CAUTION: throttle-stop testing
 

While I applaud any effort towards improved efficiency, I experienced a nagging concern, with respect to this method.
After many weeks of looking, I finally located a duel citation germane to this topic.
Rather than say a single word about it, I'll leave it up to members to interpret the data:
1) SAE Paper 810184
2) SAE Paper 830304

Neither paper appears relevant to throttle stop testing, so you’ll need to explain the point you’re trying to make.

For those not wanting to google--
1) "Formulae for the Tractive-Energy Requirements of Vehicles Driving the EPA Schedules" (1981)

A comprehensive analysis of the tractive-energy requirements of the EPA Urban and Highway driving schedules has been made. The results are compact formulae in which the specific tractive energy required to negotiate a schedule is correlated primarily with the ratio of effective aerodynamic frontal area to vehicle mass; the tire rolling resistance coefficients are additional variables. The formulae coefficients permit easy determination of the proportions of the Input energy required for overcoming the aerodynamic and rolling resistances, and for supplying the net powered increases in vehicle kinetic energy, respectively. The specific energy dissipated by braking has also been formulated.

2) "Tractive-Energy-Based Formulae for the Impact of Aerodynamics on Fuel Economy Over the EPA Driving Schedules" (1983)

A fuel consumption analysis has been made to determine the impact of changes in aerodynamic drag on fuel economy over the Environmental Protection Agency (EPA) driving schedules. It is based on the tractive energy required by vehicles to negotiate those schedules — specifically, on the fraction that is required to overcome drag. Formulae for this energy fraction that are applicable to any automobile have been previously derived. In conjunction with empirical inputs on closed-throttle fuel rate and the fuel consumption fraction for engine and vehicle accessories, an expression for the aerodynamic influence coefficient relating any percentage reduction in drag to the corresponding attainable percentage reduction in on-road fuel consumption has been formulated. The simple formula is used to show the effect of drag changes of various magnitude on EPA Urban, Highway, and Composite fuel consumptions. The equivalent reductions in vehicle mass and tire rolling-resistance coefficient required to produce the same fuel saving are also shown.

What has any of that got to do with the throttle-stop method?

The excerpt just says that you have to consider all the variables affecting efficiency to arrive at an EPA number following that protocol.

'influence coefficient'
 

This is the crux of the matter and what anyone using the throttle-stop technique must appreciate.
It's my opinion that the technique fails to quantify the known unknowns, as spelled out by Sovran et al..
Hucho devoted quite a bit of his text to address this challenge. Woe to anyone who sails without this chart.

So you actually have no points to your argument except "the technique fails to quantify the known unknowns".

1. What are the known unknowns that invalidate the test procedure?

2. How do these match with the actual results achieved when a known drag change is made?

3. How have you found the approach in actual use - or are you just theorising again?

points
 

Powertrain efficiency and BSFC are variable. BSFC dominates the scenario.
'The 'influence coefficient' is a sensitivity factor that relates a percentage change in independent variable to its corresponding percentage change dependent variable.'
'Changes in other vehicle factors may occur as a consequence of drag change, and these must be identified and considered.'
'A drag change causes the locus of the engine's operating points during the test to fall in a different region of the engine's performance map. This can alter the BSFC.'
' aerodynamic influence factor can easily deteriorate to only 60% of its potential value.'
Any load reduction

I don't think you understand the throttle stop testing technique (eg BSFC is quite irrelevant), and very clearly you've never actually used it to assess drag changes.

Never mind. To put it mildly, you've not shown yourself to be very open to new ideas, and that's OK.

(Not of course, that testing for changes in top speed to quantify drag changes is new!)

understand
 

I believe that I do understand it, and that's why I'd never use it. You may believe that you're in command of knowledge you're not in possession of. A reflection of your comments.
When it comes to testing, there's nothing more important than BSFC. I don't believe you actually understand it. That's not a cut, just an observation.
Some SAE Papers supporting your method would be instructive.

Brake Specific Fuel Consumption is irrelevant to throttle stop testing. Fuel consumption, whether specific to power being produced or distance being travelled, has nothing to do with the technique. Fuel consumed is not measured when using the technique.

Before I published the throttle stop approach, I did of course consult some aerodynamic experts - three in this case.

The only contentious point is one you have completely ignored, and that is whether the calculated change in drag depends on the square of the difference in speed, or the cube of the difference in speed.

