MPG Question (point me in the right direction)

[Edahall]

Actually the stock timing sucks. I recently learned it is set at 0 where as I have been setting it at 8* advanced forever. I'm assuming that is the reason my TBI trucks do far better than many others. The timing has to be adjusted by the computer, it isn't as easy as loosening the hold down and spinning the distributor. I have seen where people have used HEI style distributors on TBI vehicles and used a standard spring kit to adjust the timing to their liking. Apparently you have to run the tach signal from the HEI distributor to the TBI computer for the VSS to work. I haven't tried it. One thing that makes a difference is deleting the knock retardation. If the knock sensor picks anything up it will retard the timing and increase the fuel delivery, dropping the power.

Does what you describe also apply to the Vortec motor? Back when I was driving vehicles with gas engines, I used to set the timing so it was on the verge of pinging.

 

[Aloicious]

vortec motors are COMPLETELY different when it comes to timing, the timing HAS to be set as close to 0 as possible (and can only be done with a computer or scanner that can read it correctly, no timing lights)...and the spark advance is fully computer controlled as well, so you adjust it all within the tune...

 

[Chris]

I used hp because I thought Joules would be too detailed. Most of the guys here understand the term hp more than Joules. And yep, the thermal efficiency of the engine is the variable. An engine running at 25% thermal efficiency will outperform an engine at 20%, all else equal. What I was getting at is there isn't a way to significantly alter the Vortec 5.7 to achieve better economy. The best way to add some MPGs is to have the timing tuned to max ignition advance before detonation on the highest available fuel rating. If at all possible, avoid E10 with older engines. It will cause lower MPGs without exception when compared to 100% gasoline. A unit of fuel is a unit of fuel, it produces the same energy in any engine. If the fuel is being completely burned then you can't make an engine much more efficient.

That's not entirely correct. If you'll read back to what I said, EGR on older engines (and cam overlap on newer engines) is the method proven best for increasing economy of large gasoline engines. To oversimply it greatly, you're using inert gas to reduce the combustible volume of each cylinder, thereby shrinking the effective displacement of the engine.

Also, max ignition timing and high octane fuel is not the route to go for best economy. Even with high advance, a 93/94 octane fuel will still burn slower (relatively speaking here) than a fuel with an octane rating of say 87. Take the (extreme example) of Formula 1 engines. They run on what amounts to pump gas with an octane rating of less than 100. Sure, they could use something a little higher but what they've figured out is they make peak power and efficiency with the LOWEST possible octane fuel they can run, without damaging the engines from detonation. (Nowadays they don't have to worry about detonation, because the delta of the piston outruns the flamefront so it's not physically possible for there to be preignition, but the same argument was true in old days) This science has percolated down through other areas as well. The common misconception is that 87 octane has more potential energy than 93, but that is incorrect. 93 is more stable and resists preignition, and that is the only benefit over 87. However when you factor in the requirements of a street engine you can see why a "faster" burning fuel is advantageous. You still want to run as much ignition timing as possible without detonation, but coupling that with high octane fuel in a low compression engine is pointless. Heavily modified and/or high compression engines are altogether unrelated to the subject at hand.

The subject of ethanol in fuel is where things get a little......off. Less than 10% in gasoline is fine and shouldn't rob anything more than a rounding error from total fuel economy over the course of a tank. It won't melt fuel system parts or leave a residue. Quite the opposite actually, it will clean it. I've seen more stations selling E15 which is a little more tricky. It will have detrimental effects on economy but other than that, no other driveability issues abound. I was still able to see over 400 miles per tank on the stuff this summer without fail. Food for thought.

 

[Chris]

i have a 700r4 but i understand what your saying. 23 on an obs is insane how the heck are you doing that exactly?

Science, along with some creative tuning. (and very repeatable driving statistics)

 

[MOBS]

Heh, I love being able to fill up at home, drive the 150miles across the state, turn around come back, then go back again, then come back again, and still have about 50miles worth in the tank

 

[GMCTRUCKS]

At 60 my 93 GMC does much better but when I go to passing that's when it eats up more gas and also the wind direction makes a big difference hills also hurts.

 

[MOBS]

At cruising speed, the best mpg's are attained with a very steady foot....every time the pedal changes, it interrupts the smooth mix and causes ecm to lean or fatten the mix.

 

[Tempted]

And the off/on causes our converters to unlock and lock.

 

[MOBS]

^exactly right....ever notice how when you're decellerating down a hill you can feel the converter holding you from just free rolling....then when you touch the gas, on some vehicles, it'll feel like it shifts from 3rd to 4th....that's because some of our trucks use a shift-down feature with converter lockup to slow you down on hills. When you hit the pedal to accelerate again, it revs up then unlocks momentarily and shifts then locks again. Atleast that's how my 80E works haha.

 

[Tempted]

Its how the 60Es work as well. A way to find out if the converter is locking/unlocking properly is to maintain speed, keep your right foot on the throttle pedal and touch the brake pedal with you left foot. Don't release the throttle pedal. If the engine raises in RPMs when you touch the brakes then the converter lock up is working properly.