I just checked and I just got ~27 mpg on the trip back to school. 540 miles and 20 gallons at about 75 mph. I have 3.08 and the 4L60 but still, this seems really good considering it's a 330hp engine. I did notice that I stayed in overdrive all the time, even going up huge hills the thing just muscle ****ed it's way up at 2000rpm. How do I tell if it's running too lean? I didn't notice any throttle power loss (checked that a few times going 100mph on flat stretch no cars).

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, Jon (350 TBI!)
91 Red My website

 

You could use a scan tool to check for O2 crosscounts and/or O2 voltage, which should be around .450 colts or so. Or, just get a voltmeter and read the O2 voltage. Or, pull some plugs and read 'em after a highway stint. If TOO white, too lean. You're probably ok, though.

EDIT: When I run the 3.07's in my Jeep, I can lug down to 1800rpm at 65 and it pulls up most hills. Coundn't do THAT with a carb, that's for sure!!! Thank heaven for EFI and OD!!!

[This message has been edited by FastBroker (edited May 08, 2001).]

 

That sounds about right for a well tuned V-8 with your gas tank. I get about 375-400 highway miles with my 15.5 gallon tank. Even though I have a manual tranny it has no problem maintaining cruise speeds going up hills. I laugh whenever I see those small engine imports go whizzing by on flat road and then I pass em right back going up a hill.

As for checking for a lean condition, I think you can find out by monitoring your O2 sensor voltage


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92 Camaro RS, LO3, 5-spd, T-tops

Performance:
K&N Open Air Filter, Edelbrock performer TBI intake, Fastchip Prom, Timing +4 degrees, Centerforce clutch, Xact 8mm wires, SLP 1 3/4" Headers (coated), Flowmaster Catback Exhaust, Z28 Grille w/aftmkt fog lamps
Electronics:
Alpine 8030 Alarm System, Valentine One Radar Detector (How did I ever drive without one?), Pioneer DEH 7450 Head Unit w/6-pack CD changer, Pioneer DEQ 7600 Sound Processor, 2 Kenwood KAC-846 Amps powering 2 12" Pro Red subs, 2 Pioneer 6x9 and 2 MTX 4x6 speakers.
NEXT UP: TBI mods, 3.42 gears w/Torsen posi

 

These cars will go into a "highway mode" of operating after a while. The FSM that I read recently about my 9C1 said it goes into highway mode "after a certain speed has been maintained for a certain amount of time and the engine is above a certain temp". Really technical. Anyway it appears to be 50-55 mph for about 5 minutes and the engine temp over about 180*. What happens then is the car starts to lean itself out automoatically.

Then you will see O2 voltages drop all the way down to 35-45mV.

 

I might be mistaken but Highway mode doesn't come on in our cars. If you look at the constants in something like tunercat you can see that it says it comes on around 50mph but then look at what the MAP which is 98.9kpa now I don't know about you but I sure as hell don't "cruise" at WOT. I've heard that it has something to do with emissions being worse when it's a lean condition. So GM had to disable it AFTER they had drawn up the plans for the ecm.



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, Jon (350 TBI!)
91 Red My website

 

that sounds about right actually i went abotu 200 miles on about a half a tank in my 91 rs. these cars are grat on the highway.

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91 camaro RS
- 305 TBI
Mods: Performer TBI intake, 1" TBI spacer, K+N, jet fan switch ,160 thermo,Accel control module, Jet Stage 2 chip, edelbrock TES headers, Compleate 3" Flowmaster exhaust, Edelbrock springs, KYB struts and shocks, 700R4 built by TSI, SLP tq converter, b+m trans cooler
Soon to come .SLP Posi 323,Spohn LCA's and panhard bar,Ascd ram air hood, Z28 spoiler and grille,white guage faces.

 

Okay, maybe I should tell you all that I'm not running with the stock engine. It's the 330hp crate motor!

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, Jon (350 TBI!)
91 Red My website

 

I just got 23 mpg on my last tank of highway driving. Check out my sig for details. I thought that was pretty good. 27 is awesome though.

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1992 Camaro Z28; Engine:383 w/Ported Edelbrock RPM heads. Lunati Solid Roller Cam 224/232 .502/.502, Fluidampr, Comp Cams 1.5 Roller rockers and lifters, Comp Cams Triple Valvesprings, SVO 30 lb injectors. Holley/Walbro 255 lph in-tank pump, Haltech E6GM DFI, SLP 1 3/4" headers, 4" Mufflex, T56, SLP Clutch, Pro-5.0 Shifter, 4.10 Gear, Auburn Offroad Pro Posi. Super Ram. 17" X 9.5" Ronal FireHawk Rims w/ 275 40ZR17 Bridgestone RE 730's up front and Nitto Drag NT 555R's out back.

Double-Pump This! EFI Rules!

 

Jon, I don't know what values are in my ECM as I've never used Tunercat. However, I've seen "highway mode" come in to effect on my L05 Caprice, exactly as described in the FSM. I use Turbo-Link to monitor it some times and I've watched on my drive home from work. The engine temp hits about 180 after I've been moving 5 minutes or so and if I set the cruise and leave it, the O2 voltages will drop from 500-800mV all the way down to 39mV. They will flucuate between 39mV and 60mV. YMMV. I'd get a scan tool of some kind and hook it up just to seem if it does come on.

 

J Prevost, Its been my experience that a 350 will generally get better mileage than a 305 in most cases even if the 350 is more heavily modded. I have done probably a half-dozen 305 to 350 swaps and I've never seen a case where the 350 got any worse mileage than the 305 that it replaced -- especially in trucks. I put a 350, with #186 (double hump) heads and a Comp 268H cam in place of a stock LG4 305 in an 83 Z28 I used to have and I was getting 20-21 mpg even with the Q jet carb. The stock LG4 got about 16 mpg. On the highway, the 350 does not have to work nearly as hard as the 305 does to maintain a certain speed. An intellegently speced 350 will usually make as much as or more TQ at 2000 than a L03 or LG4 at its TQ peak.

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82 Z28 383, Ported 215cc Dart Iron Eagle Heads, Comp custom grind roller cam, Speed Pro .250" domes, Wheeler Motorsports 4340 I beam rods, Eagle 5140 steel crank, Weiand Team G intake, Holley 800 DP, hedman headers
Trans: Turbo 350 w/ 4000 stall -- Rearend 7.5 w/ Richmond 4.10's, Auburn Minispool
Best ET: *NEW ENGINE* untried at track!!


89 RS, L03 305, Hypertech Chip,cat delete, Dynomax exhaust,K&N open element Filter,160 stat, MSD coil --Trans:700R4 Corvette Servo -- Rearend: 7.5 GM 3.42 w/ posi-lock
New Best ET: 14.91 @92.9
Bassett Racing
Mid Atlantic F Body -82 Z28 Page

R.I.P. Dale Earnhardt #3

 

After taking the air pump off & cutting off the catalytic converter, I got 26.5 which was better than I expected considering I was driving 70-80 mph.

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1987 Z28 Black & Silver 305 LG4/5-Speed . . .
NRA/AMA/FOP/IUPA/Blue Knights