Intake, exhaust & dyno-tune yield LOWER mpg?!?
07-20-2013, 03:10 PM
#1
Melting Slicks
Thread Starter
Intake, exhaust & dyno-tune yield LOWER mpg?!?
Now, before you say, “Lay off the pedal and your mileage may improve,” please hear me out.
When my ’04 A4 coupe was bone stock, I got about 20 mpg during my commute, which averages about 45 mph city/hwy over 32 miles daily. On 500+ mile trips on the highway, with the cruise set to about 70, I’d consistently get 25 mpg.
I regularly see forum members claim an average of 28-30 highway mpg with their C5. I figured my 3.15 performance axle may have been causing some of the discrepancy. That, and a few hot starts, but who are we really kidding here?
Anyway, over the past few years, in order to boost performance (gasp!) I’ve upgraded the intake and exhaust, and had the car dyno-tuned. Don’t get me wrong – I am very pleased with the increase in HP my mods have delivered. The car really performs well over the stock configuration.
Though my weekly commute mileage stayed about the same, I kind of expected that when driven in a reasonable manner over a long, mostly highway trip, the improvements would deliver better gas mileage. Having just returned from a 2200 mile trip, I calculated that my mileage ranged between 22.7 and 24.1 with an overall average of 23.4. As always, I used 93 octane gas during the trip. We did not have any excess baggage (luggage weight) than usual.
One might naturally assume that I “exercised” the car more vigorously during the trip, given the new mods. Maybe so, but for the most part, I still had the cruise set and just drove as I always have on highway trips. My wife drove the way she always does – like a little old lady (luv ya, hon!)
The only other factors I can think of are, due to higher posted speed limits on I-95, I had the cruise set to about 77, and also, having spent most of the miles in Florida and other southern states, the AC was getting more of a workout than usual.
But are those enough to cause a 1.6 mpg DROP even after the increased airflow of the mods and increased efficiency of the tune? I know that mods vs mpg are usually inversely proportional, but am I wrong to expect that some mods should help mileage when driven decently?
What have been your experiences with gas mileage after performance mods?
.
When my ’04 A4 coupe was bone stock, I got about 20 mpg during my commute, which averages about 45 mph city/hwy over 32 miles daily. On 500+ mile trips on the highway, with the cruise set to about 70, I’d consistently get 25 mpg.
I regularly see forum members claim an average of 28-30 highway mpg with their C5. I figured my 3.15 performance axle may have been causing some of the discrepancy. That, and a few hot starts, but who are we really kidding here?
Anyway, over the past few years, in order to boost performance (gasp!) I’ve upgraded the intake and exhaust, and had the car dyno-tuned. Don’t get me wrong – I am very pleased with the increase in HP my mods have delivered. The car really performs well over the stock configuration.
Though my weekly commute mileage stayed about the same, I kind of expected that when driven in a reasonable manner over a long, mostly highway trip, the improvements would deliver better gas mileage. Having just returned from a 2200 mile trip, I calculated that my mileage ranged between 22.7 and 24.1 with an overall average of 23.4. As always, I used 93 octane gas during the trip. We did not have any excess baggage (luggage weight) than usual.
One might naturally assume that I “exercised” the car more vigorously during the trip, given the new mods. Maybe so, but for the most part, I still had the cruise set and just drove as I always have on highway trips. My wife drove the way she always does – like a little old lady (luv ya, hon!)
The only other factors I can think of are, due to higher posted speed limits on I-95, I had the cruise set to about 77, and also, having spent most of the miles in Florida and other southern states, the AC was getting more of a workout than usual.
But are those enough to cause a 1.6 mpg DROP even after the increased airflow of the mods and increased efficiency of the tune? I know that mods vs mpg are usually inversely proportional, but am I wrong to expect that some mods should help mileage when driven decently?
What have been your experiences with gas mileage after performance mods?
.
07-20-2013, 03:54 PM
#2
Melting Slicks
Interesting read. My BEST educated guess is that your MPG's should have actually increased as your motor is running more efficient. In fact part of most dyno tunes (for an N/A car) is actually leaning the fuel mixture out a little as they tend to come on the richer side from the factory.
My only explanation (my guess) is that with the mods, you are more likely to drive it spiritedly! My first mod was by Borla Stingers & while that may NOT have made the car faster or more efficient, me wanting to hear the "growl" of the exhaust definitely hurt my MPG's....LOL
My only explanation (my guess) is that with the mods, you are more likely to drive it spiritedly! My first mod was by Borla Stingers & while that may NOT have made the car faster or more efficient, me wanting to hear the "growl" of the exhaust definitely hurt my MPG's....LOL
07-20-2013, 03:55 PM
#3
Racer
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With the auto u need to get some 2.73 to bump up from where u are at. 24 is pretty good with the 3.15.
