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Old 03-16-2009, 09:51 AM
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305 vs 350 both carb which gives better mpg?

im thinking of swapping my 305 to a 350 but i want to know how much of a mpg fuel diffrence there will be both being stock engines

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Old 03-16-2009, 10:03 AM
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With some simple mods, you can make the mileage better than stock.
It comes down to a few things also such as gear ratio, the state of tune, and how you drive it. The smaller mouse of course will get better mileage, but it reflects back on what I mentioned as well. Vehicle weight plays a role in this also.
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Old 03-16-2009, 12:57 PM
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All things being equal, probably neither one. You need X hp to sustain highway cruise, and your driving style won't change. You'll still accelerate at the same seat-of-the-pants rate.

gasoline has X btus per gallon, so you'll burn as much gas to suit your driving. Your right foot will determine your MPG.

That's assuming apples to apples. You can't always make that comparison; like if you were comparing Vortec heads to non vortec, or a chevy 307 to an Olds 307... they all have different design aspects that might squeeze different levels of BTUs out of the gas you give it, but when comparing old school chevy to old school chevy, it won't really make a difference.
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Old 03-16-2009, 02:46 PM
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I have a 85 chevy van, with a 305. horrible gas mileagepulling my race car, but when its just the van about 15 mpg. I think it would get better if it were a 350,
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Old 03-16-2009, 03:12 PM
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I'd be willing to bet that a 305 (due to the bore size and the attendant valve shrouding effect) would have a higher BSFC than a 350. Chevrolet crutched this up with the 305's smaller intake valve size.

Mileage in the real world? Probably not too different between the two, IMHO.
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Old 03-16-2009, 08:25 PM
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I think all else being equal the 305 will get better mileage in theory and practice for two or maybe 3 reasons...well for every reason really. Smaller displacement equals more thermal and mechanical (friction) efficiency especially with a more undersquare bore to stroke ratio. Also a smaller engine would have less pumping loss at the same load due to a wider throttle opening. How much more the 305 would get in the real world might be small but I bet it is noticable, maybe up to 10% or more I'm guessing. Of course there's more off-the shelf cam and head choices for the 350 but most of them aren't designd as being mileage adders in mind. Also it's hard to drive a 350 as mundanely as a 305, or at least it is for me as I would be stepping into the throttle more.

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Old 03-16-2009, 09:00 PM
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my friends 350tpi gets 17 mpg city, my 305 gets 17mpg city, my 3800 camaro gets 17 mpg city...all camaros
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Old 03-17-2009, 07:27 PM
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85 k10 305/700rv10-14 3.08 gears

93 k-1500 350/nv3500 16-18.8 3.42 gears

Shane
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Old 12-10-2015, 09:42 AM
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305 vs 350

Some great reply's in this thread, and I agree with them all, especially Double Vision. MPG can be improved over factory on most engines, with a compression bump and a better cam, and fuel/exhaust work. But replacing your 350 with a 305 may get worse, depending on vehicle size and weight, driving style, and transmission/rear gear.
In my case, a stripped down short Safari van converted to manual five speed, a 305 will always be better MPG than a 350, and geared correctly will have plenty of power. However, my Safari is relatively light and sits lower than factory, and I don't do much heavy towing. A 350 will do as good or better MPG in a four wheel drive/automatic - usually, because the extra load works the 305 too hard, causing a heavy foot on the gas just to go, which is fuel economy murder. That said, a friends 77 full time 4 wheel drive pickup with 350 only got about 9 MPG.

I like smaller vehicles, and gas is expensive, so for me a 350 just isn't very efficient. I have imposed upon myself what I call "The Five Liter Law". No engines bigger than 302, 304, 305, 307 (all of which I have in stock). With Mazda getting 157 HP out of their 1.8 Miata engine, I figure I'm nobody ifn I can't make a real good runner out of any five liter V8, while retaining reasonable fuel economy. In the old days it was fun to just use the biggest engine in the smallest car, but these days - trying for better economy because of being poor like me - one must take all things in consideration (including driving style) and build the right combination.
JT
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Old 12-10-2015, 09:53 AM
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This thread is almost seven years old. That said, I recall in '74 following my brother up the ALCAN from Seattle to Anchorage, he got better mileage in his new 454 Suburban than I did in my stock '72 350 pickup, both automatics and not sure what the gear ratios were, especially in the mountain passes and hilly topo. I assume less throttle application (more torque) is part of the reason. At times we ran 80 to 90 mph where the road condition allowed it, it was winter. Mostly ice roads. And yep, the Mounties got us once in Yukon Territory.
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Old 12-10-2015, 11:04 AM
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my 77 camaro (350 roller, iron L31 heads, Quadrajet, 342 rear, intake, headers, 295/60/15 rears, LT4 hotcam cam) gets 21mpg highway - has decent power too when u mash it..

alot has to do with your cars aerodynamics, thats why my car and corvettes get gr8 mileage.. i compared with a earlier 70s nova when i was shopping and they get less, even though its a lighter car..
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Old 12-10-2015, 12:08 PM
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getting great fuel economy at normal highway speeds and reduced cruising speeds with NON computerized carbs depends on how well your idle transition circuit is tuned.. with quadrajets it depends also on the APT adjustment. with AFB'S/AVS it depends on the power valve springs. along with the well hidden transition circuits.

with tuned port injected models like L98 and LT1/optispark corvettes. it depends a lot on the minimum TPS setting being right at 0.53 volts and that the VSS is sending speed data to the computer.. along with a dozen other things.. i have met owners with 5 speed L98 corvettes that get over 27MPG on the highway with a 5 speed.. after doing a bunch of repairs to my sisters 87 town car.. 5.0EFI.. she gets 29MPG on the highway.. but she rolls slowly into and out of the throttle.

on Fords with EFI and other brands with EFI.. exhaust leaks before the first pair of oxygen sensors will really screw the fuel economy.. as the atmosphere leaking in either thru the cracks or the air injection system will force the fuel injection to go way rich at highway speeds killing your fuel economy..

when people do the 305 to 350 or 305 to 383 swaps with computer controlled carbs on GM cars.. there is an issue. these have very close limits on what the metering system can do.. so it will go lean if the engine displacement is larger than the carb is spec'd for.. there are ways to fix it.. but it gets really complicated. the computer controlled carbs can get 26MPG on a V6.. 23 on a 305 and 21 on a 350. .

oh... and OBD1 cars got better fuel economy as OBD2 cars modified the cam profiles and overlap to reduce manifold vacuum so there is more air going thru the motor and more fuel needed to keep the cat hot at idle...
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Old 12-10-2015, 02:30 PM
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If you drive the car hard the 350 can get better milage. With the 305 being down on power in a heavy car you will be thrashing it to get going the 350 may not take as much peddle to get the same amount of punch.

If your foot is heavy the 305 is better but much slower. The 350 is a better bet all around. A vortec or other good headed engine can get more mpg. Just depends on how you drive and how fast your engine is needs to spin to get you there.
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