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305 V 350 for MPG

15K views 32 replies 18 participants last post by  satdiver1 
#1 ·
Got a stock 305 and a 700R4. Was going to retro roller cam and lifters, free up some pressure on the cam by using fuel pump in the gas tank and stay with a street cam. 570 spread boar and stay out of the secondaries. After saying all this I am looking for the best fuel millage I can get. Can I get better millage out of this 305 [ which I already have ], than a 350 ? And if this is so if anyone has some more ways to free up the engine or on the cam I should use I would appreciate the help. The picture is what it's going into, still in epoxy primer and still a long way off. Thanks
 

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#3 ·
Sometimes depending on weight, load and driving habits ….. I get better milage with a bigger engine. Example for ya, once I worked a company that went out and switched us from v8 vans to straight 6 vans. I took 3 months and they had a choice, go broke or get us back our v8 vans. We used 3 times the fuel in a 6. I find the same for personal cars / trucks. Older 350's had better millage with a carb than a 305 with fuel injection. Maybe it just me, just saying I would take the 350 in a heart beat. I always hurt push rods on the 305's anyway.
 
#4 ·
If you are building the engine from scratch,then exactly the same as building a high powered engine"all" the parts need to match and work with a proper tune for the HP required and application of engine.
If you look at newer cars with good mileage you will see F.I. and computer controlled spark advance, short cams,high velocity and smaller runners in the heads,fast burn combustion chambers,,,,
research the new mazda/ford/gm etc and see if any of the new technology can be incorporated into your engine and budget. Look at some of the designs of Smokey Yunick and other such Guys that were shooting for 100 mpg,
You also need to consider how much actual power you need for your application,
Transmissions play a big part of fuel economy,as well as good vehicle that tracks straight and light weight.
areo dynamics is huge if you drive over 47 mph

what exactly do you want?
How far are you willing to go to meet your goals
research SFC (specific fuel consumption)
 
#5 ·
If I were looking for decent pulling power, but mainly fuel mileage in an early Chevy pickup, I'd find a wrecked late model pickup or van, buy the whole mess and use the whole stock drivetrain out of it, along with all the wiring and electronics. Then I'd part out and sell off what was left to ease my investment.

This might be what I'd go looking for....(copied from wikipedia).....

LR4
The Vortec 4800 LR4 (VIN code "V") is a Generation III small block V8 truck engine. Displacement is 4.8 L (290 cu in) with a 96.01 mm (3.78 inch) bore and 83 mm (3.27 inch) stroke. It is the smallest of the Generation III Vortec truck engines and was the replacement for the 5.0 L 5000 L30. The LR4 engines from 1999-2000 produced 255 hp (190 kW) while the 2001 and above models made 270–285 hp (201–213 kW) and all have a torque rating between 285–295 lb·ft (386–400 N·m), depending on the model year and application. The 2005-2006 models made 285 hp (213 kW) and 295 lb·ft (400 N·m), LR4s are manufactured at St. Catharines, Ontario and Romulus, Michigan.
LR4 applications:
2003–2006 Chevrolet Express/GMC Savana
1999–2006 Chevrolet Silverado/GMC Sierra 1500
2000–2006 Chevrolet Tahoe/GMC Yukon
 
#8 ·
why?


I dont understand your post? Why would a 3o5 have to work harder? the truck remains the same,so the load remains the same.
what ever it takes to move the truck,both engines would have the same work to do, The 350 might have a higher reserve capacity to do more work than the 305,thats all
you too should research specific fuel consumption,,,
 
#9 ·
the 305 doesn't give up much in torque to a 350 in an engine based(built) for mpg. feeding 45 less cubes if both are tuned correctly the 305 will get you more mpg. if you don't have a lead foot, and use a q-jet not a holley..
like tech said the 4.8 ls engine is a great little engine and it'll stomp on the 305 mpg..
some of that is the rest of the drivetrain but you have a 7004r so the gearing is the same.. the computer controlled trans shifts to get the most mpg it can..
a 305 with tpi(makes killer torque) will keep up with that 4.8 in a mph war..
as the long runners make ft lb down low, so you need less throttle to get moving and why gm didn't put tpi in trucks is beyond me..
but been counters said, trucks emission standards and a tbi is cheaper..
 
#10 ·
I agree totally with going with a complete setup from a 'modern' V8 as Tech said, but if you want a carb and have access to either a 350 or 305, then pick the 350. If you already have a 'properly' running 305 and money is a concern, then stick with the 305. Then I would recommend a two step process for achieving proper fuel economy.

1st, get the 305 tuned properly to get maximum mileage at the speed where the cruise vacuum is the greatest. This means driving all over the place with a vacuum gauge hooked up and watching it and the tach all the time. You'll find that this may be anywhere from 44 to 71 mph depending on your rear gearing. Once you find what rpm achieves the highest vacuum, then step 2, apply that rpm to speed that you want to cruise at. For example, if you are getting maximum mileage in the current setup at 62 mph in high gear and you want to cruise at 70 mph, then taller gears and/or tire combo is needed.

