SUV Kammback

[Baltothewolf]

Originally Posted by UnicycleDan

Thanks backpacker3. Do you know, when coasting to a red light or stop sign, would it be more fuel efficient for me to flip to neutral? Or would that not help much in an automatic while I'm still moving? I'm comparing this in my mind to a lesser version of EOC, which I shouldn't do with my automatic.

...And thanks aerohead. I'm back to the template, but I guess that's good, because the template is straightforward. I still find it very interesting, however, that even in the picture of the Golf/Rabbit, the Cd increases, then decreases again beyond a point, where a certain range of kammback angles would actually worsen aero drag compared to my chopped SUV back.

Absolutely, back when I eco-drove my 95 Camry, one of the biggest jumps in mpg was when I started popping it into neutral way back and coasting up to a stop light. Imagine this, I'm driving up to a light I see go red, so I let off the gas say, 300ft from the light, due to engine braking, I slow from 55 to 34-45, and then have to re accelerate back up to speed. Now let's flip it, I pop it in to neutral at the same distance, and instead of slowing down to 35-45, you slow to 45-50. This is a major difference and will boost your mpg more then you would think. I went from 27 average in the city to 31 because of it.

[UnicycleDan]

Originally Posted by Baltothewolf

Absolutely, back when I eco-drove my 95 Camry, one of the biggest jumps in mpg was when I started popping it into neutral way back and coasting up to a stop light. Imagine this, I'm driving up to a light I see go red, so I let off the gas say, 300ft from the light, due to engine braking, I slow from 55 to 34-45, and then have to re accelerate back up to speed. Now let's flip it, I pop it in to neutral at the same distance, and instead of slowing down to 35-45, you slow to 45-50. This is a major difference and will boost your mpg more then you would think. I went from 27 average in the city to 31 because of it.

Do you have to worry about your engine, though, when you kick it back out of neutral into drive and your vehicle's speed is very different from the neutral engine speed? The engine will be forced to ramp up to the appropriate RPMs for that speed fairly quickly. I know this is what your transmission/torque converter is for, but will this cause increased wear on these parts in the long run?

[wmjinman]

I have had similar experiences as Baltothewolf when driving automatics with a ScanGauge attached - - - popping it into neutral gave much better mpg during the coast than leaving it in gear did.

I never worrind too much about any possible engine/transmission wear from doing this, but then, I'm not the worrying type.....

[Baltothewolf]

Originally Posted by UnicycleDan

Do you have to worry about your engine, though, when you kick it back out of neutral into drive and your vehicle's speed is very different from the neutral engine speed? The engine will be forced to ramp up to the appropriate RPMs for that speed fairly quickly. I know this is what your transmission/torque converter is for, but will this cause increased wear on these parts in the long run?

Turn off radio and everything else that can make noise, and listen to your engine and transmission. Get up to 70mph and put it in Neutral, then coast down to 65 and put it in drive, you shouldn't hear it go VROOOOOOOOOOOM then go down, it should be a smooth transition. My friends 92 crown Victoria did this, so he couldn't coast in neutral, but my camry simply matched the gear appropriate to the speed of the car and it always dropped in smooth.

[backpacker3]

You might want to find out if your car has DFCO if it does you'll probably get better mileage in drive as the car will cut off fuel to the engine and keep it turning with the transmission.

[wmjinman]

Originally Posted by backpacker3

You might want to find out if your car has DFCO if it does you'll probably get better mileage in drive as the car will cut off fuel to the engine and keep it turning with the transmission.

- but it won't coast as far due to "engine braking". So you can choose - if you WANT engine braking, like when coming down a mountain or to a red light you didn't anticipate soon enough, then leave it in gear to slow down faster. But if you want to coast farther, putting it in neutral should do that.

One strange thing I found out though - - - in my fiance's '97 Buick Riviera, it seems to coast better in gear - ????? - almost the like the torque converter "disconnects" when the throttle is closed at speed. If I pop it into neutral, it seems to start slowing down faster. Strangest thing - I can't figure out what the heck's going on with that. ????

[Sven7]

Originally Posted by UnicycleDan

where a certain range of kammback angles would actually worsen aero drag compared to my chopped SUV back.

You'd still be better off with too shallow of an angle than with no kammback at all!

[freebeard]

Another question for the thread first: Do my thoughts on construction and design of the Kammback seem reasonable so far?

I can't quote the picture when it's attached instead of inlined, but you know the red thing? It only covers the top half. You'd do as well to make it half as long with the bottom half done, too.

All the examples so far use hard angles and flat planes. Your Envoy is a long way from being a box van. Consider Mair:



Four pieces rolled laterally for the top, bottom and sides, and four strips rolled longitudinally for the corners. All one material, with rivets.

[wmjinman]

But doesn't anything more than the slightest upslope on the back of the boattail (like 2.5 degrees) INCREASE drag because it tends to cause a partial vacuum between it and the ground, which tries to pull the car backwards into it?

[aerohead]

Originally Posted by UnicycleDan

Thanks backpacker3. Do you know, when coasting to a red light or stop sign, would it be more fuel efficient for me to flip to neutral? Or would that not help much in an automatic while I'm still moving? I'm comparing this in my mind to a lesser version of EOC, which I shouldn't do with my automatic.

...And thanks aerohead. I'm back to the template, but I guess that's good, because the template is straightforward. I still find it very interesting, however, that even in the picture of the Golf/Rabbit, the Cd increases, then decreases again beyond a point, where a certain range of kammback angles would actually worsen aero drag compared to my chopped SUV back.

Between 25-degrees and 30-degrees exists what Hucho refers to as a 'bistable' flow condition,where one second you have a fastback wake,then in an instant,a squareback wake,with the separation line dancing around between the two conditions.Real aerodynamic pandemonium.This range of angles are to be avoided like the plague.
Buchheim recommended an angle no steeper than 23-degrees under any circumstance.
Mair recommended 22-degrees as a maximum.
The 'Template' honors Mair's 22-degree recommendation.Here's his contour