Throttle stop testing on a diesel
 

Today, while I was waiting for the Tesla to arrive for tuft and pressure testing, I did some throttle stop testing in our W212 Mercedes.

Using a Scangage to monitor throttle position, I was able to hold a constant throttle (ie I didn't use a 'stop' as such - it's not too hard to hold a constant throttle once you get used to it.)

On the same test road that I've been using for my Insight, I recorded the following at TPS = 52 (not 52 per cent obviously, but a constant throttle).

Windows closed: 101 km/h
Windows open: 97 km/h

That calculates to an 8 per cent increase in drag - a bit lower than the change with the Insight (12 per cent) and well under the change quoted in Katz (2016) that has it at 16 per cent for a notchback.

I might do a few more tests with weird rear spoilers.

I put a throttle stop on my alh beetle years back, have one on my bew golf now, but have it screwed way down to like 80%. started trying to contrive some means to have a 2 stage stop. one for regular driving, but more of a detent that can be pushed through when more is really needed like, "right now full emergency thrust" situations.
original experimental intent was like 55%, worked for several days till it didn't. short version is that i thought i was about to die. "at your own risk" for sure.

have been thinking that a smarter cruise control might be better than trying to continue to improve the nut holding my steering wheel. PID maybe?

Just so that you're aware, the throttle stop testing I am talking about is to test for changes in aerodynamic drag, with the stop used only during the testing process.

Bad news for US (North America)
 

I followed the original Throttle Stop thread and assumed it would not work on the TDI since it is both drive-by-wire and turbocharged. I saw this thread and hoped it might give us another option besides A-B-A that Darrin has suggested for years. Alas, I tried it today without success.

I had the car mostly warmed up on a flat, straight road, at 70 mph, 16 degrees F with zero wind and speed and elevation measured by a Garmin on the dash. First, the Throttle Position Sensor on the ScanGuage (TPS) was useless as it just read 95 all the time unless I really stood on the throttle. I tried Load (LOD) but it was all over the place even with my right foot braced tightly on the footwell. I finally tried Gallons Per Hour (GPH) and was able to hold a fairly steady 1.66 @ 70 mph. However the windows up or down did not show a consistent difference in top speed.

I would say that for the most common small diesels in North America (Volkswagen Audi Group TDI) Throttle Stop testing may not be a viable option.

If anyone would like more info about A-B-A testing on my Golf TDI check out my Heresy thread.

EDIT: I just read a note from Julian that suggested that the test distance needed to be over the course of a kilometer. I may have to repeat the test under better conditions in the future. If anyone is attempting this with the TDI I still suggest bracing your right foot and using GPH on the ScanGuage

Small “thread hijack” for a shameless plug for my How to Ecodrive the TDI thread, specifically the Driving With Load (DWL) post. In hilly Colorado it doesn’t work well for me for optimum mpg. Besides why would anyone ever want to limit the power of the TDI? That is the fun of this vehicle, acceleration of a sports car but can be hypermiled like a 3 cylinder Geo Metro. :thumbup:

https://ecomodder.com/forum/showthre...sel-38306.html

Hijack over. Apologies Julian.

I just want to confirm the approach you took.

1. Work out throttle position that holds car at chosen test speed.

2. Enter test section at that speed and then hold foot constant at that previously determined throttle position (as confirmed by whatever factor you're looking at on scangauge).

3. Wait until speed settles - might be 1km or more. Need to read speed to a resolution of at least 1 per cent.

4. Repeat with windows open, using exactly the same steps 1-3 above.

Is that how you did it?

Sounds good! Got some test results that you can share with us?

Not exactly. My test was done hastily and under less than optimal conditions. It is cold and snowy here at this time of year. I was going from memory and I forgot I needed to do this over a level kilometer or more. I did not have a test section of over 1 kilometer without a slight change in grade.

That said, I found it difficult to hold a steady GPH reading even with my my foot jammed as hard as I could against the “trans tunnel”. I was hoping for a quick go/no go test system. If I need a long, level test track I think I will go back to setting the cruise control, leaving the car idling between runs while I make mods to hold the cruise setting in the car computer, and using the ScanGuage reset to current mpg to measure the change in aero drag.

I do want to try Throttle Stop testing on a ‘90 minivan I have as it is pre OBD, good old fashioned cable throttle and no cruise control. That will have to wait till spring as I do not have snow tires on it and it is parked for the winter.

What does this show when you do it windows up / windows down?

I also do this testing only on a dry road as I found that a road with sporadic puddles gave immediate inconsistencies in the results.