Can a C3500 5.7L 4.10 beat 10 MPG?

[Darrin De May]

I drove to & from Orange County from Palm Desert yesterday on the 10, 215 & 91 freeways, and including a half hour of bumper-to-bumper crawling on the way back, I calculated that I got 13.5 mpg. Without the traffic jam, it would likely have been a bit better, but that's the reality of using the 91 during afternoon rush hour.

 

[Spookers]

This is post is laughable. It could be picked apart from so many angles such that all I'm going to say is that you will not get better MPG swapping to a carburetor and that the TBI system isn't holding the vehicle back.

Agree to disagree lol thats just what I've found in my case.
from day 1 it got 10mpg and it didn't matter how you drove it or what gears or if it had a little boost, I even had a 454 tbi with the 74lb injectors on it and that didn't change the mpg either. (i figured any one of those things would have worsened it but they didn't)
All I can say is once I swapped to a carb instantly got 17mpg and it ran beautiful other than it coughed at wot when it was cold (didn't actually think it would work as well as it does just needed an engine fast and that's why it's another 305)

Considering the small block tbi is barely large enough for a 305, then yea it holds you back on a 350, a big block tbi would work on a stock 350 but not if its "hopped up" in anyway

 

[Shiny]

This may be over looked too but make sure your tires are inflated to proper pressure and that no brakes are dragging, front or rear. Check front wheel bearings for proper nut tension and make sure you do a bearing pack. Those may help some if they are out in some way.

 

[DerekTheGreat]

Agree to disagree lol thats just what I've found in my case.
from day 1 it got 10mpg and it didn't matter how you drove it or what gears or if it had a little boost, I even had a 454 tbi with the 74lb injectors on it and that didn't change the mpg either. (i figured any one of those things would have worsened it but they didn't)
All I can say is once I swapped to a carb instantly got 17mpg and it ran beautiful other than it coughed at wot when it was cold (didn't actually think it would work as well as it does just needed an engine fast and that's why it's another 305)

Considering the small block tbi is barely large enough for a 305, then yea it holds you back on a 350, a big block tbi would work on a stock 350 but not if its "hopped up" in anyway

In stock configuration the TBI holds nothing back. As a matter of fact, if you're driving at anything below full throttle it sure as hell isn't.

I wasn't the one filling your tank up or keeping track of your mileage but I very much suspect your methods. I bet your speedometer wasn't calibrated correctly and that you weren't filling the tank up to the same spot every time (I fill mine until gas stands in the filler neck, ::gasp:: it's consistent. Going until the pump clicks isn't.) Then I doubt your truck was running right when it still had it's EFI set-up. If that carbureted 305 was factory stock, it had less torque than the 350 it replaced, so it had to work harder to move the additional weight and thus burn more gas to do so. Unless I could drive it myself for a month, I refuse to believe those numbers vs TBI.

 

[89GMCJOHN]

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I realize its a Burban and 454 SS ...but typical of the mpg's

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back then ....

 

[Spookers]

In stock configuration the TBI holds nothing back. As a matter of fact, if you're driving at anything below full throttle it sure as hell isn't.

I wasn't the one filling your tank up or keeping track of your mileage but I very much suspect your methods. I bet your speedometer wasn't calibrated correctly and that you weren't filling the tank up to the same spot every time (I fill mine until gas stands in the filler neck, ::gasp:: it's consistent. Going until the pump clicks isn't.) Then I doubt your truck was running right when it still had it's EFI set-up. If that carbureted 305 was factory stock, it had less torque than the 350 it replaced, so it had to work harder to move the additional weight and thus burn more gas to do so. Unless I could drive it myself for a month, I refuse to believe those numbers vs TBI.