Changes in top speed as indicating changes in drag has been used for over 100 years.

indicating
 

There's no full accounting. You're making assumptions and your conclusions are made from incomplete knowledge. You couldn't possibly prove causality.
Many published top speeds have absolutely nothing to do with actual drag-limited top speed, which is what you'd be after. The recent Corvette would be an example.
The 2020 Corvette Stingray has 495-bhp and a top speed of 184-mph.
This Corvette will maintain 184-mph in 8th-gear, at 3870-rpm, and 327-horsepower. It's gearing will not allow a higher top speed even though it clearly has the power for it. Context!!!!!!!!!!!!!!!!!!!!!

Maybe go and read what I have written here before about throttle stop testing technique? So that you understand what it involves?

Gearing has nothing to with throttle stop testing, except in that during testing, gearing is fixed. Gearing does not limit top speed during throttle stop testing.

At least you've moved away from the irrelevancy of BSFC. If you want to actually even be in the ballpark of the discussion, BMEP would be more relevant.

If one is certifying their vehicle for EPA fuel economy purposes, I doubt anyone would disagree that a more rigorous method should be utilized.

Since we're shade tree mechanics, the trend is more important than the precision. Having a ballpark estimate of what a modification does is tremendously helpful.

gearing
 

Your presuming that the mechanical efficiency of the powertrain is constant. It's not. That's addressed in the SAE Paper.
Brake Mean Effective Pressure will impact torque and horsepower, but BSFC will determine the thermal efficiency for which that BMEP is developed.
Gearing absolutely limits top speed in the example of the Corvette Stingray, if W.O.T. is taken as the throttle stop?

trend
 

I couldn't agree more. I just wanted to make clear, that we need to understand the context of the results, and that there may be factors at play, that we'd be hard pressed to actually quantify. It was tough enough with a dedicated engine lab and dyno!

OK, you have not understood how throttle stop testing is done, and you refuse to acquaint yourself with how it is done.

That's fine, but it basically makes your criticism worthless.


- BSFC and thermal efficiency are completely irrelevant to throttle stop testing. You really have to stop applying your existing schemata and think about is actually being discussed, not what you automatically think is being discussed.

- Throttle stop testing is not done at full throttle. That's why it is called 'throttle stop testing'.

- Gearing is irrelevant to throttle stop testing, except it should be done in the highest gear.

Look, don't worry about it. You don't understand it, haven't done it and are quite confused about it.

testing
 

1) If you have reduced aerodynamic drag, you've altered the BSFC of the engine.
2) You're operating the engine at a load that it was never designed for.
3) Unless the topography of the engine's BSFC map is exceedingly 'flat', it will fall to a less efficient island of specific fuel consumption. It's not negotiable.
4) Powertrain efficiency varies as a function of transmitted power. If your aerodynamic modification lowers the road load on the car, it cannot be presumed that the powertrain will have the same mechanical efficiency as prior to the modification. It's become an unknown.
5) Between the BSFC and Powertain, you now have two unknowns.
6) If your basing aerodynamic performance upon mpg, you'll never be able to actually parse out what went to what.

BSFC is quite irrelevant.

Complete and utter rubbish. Do you know what engine load actually means?

Fuel consumption of any type is irrelevant. It is not any part of the technique!

To the tiniest, tiniest degree, yes. But quite irrelevant in the testing regime we're actually using.

We're interested in factors that will have measurable impact the results. These don't.

The technique has nothing to do with mpg!!

You seriously have no idea what throttle stop testing involves, do you? This is ridiculous. You're arguing about a technique of which you have little knowledge or understanding.

No apparently it's better to look at pictures and guess, rather than do on-road test and development - pressure measurement, throttle-stop testing... even lift/downforce testing.

I think a major underlying theme is that on-road testing soon shows that a significant number of ideas spread in this group are wrong - very embarrassing for some!

testing
 

Store's closing in a few minutes. I'll be back Friday and pick up where I left off. We'll have some things to talk about.
In the meantime, you might want to reflect on all the possible variables which could be present during testing, which could color the data.

It seems that a change in RPM at a given throttle position would affect BSFC, and therefore affect fuel efficiency.

