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#1 |
Registered User
Join Date: Jul 2006
Location: Oldenburg, Indiana
Posts: 199
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305 v8 or 250 I6 for better mpg
my 69 right now has a 355 with some kind of milder cam in it, i dunno the exact specs thats the way i bought it. it has 882 heads and a edelbrock 600 carb. the carb has the same rods and jets it came with when i bought it. it has a 350 tranny with a mild shift kit and i beleive the rear en is 3:73 gears. right now i get about 6-8 mpg and thats about a good as i can get it no matter how i drive it it stays about the same. so im thinkin of maybe swapping in either a stock 305 v8 or 250 striaght 6 which i can get both for free. i would rebuild which ever one i bought. which one would be the better choice to stick in for beter economy. i would love swap in the sm465 i have laying around but i lack all the parts to do so. so for now it will keep the 350 tranny. which motor would be the best? thanks guys
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#2 |
Senior Member
Join Date: Jan 2005
Location: Raleigh, NC
Posts: 894
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Re: 305 v8 or 250 I6 for better mpg
If it is a street rig I would consider swapping the rear axle ratio to something like a 3.07:1, since you have a decent engine and an automatic to push it. The high revs are killing your fuel mileage. I don't think an SM465 swap would save you anything because the engine will have to spin just as much to maintain the same rate of speed.
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-Chris Building a stripper, one part at a time: 1969 K5, 307, 3spd, 3 seats, hard top. Added Pwr Discs, Pwr Steering, Aux Battery, T-case Skid, Lighted Sidemarkers, HEI, Lock-Right Diff, ECE Class IV Hitch, 32" MT/Rs. Parts to Install: Hand Throttle, Console, Tow Hooks, Dual Horns, AM-FM, Dealer Swing-Away Tire Carrier, Gas Tank Skid. Also building a 1950 Willys CJ-3A and off-roading a 2001 Nissan Frontier on 1-Ton Portals... |
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#3 |
Step Brother
Join Date: Sep 2007
Location: K-town
Posts: 4,029
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Re: 305 v8 or 250 I6 for better mpg
My 69 is on it's second 250 and it has a saginaw 3 spd. and 3:73 Gears and I have never got any better then 12 to 15 mpg. Maybe my driving style but that all I get, I can't tell you about the 305 maybe someone else can, Maybe someone else gets more out of their 250, but that's best mpg. I have got with mine. George
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#4 |
Registered User
Join Date: Jan 2007
Location: Opdyke, IL
Posts: 775
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Re: 305 v8 or 250 I6 for better mpg
I used to own a 68 with a 250 and a SM465 with 3:73 rear. That sucker only got about 10 mpg. I think your best bed for increased fuel mileage is to go with a 3:08 rear end gear. It won't be as good from the line but it might help at cruising speeds. Better yet would be to get an overdrive transmission.
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1978 Chevrolet Silverado 3/4 ton 400 small block. 2000 F-250 Powerstroke 2002 GMC Yukon Denali XL |
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#5 |
Registered User
![]() Join Date: Nov 2006
Location: KINGSPORT,TN.
Posts: 3,035
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Re: 305 v8 or 250 I6 for better mpg
same milage with my 250 & 3 on the tree11-15 mpg best . i think a sm465 and 3.08,s would be worth tryin. you,ll need 1 st gear to pull out in for sure especially if your loaded my truck dont like it a speeds of 65 mph or more with the 3.73,s . i think a nv4500 and 3.73,s would be nice too .
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I BELIEVE IN JOHN 3:16 |
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#6 |
Registered User
Join Date: Mar 2007
Location: San Pedro River, SE Arizona
Posts: 734
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Re: 305 v8 or 250 I6 for better mpg
For normal driving with a near-stock engine setup that is LOW MPG no matter which engine is in there. That particular 600 CFM carb may be an issue, especially if takeoffs are racy and if one has an unsteady foot at highway speeds. To answer your question, experience here has been that a smaller cu. in. small block will yield as good or better MPG than a 250 I6. If you want some performance ability but mainly are interested in better MPG, my suggestion would be the small block with HEI (including correct wires/plugs) and a good 2 barrel carb. Have not used one, but have been told that the Holley Economaster/mizer is good. Hope this is helpful.