Again, the 1st step can't be overlooked. Before changing rear gears or tires size you have to have the engine running properly (not just 'OK').

I did this with my son's inline 250 in a 67 C20 that we put a T5 overdrive into. We started out getting 12 mpg and had up to 17 mpg max. That was with 31.5" tires and a 4.56 rear. It took a couple of months of commuting (120 miles per day) and an education on Holley tuning but it was worth it. It was alot of driving on the interstate at different speeds for entire commute and monitoring the mileage, tach and vacuum gauge. Forgot to mention this had a 390 Holley vacuum secondary on it.

Remember, you'll never learn if you don't make some mistakes along the way. I look at your project as a way to learn about making the engine run properly via timing, carb adjusting and gearing. I call that GREAT fun.

Good Luck - Jim
 
#11 · (Edited)
If you're running a carb in your vehicle and don't want to worry about electronics, then your best bet would be to run an Edelbrock Performer RPM intake, non-electronic Quadrajet Carb (best carb on the planet!!! Yes, my biased opinion ha!), World Torquer S/R 305 heads (Specify the 305 heads) and the 305. For pulling, with the 305 you'll want a little steeper gears like 3.73's. Without an ECM to run it, you'll need a Torque converter lock-up kit from TCI or equivilent for the 700R4.

If you don't mind Electronics, I'd look at getting an ECM and engine harness from say a early--mid 80's Firebird/Camaro or GM Truck. You'll get the simplicity of the Computer controlled carb but have much better part-throttle and MPG's and it'll control the transmission.

My 84 T/A, yes it's a bit lighter than your truck perhaps, was getting 23mpg with the 305 before I swapped to the 350. With the 700R4, I was running the set up I mentioned earlier except with the computer controlled Quadrajet carb. With the 350 (Dynoed 261 RWHP) and the same computer controlled carb set up, Now it's getting around 18mph on the highway.
 
#12 ·
For cruising and around town you'd probably not see enough difference to worry about. It takes X amount of force to move the vehicle at legal speeds. Whether that comes from an inline 6 where you've got you foot way in it to get into the useful power band for the resistance, or whether you're just barely tipping in the throttle on a 350 the power needed is the same. Admittedly 4 barrel carbs and big inch engines do take an excess of fuel sometimes it seems simply because they exist.

The biggest things you can do to improve mileage:

- Use modern heads like the L31 Vortec, or an aftermarket equivalent. These type heads with a full up Ricardo combustion chamber do many good things that other heads including the small chamber 305 heads just can't get to among these advantages is the ability to run fairly high compression ratios with regular fuel. An aluminum head lets you push this even further but the cost may not be recoverable from the fuel savings. But aluminum lets you run the compression ratio up and allows a higher engine operating temp which does deliver better fuel economy especially when combined with a high silicon alloy piston of the correct crown shape and good rings.

- Use flat top or D dish hyper-eutectic pistons with the minimum skirt clearance with moly rings or a one piece ring if you can afford it. This selection keeps the piston thus rings more square with the bore which in turn minimizes blow by. Blow by is fuel energy lost to the effort of turning the crankshaft.

- Use a mild roller cam, these outlive flat tappet cams reducing maintenance and replacement costs for camshaft and parts. The timing of the cam is all important for mileage; you want little to no overlap, an early closing intake valve, with an LSA around 113-116 degrees. This tends to lean you in the direction of modern factory type roller cam, imagine that!

- Match the Static Compression Ratio (SCR) to the cam's intake closing point to get a Dynamic Compression Ratio (DCR) of 8 to 1 or a little better for cast iron or 8.5 to 1 with aluminum. This is a point for me to mention that you need to think of the engine (not to mention the vehicle as a system, not just a bunch of parts). The individual parts need to bring characteristics that support the system's goals. Don't feel bad if you struggle with this, it took Detroit decades to figure this concept out.