The difference wouldn't matter much I wouldn't think, but it's an unaccounted for variable, which makes the throttle stop technique useful for average Joe, and not so useful to those in the industry that have access to more precise equipment and need to account for all the variables they possibly can.

I suggest that you actually learn about what the technique involves, then you won't be arguing against some technique that appears to exist only in your mind (mpg, BSFC, etc).

Why does fuel efficiency / BSFC matter? It doesn't at all. It's nothing to do with the technique!

I have to admit, it's been a while since I glossed over the technique. The overall concept was sufficient to me, which is that comparing the delta in max speed for a given throttle position, one could extrapolate out the difference in aero drag and efficiency.

I was going to read up in more detail when I actually employ the technique. Unfortunately I've lost most of my interest at the moment because I rarely drive, and when I do, my fuel is paid for by my employer. I'm not going to mod a company car.

The technique has nothing whatsoever to do with fuel usage, thermal efficiency or BSFC. Nothing at all.

This thread is a good example of Aerohead just leading people astray and causing confusion - bringing in irrelevant concepts, not understanding what is being described (but at the same time being sure he is against it), citing irrelevant references.

If done carefully, the throttle-stop technique works extremely well in cars with manual throttles, and is light-years ahead of coast-down testing.

I arrived at confusion without the aid of anyone else.

The requirement of a direct throttle cable is a detail not known to me, as I assumed the technique could fairly well be applied to most any modern vehicle with "throttle by wire".

I understand the tendency to get irritated by the ignorance of others, but we can't really blame people for what they don't know. Everyone is ignorant of everything until they are not.

I think you were helped into your confusion, since you picked up Aerohead's point and then ran with it. There's never been any mention of BSFC and throttle stop testing except in this thread.

I am never irritated by people who are ignorant. I am irritated when people refuse to improve their understanding, even when they are directed to sources that would allow them to do so. And I am more than irritated when people who are ignorant, and who refuse to improve their understanding, then lead others astray.

I'll lay out the rough process my mind went through upon encountering this thread:

1. I found a provocative title saying I should question the throttle-stop method of testing mods.

2. The OP contained no information other than links, which I despise. Links should have commentary to a. take a position b. illuminate the relevant part of the link c. summarize the point

3. Someone posted an excerpt that was meant to be relevant, which I did not find to be relevant. I commented on that.

4. I glossed over and skipped most of the posts other than those in response to me.

5. I responded to your post since it was relevant to my misunderstanding.

I've got plenty of cause to dismiss aerohead, but I don't because I appreciate a good-faith representation of ideas. That isn't to say the idea is fully formulated, articulated, or even correct, but it comes from a place of candor. Candor I respect above all else.

I'm as irritated by others who don't agree with my careful illumination of the obvious, but even a refusal to "see the light" is baked into existence. Ultimately I have to end with saying I disagree, I have provided sufficient evidence to support my point of view, and you'll need to be specific with the thing you disagree with or don't quite understand.

Skipping posts is always a problem in a thread, because (as in this case) your point was answered multiple times before you made it.

And I can't see how Aerohead can possibly be regarded as acting in good faith when he didn't understand the approach (or even try to understand, it appears - it's a pretty simple method) and then told everyone it didn't work, shouldn't be used, etc.

Good faith would have been to ask questions so that he could understand the method, and then perhaps test it for himself - not pass judgement in complete ignorance.

I was just going to post a detailed discussion of throttle stop testing (especially versus other amateur testing techniques for drag) but I just watched this video for the first time since I made it 9 months ago.

https://www.youtube.com/watch?v=tVAokIdaXm0

It the longest I have done but it covers quite clearly:

- how throttle stop testing is done

- what its advantages and disadvantages are

- and gives examples of its use with aerodynamic changes that are going to give clear changes in drag (eg ride height, windows up /down, wing in air brake position).

Very obviously Aerohead either never watched the video, didn't understand it, or watched it and forgot its contents. (And that's OK, but you'd think he'd ask for details of the testing if he didn't get it.)

The other video that is germane is this one, which includes reverting to standard aero configuration no less than three times during the test session to ensure the results in this form were consistent.

This video also shows the step-by-step of the development of the Edgarwit external air curtains' position and their tuning. (And I must have been enthusiastic. I did a lot of testing in the one session!)

