Just returned from a 1300 mi+ road trip in my '07 A6 and averaged 24.3 (Highway-85%, Town-15%) w/ 93oct. Car has 6000+ on the clock. Highway driving conservative: 75 avg. Seems like I should have gotten a little more based on what I've read here. Also disappointed in dyno results: 342rwhp/340rwtq. Car has headers, 160 ts, and tune. Am I missing something here?

 

Milage doesn't sound that bad. I generally average about 19 mpg for city and highway. 342 hp is low with headers.

I'd like to see if there is a difference in dyno results for cars with the 799 head castings.

 

I run 14 in town up to 30 on the highway. Your mod's may be the problem. It will be interesting to read the tuners responses.

 

Was the top down? You'll probably lose 2+mpg with the top down.

 

I do a 200 mi frequently. Set the cruise at 77 and get 28 MPG. MN5 Z51

 

you should port your intake & tb...I just got a tune w/ header, port intake & tb, cai & 160 t & got 371hp & 379t.....& got 16-18 mpg 50% freeway & 50% street.....that with alot of hills (hills kill me on the mpg, san francisco)

 

I got 28MPG on non ethanol gas and 25MPG with ethanol gas. With a tune and 4.10 gears, I got 23.7 on ethanol gas. (all freeway driving) Ethanol cost about 10% in fuel mileage it seems. All that said, it is still better than my BMW

 

What type of dyno was the car on? If it was Mustang, then your numbers aren't too bad. If it was a Dynojet, then that's pretty low considering the mods you have. My brother's C6 A6 did 338 RWHP on a Dynojet bone stock with ambient temps in the low 60's/upper 50's. If your ambient temps were in the 80's, that'll affect the numbers also as the computer will pull timing as IAT goes up. I wouldn't be too concerned about the absolute numbers as long as the before and after numbers showed an improvement. Also, are those corrected numbers or raw numbers?

I'm not surprised at your low MPG though. When you start modding, the first thing you have to realize is you don't get something for nothing. Like with the headers/exhaust, you gain HP but at the expense of increased noise. Some people like the sound and see it as a double bonus, others don't like it and now view the car as obnoxious. The problem with your MPG is directly attributed to your 160 t-stat. While you gain full throttle HP by running more timing/avoiding knock retard due to lower combustion chamber temps, you lose part throttle efficiency due to the exact same reason. People don't realize HP is heat...1 HP=746 watts, for a demonstration of that concept, grab a 60 watt light bulb. For every gallon of gas burned, there is a certain heat content released during the combustion process. A very rough estimate (for demonstration purposes only) has about 1/4 of the heat going out the exhaust, 1/4 going to friction, 1/4 going to the cooling system, and 1/4 going to the rear wheels to drive the car. Changing the t-stat to a lower temp means there is a higher difference in temperature between the combustion chamber temps and coolant temps. This causes a greater amount of heat to go to the cooling system and with the friction and exhaust components a constant, this reduces the amount available to drive the wheels. Since the HP to go a constant speed doesn't change, more gas has to be burned to compensate for the extra heat rejected to the cooling system. You see it as a reduction in MPG...something you probably weren't told when you bought the t-stat or you wouldn't be wondering where your fantastic MPG went to that you use to get. There are no free lunches, every design element on the car is a compromise between EPA (emissions, MPG, and noise)/safety standards/reliability/costs/drivability/consumer desires and performance that GM has to walk the fine line on. Any mods will skew that compromise and may result in an undesirable side effect to the consumer...always ask what the negative side is to a mod before blindly buying into the positive side.

Edit: Please note I'm not saying don't ever mod...just don't go blindly into a mod. I've done my share of mods over the years including removing the engine driven cooling fan on a '71 'Vette I autocrossed. While it's not much of a mod, it certainly had it's negative side as it was my daily driver...any traffic jams involved one eye on the car in front of me and one eye on the temp gauge in a nail-biting race between the needle hitting the red zone and traffic clearing.

 

Thanx for the responses! My mistake was that I didn't get a baseline dyno BEFORE adding the headers, wires, and ts. The car baselined @ 326hp/335rwtq WITH headers, wires and ts (before tune) on a DYNOJET. VERY DISAPPOINTING after springing for the mods. The gain from the tune was OK (+14rwhp), Based on what I've read on this forum, I should be putting out 360+ rwhp with the headers and tune.

Seems like I should have picked up a couple of mpg w/ the headers. Makes me wonder what is wrong w/ this car...I am missing 20+hp and 2-3mpg. Any suggestions for diagnosis??