Have you ever ran a vacuum gauge? At wot above 3500rpm it's starts pulling vacuum on the 305 tbi (never had a 350)that had the "ultimate tbi mods" and the "salad bowl", by 4000rpm I was seeing 3inhg.
Sure I only filled till the nozzle shut off (like everyone else), but I went to the same pump everytime, and recorded the trip meter and reset it everytime, I had a GPS that confirmed my Speedo was right (i did swap the vssb when I swapped gears) but sure I never had it taken to a shop to calibrate it like they do for police vehicles, it may or may not have been filled to the exact same point right down to the last drop but when the data I collected was being very consistent I don't see a reason adding risk of spilling fuel just to see it in my filler tube, which if you don't have a line that you fill to can be just as "inaccurate" and your vent wont work properly with it that full and on top of that unless you took the flap off the filler neck you can't even see in the tube.
And like I said agree to disagree, it can be hard to be told that what you believe in to be wrong. I only found out how much better the carb was by swapping to it (not my first choice but was the only choice I had in a short notice) I too did not expect it to drive the way it does and get the fuel economy it does.

This is getting off topic, the point I was making is The tbi injection system was a very inefficient design. if better fuel economy is wanted try anything else, carb is likely the easier to swap but going to a vortec with the updated injectors and a 411 pm to tune it in or to an ls would also get much better fuel economy

 

[94burbk1500]

Have you ever ran a vacuum gauge? At wot above 3500rpm it's starts pulling vacuum on the 305 tbi (never had a 350)that had the "ultimate tbi mods" and the "salad bowl", by 4000rpm I was seeing 3inhg.
Sure I only filled till the nozzle shut off (like everyone else), but I went to the same pump everytime, and recorded the trip meter and reset it everytime, I had a GPS that confirmed my Speedo was right (i did swap the vssb when I swapped gears) but sure I never had it taken to a shop to calibrate it like they do for police vehicles, it may or may not have been filled to the exact same point right down to the last drop but when the data I collected was being very consistent I don't see a reason adding risk of spilling fuel just to see it in my filler tube, which if you don't have a line that you fill to can be just as "inaccurate" and your vent wont work properly with it that full and on top of that unless you took the flap off the filler neck you can't even see in the tube.
And like I said agree to disagree, it can be hard to be told that what you believe in to be wrong. I only found out how much better the carb was by swapping to it (not my first choice but was the only choice I had in a short notice) I too did not expect it to drive the way it does and get the fuel economy it does.

This is getting off topic, the point I was making is The tbi injection system was a very inefficient design. if better fuel economy is wanted try anything else, carb is likely the easier to swap but going to a vortec with the updated injectors and a 411 pm to tune it in or to an ls would also get much better fuel economy

Dude, you're wrong, just admit it. GM went to TBI because it is more efficient than a carburetor. If what you're saying were true, why were carbs killed off in the late 80's? Next you're going to tell us that an ebay "cold air intake" woke up your engine. Find me one article from a reputable source backing up your claim.

 

[PlayingWithTBI]

All I know is I'm getting 12.7 MPG with my bored (46mm) and "ultimate mods" TBI system with 170cc aluminum heads, 3704 intake manifold, cam, headers, and 3" exhaust on my 350 - DRIVING IT HARD! This TB flows 650 CFM which is plenty for 6000 RPM! I data log quite often and my MAP at WOT up through 5400 RPM stays flat.

 

[DerekTheGreat]

Have you ever ran a vacuum gauge?...

Have you ever admitted you're wrong?

...Sure I only filled till the nozzle shut off (like everyone else)…

Guess you haven't heard what they say about assumption either.

...This is getting off topic...

Ah, the words of someone whose face is starting to turn red from embarrassment.

 

[L31MaxExpress]

Something is definately wrong. I have an Express conversion van with a 450 hp 5.7 and full 1-ton running gear with a 5.13 gear amd 32" tall tires. 13 mpg city and 17 mpg highway running at 70 mph @ 2,700 rpm. With the relatively stock engine, 4L85E and 3.73s it would get 18-19 mpg highway cruising 75 mph @ 2,200 rpm. With the tall gears and much less power I was foot to the floor alot in the city and still averaged 10-11 mpg. I think the worst tank I ever had with alot of idle time and running hard around town was 9.7.