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CCCC 67 Sting Ray Coupe 327 4sp; 68 K10 Suburban 327 4sp; 69 K20 Lifted 383 4sp; 69 C80 Fire Truck 478 V6 5sp/2sp; 69 C20 Longhorn 327 4sp; 70 C30 Wrecker 350 4sp; 70 C30 Flat Bed 350 4sp; 71 C20 LWB 350 4sp; 72 K5 Blazer 350 4sp; 72 C30 Flat Bed Gooseneck Hauler with sleeper 350/TH400; 72 C20 Longhorn 402 4sp; 52 Mack LTL Supercharged Cummins 5sp x 3sp; 01 Dodge 5.9 HO Cummins 6spd; 02 Safari Trek 2430 8.1 Vortec on Workhorse Chassis; 03 Silverado 2500 HD 4wd Duramax/Allison Moved but still in the family 70 C20 Custom Camper 350/TH350; 72 K5 Blazer CST Highlander 350/TH350 Recently sent to new homes 72 C30 MoHome 402 4sp; 72 K5 Blazer Conv 350 4sp; 75 GMC 6000 Aerial 350 w Allison; 72 GMC 3500 Utility 350 4sp; 70 GMC 3500, 350 4sp |
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#7 |
Registered User
![]() Join Date: Jul 2006
Location: Ozark, MO
Posts: 4,887
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Re: 305 v8 or 250 I6 for better mpg
Sounds like a carb tuning issue. That thing has to be running rich to get 6-8mpg. I second the 2bbl and 3:08 gears and HEI, which should be good for at least 15mpg if not better. It won't be a powerhouse, but at least you'll still have the sound of the good ole' 350
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'20 Silverado Trail Boss ~ '17 Tahoe ~ '79 K15 Sierra Grande ~ '76 Blazer 2wd ~ '71 Cheyenne swb ~ '55 Pontiac Safari ~'50 3100 bagged ~ '80 Wife ~ Late model kids
Last edited by LEEVON; 09-17-2007 at 01:17 AM. |
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#8 |
Registered User
Join Date: Mar 2004
Location: CA
Posts: 2,057
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Re: 305 v8 or 250 I6 for better mpg
stay away from a 2bbl (the 2bbl is actully bigger than the primarys on a q-jet)... get a rebiult q-jet and put it on your 355, and then put some 3.07 gears in there and you'll prolly get 12-15mpg
my 71 k10 with 3.07 gears, q-jet and a fully loaded car trailor, and a fully loaded bed got 16mpg last swap meet i went to... it was mostly hwy driving, but i was still impressed |
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#9 |
Registered User
Join Date: Jan 2007
Location: Sonoma, CA
Posts: 521
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Re: 305 v8 or 250 I6 for better mpg
I had a '60 long step Apache 10. It had a 350/465/3:08 Quadrajet on an Edelbrock Performer. I averaged 16 - 18 mpg in that heavy beast. My '72 SWB has a 350/350/? Edelbrock 1506, best I can get is ~14. Both trucks had/have headers and dual exhausts.
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Oscar '72 Shortbed Fleetside '95 Jimmy 4X4 '69 GMC Custom Camper |
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#10 |
Out of the carpool lane.
Join Date: Jan 2000
Location: Clark Co, WA
Posts: 5,673
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Re: 305 v8 or 250 I6 for better mpg
Since you're going to rebuild either one, it really comes down to personal preference. I also wanted to say that it'll probably be cheaper overall to build the 305. I would make sure that either have a minimum of 9.0 compression. The reason all these old tired L6's get bad mileage is due to the poor compression ratios. Bump it up to about 9.5 and you'll get awesome mileage. That's what I'll be doing when the day comes. The trouble is, I'll be old and grey before my 6 needs a rebuild.
Btw, I'm getting 16.3 around town with an occasional 65mph highway trip and my engine has about 130K on the clock.
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1968 C-10 SWB, 5.7 Vortec/700R4/3.73 posi, Torch Red 1968 Camaro, 250/Powerglide, all original (No, I'm not gonna drop a 350 in it!...Jeez!) 2000 Honda VFR in the faster yellow! 2008 Husqvarna TE-610 1967 C-10 SWB 'Six Appeal'-Gone but not forgotten... Last edited by Brad; 09-17-2007 at 02:16 AM. |
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#11 |
Active Member
![]() Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
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Re: 305 v8 or 250 I6 for better mpg
I agree with Brad. I, too, would go with the 305. It's more versatile in the carb selections and parts for upgrading are readily available.
