Unusually Poor Mileage

[Step'67]

Another mileage thread, I'm sure you guys are thrilled. Oh well, here we go. Maybe it's just me, but my truck has been sporting some pretty poor mileage since the trans swap/engine refresh. Here's my current setup: 355, 9.5:1, lunati voodoo 262 cam, vortec heads, performer intake, headers, glasspacks, edelbrock 1406 carb, hei, and a 4l80e.

Prior to the trans swap (and engine refresh), it had the old truck 4-speed, and was burning quite a bit of oil due to a bad intake gasket seal, yet even with this it still pulled 13mpg on average, driving spiritedly. Tore it back apart, bored it out a bit, decked the block, new pistons, brought up the CR to the above spec, etc etc.

So my question is, I seem to be averaging 8-10 mpg city, not much better if any highway, with the current setup, 10 being fairly rare. I just filled up, drove her pretty easy over the past week, 9.5 mpg. I've got a wideband O2 on it, and have the carb tuned to 15.3 afr on average for cruise, which is where it stayed this past fill up. Engine runs great, and timing was set up following the "It's just a pickup" thread. I know these things are like bricks dragging a parachute down the freeway, but come on... No fuel leaks to speak of, is it just that big trans sucking up all my gas?

Maybe I'm just expecting more than I should out of it, but any suggestions on what to try and bump up the mileage? I've been seriously considering megasquirting it with a 454 TBI that I already have, mainly because of the tuning capabilities it provides, and looks pretty easy to solder (do this for a living on cell phones) and tune, but also could I expect better mileage? Sorry for the long post guys, any thoughts? Input is appreciated, thanks.

[buckskins4ever]

I have a 355 Built Vortec as well but backed it with the 700r4 and 3.73 rear posi, Holley Ultra Avenger 670 dual feed double pumper and I average right at 14mpg. I could only guess your 4l80e is dragging you down a bit but shouldnt be 3-4mpg difference.
Did you get it dyno tuned?

[Step'67]

Forgot to mention I have 3.73 rear end also. No, no dyno tune, still on my list. I actually have a new Holley Street Avenger 670 that hasn't been touched yet. Maybe this week or so I'll slap that on and see if it makes a difference, even without being AFR tuned.

[GASoline71]

What torque converter are you using in the 4L80E? The first 4L80E transmissions in the early 90's had HUGE torque converters in them weighing 50 pounds or more.

I had a 1991 3/4 ton Suburban eons ago. It had the original stock 5.7L with TBI, and a huge honkin' 4L80E behind it with a massive torque converter. Gas mileage was around 6mpg. I had to have the 4L80E rebuilt, and the shop that did the instal recommended a different lockup converter, and it was like night and day with that rig. Gas mileage improved to 11mpg, which is damn good for a 3/4ton Suburban with a Small Block in it.

Gary

[Step'67]

Wow, big improvement on the mileage, didn't consider the torque converter. The trans was out of an '06 GMC Savana, very few miles on it. I couldn't recall the dimensions on the converter, but she definitely was big. I don't know about 50lbs, but it sure was heavy. I guess that is a lot of mass to get spinning...

[Rufton]

Don't think TBI is solution because 15.3 AFR is already very lean.
I get good cruise mileage at 14 A/F.
What rod/jets are you running in the 1406?
I'm surprised you can cruise 1406 at 15.3 AFR and tune rich enough for WOT.
I'd double check fuel leaks and transmission slippage as mentioned.
And yes the torque curve would be fun.

[Step'67]

I played around with different rod/jet combinations per the chart listed in the manual, and actually found the stock setup to work best. Like I said, its usually in the low 15's afr cruise mode, power mode gets it into around low 13's afr, with I believe the 5" springs (changed from a heavier spring in attempt for mileage). Keeps me in cruise, until I really want it to kick into "power mode". With those springs there is a bit of dead area between cruise and power, so I'll probably go back to the last springs I had to bring back the response, but we shall see.

[piecesparts]

Can you tell us, what your timing is set at? If you discuss the timing with many tuners, they will tell you that timing is important for fuel economy. Also, what size tires are you running with your OD tranny. You may be working your engine to hard to get the truck rolling and then working it harder to get up and down grades. The OD tranny and the 3.73 gears is a good combo, but I agree, the truck should at least get 14 to 15 MPG. I have a stroker 385 engine, running a comp roller cam, with a 700 R4 with a 2300 stall non-lockup converter and a set of 3.73 gears. I get 15 MPG on the highway. Did you get all of the wiring connected on the 4L80E tranny, so that it is doing what it should?

