#5094472 - 10/10/10 04:13 PM
Re: The pros and cons of Audi S4 ownership
[Re: NOT spotch...]
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UglyValiant
Post Master Sr
Registered: 10/09/02
Posts: 6364
Loc: Milwaukee, WI
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Ok, so just comparing a 98 neon with a .72 fifth gear to one with a .81, the .72 is going to be the mileage master even though it will be way slower when you punch it on the highway.
My opinion of what's important for most efficient rpm for a given engine in a given car is this is short form: For a given car, the highest gear ratio that will allow it to cruise at typical cruising speeds at the lowest rpm/highest engine load while still being able to run w/ a lean A/F ratio. Different engines in the same car will have different points, likewise same engine different cars will have vastly different points.
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1989 Shelby CSX (#500/500)
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#5094504 - 10/10/10 04:37 PM
Re: The pros and cons of Audi S4 ownership
[Re: UglyValiant]
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Euphoricuck
Post Master Supreme
Registered: 11/05/03
Posts: 92703
Loc: Canadistan
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For a given car, the highest gear ratio that will allow it to cruise at typical cruising speeds at the lowest rpm/highest engine load while still being able to run w/ a lean A/F ratio. Different engines in the same car will have different points, likewise same engine different cars will have vastly different points.
give certain engines to low of a ratio and you need more load to move the car while cruising, thus using more gas.
load has a bigger effect on mpg than an extra 1000 rpms do.
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#5094512 - 10/10/10 04:41 PM
Re: The pros and cons of Audi S4 ownership
[Re: Euphoricuck]
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NOT spotch...
Post Master Supreme
Registered: 07/25/00
Posts: 57155
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Don't forget, neons w/ .72 5th gears used a 3.55:1 differential while the .81 5th gears used a 3.91 diff...making the disparity even greater.
The sports+expressos had the 3.94/.72 mix... perfection for the less torquey DOHC imho.
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#5095117 - 10/10/10 11:16 PM
Re: The pros and cons of Audi S4 ownership
[Re: Euphoricuck]
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LNXGUY
Post Master Supreme
Registered: 08/06/00
Posts: 107013
Loc: Barrie, Ont,
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The closer you are to the torque peak while cruising will give you better mileage.
Shit, there are guys that running .658's in TDI's because they want better mpgs, they gain about 10-12% on mileage because of it..
It's all about the torque curve.
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#5095453 - 10/11/10 08:51 AM
Re: The pros and cons of Audi S4 ownership
[Re: Euphoricuck]
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FCobra94
Post Master Supreme
Registered: 10/22/01
Posts: 19689
Loc: MD
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like all those who whine about hondas revving high on the highway. giving it another gear that dropped the revs 1k or more(for example) would probably make it drink more fuel since it would need more load (throttle) to maintain the same speed since it makes lower torch.
Not true. My Si spins @ 4K RPM when doing 75 too. When owners drop in an RSX 6th gear cog into the stock Si trans (no change to final drive) they see a consistant gain of 2 mpg on the highway.
The right car is worth a few bucks at fill up...Shit, my G35 gets about 24-25 highway when driven nicely, but 16-17 in the city.
I tend to agree with this, but that decision gets harder when you see motors like the new Mustang's V6 putting down more power and returning 30+ mpg on the highway.
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#5097431 - 10/11/10 11:42 PM
Re: The pros and cons of Audi S4 ownership
[Re: NOT spotch...]
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grkthugisback
Jr Poster
Registered: 09/23/10
Posts: 84
Loc: Brooklyn , Ny
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40 mpg is FUCKING awesome lol .
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#5102036 - 10/13/10 06:19 PM
Re: The pros and cons of Audi S4 ownership
[Re: Euphoricuck]
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Vogz
Post Master Sr
Registered: 11/06/01
Posts: 8233
Loc: Chicago Burbs
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its a balance between passing power in top gear and mpg for the audi , more than likely.
Maybe, but 3500rpm @75mph is REALLY high for a 4.2L V8
For comparison my S60R turns 2700rpm in 6th @ 75mph and still has PLENTY of passing power with it's 2.5L I5T. I get around 29-30mpg @ 70-75mph.
My R is going 97mph @ 3500rpm in 6th.
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#5102093 - 10/13/10 06:49 PM
Re: The pros and cons of Audi S4 ownership
[Re: NOT spotch...]