With the engine conditon being at it's maximum efficiency, size does not determine mpg, unless, of course, you are in the big block category. A six cylinder has to work harder to get the truck going than a v8 so it may not buy you much. A lot of other factors are involved here. As already mentioned, rear end ratios, driving conditions (city or rural), carb efficiency, tires, weight of vehicle, etc... And as mentioned also, a single and 2 barrel carb are not the most economical. A well tuned quadrajet with the small primaries can give you as good or maybe even better mpg's than a 2 barrel. Plus you have the option of the additonal power of the secondaries when needed. |
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#12 |
Registered User
Join Date: May 2007
Location: CA
Posts: 39
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Re: 305 v8 or 250 I6 for better mpg
Step it up with a 305 throttle body injection with 3.08s
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67 3/4 Lwb Step side X San Diego County truck 70 3/4 berb was 402 Th400 Dana 60 now just the dana 70 3/4 lwb Fleet Choptop bagged th350 no eng yet 79 rx7 Wide body 350 TPI t5 flat black(The interceptor) 64 comet mounted on a 93 Thunderbird chassis. |
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#13 |
Registered User
Join Date: May 2007
Location: Moore, Oklahoma
Posts: 2,396
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Re: 305 v8 or 250 I6 for better mpg
Something is wrong with your setup if you are getting 6 to 8 mpg. 3.73 is not that wild a gear ratio.
I assume you have checked the obvious things already, like fouled plugs, old wires, exhaust leaks, etc. and if so, it has to point at the carb My 69 GMC with a 307, 2bbl and Turbo400 only got 11 mpg on its best day-- i swapped a new Holley 600 and intake, and it still got 11. However, my 75 with a 350, th350, Edelbrock Performer 650 carb and more modern HEI ignition gets12-14 . Bigger motor, better mileage - because of a lighter duty tranny and more modern ignition ---- yours already has a T350 too, so it should be in the same ballpark --- swap the carb, you'll need one if you do another engine swap later anyway |
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#14 |
Hittin E-Z Street on Mud Tires
![]() Join Date: Nov 2004
Location: Greenville, SC
Posts: 23,090
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Re: 305 v8 or 250 I6 for better mpg
There must have been something seriously wrong with my truck. I averaged about 10 with my 67 back when it was a 250/3spd/3.73 with a "highway cruise" speed of 55 I could pull off 12 mpg.
Even with my 350/700R4/3.73 I'm getting 12.5 city avg and about 16 highway. Personally, I'd rather have the 250 over the 305. I had one in a Z28 and hated the gutless wonder. At least the 250 has lots of peppy torque. They would both probably get very similar fuel mileage #'s.
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Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride ![]() American Born, Country by the Grace of God ![]() 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications! Last edited by 67ChevyRedneck; 09-17-2007 at 10:11 AM. |
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#15 |
Registered User
Join Date: Jul 2006
Location: Fair Oaks, CA
Posts: 475
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Re: 305 v8 or 250 I6 for better mpg
Keep the 350 and put a q jet on it maybe an overdrive tranny, fuel injection, new gears. If you go with a 305 youll just have your foot in it farther to get it down the road, I had a 305 in my old malibu and swapped in a 450hp 350 and my milage didn't change much until I swapped out the 2.73 gears for 3.42 thats when my milage suffered. Even the bigger jets in the carb didn't kill it if you kept your foot out of it
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1968 Short Fleet 350/700r4 http://67-72chevytrucks.com/vboard/s...d.php?t=249435 1997 Bagged XtdCab Vortec350/4L60e http://67-72chevytrucks.com/vboard/s...44#post2286144 1993 XtdCab Longbed 4x4 6.5Turbo/NV4500 "Daily Driver" 1985 Toyota XtdCab 4x4 22re/W-56/Dual T-cases, Doug Thorley Header, Homebrew front and rear spring packs, Tons of Mods to suspension and drivetrain, 35" boggers, coming soon ARB's and 5.29's, Possible 4.3/700r4 swap |
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#16 |
Registered User
Join Date: Jul 2006
Location: Oldenburg, Indiana
Posts: 199
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Re: 305 v8 or 250 I6 for better mpg
it has hei and im running 45TS plugs. if the engine is warm and i shut it off and try to start it after about 20 minutes or so the engine will crank over many times before it finally takes off, but if i give it any gas when its cranking its starts but it runs really rough and smokes black out the tail pipe i know it runs pretty rich because i can smell it out the exhaust. do you think i should change jets and rods. if so what size should i go with. it has the ones it came with in it now.