[Step'67]

I guess I should have listed my timing specs. 14* initial, 19* mechanical in by 2800, and 10* vacuum. I did this a while back, but I am fairly certain those are the correct numbers. According to that thread, ideally I'd have more vacuum advance, but the stock canister doesn't allow for much, and I never got a replacement one with more advance capability. Could that also be hurting the cruise efficiency? As for tires, they are 275-60-r15 on all four corners. Transmission is being controlled by a FAST tcu, which has been working flawlessly since I installed it, and is wired properly as instructed.

[Rufton]

Originally Posted by Step'67

I played around with different rod/jet combinations per the chart listed in the manual, and actually found the stock setup to work best. Like I said, its usually in the low 15's afr cruise mode, power mode gets it into around low 13's afr, with I believe the 5" springs (changed from a heavier spring in attempt for mileage). Keeps me in cruise, until I really want it to kick into "power mode". With those springs there is a bit of dead area between cruise and power, so I'll probably go back to the last springs I had to bring back the response, but we shall see.

I have 357 CI and edelbrock 1405. I once tried the 1406 rod/jet combo and got lean stumble and bright white plugs; too lean for me. 1406 did produce 18.5 avg hwy mpg (380 mile hwy trip) but I was afraid I'd melt the pistons.

Now I'm using:

Main Jet=0.100"
Rod Cruise=0.075"
Rod Power=0.037"
Sec Jet=0.110"
Spring: Silver (power rod lifts a 7 Hg at about 1700 RPM)
Cruise AFR=approx 14
WOT AFR=approx 12
Accelerator pump linkage position=minimum squirt w/ no stumble throttle response.
Torque curve (Eng Analz Pro)= 340@1500, 380@2000, 420@2500.
Hwy Mileage=17mpg @ 3000lbs w/ 3.0 rear and 24" tires.

If your engine is in the ballpark as mine, 1406 OEM rod/jet should produce good hwy mpg if in meaty part of torque band but may be lean enough to produce excess piston heat.

[Step'67]

Im happy with the carb setup currently, so I'll probably stick with that unless someone strongly disagrees. I'm making about the same torque, just a tad higher in the rpm band. I know I could lean it out a bit more, but I don't believe that would return significantly higher mileage. Two things on my mind right now, vacuum advance and torque converter. Would a higher advancing canister earn me better city mileage? And what lighter mass torque converter would you guys recommend for my setup, if you think it would improve anything?

[Rufton]

I've run w/ all in advance from 32 to 38 and didn't notice significant hwy mpg difference.
Hard to judge city mpg because function of foot weight.
I agree,my money is on transmission slippage in your case unless your foot is made of lead.
Have you looked at tire tread travel per engine rpm to see how much transmission slippage you have?

[Step'67]

Pardon my lack of knowledge on this, but how would I determine if the trans is slipping? The trans only has like 15k miles on it, I'd be disappointed if it had an issue, Haha.

[Rufton]

Originally Posted by Step'67

Pardon my lack of knowledge on this, but how would I determine if the trans is slipping? The trans only has like 15k miles on it, I'd be disappointed if it had an issue, Haha.

Do the same calculation used to determine mph vs rpm. Function of engine rpm, rear ratio and tire diameter. Compare calculated to actual to see if transmission is slipping. For actual you can use mile marker, GPS, etc.

[Step'67]

I've done that, no slippage to speak of. Rpm is right where it should be according to speed, as verified by GPS. Also, the TCU calculates speed from internal speed sensors in the trans, rpm, tires, and rear end ratio, its always dead on the GPS.

[MSHax]

Originally Posted by Rufton

Don't think TBI is solution because 15.3 AFR is already very lean.
I get good cruise mileage at 14 A/F.
What rod/jets are you running in the 1406?
I'm surprised you can cruise 1406 at 15.3 AFR and tune rich enough for WOT.
I'd double check fuel leaks and transmission slippage as mentioned.
And yes the torque curve would be fun.

I agree with Rufton, check for leaks, and torque converter slippage. That was the first sign my trans was going out in my Hummer, I went from the traditional 10mpg down to about 6mpg. More recently on my 70 I was averaging about 5mpg and just rolled with it. One day I happen to take a look at my fuel pump, it was leaking fuel around the body, not enough to leave some one the ground, but enough to be visible. I started the truck and let it run in place for a little bit and saw it leaking. After replacing the pump, I'm up to 18mpg. I was living with the 5mpg, but am very pleased with the 18.

