Better gas mileage and performance.

[Sean Buick 76]

So how many guys are running on the original 02 sensors at 200,000+ miles?

 

[Drunkcanuk]

So how many guys are running on the original 02 sensors at 200,000+ miles?

Lol....this guy!!!! At 330 000km!!! On the to do list in the near future.

 

[Scooterwrench]

So how many guys are running on the original 02 sensors at 200,000+ miles?

Many I would guess. Nobody thinks to replace them until they go bad entirely. The two parts that have done the most for performance in my tired ol truck motor have been the O2 and ECT sensor. Both worked but not optimally.

 

[L31MaxExpress]

Without the 02 sensor the engine runs off a default air fuel and timing trim which is super rich and will burn 30-40% more fuel.

I can tell you that is not correct at all provided it is a stock engine and stock ECM or PCM calibration. The open loop air/fuel ratio lockup table will still command a 14.7:1 air/fuel mixture when the engine reaches ~150F and the fueling is still reasonably close. Some of the TBI and Vortecs even came from the factory without 02 sensors at all. I have owned an Innovative LM1 with dual widebands for years, when I tune these trucks in person, I pull the front narrowbands out, slip the widebands in, clear the long term fuel trims and tune the VE table and MAF table in open loop. It will throw codes for lack of 02 activity, but it does not signifigantly change the air/fuel ratio.

 

[Sean Buick 76]

I can tell you that is not correct at all provided it is a stock engine and stock ECM or PCM calibration. The open loop air/fuel ratio lockup table will still command a 14.7:1 air/fuel mixture when the engine reaches ~150F and the fueling is still reasonably close. Some of the TBI and Vortecs even came from the factory without 02 sensors at all. I have owned an Innovative LM1 with dual widebands for years, when I tune these trucks in person, I pull the front narrowbands out, slip the widebands in, clear the long term fuel trims and tune the VE table and MAF table in open loop. It will throw codes for lack of 02 activity, but it does not signifigantly change the air/fuel ratio.

I will take your word for it but I am surprised. How does the computer keep the engine at 14.7ish without the input from the 02 sensors?

 

[95Noobie]

So the 305 is underpowered for the work we are asking it to do. I agree that if this motor gets tired or gives us issues we can easily put a 350 in, or I have read about many putting the next generation 5.3 in with some modifications to wiring and computer system.

What does it take to put different gears in the rear? You get a donor rear axle and attach drive shaft and rear suspension to it? Foolish question but if the gear ratio is 3.73 then that is too 'tall' meaning not enough torque and the engine works harder to get it going? I thought higher gears meant lower rpms at higher speeds and better mpg (like adding a overdrive to a 3 speed) But then if we are using fuel trying to get the heavy truck going at low speeds then we need a smaller gear, like a 3.4?

 

[PlayingWithTBI]

I will take your word for it but I am surprised. How does the computer keep the engine at 14.7ish without the input from the 02 sensors?

I think what he's saying is, the VE table was developed to achieve 14.7 AFR as long as the conditions are optimal like, elevation, temperature, vacuum leaks (to a certain extent), etc. But, if they vary at all, it doesn't have the feedback from the O2 sensor to correct the VE settings. GM did not set the table to run rich when in open loop, it's as close as they can get it, not a target of running rich.

 

[RDF1]

I will take your word for it but I am surprised. How does the computer keep the engine at 14.7ish without the input from the 02 sensors?

If the truck is all factory (intake and exhaust) The MAF and VE tables are based around that. So if its stock, you can remove the Oxygen Sesnors and the goes back into Open Loop which is VE and MAF without any fueling corrections.
If the engine is healthy and all sesnors/injectors are in good health and the PCM is in closed loop it shouldnt be much more than 5% Fuel Trims.

When you cold start just about anything, it will be in open loop till a certian coolant temp based off Intake air temp. This is from a 2004 Silverado 5.3

 

[Road Trip]

Back when I had 33s, 3.42s and no lift I was getting 13-14 mpg. If I remember right I lost about 1-2 mpgs going from a 285 A/T tire to a 12.50 M/T tire for what it's worth. This was all tank average at the pump but more highway than city most of the time.