People getting 28-30 have m6 not a4.
And the ac and 77 mph is def enough to cause a 1.6 mpg drop. As well as the brand of gas you got could do that much change as well
People getting 28-30 have m6 not a4.
And the ac and 77 mph is def enough to cause a 1.6 mpg drop. As well as the brand of gas you got could do that much change as well
Last edited by Bent98c5; 07-20-2013 at 03:57 PM.
07-20-2013, 03:57 PM
#4
Melting Slicks
07-20-2013, 04:25 PM
#6
Melting Slicks
Thread Starter
07-20-2013, 05:20 PM
#7
Le Mans Master
The 2.73 will lower the highway rpm and result in better mileage, but I didn't think that was the question.
The mods upgrading the intake and exhaust should have helped not hurt the mileage if the tune was set up for them.
My guess is that the tune is not the best. It seems like frequently a dyno tune tries to set up the car for max power, but misses the driveability and mileage at highway cruise. If possible try to datalog at your car at highway cruise speed and check.
My '99 has a Fast intake, Vararam, LG longtube pros with a catless X-pipe and B&B route 66 mufflers. Granted that mine is a 6 speed, but with all the mods, I easily get over 30 mpg on the highway. My best when I was trying for mileage was 37.7 mpg on the DIC (calculated was 36.2), and recently on a 1700 mile trip to the Corvette Museum and the Tail of the Dragon the DIC showed 32.0 mpg (calculated was 30.5) even with a lot of lower gear playing on the nice roads.
Mine with the intake and exhaust mods does not suffer for mpg's, so I suspect the tuning is not set up right on yours. Mine was an HPTuners tune after datalogging.
Good luck.
The mods upgrading the intake and exhaust should have helped not hurt the mileage if the tune was set up for them.
My guess is that the tune is not the best. It seems like frequently a dyno tune tries to set up the car for max power, but misses the driveability and mileage at highway cruise. If possible try to datalog at your car at highway cruise speed and check.
My '99 has a Fast intake, Vararam, LG longtube pros with a catless X-pipe and B&B route 66 mufflers. Granted that mine is a 6 speed, but with all the mods, I easily get over 30 mpg on the highway. My best when I was trying for mileage was 37.7 mpg on the DIC (calculated was 36.2), and recently on a 1700 mile trip to the Corvette Museum and the Tail of the Dragon the DIC showed 32.0 mpg (calculated was 30.5) even with a lot of lower gear playing on the nice roads.
Mine with the intake and exhaust mods does not suffer for mpg's, so I suspect the tuning is not set up right on yours. Mine was an HPTuners tune after datalogging.
Good luck.
07-20-2013, 09:20 PM
#9
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Average mpgs can vary widely depending on a number of factors - wind, lay of the road (hilly, flat, etc.), average speed, and so on. I know my car's mpg varies almost every time I take a trip because there are variables that are different each time.
07-20-2013, 10:14 PM
#10
Race Director
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2003 A4 with 3.15 rear, Honker intake, Corsa exhaust (all else stock).
With original run flats got mid 26 running with AC @ 70 mph.
With new non run flats that dropped to 24-26 max. (mostly in the 25's).
Added the Predator tune, and essentially no mpg change (but the performance is better, and no A4 torque management lag).
This is all with Shell 93 octane.
Note that we occasionally run north (of FL) in wife's Dodge Magnum Hemi, and we see a wide range in fuel mileage (while running same speeds) depending on last fuel stop. This is with consistant fuel top off procedure and full gauge reading. It seems to vary by states by up to 3 mpg (example is 24+ mpg in N.C., and we are lucky to get 21 in FL). I always kid that we get better mpg is because we are running uphill. Of course we get the same variance when returning, so there goes that terrible theory......
Plasticman
With original run flats got mid 26 running with AC @ 70 mph.
With new non run flats that dropped to 24-26 max. (mostly in the 25's).
Added the Predator tune, and essentially no mpg change (but the performance is better, and no A4 torque management lag).
This is all with Shell 93 octane.