Note that the 8 per cent reduction in drag with the Edgarwits was only in dead-calm conditions. It was about 4-5 per cent in windier conditions (and again, note that the throttle stop testing shows this degree of variation).

https://www.youtube.com/watch?v=I4O2Bp2RnoA

throttle-stop research database
 

Having reviewed the video of the throttle-stop testing, I've spent, since Wednesday, going over my archive, looking at official SAE, MIRA, and EPA road test protocols. References are ( and yes. they are all dated ) :
* Internal Combustion Engines and Air Pollution, Professor Edward E. Obert, Harper & Row Publishers, 1973.
* Automotive Fuel Economy, Progress in Technology Series, Number 15 & 18, Selected SAE Papers, 1965-1979, Volumes 1 & 2, SAE, 1976, 1979:
1) SAE Paper 770844
2) SAE Paper 780346
3) SAE Paper 760187
4) SAE Paper 730790
5) SAE Paper 740969
6) SAE Paper 740594
7) SAE Paper 750956
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8) SAE J-1082, ' Fuel Economy Measurement - Road Test Procedure '
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9) SAE Paper 830384

Automotive Engineering Road Test Protocols
 

The following are gleaned from the aforementioned literature:
1) Pre-testing requires approximately 30-miles @ 50-mph,thermal ambient condition equilibrium stabilization for:
* Tire temperature-related R-R variability
* Temperature / viscosity - related hydrodynamic tribological mpg- variability to guaranty repeatability.
--------------------------------------------------------------------------------------
2) constant grade test course ( coast-up / coast-down ) prevention
3) wind less than 15-km/h ( 9.3- mph )
4) peak wind speed less than 20-km/h ( 12.4-mph )
5) continuous data-logging of :
- ambient temperature
- barometric pressure
- relative humidity
- road temperature
6) Reid Vapor Pressure ( RVP ) of fuel
7) Distillation temperature of fuel
8) No accessories allowed ( HVAC or Blower fan ) [alternator loading]
9) Recommended 68-F through 86-F ambient temperature window
10) 1/10th-gallon, graduated Burette fuel supply
11) Fuel mass, thermal - expansion compensation factoring
12) Instantaneous fuel rate by mass
13) Ambient, environmental, axial and crosswind aerodynamic loading
14) Uniform pavement surface
15) Straight course
16) Dry course

SAE ' Throttle-Stop' testing
 

From Donald L. Stivender, Engine Research Department, General Motors Research Laboratory, General Motors Corp, SAE Paper 780346
1) At a given constant throttle, essentially constant airflow is experienced.
2) At any given constant throttle position, the engine will exhibit a parametric BSFC ' fishhook', based upon F/A ratio ( oxygen balance ).
3) Engine torque = BMEP
4) Mechanical efficiency = BSFC
5) BSFC is inversely proportional to thermal efficiency
6) Optimum torque occurs @ ' Minimum Best Torque' - Spark Timing
7) BMEP is highly dependent upon A/F ratio
8) Minimum BSFC is defined by all throttle settings tangent to the throttle-stop fishhook
9) Fuel consumption- load characteristics generally apply at all engine rpm
10) Torque - A/F - efficiency, are all interrelated characteristics ( BMEP - BSFC - A/F )
11) Maximum torque occurs @ approx. A/F = 13:1 ( 'best power mixture' )
12) BMEP decreases with fuel flow at throttle - stop, torque per unit fuel increases to a maximum as fuel flow is reduced ( A/F = 18:1 )
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13) Throttle position is the only link between engine operation and the CPU
14) Any change in road load is unrecognized by the CPU, as it's 'eyes' have been removed, via the Vernier Rheostat in the throttle position sensor fixed position.
15 The CPU cannot detect 'driver intent'.
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16 Reduction in Road Load Horsepower forces the engine to shift it's BSFC map island, to that of a higher BSFC island, as friction effects now represent a larger fraction, increasing overall pumping losses.
17) A/F ratio experiences an excursion, taking BSFC with it, for overall lower mpg.
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18) Aerodynamic drag reduction will precipitate the excursion, unless the original engine-map load is maintained ( gear-matching ).
19) Failure to maintain original engine load is the setup for the 'influence factors' reported by Gino Sovran, introducing 'hidden' losses within the vehicle's internal fuel management system. It's flying blind.
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20) Any mpg attribution to an add-on aerodynamic appliance cannot be completely valid, by default.
Causality is indeterminant .
21) An 'observed' mpg improvement of say 0.6-mpg could actually be 1.0-mpg, only if the gear-matching is accomplished.
I've experienced this as a first-order-reality experience with 'Spirit.'
The highest mpgs ever returned were with 'rubber gearing', which reduced revolutions per mile, from 766, to 698. A $ 290 exercise.
A 'sixth-gear' would no doubt have helped even more. They don't make those.
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I like the throttle-stop testing technique!
It covers so many of the published variables that, I'd be surprised if it didn't indicate aerodynamic trends.
As to the degree of precision we can expect, with respect to quantifying the actual aerodynamic performance of any given modification, Julian has already adequately addressed that issue.
The information I've presented is just to add some seasoning to a fairly tasty recipe.

