?

4x4ord wrote:
I took my Ford out this morning and conducted a test that I think demonstrates the effectiveness of an exhaust brake.

On a level stretch of highway I revved the truck to 3500 rpm in 3rd gear before backing off on the accelerator. I watched the speedometer as the truck started to slow down and with the exhaust brake off I timed how long it took for the truck to slow from 100 kph to 80 kph. I then followed the same procedure with the exhaust brake on. Then I timed how long it took for the truck to slow from 100 to 80 kph with the transmission in neutral. After recording the results I found an online calculator that allowed me to calculate the negative horsepower acting on the truck in each of the three cases. Here are the results:

Exhaust brake off ............. 5.25 seconds ...... negative 142 horsepower
Exhaust brake on ............. 3.98 seconds ...... negative 187 horsepower
Transmission in neutral .... 16.5 seconds ...... negative 45 horsepower

So the results demonstrate that on my Ford turning the exhaust brake on gives about 45 additional braking HP over transmission braking alone with the engine revved in the range of 3200 to 2600 rpm. I agree with Me Again in that you can feel the difference. To use the full potential of the exhaust brake my truck would have to be revved to 4000 rpm.

Here is a link to the online calculator: click

You need to repeat your little test with the trailer behind you!

4x4ord, neat test and information. 4K rpm is probably about right with the V-8, as 2800 to 3000 appears to give maximum braking with our Cummins. I’ve never done a test as you discribed......just “seat of the pants” input! :). The tried and proven ...SWAG test system!

I took my Ford out this morning and conducted a test that I think demonstrates the effectiveness of an exhaust brake.

On a level stretch of highway I revved the truck to 3500 rpm in 3rd gear before backing off on the accelerator. I watched the speedometer as the truck started to slow down and with the exhaust brake off I timed how long it took for the truck to slow from 100 kph to 80 kph. I then followed the same procedure with the exhaust brake on. Then I timed how long it took for the truck to slow from 100 to 80 kph with the transmission in neutral. After recording the results I found an online calculator that allowed me to calculate the negative horsepower acting on the truck in each of the three cases. Here are the results:

Exhaust brake off ............. 5.25 seconds ...... negative 142 horsepower (subsequent tests revealed this number to be false. A more accurate value is 7.3 seconds and 102 negative Hp with the exhaust brake off. See edit)
Exhaust brake on ............. 3.98 seconds ...... negative 187 horsepower
Transmission in neutral .... 16.5 seconds ...... negative 45 horsepower

So the results demonstrate that on my Ford turning the exhaust brake on gives about 45 additional braking HP over transmission braking alone with the engine revved in the range of 3200 to 2600 rpm. I agree with Me Again in that you can feel the difference. To use the full potential of the exhaust brake my truck would have to be revved to 4000 rpm.

Here is a link to the online calculator: click


Edit: I did a redo on this experiment. The first time I conducted this test I went out early in the morning while the truck had not yet warmed up. Tonight with the truck thoroughly warmed up the time to decelerate from 100 to 80 without the use of the exhaust brake was 7.3 seconds as opposed to 5.25 seconds. I wonder if when truck was cold the turbo could have been partially restricted to aid in warning the engine? This time testing I conducted the test over several times. With the exhaust brake on the time to slow down was very consistent. With it off there was much more variation. 7.3 was pretty typical and once it slowed down in 5.8 seconds. Anyway 7.3 seconds means 102 negative horsepower without the exhaust brake on.

Lol

Me Again wrote:

Me Again wrote:
Well this morning on the way to the RC field I tried and tried to see boost on the exhaust brake.

Finally I hit a down hill exit ramp at 70 MPH in 4th gear and saw boost come up the the claimed 5-7 PSI range for a second or two. Braking did not change when this happen. We will not be towing until May 1st. Chris

So today I hit the same exit ramp at 70 MPH in 4th gear in TH and exhaust brake off. Result, the truck with no trailer did not slow down very much!!! Which I already knew would occur. I can just imagine what it would be like with 16K of 5th wheel behind me. Chris

So with the exhaust brake on you felt it slow down and without it on it didn't slow down much. Ya..... I guess that means a lot. Good experiment!

Me Again wrote:
Well this morning on the way to the RC field I tried and tried to see boost on the exhaust brake.

Finally I hit a down hill exit ramp at 70 MPH in 4th gear and saw boost come up the the claimed 5-7 PSI range for a second or two. Braking did not change when this happen. We will not be towing until May 1st. Chris

So today I hit the same exit ramp at 70 MPH in 4th gear in TH and exhaust brake off. Result, the truck with no trailer did not slow down very much!!! Which I already knew would occur. I can just imagine what it would be like with 16K of 5th wheel behind me. Chris

larry barnhart wrote:
I know it would be great. After what my wife has been though for over one month and life threating we might have to end rving. This summer will give us a clue. Cabbage patch downgrade next week. Piece of cake tow. Chevman

Have a safe trip North! We could name many in the West like Wolf Creek that are more challenging than Cabbage Patch.

I know it would be great. After what my wife has been though for over one month and life threating we might have to end rving. This summer will give us a clue. Cabbage patch downgrade next week. Piece of cake tow. Chevman

larry barnhart wrote:

Me Again wrote:

larry barnhart wrote:
We use the cabbage patch going home and have a very controled tow just using the Allison. More trailer weight would want me to have an exhaust brake. Chevman

You confirm that the grade braking with the V-8's works better than the Cummins I-6. Chris

Same for the 3500 8.1 dually with the same fifth. Same hill. I have been impressed with grade braking on a gas and then diesel.
chevman

Then one day you will do it with an exhaust brake and say wow, as long as mother time catches up with you first!