Terrible Gas Mileage

[BBBURB]

I get 8.8 mpg after replacing all the electronics related to combustion (spark wires, ignition, Distributor cap, etc) and rebuilding the throttle body with new injectors and doing a nice valve job. I was previously getting only 6mpg. An extra 2.8 mpg is a big improvment when I add 10 gallons. Thats 28 miles Free! Woohoo!

 

[Bextreme04]

I got a 350 for free from a local squarebody group member that does scrap/salvage stuff for a living. He just wanted it gone out of one of his project trucks so he could go back to an I6. It ran, but not well and had no pedigree.

Anyways, I've always wondered why these old truck motors just didn't really perform well and left so much power on the table at only 165-195hp stock. Until I started tearing into this motor that is... It is a 1973 Truck 350(LT9) that appears to have never been touched. No rebuild or machine work that I can identify. I was planning on just throwing new seals on it and maybe a quick bearing replacement... but when I ran a compression test I found one cylinder way down on compression and a leakdown showed it was the intake valve. So then I tore it down to see what I needed to do.

The bearings were pretty trashed and there was lots of buildup on the valves. They only had the crappy o-ring valve seals and they were totally rotted off. Then I started to tear it down to take it to get it hot-tanked and honed... measured the deck clearance and found the stupid pistons are dished with a 13cc dish... and are .038" under the deck

Which gives a ridiculous 8.04:1 CR and .067 quench with the factory GM 0.029 thickness gasket. Since the standard Fel-Pro Perma-torque gasket is .039 compressed thickness, anyone doing just a head gasket replacement would be dropping their CR to 7.9:1 and .077 quench. Which of course is horrible. So now I'm looking at just having it bored .020 over... decking the block .020" and then getting some flat top pistons with a 1.55" compression height so that the piston is only down in the bore .015" and a quench of .054" for an 8.75:1 CR.

All of that to say... we get **** gas mileage because the motors had terribly inefficient setups from the factory... a simple bowl blend, better cam, and reconfiguration of the CR/quench setup should get big gains in power AND efficiency. We'll see just how much when I get it back together, but I'm betting it gets significantly better mileage and has TONS more power than the old 350 I had, which was only getting between 6-8mpg and couldn't even spin the single tire with 4.10's.

 

[DoubleDingo]

1981 C10, 700R4. Not sure of the rear axle ratio however was a diesel. Now have a remanufactured 350 from Mabbco Motors. Quadrajet 4bbl, turning 2K rpm at 73 mph. Maybe getting 10 mpg. Replaced a 350 burning a quart of oil every 200 miles, that was getting 8 - 9 mpg. Mileage doesn’t change whether it was highway or city driving.

That is disappointing to hear. I figured a fresh, more powerful engine would not work as hard an return better mileage. Well, better than a 1 MPG improvement.

2K at 73 MPH is just barely turning - wonder if it is out of it's desired powerband.

Which could be the case w/ Goldie trying to sling 33's with 3:42's and a weak ass 175 HP motor - when brand new, so 112K ago. LOL

Yep, 2,000 at 73 is too low. It should be 2,200 minimum. 2,400-2,600 would be better. In my old Mean Green, 1965 C20, it was tuned to run smooth. Basically a stock 350, no frills, sm420, 3.54 gears, and 29 inch diameter tires. Got 16 out of it cruising 70, which was 2,600 rpms, but that damned engine loved 3,000 rpms, which was 80 and the economy went down the q-jet...lol...

 

[Raybo135]

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[Raybo135]

Yeah, Eagle Drive on the way to my Moms house has one of those. For me, 35mph indicated = 40 mph on radar.

Always good to know.


What was your factory tire size ?

There are a bunch of these online, but I have this one saved as a favorite.

https://www.tacomaworld.com/tirecalc?tires=235-75r15-33-9.5r15

Yeah, those dinky things are what the OE tires were...

Original size tires were L78-15. How do I convert that to this chart?

 

[Goldie Driver]

Original size tires were L78-15. How do I convert that to this chart?

That's a good question.

If this is accurate -

TireConversion (tireimages.com)

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[Raider L]

@Goldie Driver,

I agree. Something needs tending to. I always know when mine needs to have the plugs changed.
Actually I'm cheap I pull mine, clean them, regap them, and put them back in. I don't like spending $24 for a set of eight AC's I'm 71 and I'm not going to start out saying, " Back in my day I can remember...". Yeah what, grand paw?? This is 2021 not 1971!!

 

[Raider L]

@just_some_guy,

Well, there's your problem right there! You're using them dang Ford spark plugs!! Ha, ha, ha.