...The dual exhaust and big (heavy) wheels aren't helping either, taxing the engine every time you take off from a stop.

Anyone who has changed from a stock weight cast iron flywheel to a lighter aluminum one
on their engine knows the difference that the resulting difference in rotational inertia affects
the powerplant dynamics.

Essentially, the 4 (or 6) tire/wheel assemblies are large flywheels. And the closer the weight is
to the outside diameter the more the inertial effect it has. When it comes to MPG, a larger/heavier
tire/wheel combo has a varying effect, depending upon how it's driven.

At one extreme, if the truck is used 'long haul' style (fill up, then get to cruising and stay
there for the entire tank) then the difference in MPG will actually be minimal.
(Assuming a good alignment, tires not scrubbing, aired up properly, etc)

At the other extreme, let's make the same tire change and then start using the truck
as a UPS or FedEx style city delivery vehicle. Due to the greater rotational inertia of the
larger tires/wheels versus the constant starting/stopping there will be a significant MPG
penalty. It's just the way that the inertial component of flywheels work.

In English, to maintain the same level of acceleration and stopping power, you need
more motor and better brakes to counter the net effect of the new, larger sneakers.

Given the above, it's entirely possible for 2 different people to experience different MPG
changes when moving to larger/heavier tire/wheel assemblies. It all depends on how
they task the truck post upgrade.

So yeah, I thought that boy&hisdogs was spot on by noting how he used his truck
when describing the MPG change he observed with the tire change. YMMV.

 

[Scooterwrench]

So the 305 is underpowered for the work we are asking it to do. I agree that if this motor gets tired or gives us issues we can easily put a 350 in, or I have read about many putting the next generation 5.3 in with some modifications to wiring and computer system.

If you're gonna go LS go 6.0,better torque.

What does it take to put different gears in the rear? You get a donor rear axle and attach drive shaft and rear suspension to it? Foolish question but if the gear ratio is 3.73 then that is too 'tall' meaning not enough torque and the engine works harder to get it going? I thought higher gears meant lower rpms at higher speeds and better mpg (like adding a overdrive to a 3 speed) But then if we are using fuel trying to get the heavy truck going at low speeds then we need a smaller gear, like a 3.4?

Your truck is 4WD so you would want to change the gears in the front too.
It seems that gear ratio description has changed. People now say a 3.73 is lower than a 4.10 because the numbers of the 3.73 are lower. Us old graybeards would refer to the 4.10 being the lower gear and the 3.73 the taller one. Low gears(4.10's)will get you up to speed quicker and the engine will be turning faster at the same vehicle speed as the 3.73(taller)gear. The best scenario is to find the right gear that puts your engine in its torque range at cruise with the size tires you want to run.
The math is,
One mile(63360") divided by tire circumference = tire RPM. Tire circumference = tire diameter times Pi(3.1416)
Tire RPM times gear ratio = driveshaft RPM and engine RPM in direct drive.
Our OD trannys have a .7 ratio so you would multiply your driveshaft revs times .7 to get your engine speed in OD.
Lets say your tires are 34" in dia.
34" x 3.1416(Pi)=106.8"(circumference)
63,360(inches in a mile)/106.8=593.25(tire RPM)
593.25(TRPM)X 3.73(gear ratio)=2212.85(driveshaft RPM) and the engine RPM in direct drive @60 MPH
2212.85(DRPM) X .7(OD ratio) = 1549(engine RPM in OD) @60 MPH.
1550 is bogging your engine down with that tire and gear so to get the engine back up in the sweet spot you would want to lower the gear to 4.10.

Doing the math with a lower 4.10 gear brings your engine up to 1702 RPM in OD. Better but the sweet spot is 1800-2200 for your engine. That is where your going to get your best fuel economy and drivability.