No, as always, the information you've added a mish-mash of irrelevancies, misunderstandings, confusion and weird theories.

In addition, again as always, you are quoting material that shows you have a poor / zero understanding of the topic. There are so many mistakes in what you have written that it would be far easier to pick out the one or two correct points than address all the errors.

I must remember to take a copy of some of these posts so I can use them in a book one day about how much rubbish is disseminated on automotive discussion groups. But to be honest, I don't think readers would believe me. (I'll just use "Maximum torque occurs @ approx. A/F = 13:1" - that will cause anyone who has ever done any dyno tuning to just fall over laughing.)

If you want to write about engine management, I suggest that you actually learn something about the subject first. This will help you, it starts at the very basics and then goes right through to Bosch Motronic, etc. (The version that's just been published in Korean on the left.)

https://i.postimg.cc/P5nJ2dct/Korean-English-covers.png

No, as always
 

Nice reward for dedicating the last two days of my time off to your thread.
If you'd like to parse out your points of contention, we're all ears ( eyes ).
I'm uncertain how the physics of internal combustion engines and electronic fuel injection has changed since 1979, however it must be astonishing, and I'm very anxious to hear all there is about it.
Thanks in advance.

There's honestly no point. I wasn't joking when I said far more of what you have written is wrong than right. Your knowledge of engine management is obviously very poor, so the first step would be to learn something about it.

If you could then add some practical experience (eg in engine management modification or tuning) that would again help your understanding a great deal. It certainly helped mine.

(And, pretty darn obviously, there have been huge changes in engine management in the last 42 years!)

engine management
 

Allegedly, two capacitors in the T-100's mother board ultimately gave up the ghost, leaking acid, and eating away two traces of the printed circuit board.
The computer was sent off for repair. When it was returned and installed, the truck failed to start.
Presuming the computer was 'fixed' I went on a month's spree, chasing through the entire engine management system before coming to the realization that, the computer had not been repaired at all.
After sending the computer back, I got a call from the technician, apologizing for having missed two additional dead caps, and dissolved circuit board traces.
I got the CPU back, installed it, and the truck fired right up.
Over the course of all that, I got a pretty intimate look at all the logic and hardware from Toyota's official training manual.
The vernier-rheostat inside the throttle-position-sensor, and its variable resistance signal is a vital component of, and the CPU hasn't a clue what's happening without it's signal.
That may not be germane to your cars.

1. No car uses a rheostat as the throttle position sensor (TPS). They use a potentiometer. Different device, three wires versus two, with a different output (voltage versus current). There is no 'varying resistance signal', there is a varying voltage signal.

2. In cars without electronic throttle, and excluding the earliest L-Jetronic systems that used a full-load throttle switch and the 'carby-like' Bosch Mono-Jetronic system, the TPS is not used to indicate load to the Electronic Control Unit (ECU).

3. Engine load is determined by either:

- the combination of a MAP (Manifold Absolute Pressure), engine RPM and look-up tables that correlate these with engine volumetric efficiency

or

- the output of the airflow meter

The main function of the TPS is to control transients - eg fuel enrichment / enleanment on throttle increase / decrease, and the same for ignition timing advance / retard.

Note that in throttle stop testing, the throttle is not moved.

As I said, you obviously don't have much understanding of engine management systems. That's OK, but it makes criticism of the throttle stop method based on your misunderstandings pretty problematic.

know
 

You may read all the listed papers and find out for yourself what they knew.
You may re-read my remarks. Obviously you never have, or you wouldn't be making the asinine remarks that you are.