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#17 |
Active Member
![]() Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
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Re: 305 v8 or 250 I6 for better mpg
Sounds like you may be leaking raw gas into the intake while it's sitting. Could be float adjustment or something else. Let it sit for 20 minutes and remove the breather and look in the throat to see if you can see gas leaking or smell it.
Check you choke adjustment. It shouldn't close too tight while sitting for 20 minutes. If it's off, adjust it so that it barely closes when the engine is cold. Here in Texas, I back it off ever so slightly after I do this adjustment. |
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#18 |
I dun gradjiated collij!!!
Join Date: Jan 2006
Location: St. Helens Oregon
Posts: 1,751
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Re: 305 v8 or 250 I6 for better mpg
You need to rebuild that 1406. I had the exact same symptoms you mentioned with my 1406. These old fuel tanks have years of crap flaking off into the fuel and it WILL eventually clog up your carb. replace the filter, clean out/ rebuild/ replace the carb, save your money for a 700R4 transplant and you should see 12-15MPG easy.
I'd personally stay away from the 305 unless you want a new boat anchor. If you really want a good low cube small block that is cheap to build and run, look at the original 307s. ![]()
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Built NOT Bought. Viper Red 1971 LWB C10 355 Vortec W/700R4 4/5 drop AKA Screamin' You are missed!
2006 F150 XLT 5.4L Triton Rollin' on 22s the "F1" 67 C20 PS/AC/327/TH400 we call 'er "Bessie" 67 Malibu PS/ZR1-LS1 Brakes/350 Vortec/700R4 2/2 drop on 18" 338s - Tracy's "Boo" Boosted '67 Cougar EFI 5.0 HO roller motor/T5/8.8 Tracloc/ 4 wheel disks - "Project Panther" 79 Honda Z50 - stretched, cafe'd and hotrodded 125CC Lifan/5spd FEAR THE MONKEY! |
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#19 |
Registered User
Join Date: Feb 2000
Location: Center City, MN, USA
Posts: 3,254
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Re: 305 v8 or 250 I6 for better mpg
The first thing I would do is verify your data. If you don't own one, beg, buy, or steal a GPS. On a highway, check your speedo against the GPS. In relative terms, the speed on your GPS is more accurate than MOST speedometers. Check it at 55, 60, and 70 (where legal). Then in town check your speed at 30 and 45.
If all positions check out, and you REALLY are getting 6-8 mpg, you have problems. Get that carb working right. Tune up the engine. New air filter. Do not immediately jump to throwing new and expensive parts at it like an overdrive tranny or higher (lower numerical) rear end gears. The OD in a 700r4 is .7:1 where the final drive in your th350 is 1:1. The BEST you can expect switching from a th350 to a 700r4 is 30% or 2.4 mpg - assuming you really get 8 mpg now and you ONLY drive in OD - which doesn't happen. Swapping gears will have similar results. Going between each factory gear set you gain or loose 10% of what you get now. So if you're running 2800 rpms at highway speed with your 3:73s, and switch to 3:55s, you'll take off approximately 10% so subtract 280 r's - you'll be running 2520 rpms and should see approximately .8 more miles per gallon. Drop to 3:08s, take off 560 r's you SHOULD see around 2240 r's and APPROXIMATELY 1.6 more mpgs. So AFTER verifying your data (check your speedo), and tuning up your engine, you need to figure out how much you drive, how much you'll save with 10%, 20%, ... better mileage, THEN decide what YOU should to to improve the mileage on YOUR truck.
