MPG

[Edahall]

Wouldn't a higher compression ratio and high lift cam by themselves lower mpg's?  If you did those two things to allow you to run an ultra high gear maybe?

Higher duration cams usually lower mpg's but not high lift.  High lift cams help with breathing.

[Edahall]

UPdate:  New strategies for increased mpg, in my opinion:

Think ----higher compression and/or cam---mega rv type cam with massive bottom end torque and no top end.

-----jet down smaller in the carb.

-----electronic fuel injection is getting a little cheaper.

-----keep it at or below 65mph.

I would also add aerodynamic mods.  There's much more to be gained with aerodynamic mods on our square trucks than with the powertrain.  I would start off with the simple first such as:

-----Lower resistant tires (think tall and skinny)

-----Lowering the vehicle

-----Extending the bumper down

-----Canopy that angles down towards the rear

[malibu795]

i swapped in a 468bbc in my 79 malibu(brick square nose) over the 305sbc i had in there... town mpg nearly doubled and i picked up ~2mpg on the freeway i avg 18-19mpg @ 75
dynod it it does 305hp @ ~4800 and 435lb@~2400 i still have oem 2.41 gears i cruise about 1900rpm

short duration big lift create torque and inproves effciency.. hydrualic roller cams are the best for that... but thats ~1000.00 to retro fit a gen 1 block... simple conversont would be find a 88-98? gen 2 or 1 piece main block that had a hr cam already in it.. flywheel bolt patern is the only real difference.. some had mech fuel bose newer ones dont.

anything more need a diesel 6.2/6.5 or dmax swap  6.2/5 work very nice in DD applications and put out almost with peak tq about 2000rpm.. on most the the vairants. you arent goign to go very fast  but you will pas alot of fuel stations.

[84scottsdale454]

sounds to me like your where you need to bed! there is not much you can change to make a difference really! i get 15mpg in my 454/th350! it feels real nice getting as good as mpg as some newer trucks!

[84 K20]

Wish I could get that kinda mileage! I get about 11MPG average! 350/TH400 combo with 4.10 gears (K20). Leaning heavily toward a higher gearset since I don't tow often enough to need the 4.10's.

[1979C20]

You guys are all lucky. My truck gets 8 on a good day. 350, stock qjet, sm465, and 4.11's. It usual gets 6 or 7.

[1980c10]

I checked my mpg today for the first time in my '80 2wd swb.
355 cid, crower torque beast cam, flat top pistons, vortech heads, efan, no a/c,
holly street avenger; jetted f-68 r-72, edelbrock performer vortech intake, 1" spacer, timing 32 degrees advanced,
vacuum advance connected, no egr or any polution emission stuff, flotech headers, 2.5 magnaflow dual in/dual out exhaust, 
dyno tuned to 393 hp and 432 tq peak
nv 3500 5 speed trans.
lowered 2/4, bed cover.
trip of 104 miles combination interstate, hwy and city driving equaled 20.9 mpg today. 

[TexasRed]

1980c10, what rear end ratio are you running?

I'm running a 383 with somewhat dished pistons with vortec heads, isky 264 megacam, quadrajet carb, 1" spacer, should be close to the same timing and getting less than 8 mpg. I'm going to try a new coil and maybe pull the module to see if the coil damaged it somehow. I've got a 700r4 and 2.73 gears but I've been driving in 3rd to keep the RPMs up but the MPG has gone down.

[1980c10]

I don't know what ratio i have but it's probably pretty high(it's stock) as i can pull 65+ in second gear. This truck is still a work in progress, getting to the rearend sometime as I need to replace the pinion seal and a few other things-so I will check then.

You do have a little more cam than I do, more cubes, less compression and an automatic transmission.
Why you would want to keep the rpms up?
Do you have your vacuum advance hooked up?
Is you spacer open in the middle? Mine is open and is a phenolic spacer.
What spark plugs are you running?

[Irish_Alley]

in my 79k20 i have 4.10 and a 350/sm465 with that set up when i had my 38x16x16 i was running about 8 or 9 i switched it to 37.5x12x16.5 now im 11. next mods will be electric fans and cleaning out the tool box of all the unused junk

[TexasRed]

1980c10,

Actually, our compression is fairly close at 9.4 to 9.5:1.

The cam makes more torque at higher RPM and the overdrive can cause a too low of an RPM to generate sufficient vacuum to keep the carb out of enrichment.

My vacuum advance is hooked up to manifold, I may switch to ported.

I'm running some kind of autolite plats for vortec heads. My spacer is a four hole spacer for the qjet.

I'm going to see if I can get an external coil adapter and get my module checked. That should eliminate some doubts.

[1980c10]

TexasRed
just some thoughts
I forgot to mention I am running flat top pistons with 4 valve reliefs. I estimate a 9.7+ to 1 CR
when I had the engine on the dyno is when I added the open spacer. the object wasn't to streemline the air or to create more space.
The objective was to even out the right and left banks air fuel ratio and it did also added hp.(one bank was running lean and one was running rich)
I don't see any reason you couldn't use an open spacer with the q-jet.
btw at 65 mph I am running pretty low rpms at 50 Is borderline for having to downshift.
Also I am running a/c delco platinum plugs (probably very similar to what you are running)
Why are you having doubts about your coil? Anytime i have had a bad one it was very noticable.
Just a thought also check your choke to be sure it is completely opening.
also note that my truck is a non a/c truck, the only acc are the water pump, alt and ps pump.
I also was running 91 octane fuel-(don't know if this hurts or helps)
my dist is a Proform hei and wires are msd streetfire.

[TexasRed]

Didn't mean to hijack the thread.

Well, I suspect the coil because it's one of the few things that hasn't been replaced yet. It's a jeg's high performance one. I read on another forum that you really can't test them with an ohm meter anymore and it can cause the module to fail which mine was bad but the engine still ran. So I'm thinking it caused the earlier module failure and maybe causing it's replacement to fail. I need to pull the module and get it checked, then I'll be pretty sure.

[Irish_Alley]

If you weren’t supposed to test them parts stores would touch them. Cause then the whole reliability thing would be pointed at them. Never had a coil work but not be good they either work or not but temp also affects that, that’s my experience with them. Now i would point more towards plugs wires. But pretty much anything in the coil can be tested.

[TexasRed]

New MSD wires.

The module failed. Now I'm collecting parts to convert to external coil.