Anyone know why the 4.2 gets better mileage than the 3.6? What's the official Audi reasoning for the 4.2?

different fueling and timing maps on the ECU is my guess.

This is a very interesting hypothesis; but why go to 4.2 litters? I don't buy that, Audi's main reason must be something else. A big leap in injection tech that then causes Audi to go to a 4.2 litters for the hell of it, I don't think so. The 3.6 is two 1.8 vw motors. How similar is the 4.2 to two 2.1's.......very interesting.

Although for the record I like the short stroke 3.6, 4.2 -5mm.

Too

what about the transmissions and aerodynamics... that plays a HUGE part in it...

perhaps the 4.2 with the longer stroke runs a much lower cruising rpm then the 3.6 reducing parasitic losses and improving economy.

Well now, it's the Natrix...... I honestly like this solution more. It simply makes more sense that the 4.2 could use it's torque to run the car at lower rpm. So the 4.2 is more efficent at going 60. That is a side effect of thermal efficency.
It's kind of amazing how a bigger motor can use less fuel to go the same speed. (obviously this is not always the case)
In the V8 quattro example areodynamics play no role in overall economy as the 3.6 and 4.2 are identical body styles. As to gearing I run 255 45 18's on my car so it spins less at speed. I don't get better fuel economy.

where did you get your economy #'s from?

Does this come from years of reserch on your part? I get about 24 highway 18-20 around town. That in a combo of Sport mode, Economy, & Manual modes.
The only thing thats different is the ECU, it fires the injectors differently. From my seat of the pants the 3.6 wants to go fast & the 4.2 is more of a brute. Kinda like the tortus & the hair.
Thanks
Ron

The economy numbers come from across the board. Audi has been pushing for better and better economy out of it's V8 engines. The ABH 4.2 get 2 or 3 mpg better on the highway than the PT, depending on where you find the info. I have never seen the U.S. certified numbers ( gas guzzler tax window sticker) for the manual; however I'd expect at least a few better mpg in town out of the manual.

Ron what configuration is your V8 (3.6, 4.2, manual, automatic)? As I'm sure you are aware there are some key differences between the 4.2 and the 3.6. It seems clear Audi likes the 4.2 bore and stroke.
I am a bit unclear which you consider the tortus and which the hair.
I have a deep respect for both engines. I just wonder why Audi seems to favor the 4.2.

probably because they could get mroe power out of it easier. kinda like asking why audi went from the 2.1 JT to the 2.3 NG.

So is the 3.6 the same bore as the 4.2 just shorter stroked?

[quote="Type44V8"

Ron what configuration is your V8 (3.6, 4.2, manual, automatic)? As I'm sure you are aware there are some key differences between the 4.2 and the 3.6. It seems clear Audi likes the 4.2 bore and stroke.
I am a bit unclear which you consider the tortus and which the hair.
I have a deep respect for both engines. I just wonder why Audi seems to favor the 4.2.[/quote]

My car is a 90 with the 3.6 & auto. Yeah some things are different but the main thing is the ECU.
I saie the 4.2 is the tortus & the 3.6 is the hair. The 3.6 just feels like it wants to go like hell as opposed to the 4.2. Yeah it's got 30 more HP but in my opinion it's a tame 30 more HP. The 4.2 is limited by the ECU while the 3.6 isn't.
Now get a port & polish & open up that intake & now were talking.
+ whens the last time you found a 4.2 motor for anything under $2k?
This is America 4.2 bigger motor Americans gobble that kinda Cubic Inchs up.
The later 3.7 motors in the A8's were 300HP & the 4.2's were 280. Tweek, Tweek, Tweek. Anything can be accomplished.
Thanks
Ron

Skiddmark,
PT = 81 mm bore, 86.4 mm stroke.
ABH = 84.5mm bore, 93mm stroke.

As to thermal efficiency here an equation,

.0223 x HP/Gallons of fuel per hour = TE

Basically not all engines are created equally, some inherently generate more energy as heat in the heat to hp ratio than others. Some make more power and less heat on the same amount of fuel. There are many, many factors that are involved in this. Bore and stroke are two of the factors.

I do not feel Audi added more displacement so that Americans would buy there cars. It seems unlikely for Audi, perhaps more likely for GM.

I was building a 3.6 turbo, I feel the short stroke motor will make one hell of a turbo engine. However I did find an ABH a couple days ago at well below the 2k mark...Good luck, and the good word from a friend.
I feel the 9lb flywheel will help compensate for alittle more stroke. What the hell, looks like the extra 3.6 is already gone, so it's working out well on the pocket book.
But I digress.....

Derr I don't think it could have been said any simpler.

"probably because they could get more power out of it easier."

yeah, but the driveability of the motor and cam specs and all that jazz can make a more powerful engine seem like a dog. ha! i've never driven either 3.6 OR 4.2. so i really can't say what either are like.

but one thing i could compare it to is this, i've driven a lot of mercedes benzes, and i've one particular worn out smoking 3.8L v8 i've driven still screams and seems to have more giddy up and go than some of the 5.0l and 5.6l engine cars i've driven........hmmmm

Well no doubt you will enjoy the PT it's a nice engine!

Another simple factor to remember is port velocity is dictated by piston velocity and piston diameter, rod ratio and stroke can determine when peak airflow occur (crank angle). if head and cam etc. are equal. generally a larger engine will run more efficient at a lower rpm given all else equal. ports that are LITTLE too big may be just right with more displacement. also the rpm the engine runs at is a key element. a opposite case is the jaguar 6.0 liter. it is not liked as well as the 5.3liter. it isn't more efficent and it doesn't run as smooth. it is how all of the components work together that make it happy.
some of us mere mortals don't have all of the toys to get these answers. Those toys cost millions and only the manufactures can afford them.

Nicely put Doc,
Thermodynamics are wonderful but expensive to test in the basement, or garage for that matter. My money say 's Udi knows what they are doing.

Yes, velocity is key. It's tricky thought. However all the Audi ports I"ve seen are in need of aLITTLE to big.

The gear ratio is numerically lower resulting in less highway rpms.