Note that we occasionally run north (of FL) in wife's Dodge Magnum Hemi, and we see a wide range in fuel mileage (while running same speeds) depending on last fuel stop. This is with consistant fuel top off procedure and full gauge reading. It seems to vary by states by up to 3 mpg (example is 24+ mpg in N.C., and we are lucky to get 21 in FL). I always kid that we get better mpg is because we are running uphill. Of course we get the same variance when returning, so there goes that terrible theory......
Plasticman
07-21-2013, 07:56 AM
#11
Drifting
[QUOTE=Vetteman Jack;1584459525]Mind sharing your secret? I've got an A4 with 3.15 gears and on a good day I get 26-27 on the highway with cruise set - normally it is 25-26 mpg. I could hit 30 mpg with my '00 MN6 car, but the auto is not rated anywhere near the same as the six speed car.
Yep, My secret is I drive like an old man! Maybe because I am one
I drive the speed limit which is 55 here on the OBX and I have never had a speeding ticket or any moving violation in my 50+ years of driving. I do not drive like an old man on the dragstrip however 
! I always get better than e.p.a. milage on my cars as I am also a "tightwad" [wife's description] who doesn't like to buy gas!
Yep, My secret is I drive like an old man! Maybe because I am one
I drive the speed limit which is 55 here on the OBX and I have never had a speeding ticket or any moving violation in my 50+ years of driving. I do not drive like an old man on the dragstrip however ! I always get better than e.p.a. milage on my cars as I am also a "tightwad" [wife's description] who doesn't like to buy gas!
07-21-2013, 10:46 AM
#12
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When I bought mine and drove it home from Tampa to CT, I averaged 29.5 auto cruise at 75. 3:15 gears was showing RPM at 2100.
With all the mods I've done I still show RPM of 2100 at 75. on highway Im around 24-26MPG in CT. on a flatter surface Im sure it would be higher.
Im fine tuning the EFI Live tunes to play with it, turn some things on after the tune, boost timing in area's that can handle it all should increase mileage.
With all the mods I've done I still show RPM of 2100 at 75. on highway Im around 24-26MPG in CT. on a flatter surface Im sure it would be higher.
Im fine tuning the EFI Live tunes to play with it, turn some things on after the tune, boost timing in area's that can handle it all should increase mileage.
03-13-2014, 05:30 PM
#13
Melting Slicks
Thread Starter
UPDATE: The following dialog from another thread helps to further describe parameters that may affect the topic discussion:
There’s good theory that *more* restriction in the exhaust will *help* cruise mileage!
At steady-state cruise, the car’s computer watches the O2 sensor and maintains stoichiometric ratio of 14.7:1 air/fuel.
Low-restriction intake & exhaust are designed to better fill & empty the combustion chambers. At steady-state cruise, the throttle plate is far & away the restriction in the air system; it’s almost closed. The amount of vacuum in the intake is measured by the MAP sensor.
A low-restriction intake ‘might’ ever so slightly reduce the airbox system vacuum relative to the ambient pressure. However the throttle will then stay 1 or 2 % more closed to restrict the airflow.
So – high-flow intake does absolutely squat to cruise mileage.
The stock exhaust restricts the amount of air leaving the engine. Say the stock exhaust allows the 5.7 liter to be 90% efficient at cruise (i.e. 90% of the exhaust gas leaves the cylinder on the exhaust stroke, or 5.13 liters).
Aftermarket exhaust might allow it to be 95% efficient (5.42 liter), just pulling these numbers out of my hat. At the same rpm, the car is then using 5% MORE gas to hit the stoichiometric ratio of 14.7:1 air/fuel, because the effective engine displacement just went up.
It also in theory makes 5% more power at the same rpm & stoichiometric ratio. So what happens – the throttle plate stays another 1-2% closed.
So how to improve mileage? Reduce the effective engine displacement.
Add restriction to the exhaust; make it say 80% exhaust scavenging effective. The 5.7 then ‘acts’ like a 4.56 liter. Some engines have EGR and other systems for this & other purposes.
Or increase the gear ratio, say from 3.42 to 2.73, so the engine spins over fewer times, throttle is more % open, burns less O2 & fuel for the same distance.
The engine (which is just a pump) is most efficient when the pumping loss is smallest. The throttle plate is just an enormous restriction on the pump. This is why a lot of hybrids, etc are designed to run their engines at a fairly high RMP/open throttle position, while maintaining stoichiometric ratio of 14.7:1: to reduce pumping loss. And part of the reason why diesels (no throttle plate) are more efficient. ...
At steady-state cruise, the car’s computer watches the O2 sensor and maintains stoichiometric ratio of 14.7:1 air/fuel.
