Thinking about taking my Vette ('66 but doesn't the HAMB have a "like body" clause?) on a trip. 454 with lumpy cam and 850 DP Holley, TH400 and 3:08s. Last I checked it got about 9mpg. Anyone think mileage would improve noticeably if I plopped a spread bore Holley or Q-jet on it and kept my right foot under control? I saw some published figures for a 454 Impala at 8-10 mpg, maybe that's the best one could expect.
I tried that with an adapter plate. I think you need to keep the four holes separate to keep air velocity up. The spread bore adapter has one big hole. My fuel mileage was worse, and throttle response really sucked. It came off right away.
Probably aint gonna be any better.....You could try a 2bbl with adapter......might get 9 1/2mpg....lol
I am real interested in the responses to this thread, and also curious if our old cars will ever get decent mileage with todays gasoline. It is a big block Vette, the anti-mileage car...lol
The exhaust note will sound the same with a smaller carb.... Go with a Holley List# 4777 650 cfm DP carb...
Road and Track, 1970 Vette 454, Q-jet, automatic and 3:08s. 9 mpg. Guess that answers that question. http://www.roadandtrack.com/car-reviews/1970-chevrolet-454-corvette
double pumpers are mileage killers but if you switch to a spread bore it may not help if the tune is off. I would get a fuel air ratio meter with a wide band oxygen sensor in the exhaust. I've looked into them and they can be bought for about $200.00 this would let you know what circuit in the carb is rich or lean. I've not gotten one yet but I've read about them several times (one article was by Jeff Smith in Car Craft) and they sound like a great tuning tool to really dial a carb in. also check out your dist. advance curve as that can make a huge diff. in drivability and MPG . the 3.08's should keep the rpm's down enough to allow some improvement, good luck
I had 73 Caprice wagon with the 454. It would get 15 mpg at 55. It weighed 7000 lbs. This was with the smog stuff still on it. If I had tuned it, I feel that I could have gotten near 20 on the highway. I know it always ran rich.
Getting 9mpg @ $4 a gal will cost you 0.44 per mile. The same at 12mpg will cost 0.33 per mile so just the increase of 3 more miles per gallon will result in a total savings of $3.66 for 100 miles. So if you were to change out the carb and the new one cost $366.00 it would take you 1000 miles to break even before you see any savings.
You need a vacuum advance calibrated for that engines vacuum level and a Power Valve that doesn't move until you really accelerate. Also high gain boosters will help the primaries at part throttle.
That's darn good! Hmm, I'm calculating more like $110 savings on a 1,000 mile trip, about the length I'm contemplating. Still, that's steaks instead of burgers for the whole trip. Interesting, the adjustable advance and power valve wouldn't cost much. Ported or engine vacuum? I've never understood the benefits of each, or why they switched.
Well if you're going to get all logical about it, it sort of sucks all the fun out of it. With an auto trans and 3.08s I think I'd lean toward vacuum secondaries instead of a double pumper. With a sharp tune-up and a lighter foot a spread bore should help your mileage too. And try driving with a manifold vacuum gauge installed where you can see it while driving. Try to keep the vacuum reading as high as possible while cruising.
Hmm, I'm calculating more like $110 savings on a 1,000 mile trip, about the length I'm contemplating. Still, that's steaks instead of burgers for the whole trip. Which makes it worse if you have to buy a new carb.
Why not lean out the primaries on that carb... I am thinkin that carb is 84 sq with a 6.5 PV. You can drop the primaries down to 80 before you loose to much fuel atomization. Plus keeping your foot off the floorboards with the peddle sandwiched between the 2 will also help. Make sure your distributer advance is working and right. It amaizing how many MPG that will rob
Ported/Manifold doesn't affect mileage. Call Quick Fuel and tell them what you are trying to do, they can calibrate a carb that will make more power and help you get 11-13 MPG. it's about a better mixture formation at part throttle. In the mean time get a new power valve, 5''. Get an adjustable vacuum advance and increase advance until you get ping going up hill at part throttle. Before that make sure your mechanical advance is working and you have a total of initial plus mechanical in the 32-36 range. Get a vacuum gauge and draw a red line at 6. Good luck, 12 is out there maybe better.
A properly selected spread-bore WILL give you better mileage IF you also have a spread-bore intake. A spread-bore carb with an adapter on a square-bore intake will give you WORSE mileage. Do the math on a conversion as others have. It would take a lot of miles to break even if you have to do both a different carb and a different intake. As others have suggested, try to improve the efficiency of what you have. And don't overlook either a dragging brake piston or tire pressure. You would be shocked at how much mileage low tire pressure can steal. Jon.
Plus 1 And a wide band A/F ratio meter to see the fat and lean spots. Most likely your fat while cruising.
It has an old Holley open plenum single plane manifold which I believe has both bolt patterns and a trapezoid shaped opening, if memory serves. I wouldn't buy a new carb, thought I might go sniffing in friend's garages for an old Q-jet. But I'll try the jets, timing and power valve first. Good point on the tire pressures. And vacuum gauge. And A/F gauge.
78 chevy pickup with a fairly low milegae target master 350.....8-10 mpg no matter what. 76 ford van with a 351.....10 mpg no matter how or what or where 51 ford shoebox with 305 built by me, turbo 350, 2.79 rear gear, quadrajet tuned by me....24.4 and 24.6 mpg running 70 mph on the interstate. My 327-350hp on the highway....about 14mpg running 70 or better and playing a little...cruzing 45 mph on the parkway on sunday pm...about 20-22 mpg or better with a quadrajet.
You can use this 6" switch STANDARD MOTOR PRODUCTS SWITCH VOES 6" HG to turn on a warning light after you install the 5 inch PV. This will warn you your mileage is about to drop 35 pct. As far as drag and measurement go here...to learn how to do coastdown measurements yourself http://www.instructables.com/id/Measure-the-drag-coefficient-of-your-car/