Zulu's Build Thread (Honda CR-Z)

[CigaR007]

Is there a way to roughly calculate or derive the reduction of Cd based on preliminary increase (percentage) in fuel economy, especially in the context of highway cruising at a specific speed ?

[freebeard]

The received folk wisdom of Ecomodder is 0.1 worth of Cd nets 2mpg, or the other way around — search failure.

But the popular new view is you can't trust anything but testing.

[M_a_t_t]

Rules of thumb are guidelines at best.

Sorry, just reminded me of this scene.

The only way to know for sure is testing.

That's my take.

I think there is merit to comparing fuel economy increase to cd reduction, but not a steadfast rule. There is just so much that could contribute on different levels, for example bsfc for a different load %.

Expanding on that, the gearing at your selected speed is fixed, but your load would change. Which means your engine efficiency is now changing at a non-linear rate. Typically when compared it is something like "10% cd reduction = 5% fuel economy increase". Which is a linear rate change.

For example: A BSFC chart for a 1L metro engine


If your speed puts you at 3000 rpm and you lower your load (or required torque output) by ~30%, from 60 to 40, you can see it might be linear(ish), which might fit one rule of thumb. Then you reduce it more and now that first rule of thumb is inaccurate because of a different bsfc island. So now you would have to use a different rule of thumb based on the average of the 2, etc.

[M_a_t_t]

To answer your question though you might try the toned down top speed test. The throttle stop test. Just limit your throttle to (just guessing here) 50% and see what that top speed is. Then mod your car and re-test using the same limitations.

I am curious if this would be accurate over long periods of time. I wouldn't want to have to remove all of my mods and go back to a stock car to compare a new mod, so maybe you could collect some speed data and use that as a base forever, or just work backward from your previous change.

Then you can do the math to calculate the change* in cd based on change in top speed.

I don't find the math very difficult to do, just need the data mostly.

*which means you would need a starting point to be able to say your cars cd went from .3 to .25 rather than 17%

[CigaR007]

Interesting take with the BSFC chart for engine efficiency at specific loads. Are BSFC charts readily available for most engines ? I did a quick search but couldn't find anything useful for my car.

[M_a_t_t]

It seems they can be difficult to find if available. I have checked for a few of the engines in my cars and found nothing. If I didn't find anything for mine within like 30 minutes I gave up. I didn't see much practical use for it so it was just a curiosity thing, which is why I gave up so quickly.

Also to be clear, that is just something I was thinking of and have never tested or verified. I think it makes sense though.

[JulianEdgar]

Originally Posted by M_a_t_t

Rules of thumb are guidelines at best.

Sorry, just reminded me of this scene.

The only way to know for sure is testing.

That's my take.

I think there is merit to comparing fuel economy increase to cd reduction, but not a steadfast rule. There is just so much that could contribute on different levels, for example bsfc for a different load %.

Expanding on that, the gearing at your selected speed is fixed, but your load would change. Which means your engine efficiency is now changing at a non-linear rate. Typically when compared it is something like "10% cd reduction = 5% fuel economy increase". Which is a linear rate change.

For example: A BSFC chart for a 1L metro engine


If your speed puts you at 3000 rpm and you lower your load (or required torque output) by ~30%, from 60 to 40, you can see it might be linear(ish), which might fit one rule of thumb. Then you reduce it more and now that first rule of thumb is inaccurate because of a different bsfc island. So now you would have to use a different rule of thumb based on the average of the 2, etc.

All good points.

But there's another major difficulty with the rule of thumb of a drag change = a percentage change in fuel economy, too.

Resistance to forward motion is made of rolling resistance plus drag. Rolling resistance hasn't changed a huge amount over time - or at least not like drag has changed. So if the rule applies to cars with (say) 0.45 Cd, how can the rule equally apply to cars with (say) 0.25 Cd? In the latter car, aero drag makes up a far smaller proportion of the overall resistance, but apparently a percentage change in Cd gives the same change in fuel economy?

[JulianEdgar]

Originally Posted by CigaR007

Is there a way to roughly calculate or derive the reduction of Cd based on preliminary increase (percentage) in fuel economy, especially in the context of highway cruising at a specific speed ?

No there isn't.

It's my belief that all Cd figures that people quote for modified cars, that aren't measured in a modern, full-size wind tunnel, are almost certainly wrong. Sometimes, I feel, they are ludicrously in error.

[jakobnev]

Originally Posted by Wikipedia

The English phrase rule of thumb refers to a principle with broad application that is not intended to be strictly accurate or reliable for every situation.

0.45 and 0.25 are pretty much the extremes.

[freebeard]

Sorry, just reminded me of this scene.

I pointed to two different perspectives. Change 'rule of thumb' to 'setting expectations'.

It's the same thing.