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LNXGUY
Post Master Supreme
Registered: 08/06/00
Posts: 107013
Loc: Barrie, Ont,
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The closer you are to the torque peak while cruising will give you better mileage.
Shit, there are guys that running .658's in TDI's because they want better mpgs, they gain about 10-12% on mileage because of it..
It's all about the torque curve. I think this is probably more true for diesels or something... I have a feeling if I drove an s2k or an ls1 fbody or even my mazda 3 at their actual tq peaks, I'd be burning a *fuckload* more gas than if I was cruising at their overdrive gears, barely touching the gas pedal at a low-ish rpm to maintain speed.
Yeah, you're most likely correct. I've been spoiled with cruising at 2k rpms 
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#5118562 - 10/21/10 08:29 AM
Re: The pros and cons of Audi S4 ownership
[Re: PurduinaSi]
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MrManGuy
Poster
Registered: 07/23/02
Posts: 320
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What you needed was an 04-05 A4 1.8T 6 speed with the Ultra Sport Package -- essentially an S4 with shitty brakes and the 4.2 swapped out for the 1.8T. Injectors and a tune make for an APR claimed 235hp at the crank. Exhaust and a test pipe are probably worth another 20hp. It's not S4 punchy, but it moves pretty good and gets over 30mpg on the highway easy. The 1.8T is also much cheaper to maintain and much easier to work on.
Speaking of the gear ratios/fuel efficiency discussion, I have some real world evidence. I own an 02 A4 1.8T Quattro 5 speed and my buddy owns an 04 A4 1.8T Quattro with the 6 speed, both APR tuned -- identical car, same exact engine, only difference being the extra gear. Here are the ratios:
2002 5 Speed
1st 3.778:1
2nd 2.176:1
3rd 1.429.1
4th 1.091:1
5th 0.865:1
R 3.700:1
2004 6 Speed
1st 3.667:1
2nd 2.053:1
3rd 1.423:1
4th 1.032:1
5th 0.800:1
6th 0.658:1
R 3.889:1
Gears 1-5 are pretty similar. IIRC, the RPM difference is 800-1000 at highway speeds. The 5 speed is rated for 29 on the highway, the 6 speed 30. At speeds of 60-65, the difference is pretty small, 1-2mpg if that, as indicated by the ratings. The difference shows up as speeds increase. Beyond 70, my car really struggles to get the 29mpg, and really starts to dip below that at 75+ (It's turning 4k at 80). On the other hand, the 6 speed car can do 75 all day and get 32-33mpg.
I suppose that is only a ~15% difference but it shows that it can make a difference. I would think that a V8 car would have enough torque that they could elongate the ratios without losing performance.
Edited by MrManGuy (10/21/10 08:31 AM)
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#5119001 - 10/21/10 12:49 PM
Re: The pros and cons of Audi S4 ownership
[Re: FCobra94]
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UglyValiant
Post Master Sr
Registered: 10/09/02
Posts: 6364
Loc: Milwaukee, WI
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Diesels operate on a different plane altogether, not because of where they make their torque peak or anything...but they don't respond to higher loads the same way as a gas engine does.
At a light engine load, a gas engine can run fairly lean AFR's...but once a load is introduced, the AFR's have to get progressively richer to a point to avoid detonation.
Diesel, on the the other hand, requires spontaneous combustion...IE detonation for the gas engine. So one CAN load down/lug a diesel engine without going from 16-17:1 AFR down to a 12.5:1 AFR like a gas engine,IE, burning significantly more fuel.
Torque peak is where the engine makes it's most power efficiently...and at WOT, utilizes gas most efficiently. But when going for fuel economy, we are not at WOT...so buzzing around in a Hemi Ram @ 4000 rpm is NOT the most efficient way to travel down the hwy.
Here's how I look at it:
We'll use a Generic car with a 5.0 v8, 4 speed auto w/ 1:1 third and a .7 4th gear, w/ 3.08 rear-end w/ 28inch tires. This engine produces 300ft-lbs of torque at 2500rpm.
Let's use a cruising speed of 75 mph.
In third gear, it's cruising rpm is rounded up to 2700rpm. In 4th, it's 2000rpm.
