Is decent gas mileage possible with this combo?

[Barry Donovan]

http://www.fueleconomy.gov/feg/bymak...olet1986.shtml
Funny how a couple models from 1986 are as good as todays hybrids. My truck came with a 305 from the factory so I can't find anything here to compare it to directly but it looks like 15mpg is what I should expect.

that is a site to find where the goals for oem are.

the only time I have used it is to find if I am too lean for an engine.

the big cam on the rochester...tricky carb.
a jet too small could suck in more. beyond lit barrels vacuum tease gets it going.
just right exists.
there is a guru for this..
trial end error if to be as custom as your engine.

[Mr4Speed]

My 86 C10 longbed would get 15 with my normal mix of half surface streets and half in town freeway driving. It would swing up or down a little depending how much hiway driving was in the mix. 305/700R4/3.08's and the stock 29" tall tires. Bone stock except for a non-ESC distributor. It too would have gotten better milage with deeper gears-it was much happier at 75 mph than it was at 60.

Swapping the TH350 out for a 350C in my 80 Caprice with 2.41's was good for a 2 mpg increase. Good chance yours came with a lockup trans of some sort new so maybe the vacuum switch and wiring are still on it.

Agree on the cam swap-that one won't be happy at 1200-1300 rpm.

[SuperBuickGuy]

that's crappy mileage for that combination. What's the timing set at? is the electronic advance working? Do you still have that terrible (when it doesn't work right) Qjet that's half computer half carburator? I had an 86 Suburban that had much the same problem - the fix was a Qjet from an 84 - solved all (including passing smog) problems.

[1988montecarloss 2]

swap a 5.3 or 4.8 into it and keep it efi, it can be done for a little over $1000 if you shop around, and you can sell the motor thats in it now to get back some of the money, the 350 will never get the mileage the ls motor will get.

[SuperBuickGuy]

swap a 5.3 or 4.8 into it and keep it efi, it can be done for a little over $1000 if you shop around, and you can sell the motor thats in it now to get back some of the money, the 350 will never get the mileage the ls motor will get.

nope, don't believe that. my limited experience with the LS motor is they don't do any better than a similar SBC

[squirrel]

nope, don't believe that. my limited experience with the LS motor is they don't do any better than a similar SBC

My limited experience says they do a little better. I'd play with the existing engine first and see how it does, though.

[1988montecarloss 2]

nope, don't believe that. my limited experience with the LS motor is they don't do any better than a similar SBC

my moms fullsize extended cab silverado gets 20mpg all day and weighs a good bit more than a SWB square body

[Bamfster]

I'm thinking the key w/the LS is all of the engine management systems (A/F, timing, etc), although the design is also more efficient as well. Add that to a traditional SBC, and you'll still see a huge improvement.

And for the record ..... I think you should fine tune what you have before swapping engines, etc.

[Beagle]

Ask Todd what it costs to put Vortecs on the 350, they were a better head and did pretty decent things for mileage.

Aside from the newer trucks being a hell of a lot more aero (see the lip on the new tailgates? I invented that. hahahahaha. but really, it does help) I think the heads are the biggest improvement on the LS. Low tension rings, I haven't checked but probably a smaller ring package, roller cam, VVT, all good things for mileage.

Still, you can get decent mpg out of a 350 (there is an echo in here?). I liked Scott's plan too. What kind of cranking compression numbers are you getting?

[Cyclone03]

If I can get 17.8 from a 433ci FE ford,3.70 gear,TKO with .68 od,850 Speed Demon at 75-80mph in a 68 Mustang surely you can get 15mpg from a 350 Chevy.

The key is tuning,o2 indicator and vacuum gauge can be the key to nailing the mpg tune.

[ka67_72]

I agree that tuning should be your first step and should help. I couldn't figure out how to upload an excel file so I did a screen shot. You can make one like this to compare the new/used vehicle or current vehicle with upgrades to see which is most cost effective. This is comparing my 59 Chevy PU with a Holley, phase II Bowtie headed 350 with a 5 speed vs the P word.

Kevin

[Huskinhano]

The funny thing was, without the tonneau cover, highway was the same as city.

Good thought, look to improve aerodynamics. Some of the things I've read years ago in magazines on trucks found making a air dam below the front bumper. Underneath is very un aerodynamic with all that junk hanging down. The air dam kinda masks this and improves things. Even your side mirrors, can you get away with smaller ones? I've read time and time again about pick up trucks and their beds. A tonneau cover isn't going to do anything, there's no drag inside the box with the gate up. Have you ever had a piece of paper or cardboard in the box with the tail gate up driving down the road? What happens? The paper just floats around, it doesn't get plastered to the tail gate, there's no air pressure or drag. The air is basically going over the whole bed with the gate up. It's quite possible with the gate down or no gate would just increase drag with out that cushion of trapped air in the box to force air over smoothly.

Maybe look to get a good running 305? I'm sure you could get one of those cheap, you think? Watch your tire pressure. In general look at other areas then the engine or trans.

[CDMBill]

A few other ideas are tire pressure, type of tire (mudders don't help) alignment, wheel bearings, gear lube (sythetic can make a noticable difference as can the type of tranny fluid. Is it lifted?

Timig events, vacuum advance set correctly, good solid plugs and wires all the usual tune-up items including carb rebuild. Has anyone made some 'high perfomance" carb adjustments which should be reversed?

[FoFittyFoSS]

A few other ideas are tire pressure, type of tire (mudders don't help) alignment, wheel bearings, gear lube (sythetic can make a noticable difference as can the type of tranny fluid. Is it lifted?

Timig events, vacuum advance set correctly, good solid plugs and wires all the usual tune-up items including carb rebuild. Has anyone made some 'high perfomance" carb adjustments which should be reversed?

Thanks for all the advice guys. The carb was laying around my cousins shop and he gave it to me and I put a rebuild kit in it. That's all I know about it. An engine swap is not in the near future but down the road, who knows? I'll have to wait for this weekend in order to answer some of you questions such as cranking comp. and timing. I drove to work in drive (3rd gear) and the truck felt good but it wasn't a long enough trip to check mileage. I talked to my buddy and told him I was thinking about putting a smaller cam back in the truck to gain MPG. He said "I never thought I would see the day where you would de-hotrod one of your vehicles". I am starting to lean toward buying a beater to drive to work and setting up the truck to work with the current cam. The truck needs gears, headers, heads, and dialed in. The distributor was new and untouched and all I set was the base timing. I don't know anything about tuning a Quadrajet as all I ever had was Holley's. I can pull a plug and see what it looks like. From what I am hearing from some of you guys and your vehicles, I may be able to make the truck faster and increase mileage at the same time. If I buy a beater I am going to lower the truck another couple inches which gave me 2mpg highway the first time.

[FoFittyFoSS]

I probably need to add that the truck doesn't puff any black smoke, and other than pumping the gas a couple times in the morning to start, it drives as well as any fuel injected vehicle I have owned.