Can a C3500 5.7L 4.10 beat 10 MPG?

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89GMCJOHN

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Spookers says :" Puked the motor and swapped in a stock 305 out of an 86 caprice with 400000 on the clock and instantly got 17mpg with my foot to the rug most of the time.
Though that's without towing a trailer but had a pair of 3 wheelers in the box from time to time and that didn't seem to change the mpg."


Only thing I can add is in fact my dad had an 86 carbd 305 Caprice as his daily commuter car for many years ..he always talked about how it got great mileage for what it was - 17 he told me many times ....I think the secret with that Carbd Caprice was its "whopping" camshaft Intake Duration 228 .351 Exhaust Duration 244 .385 so an old carb'd 305 can most assuredly put down 17-18 in stock trim...FWIW
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alpinecrick

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Now wait a minute....

I bought a brand new 1991 K2500 "light Duty" ECSB, 5.7, NV3500, 3.73, LT245/75 tires.

I live at 5900 ft, and most places I go from there is UP.

Drving it very nicely when it was new, and empty without camper shell and usual gear, I got in the high 17's mpg with the truck on several trips to Denver. I learned it got better mileage with mid-grade, so that mpg was with mid grade fuel.

Remember, the carburetor 350's the TBI replaced made 30-40 hp less, and most got 12-13 mpg. Yeah, in Colorado the TBI 5.7 wasn't enough motor for the heavier GMT 400's--especially when ECSB's became so popular.

But with apologies to the MOPAR 318 and the FOMOCO 1st gen 302, those low rpm, swirl port TBI's are probably the longest lasting cast iron V-8's ever built.

Of course, I much prefer my Vortec 350's..........:D
 

Spookers

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If the intent was keeping the tbi on your 350 what can be done to help with fuel economy is if you ran between 10-12* base timing (I had ran 14* n/a but seeing as you'll be towing try it at a lower advance) and run 305 54lb white and green injectors and use an adjustable fuel pressure regulator to up the pressure to 14-15psi,(max pressure for the tbi injectors is 20psi) this will help atomize the fuel better while still flowing enough for the 350, the timing advance also helps with getting the fuel to burn more efficiently.
The reason the tbi was so bad for fuel economy is because the fuel wouldn't atomize properly due to being only using 2 large injectors and low fuel pressure and no venturi. This is why the vortecs and ls's got much better power and fuel economy since they run about 50psi fuel pressure and use 8 smaller injectors, this atomized the fuel a lot better. The carb uses venturies to help atomize the fuel, not as precise but still did the job.
 

PlayingWithTBI

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(max pressure for the tbi injectors is 20psi

That's not true - I'm running 61lb/hr injectors @28psi (89.5lb/hr now) with a vortec fuel pump. Stock 1995 7.4s run @ 28psi that's where I got the spring.

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PlayingWithTBI

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Because when I had an 18lb spring my injector Duty Cycle was 99% in this screen shot meaning they were open all the time.

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Darrin De May

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Spookers says :" Puked the motor and swapped in a stock 305 out of an 86 caprice with 400000 on the clock and instantly got 17mpg with my foot to the rug most of the time.
Though that's without towing a trailer but had a pair of 3 wheelers in the box from time to time and that didn't seem to change the mpg."


Only thing I can add is in fact my dad had an 86 carbd 305 Caprice as his daily commuter car for many years ..he always talked about how it got great mileage for what it was - 17 he told me many times ....I think the secret with that Carbd Caprice was its "whopping" camshaft Intake Duration 228 .351 Exhaust Duration 244 .385 so an old carb'd 305 can most assuredly put down 17-18 in stock trim...FWIW
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My dad also drove a Caprice like that in the Eighties, his was a company car that he had when he lived in Newport Beach and worked for Fluor. He liked it so much that he considered buying it from the company when they went to replace it with a new model. He'd always comment on the great mileage that it got and how it had been trouble free. Of course he was comparing the fuel economy to that of his 1978 Cadillac Eldorado, a car that he eventually passed along to me, so I found out for myself what a gas guzzler that it was. I still miss it, though.

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