With all the talk about gas prices, did any of your mods increase mpg?
03-01-2008, 12:24 PM
#22
Burning Brakes
Member Since: Mar 2006
Location: San Jose CA
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Theoretically speaking, not many of the above mentioned reasons should provide better mileage.
In most ECUs, when going from NA to FI and injectors are changed, the ECU's idea of fuel usage is linked to the injector pulse width, regardless of injector. And thermodynamically speaking, there's no way a procharger can make the engine more efficient at highway speeds. So I'd be curious to see the mileage measured the old school way - by dividing miles traveled by amount of gas to fill the tank.
Also, with a tune, the engine is supposed to run in open (closed?, can't remember terminology) loop mode under part throttle, which means the ECU is trying to run 12.7 AFR. It's only under full throttle (closed loop mode) that the engine goes richer / changes timing to prevent detonation and make good power. A tune leans out that condition to make more power with less margin of safety.
I don't build SBC for GM or anything, but I am an engineer and am overly passionate about the laws of thermodynamics (NERD!).
In most ECUs, when going from NA to FI and injectors are changed, the ECU's idea of fuel usage is linked to the injector pulse width, regardless of injector. And thermodynamically speaking, there's no way a procharger can make the engine more efficient at highway speeds. So I'd be curious to see the mileage measured the old school way - by dividing miles traveled by amount of gas to fill the tank.
Also, with a tune, the engine is supposed to run in open (closed?, can't remember terminology) loop mode under part throttle, which means the ECU is trying to run 12.7 AFR. It's only under full throttle (closed loop mode) that the engine goes richer / changes timing to prevent detonation and make good power. A tune leans out that condition to make more power with less margin of safety.
I don't build SBC for GM or anything, but I am an engineer and am overly passionate about the laws of thermodynamics (NERD!).
In a very simplified explanation (cuz it's a lot more involved) PCM's run in closed loop during part throttle attempting to maintain a 14.7:1 air fuel ratio (AFR). Under full throttle, the PCM switches to open loop and uses a separate table for WOT AFR and does not rely on sensors like O2's as it does in closed loop mode.
This is where the tune (FI or NA) is so significant, especially with addons. The factory tune is a flat estimate that best estimates what would work for mass production. Unfortuntaly, all engines and their components are not manufactured the same and, therefore, vary in performance. I.E. Just because the computer commands 14.7:1 doesn't necessarily mean that's what the actual AFR is.
Tunes increase the thermodynamic efficiency of the motor by getting the commanded AFR to match the actual AFR. You can actually "cheat" a little and safely lean out a motor during closed loop daily driving for better gas mileage while keeping the WOT AFR in a safe range. Gas is better burned which means less throttle is required.
FI magnifies the extremes of good and bad gas mileage because it exceeds the theoretical and real thermodynamic efficiencies of an NA more >>100%. Of course, when you bury your foot in an FI car you have a ton more air and, subsequently, dump a lot more fuel at WOT.
03-01-2008, 03:39 PM
#24
Tech Contributor
Almost all mods increase efficiency and the only reason MPG drops when you get a H/C install is that the driver is a bit overzealous.
The most gains would be with a head swap and the highest compression that wourks on your cam and 93 octane. Spare the hot weather statements; the tune compensates fully for the hottest weather there is. If its 107degrees and 98% humidity you will have less timing running. The car pulls timing for intake air temps and engine coolant temps.
An LS2 engine running LS3 hgeads and 11.3:1 compression will see 2-3 mph gain with the stock cam.
The most gains would be with a head swap and the highest compression that wourks on your cam and 93 octane. Spare the hot weather statements; the tune compensates fully for the hottest weather there is. If its 107degrees and 98% humidity you will have less timing running. The car pulls timing for intake air temps and engine coolant temps.
An LS2 engine running LS3 hgeads and 11.3:1 compression will see 2-3 mph gain with the stock cam.
03-01-2008, 04:30 PM
#25
Safety Car
Member Since: Dec 2004
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St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I couldn't tell ya ~ gas milage has never been high on my list of priorities....but I can tell you with the mods I have made to my car, I have more of a tendancy to put my foot into it ~ that can't be good for the gas milage....I'll just continue to pay at the pump and enjoy the ride!
03-01-2008, 08:24 PM
#28
Melting Slicks
Can't say anything on here about gas mileage or the cost of gas or everyone will think your a dork. You know, it's the old "if you can't afford the gas then you shouldn't own a vette" thing. So be careful.
03-04-2008, 12:51 PM
#33
Drifting
Got 28.5 mpg with my LS2 making 461rwhp. My forged LS3/L92 427 gets 25.5 mpg @70-75 mph in 6th, and it makes 525rwhp. Only thing is, I'm usually driving a little faster than that. So I think most mods help gas milage, to a degree.
03-04-2008, 05:48 PM
#35
Heel & Toe
Member Since: Apr 2007
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I have a Halletech intake, tune & have the car lowered 2". Yesterday I had magnaflow 3" mufflers (welded in) installed with 3" inlet tips. On the way to work today (30 min drive) and I got 35.5 mpg highway! Not sure if this is temporary or what what I was impressed. Best I have had before that has been 32/33.
I hope the highway MPG stays.
The new mufflers were 20 lbs less than the factory also!
I hope the highway MPG stays.
The new mufflers were 20 lbs less than the factory also!
03-04-2008, 09:40 PM
#38
Instructor
Member Since: Apr 2007
Location: Edmond OK
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Our C6 Auto-6 best mpg has been 29.4 at 75mph or so on highway.
We have Lingenfelter CAI and Borla cat backs. Don't think the Borla
cat backs open up the exhaust a lot, but I love the sound. I am going
to have mine tuned this summer. I am interested in more mpg when I
am crusing on the highway. I also like lower ET's at the Sunday drags.
I am not a techie but if an engine runs efficiently at 1500 rpm, it stands to reason it should be running just as efficiently at 6500 rpm. I think MPG's and Low ET's can co exist.
Stay "tuned" for more. . . .
We have Lingenfelter CAI and Borla cat backs. Don't think the Borla
cat backs open up the exhaust a lot, but I love the sound. I am going
to have mine tuned this summer. I am interested in more mpg when I
am crusing on the highway. I also like lower ET's at the Sunday drags.
I am not a techie but if an engine runs efficiently at 1500 rpm, it stands to reason it should be running just as efficiently at 6500 rpm. I think MPG's and Low ET's can co exist.
Stay "tuned" for more. . . .
04-13-2008, 07:22 PM
#39
Burning Brakes
On a drive from Florida to VA, Procharged, 3.15 axle, A4 Rebuilt with stall.
37.5 gallons,
811.4 miles
about 21.9 MPG real world driving at 70 to 75 freeway. The DIC displayed 22.5 MPG.
Seems low.
37.5 gallons,
811.4 miles
about 21.9 MPG real world driving at 70 to 75 freeway. The DIC displayed 22.5 MPG.
Seems low.
04-13-2008, 07:35 PM
#40
Burning Brakes
Member Since: Apr 2008
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St. Jude Donor '08
I noticed a difference with the 160 stat and K&N I never got 29 to 34 in the rain I did on thurs 3 mpg more than I did before. Never expected it.