I know you did the math for the trans, so check to see if the torque converter isn't locking up. Since you're running a 4l80, that can make an impact on the freeway mpg, and since you're in town and freeway are the same, seems like freeway is the issue.

[Step'67]

Converter locks up as it should, nice and firm like it should be, and doesn't budge while locked. Maybe I'll pull the fuel lines, check for leaks, and tighten all the fittings. Guess that's where I'll start.

[Rufton]

Originally Posted by Step'67

I've done that, no slippage to speak of. Rpm is right where it should be according to speed, as verified by GPS. Also, the TCU calculates speed from internal speed sensors in the trans, rpm, tires, and rear end ratio, its always dead on the GPS.

I guess I'd double check fuel leaks and mpg and follow other's leads on their transmission experiences.

[buckskins4ever]

Originally Posted by Step'67

Im happy with the carb setup currently, so I'll probably stick with that unless someone strongly disagrees. I'm making about the same torque, just a tad higher in the rpm band. I know I could lean it out a bit more, but I don't believe that would return significantly higher mileage. Two things on my mind right now, vacuum advance and torque converter. Would a higher advancing canister earn me better city mileage? And what lighter mass torque converter would you guys recommend for my setup, if you think it would improve anything?

I would recommend a mild 1900rpm converter. But unless your trans came out of a 3/4 ton truck, I would have thought thats what it would have had from factory. The 700r4 comes with a 1900 as wel, but I upgraded to a 2200+ as recommended by the cam.
I dont look at weight as much as I do the rpm range. Your cam sheet would have told you what converter you should have installed. Unfortunately I dont know as much about the newer 4l80 and 4l60 trans. I had my 700 built to perform lockups and shift points automatically without vacuum.

[buckskins4ever]

Originally Posted by Step'67

I've done that, no slippage to speak of. Rpm is right where it should be according to speed, as verified by GPS. Also, the TCU calculates speed from internal speed sensors in the trans, rpm, tires, and rear end ratio, its always dead on the GPS.

You can have slippage and still perform normally according to computer specs. There are a few mechanical parts on vehicles that really need to be looked at by a professional and the Trans is one of them. you can try to trouble shoot with specs all day but a true professional who is DEDICATED to tranny repair, can diagnose by driving, then dropping the pan to check valve body. Everything from Vacuum pressure, Valve body fluid flow restriction, intermittent solenoid, and many more diagnoses can cause both intermittent issues or flaws without setting any codes or noticeable driving problems.

Most transmission shops can and will give you a free diagnosis if anything is showing wear or problems.

[Rufton]

I had a mechanical fuel pump that leaked out diaphragm flange when hot but didn't drip cold.

[Step'67]

Thanks for converter recommendations. Looks like I'll first check everything for fuel leaks to start off with and go from there.

[GASoline71]

Originally Posted by Step'67

I guess I should have listed my timing specs. 14* initial, 19* mechanical in by 2800, and 10* vacuum. I did this a while back, but I am fairly certain those are the correct numbers. According to that thread, ideally I'd have more vacuum advance, but the stock canister doesn't allow for much, and I never got a replacement one with more advance capability. Could that also be hurting the cruise efficiency? As for tires, they are 275-60-r15 on all four corners. Transmission is being controlled by a FAST tcu, which has been working flawlessly since I installed it, and is wired properly as instructed.

So if I read this correctly you have only 29 degrees total timing in by 2,800 RPM's? It should probably be close to 32 degrees or 35 degrees. A few more degrees of total timing should make a difference.

Gary

[Rufton]

Originally Posted by GASoline71

So if I read this correctly you have only 29 degrees total timing in by 2,800 RPM's? It should probably be close to 32 degrees or 35 degrees. A few more degrees of total timing should make a difference.

Gary

I think your reading glasses different than mine.

[wibilly]

it seems you guys are missing the 14degrees of initial advance. fresh new engines and trannys are not the key to fuel efficency. just as loose is fast in racing engines it is also the key to fuel effency.loose bearing clearences lots ofpiston sidewall clearance and a good sealing valve job with thin rings. throw in a light manual gear box and you will be on the right track. new bearings , brakes and tires all hurt milage