Awful gas milage.

[Obwonkonobe]

Well, your post actually saddens me. You're running what I wish I had. Would like the reliable street built 350 with efi and I already have the 700r4, unfortunately with an open rear as well. But my busted 305 gets the mileage you are getting.

Is your efi one that learns or is tunable?

It is advertised to be self learning, and I can tune the target airfuel ratios, and if I bought an msd distributor setup I could do timing control with it too, but I’ll stick to my cheap cdi and timing light for now. It’s taken me a long time to get here but even back when I had a turbo 350, manifolds, stock top end and no idea how it was running with a stock tune and nasty carb I had fun

 

[QBuff02]

Although mine is a toy, and a Big Block it only gets about 6-6.5 mpg if I'm conservative. blended down the middle with 100 octane makes for expensive driving. Lol We had just left the local gas station from topping off and I was calculating the mileage out loud and My girlfriend cuts in and says "pretty expensive, isn't that like a $1 a mile to drive this old girl?" My response was rolling into the throttle and setting her back into the seat and all I said was "Smiles per gallon, Baby."

 

[SquareRoot]

Square body and gas mileage? Lol

 

[1987 GMC Jimmy]

At least as pronounced by a cheese eating surrender monkey.

Bonjourrrrrrrrr

 

[4WDKC]

the 308s keep the rpm low where there is little power causing a load to be placed on the engine, requiring more fuel to push it than higher rpm (not high) with more power and less load. The lack of lock up hurts mileage by allowing the torque converter to slip. My truck would lock up in 3rd to help with mileage. I changed axles instead gears it was cheaper.

 

[Obwonkonobe]

the 308s keep the rpm low where there is little power causing a load to be placed on the engine, requiring more fuel to push it than higher rpm (not high) with more power and less load. The lack of lock up hurts mileage by allowing the torque converter to slip. My truck would lock up in 3rd to help with mileage. I changed axles instead gears it was cheaper.

So you’re saying that even though engine rpm is low, I have to compensate with lack of power with throttle, makes sense. I’ve been looking around for axles on Craigslist but they’re all junk 3.08s like mine. I’ve got a quote I like, so 3.73s are going in soon. And while I don’t have the money or knowledge to “properly” set up the lockup solenoid, would it be beneficial for now to just wire it directly to a switch for myself to hit when I’m just cruising?

 

[legopnuematic]

My 76 at 40mph (3.08, th350, 28inch tires) is right at stall speed, and the engine vacuum is around 15lbs (depending on gradient), where at 55 is pulling 20lbs of vacuum, which is also around the beginning of my cams power band (~2000 rpm), it is alot happier at 55 than 40 due to that reason

Sent from my Moto Z (2) using Tapatalk

 

[4WDKC]

So you’re saying that even though engine rpm is low, I have to compensate with lack of power with throttle, makes sense. I’ve been looking around for axles on Craigslist but they’re all junk 3.08s like mine. I’ve got a quote I like, so 3.73s are going in soon. And while I don’t have the money or knowledge to “properly” set up the lockup solenoid, would it be beneficial for now to just wire it directly to a switch for myself to hit when I’m just cruising?

its more of a math equation but you get it. It depends if they knew what they were doing and removed all the wiring inside the trans and used a non lock up converter
or if the just hacked the wiring out of the truck.

 

[Obwonkonobe]

its more of a math equation but you get it. It depends if they knew what they were doing and removed all the wiring inside the trans and used a non lock up converter
or if the just hacked the wiring out of the truck.

The truck is a turbo 350 truck, but the trans was pulled from an 89 and I swapped it in. There is 0 wiring in the truck for lockup, but I pulled the pan before install and DID see the solenoid, just with 2 clipped wires... and the converter is the stock one so it should be lockup. I can just give that solenoid 12v and ground right?

Is there a chance that he built some of the internals to not handle lockup as well?

 

[Red Rex]

My 84 4 wd th350 trans, 3.08 gears, Q jet aspirated 350 gets right on 10 mpg. 2.25 dual exhaust dual plane weiand intake.

So a 4L60 or 700r4 would not improve either driveability or fuel economy? 3:73 gears will?

 

[SquareRoot]

My 84 4 wd th350 trans, 3.08 gears, Q jet aspirated 350 gets right on 10 mpg. 2.25 dual exhaust dual plane weiand intake.

So a 4L60 or 700r4 would not improve either driveability or fuel economy? 3:73 gears will?

You want the engine's powerband to compliment the gearing. You need to decide what speed that happens at (55/65/70 mph)? You can build an engine with more low-end torque as another approach. Point is, effeciency, aka gas mileage, requires a system that works together. In this case, I agree that the 3:73 gears are the better solution than overdrive.

 

[4WDKC]

The truck is a turbo 350 truck, but the trans was pulled from an 89 and I swapped it in. There is 0 wiring in the truck for lockup, but I pulled the pan before install and DID see the solenoid, just with 2 clipped wires... and the converter is the stock one so it should be lockup. I can just give that solenoid 12v and ground right?

Is there a chance that he built some of the internals to not handle lockup as well?

uh, if he just cut the wires that transmission would be running hot..a normal 700 disengages the pump in OD for efficiency, that is why it needs lock up to keep the converter from slipping and heating up the trans. If all he did was clip the wires I would fix them and not buy a trans from them again. get you a cruise control brake light switch. Supple power from that to the trigger of solenoid, add a manual switch to the ground of the trigger, then ign hot fused supply to pos of solenoid and neg to ground. Using the brake switch will unlock converter anytime brakes are pushed to prevent running into someone during panic stops.

 

[4WDKC]

My 84 4 wd th350 trans, 3.08 gears, Q jet aspirated 350 gets right on 10 mpg. 2.25 dual exhaust dual plane weiand intake.

So a 4L60 or 700r4 would not improve either driveability or fuel economy? 3:73 gears will?

its not a 1:1 comparison, your truck doesnt have .7:1 overdrive ratio of the 700, your truck has a 1:1. 373 will make your truck accelerate faster, be higher rpm on the interstate and use more gas because its over the small rpm window where the engine is efficient. The truck with the 700r4 and the 308s is below the efficient rpm window. make sense?

With 3.08s your truck is at 2500RPM his is at 1700 rpm at 70 mph
With 3.73s your truck will be at 3k rpm while his will be 2047 at 70 mph

 

[Obwonkonobe]

uh, if he just cut the wires that transmission would be running hot..a normal 700 disengages the pump in OD for efficiency, that is why it needs lock up to keep the converter from slipping and heating up the trans. If all he did was clip the wires I would fix them and not buy a trans from them again. get you a cruise control brake light switch. Supple power from that to the trigger of solenoid, add a manual switch to the ground of the trigger, then ign hot fused supply to pos of solenoid and neg to ground. Using the brake switch will unlock converter anytime brakes are pushed to prevent running into someone during panic stops.

I’ll have to give him a call and ask a few quotations then.. and I’m running a rad cooler plus an external, so hopefully it’s not 100% cooked. And I’m in luck! It was factory cruise so the switch is still there, I’ll try to drop the pan and wire it up tomorrow morning, I’ll run a manual switch the way you said, it seems logical. As for tuning, what’s the leanest I’d want to run the motor cruising down the road at about 2000rpm?

 

[Obwonkonobe]

Right now I see mid 14s, every now and then the afr dips up to 15, depends on my foot