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'70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN |
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#20 | |
Registered User
Join Date: Dec 2006
Location: Rhode Island
Posts: 602
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Re: 305 v8 or 250 I6 for better mpg
Quote:
I agree. Before changing ring and pinions, or engines, or trannys, I'd start with the carb and HEI. Regarding carb's: A) Visit a chassis dyno and have them jet it for economy when cruising at highway speeds (NOT W.O.T. HP). Or, B) Try carb tuning yourself by changing carb jets or needles and watching the plug colors (An O2 sensor in the exhaust pipe is a big plus in this regard). Or, C) Replace the carb with a rebuilt Q-jet or Edelbrock #1405 or similar. I'd think 12-13MPG isn't an unreasonable expectation for a V8/373 geared truck (once the air/fuel ratio is adjusted properly). The 50% reduction in fuel consumption will repay any hardware investment costs in short order. To get 16MPG or more, you're likely going to have to drop to a 3.08 R&P or install an overdrive tranny (manual or automatic, you choose). Upgrading to EFI and an overdrive tranny should net you over 18, but it's a lot of work and maybe cost to convert. of course, all this assumes you don't have a gas guzzling cam, really low compression or the emergency brakes frozen up ('Been there. Done that. Got the T-shirt).
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69 C-10 SWB Fleetside 454/TH400 (For Sale) http://67-72chevytrucks.com/vboard/c...ct=1299&cat=12 70 El Camino 454 TH700 29 Model A 4-DR Sedan Deluxe Last edited by mnunn454; 09-18-2007 at 07:57 PM. |
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#21 |
Registered User
Join Date: Jul 2006
Location: Oldenburg, Indiana
Posts: 199
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Re: 305 v8 or 250 I6 for better mpg
sounds like the first thing i need to do is get my carb tuned right. i just bought it brand new a few months ago so i surly shouldnt need rebuilt. the engine already has HEI and i wish i knew the specs on the cam but i dont. it had that cam in it when i bought the truck. how do i know what size rods and jets i have now and how do i determine what size i need to put in. if thats even the problem.
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#22 |
Senior Member
Join Date: Mar 2004
Location: Nebraska
Posts: 583
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Re: 305 v8 or 250 I6 for better mpg
i am pretty sure there is an instruction sheet at edelbrock on how to tune your carb. this sounds like your problem as these carbs are set rich so they will run a variety of combo's. if they were to set these carbs up lean from the factory they would probably be liable for detonation problems.
http://www.edelbrock.com/automotive_...ess_main.shtml Last edited by 57sailplane; 09-18-2007 at 09:58 PM. |
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#23 | |
Registered User
Join Date: Dec 2006
Location: Rhode Island
Posts: 602
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Re: 305 v8 or 250 I6 for better mpg
Quote:
I just posted the response below on another thread. Pasted it here as well because I think it addresses this problem too. Here's the reasoning behind my belief that greatly improved mileage can be had with a good carb tune (not just a simpe rebuild). It's a Chevy High Performance article from a year or so ago called "Old school meets new school". It's on-line at http://www.chevyhiperformance.com/te...age/index.html In short, using some tech tools, this guy has a 500+ HP SBC and adjusted his Mighty Demon carb's idle air speed and high speed air bleeds to increase the operating range (RPM and load) of the idle circuit and delay the onset of the primary's main metering circuit. Result was smoother operation and 21 MPG on the trip home following a drag racing outing (granted he has an AOD tranny). It's a technical article, so grab a cup of coffee before you sit down to read it. I think once you read it you'll agree that a lot can be done to improve mileage as long as you can see what your air/fuel ratio's doing.
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69 C-10 SWB Fleetside 454/TH400 (For Sale) http://67-72chevytrucks.com/vboard/c...ct=1299&cat=12 70 El Camino 454 TH700 29 Model A 4-DR Sedan Deluxe |
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#24 |
Java Mechanic
Join Date: Jun 2005
Location: Martinsburg, WV
Posts: 6,764
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Re: 305 v8 or 250 I6 for better mpg
I'll just say "me too" to a couple of posts here. It's absolutely a carb tuning issue *AND* -- a q-jet that is in good shape and tuned right will beat just about anything for *overall* performance (which includes economy.)
Q-jets do *everything* pretty good. Most specialty carbs do a *couple* things better than a q-jet but give up performance in other respects to do it. For instance --nothing makes top-end power like a big bore Holley type carb, but the damn things thing gas is still 38 cents a gallon.
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Keith 11 Lincoln MKT -- Momma's wagon ![]() 13 G37xS -- middle age crazy car 68 C20 Fleetside -- RIP Decorating the whole town up at a cost of $27 .... |
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