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Old 01-13-2021, 06:02 PM   #11 (permalink)
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Originally Posted by JulianEdgar View Post
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.
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!!!!!!!!!!!!!!!!!!!!!

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Old 01-13-2021, 06:10 PM   #12 (permalink)
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Quote:
Originally Posted by aerohead View Post
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.
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Old 01-13-2021, 06:17 PM   #13 (permalink)
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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.
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Old 01-13-2021, 06:28 PM   #14 (permalink)
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Originally Posted by JulianEdgar View Post
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.
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?
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Old 01-13-2021, 06:36 PM   #15 (permalink)
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Originally Posted by redpoint5 View Post
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.
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!
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Old 01-13-2021, 06:38 PM   #16 (permalink)
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Quote:
Originally Posted by aerohead View Post
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?
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.
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Old 01-13-2021, 06:51 PM   #17 (permalink)
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Originally Posted by JulianEdgar View Post
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.
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.
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Old 01-13-2021, 07:12 PM   #18 (permalink)
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Quote:
Originally Posted by aerohead View Post
1) If you have reduced aerodynamic drag, you've altered the BSFC of the engine.
BSFC is quite irrelevant.

Quote:
2) You're operating the engine at a load that it was never designed for.
Complete and utter rubbish. Do you know what engine load actually means?

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

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

Quote:
5) Between the BSFC and Powertain, you now have two unknowns.
We're interested in factors that will have measurable impact the results. These don't.

Quote:
6) If your basing aerodynamic performance upon mpg, you'll never be able to actually parse out what went to what.
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.
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Old 01-13-2021, 07:20 PM   #19 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
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.
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!
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Old 01-13-2021, 07:28 PM   #20 (permalink)
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Originally Posted by JulianEdgar View Post
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.
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.

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