 

I agree with Glass Slipper on the thermostat. Reduced combustion efficiency. But I also agree that the headers alone should have gotten you more hp than that. I'd ditch the thermostat. It's of little use unless you're going to the dragstrip every week. And of course the automatic will not be as good for mileage as a manual.

You still didn't mention what kind of dyno you used and what were the engine temps when you made the pull.

 

Did you reset the miles and avg. MPG each tank? What avg. speed?

What kiind of Dyno? SAE corrected?

 

I have an 06 A6 and I'm a little dissappointed by the gas mileage. Especially since my 99 six speed would get 30/31 on a trip & average 23/24 all around town. Even after racing all day at E town I'd still get 21. My A6 is lucky to get 18 doing the same. However, it still is an awesome car and I hope the mileage will get better. I only have 4000 miles. It does run well. 12.4, 114.4 on street tires ain't too shabby.

 

Avg mileage for trip: 24.3, conservative driving 85% freeway, 75mph avg.; 15% city. Dyno was a DYNOJET, SAE corrected #'s. Performance disappointing all the way around. Dyno tune was done on a perfect FLA day with humidity below 45%, Temp low 70's.

 

vetteguy, if i were you, i wouldn't take that 160 stat out. i understand glass slipper point from a engineering perspective. but at this lower temp it allows more timing advance / or not having ecm pull as much timing. this extra timing will make up the lost in efficiency glass slipper has point out. let you give you some recent on road data, glennhl and i had completed over 12 hptuners scans on our cars the last few days. what we discovered is these l2s loves to ping, kept in mind we only have 91 octane gas here. i didn't even know my car was in the low octane table the entire time, glenn's car did a little better, but he was still 5* or so under the high octane table. kept in mind both of us already has 160*. i don't even want to know how much timing is getting pull if we had the stock stat. a couple days ago glennhl put in some 101 race gas to get an avg of 95 octane and rescan his car. guess what, all the knocking went away, under wot he gain almost 10* of timing. anyone wants to see the hp log, i am sure he will sent it to you or i can. take a look at this thread on hptuner site, these guys all tune / data log or are tuners. you will realize 160 stat is a must.

http://www.hptuners.com/forum/showthread.php?t=12200


most of us don't data log or/and track our cars will never know these facts. since you already has a tune & i am sure your tuner base it on the 160 stat. if you were to remove it, i am certain you will haven even less timing advance. if you think your hp is low now, you will even see lower. i truely don't believe your car is only 342hp, perhaps you car was heat soak majorly on the dyno, this will kill your hp/tq. glennhl's a6 has lg, k&n intake w 160 stat & no tune dyno at 360rwhp. if you do track your car, take it down to your local track and you will find out.

regarding your gas mileage. if i do nothing but free way, i get over 28+ mph with 160 stat. you stated 15% of your driving was city, this is a major vaiable, what kind of city mileage was it? bumper to bumper, lots of sitting around idleing.......ect these all will make some difference. with your 24 mpg and that includes some city driving, i don't think it is that bad at all. enough of me.......

 

If you want better mileage, talk to your tuner. Keep the car as is.

 

Great post and thanks for the HP thread. Sure was an eye opener regarding stats..... Cooler is better and gas mileage be damned....

 

Ok. my A4 is not apples to apples but. we averaged between 24 and 28. 28 was the high ,, on 250 miles all highway, 24 was a good mix of 70 /30. highway/city. I think your numbers are o.k.

the HP numbers thats for the other fellas to debate. Mine is stock front to back running a 3.15 axle and 93 octane fuel from central Missouri to Toledo Ohio and back. Car got the best mileage on the way back, I do think Hills/ Wind/ Rain/ Humidity/ Air Pressure in the tires/ Air Filters/ all play a role in the MPG game.

Now if our government/or somebody would do something about the gas prices we would all be happier.

 

Thanx for all the input! I'm determined to find the missing 20hp...will remove the mods if I have to and re-dyno. Will share this info in another post when I have the answer

 

If gas mileage were important to me, I wouldn't have bought a Vette, even if I were getting 1 city & 2 on the road.

 

your numbers aren't that far off with an A6. As for gas milage and a 160 stat. That's all I've ever run in my LS type motors since 1999. Never has it ever negatively affect my milage. If anything it it's always helped. My current Motor a stroked L92 427 in an o5 Z51, making 525rwhp, at 70-75 mph in 6th gear gets 26.5mpg. Even though you have an A6 with a lock-up converter your milage should be a little better but not by much. I don't think it's a any real concern. Just drive it and enjoy.