Low-restriction intake & exhaust are designed to better fill & empty the combustion chambers. At steady-state cruise, the throttle plate is far & away the restriction in the air system; it’s almost closed. The amount of vacuum in the intake is measured by the MAP sensor.
A low-restriction intake ‘might’ ever so slightly reduce the airbox system vacuum relative to the ambient pressure. However the throttle will then stay 1 or 2 % more closed to restrict the airflow.
So – high-flow intake does absolutely squat to cruise mileage.
The stock exhaust restricts the amount of air leaving the engine. Say the stock exhaust allows the 5.7 liter to be 90% efficient at cruise (i.e. 90% of the exhaust gas leaves the cylinder on the exhaust stroke, or 5.13 liters).
Aftermarket exhaust might allow it to be 95% efficient (5.42 liter), just pulling these numbers out of my hat. At the same rpm, the car is then using 5% MORE gas to hit the stoichiometric ratio of 14.7:1 air/fuel, because the effective engine displacement just went up.
It also in theory makes 5% more power at the same rpm & stoichiometric ratio. So what happens – the throttle plate stays another 1-2% closed.
So how to improve mileage? Reduce the effective engine displacement.
Add restriction to the exhaust; make it say 80% exhaust scavenging effective. The 5.7 then ‘acts’ like a 4.56 liter. Some engines have EGR and other systems for this & other purposes.
Or increase the gear ratio, say from 3.42 to 2.73, so the engine spins over fewer times, throttle is more % open, burns less O2 & fuel for the same distance.
The engine (which is just a pump) is most efficient when the pumping loss is smallest. The throttle plate is just an enormous restriction on the pump. This is why a lot of hybrids, etc are designed to run their engines at a fairly high RMP/open throttle position, while maintaining stoichiometric ratio of 14.7:1: to reduce pumping loss. And part of the reason why diesels (no throttle plate) are more efficient. ...
Yep - For power production the limiting factor is how much O2 can get jammed in the cylinder. Thus high-flow intake & exhaust, cam, turbo, etc.
To maintain the closed-loop cruise 14.7:1 ratio, more O2 = more fuel required = less mpg...everything else being equal
I suspect some of the HP gains from the standard upgrades of intake, exhaust etc is that the engine computer operates in open-loop under heavy throttle. The factory calibration might be 12:1 under load (again just making this up). The new parts let more air in& out, ratio might go to 13:1.
This will make more power on the dyno but at the risk of higher combustion temp, detonation etc. So having the car competently tuned after any major mod is extremely important for the engine's longevity. ...
To maintain the closed-loop cruise 14.7:1 ratio, more O2 = more fuel required = less mpg...everything else being equal
I suspect some of the HP gains from the standard upgrades of intake, exhaust etc is that the engine computer operates in open-loop under heavy throttle. The factory calibration might be 12:1 under load (again just making this up). The new parts let more air in& out, ratio might go to 13:1.
This will make more power on the dyno but at the risk of higher combustion temp, detonation etc. So having the car competently tuned after any major mod is extremely important for the engine's longevity. ...
03-13-2014, 07:45 PM
#14
Burning Brakes
2001 convertible with top down,300 miles over the cascade mountains at about 75 MPH with 3.15s and A4 trans.I got 26.2 MPG.That is the best I have ever done.
Fred
Fred
Last edited by memmer; 03-13-2014 at 07:50 PM.
03-14-2014, 03:05 AM
#15
Burning Brakes
I have a K&N intake, LG long tube headers and a tune. Went on a 3000 mile road trip across the country in July drove 75 on cruise almost the whole way and back and averaged 31.7mpg with 91 oct. A/C blasted the whole way, including setting in city traffic and road work occationally.
But I have a 6 speed =)
But I have a 6 speed =)
03-14-2014, 05:40 AM
#16
Melting Slicks
Thread Starter
03-14-2014, 10:20 AM
#18
Melting Slicks
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Man, the difference in peoples ave. fuel economy is crazy! I averaged around 30 on the freeway with my A4 (2.73 rear). Even now with 3.42's, higher stall converter, H/C/I build, etc. I am averaging 24-25mpg on the freeway. Thats 91 octane too. Must be that Im at sea level... ?
03-14-2014, 10:41 AM
#19
Averaging 25-27mpg hwy with H/C/I A4, 3200 stall, full exhaust no cats, 3.15 rear, 36lb injectors. Around town, completely different story haha, haven't averaged it but id guess about 12mpg or so