Okay, so it's torque peak is more closely achieved in third gear...right? But then why ever have a 4th gear...wouldn't this make it's fuel mileage always worse in pretty much every driving situation? What were those dumb engineers thinking?
It really has everything to do with what the engine is doing dynamically. We all know that as compression goes up, we generally associate that with an increase in power...right? It takes the same volume of air, same mixture of gas, and yet yields increased power...this is due in part to the quicker/more aggressive burn characteristics of the mixture under higher pressures.
When we are at low load/mostly closed throttle situations, this 5.0 v8 is NOT displacing 5.0 liters every two engine revolutions. The throttle body is restricting airflow, so perhaps it's only displacing 1.8 or 2.0 liters every two revolutions...how do we know this? Look simply to the HP formula!
We all know that a car doesn't need 225hp to maintain 75mph down the hwy, right? No...the actual figure is probably around 70hp or so for this generic, nameless car. It has to overcome basically air and mechanical drag, and at those speeds, we'll for the sake of argument just call it 70hp....
So, how much torque is it producing at 70hp? Well, in third gear, when the engine is spinning at 2700rpm, the torque required to produce 70 hp is roughly 136 ft-lbs. The same HP is need whether it's in 4th or 3rd, so 70hp while spinning at 2000 rpm requires the engine to produce 184 ft-lbs of torque.
So what's happening? The same power is being produced...therefore one can envision how the same amount of air/fuel is being burned in the same minute. But, in the lower revving engine, more air and fuel is being processed per engine stroke. So every time that piston is moving up during the compression stroke, there is a higher dynamic compression being generated, so power is being more efficiently generated...so slightly less fuel/air is needed to produce the same amount of power. This is the why/how engines using cylinder deactivation see any advantage...
Of course, this assumes that there is no penalty to pay by having to overcome 35% more revolutions per minute and the drag/windage within the engine itself and the various accessories driven by the engine. Of course, in the real world, that's not the case, and this only favors the lower rpm engine more.
As long as the loads are not so great that one can't go from an advantageous AFR to a rich one to avoid detonation, the lower the rpm one can get, usually the better. Diesels of course benefit from the same low rpms not because they usually produce their peak torque at low rpms, but because when running the low rpms they develop better dynamic compression, less mechanical drag/pumping losses, AND don't have to offset higher loads with richer AFRs.
And that generic car should be one of the easiest ones in the world to identify 
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1979 Dodge Lil' Red Express Truck
1989 Shelby CSX (#500/500)
The most powerful production minivan money can buy.
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#5121641 - 10/22/10 11:35 AM
Re: The pros and cons of Audi S4 ownership
[Re: UglyValiant]
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ElectronVTEC2
Post Master Sr
Registered: 07/10/08
Posts: 5915
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The RX has been a tank. It's an appliance, but it's been the most reliable car I've ever owned. Had to replace one coil pack, but otherwise it's just been general maintenance which, all told, has been very reasonable.
Todd
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#5124505 - 10/24/10 10:07 AM
Re: The pros and cons of Audi S4 ownership
[Re: bren si]
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ElectronVTEC2
Post Master Sr
Registered: 07/10/08
Posts: 5915
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Not being ready for a fun car? That's BS, but I would totally agree that I wasn't "ready" for the ass-raping of German car dealers. The S4 is wonderful, but woefully overcomplicated...
Todd
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#5126754 - 10/25/10 02:47 PM
Re: The pros and cons of Audi S4 ownership
[Re: bren si]
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ElectronVTEC2
Post Master Sr
Registered: 07/10/08
Posts: 5915
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Not sure what you're on lately, but I haven't had a warrantied car since I bought my new Si in 1999. Since then I've had several fun cars and have done most, if not all, of my own work on them.
Not sure why you fail to get this, but I did not buy the S4 for it's gas mileage ratings. But when life throws you a little curveball and all of a sudden acceptable MPG practicality turns into "you'll be paying almost $60 a week for gas", well things change. I have a car now that works for what I need - a practical, reliable car to get me from A to B. And next summer, I'll pick up something I'm more comfortable working on myself. And I'm happy knowing I hardly spend $60 a month in gas now, instead of $60 a week. I don't care who you are or how much money you have - that kind of ownership cost adjustment is going to change how you feel about a car, no matter how fun it is to drive or how luxurious it is. Get off your German car high horse for once, if that's even possible.
Todd
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