- Keeping the intake system small in size will improve bottom end through mid range torque which allow you to work the engine with less throttle that rewards you with better mileage. A small bore 4 barrel like the Q-Jet or Edlebrock Carter of about 600 cfm is plenty. The intake should be an Edlebrock Performer EPS with exhaust heat under the plenum, if you like the look I'd recommend a polished finish. These finishes do not transfer heat well which will keep you intake warmer. The problem with a mild street engine is vaporizing the fuel, a high speed engine can use port velocity to force this, however, a low RPM street engine does not enjoy this so you have to apply heat to force the needed result, especially if you live where it gets cold and damp. Vaporized fuel burns well and completely, wet fuel does not. The latter situation is another the L31 style head helps with as it is designed to take a pool of fuel sitting on the backside of the intake valve and thoroughly mix it with air on the intake and compression strokes before spark is applied so these type heads make a darn good back up to the purpose of mixing fuel and air. If you can avoid EGR do so, it adds a complication to mixture control that a carb can't be tuned to handle as fuel injection can, so dispense with it if you can. Also, avoid air gap intakes; these are top RPM power producing stuff not useful to extracting high levels of fuel economy at low to moderate revs. Well vaporized fuel is not only good for improved fuel economy but also for engine life. Most wear occurs with startup and warm up where the upper cylinder lubrication is minimal and the excess fuel wash on the cylinder walls high. Fuel wash comes when "choke" is applied. Whether it's a carb that actually chokes the inlet to force an extra rich mixture or with fuel injection that programs the extra rich mixture based on temperature the result is the same. They throw extra fuel to get enough that will vaporize to run the engine. The unvaporized portion will have some carried out the exhaust and some forced through the rings. So the sooner you can get the engine out of this situation the better will be your fuel costs and the longer the engine will live. To this end a great help is to use an engine oil heat exchanger in the radiator as modern vehicles do. It turns out that warming the engine oil with the coolant cuts engine warm up time by 60-70 percent.

- I really would recommend the use of a T5 or T56 manual transmission with overdrives and deep low gears. However, use an automatic transmission with a deep low and an overdrive high like the 700R4 that you have will help mileage a lot. The deep low gives the engine a lot of mechanical advantage in starting so less throttle opening is needed to get moving. The overdrive drops the cruise revs by 20% which on flat to rolling hill terrain where it can be used with little throttle opening results in significant fuel savings. Plus the use of a lock up converter in cruise can save another 10% in RPM.

Bogie
 
#13 ·
a lot of people are advising building an engine for mileage...

Consider that a complete takeout 5.3L drivetrain is about a grand, and that buying a new intake, set of heads, and a cam is about a grand.

I wouldn't waste any money building a sbc for mileage when better options cost less money.
 
#17 ·
a lot of people are advising building an engine for mileage...

Consider that a complete takeout 5.3L drivetrain is about a grand, and that buying a new intake, set of heads, and a cam is about a grand.

I wouldn't waste any money building a sbc for mileage when better options cost less money.
love all these internet post of complete 5.3 take outs for a grand

you can't touch a 5.3 (engine only no alt/ps pump/ac compressor,starter) for under 800.oo want a complete engine fork over 1200-1400 want ecu and harness and senders 1500 min, forget the trans
from a known good running, it's there because of body damage engine..
 
#23 ·
You have a point, vinnie. In the long run, OP, saving as much money as you can on your current engine (not replacing things that don't need replacing and simply rebuilding with what you have) is the biggest way to save money in the long run. I assume that's what you're trying to do by building a mileage-oriented small block in a 60's pickup?

Where are you with this 305? Is it a shortblock, longblock? What heads? How many miles? What transmission and ring and pinion ratio do you currently have? If you just have a rolling chassis truck from the 60s and some 305 short block, I'd say sell off the 305 and start fresh. As suggested, you'd be way ahead of the game to have a complete drivetrain from a more modern vehicle. It doesn't even have to be a "fancy" "brand new" gen III truck motor. Even a 350 tbi and 700r4 from an early 90s pickup or car would be a decent package, so long as you kept your foot out of it - even if it wouldn't make the same returns on mileage as the newer setup.

If you sink your money into building an engine for maximum MPGs, you'll likely never see a return on your investment.

EDIT: Missed in first post that he has a 700r4.
 
#29 ·
Basically, you want a high velocity engine with lots of bottom end torque to get that weight moving.
Back in the 80's, "Super Chevy Magazine" had the "Econperformer" project going where a 4100 pound '70 Monte Carlo with a 350 (Crower Baja cam, SP2P intake, and a leaned out Q-Jet), a TH-400, and 3.31 gears was running low 14's@95 mph and getting 25 mpg
And that was with 80's technology; I suspect you could do better applying the same ideas with Vortec heads, a 700-R4, and TPI
 
#31 ·
vortec heads and tpi is an expensive combination , the vortec tpi base will run u around 600$'s just for the manifold base , by the time u pay for heads and intake etc i would just do an lsx and t56

for the ultimate budget i would do a stockish tpi engine out of a camaro or vette, they can be found cheap for comeplete good engines, hell i just sold a complete 40k mile ( original not rebuilt) l98 and 700r4 combo for 300 bucks

tpi is just that lots of bottom end and falls off fast in the upper midrange
 
#33 ·
I replaced a stock 350 engine with a 383 stroker and got an honest 2-3 mpg improvement. The stronger engine does not have to work as hard nor does it downshift when going up hills on the interstate. The 350 always down shifted when going up the same hills at the same speed. My conclusion is that with more torque going up in size within reason will move your vehicle with less effort to the motor. I think you will be pleased with the 350 but I recommend using the later model vortec heads.
 
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