20 plus mpg question

[cortcomp]

With the stock (well worn) 327, 3.73, and factory 3 spd, we drove home from fresno, ca to youngstown, OH, in my longbed 66. The carb leaked so bad the whole time that there was always a small puddle in the front of the carb on the manifold. Now, think after driving 12-15 hours a day, how warm it is under the hood right on the engine, and how much you need to leak to keep a puddle there from evaporating. We also found later that the fuel line from the hardline to to the pump (little flex line) was leaking air and sucking so bad, that if we pulled off the highway, sometimes it would stall and lose prime and we'd be lucky to get it going again before rolling to a stop. (we got pretty good at it.) We didn't trust the fuel gauge, we'd stop about every 100 miles.

So, driving 99% interstate, at a dead 70MPH (per gps, 75 per speedo) all day for hours, we had plenty of tankfulls to do the math with. We averaged 12.5-13.5 per tank. That's the 4bbl, and i know all 4 bbls were cracked (not WOT but open) to maintain that speed. When the motor was tore down, the builder said the walls were well scored and needed to go to .060 over, and the valves were beat back into the head. It ran great, but obviously well wore.

With a newly rebuilt tight motor, adding stick over drive (so can cruise staying out of the secondaries), keeping a factory style cam for good low end and low hp=less fuel, fixing leaks, pro rebuilt carb, etc, i see no issue why i can't come up with a solid 15-17 or even close to 20 the way i baby cruise around.

The key is to have a 4bbl and stay in the primaries (more throttle response and economy with front 2 bbls of a 4bbl than the 2 larger bbls of a 2bbl.) or FI but i don't think the standard 2 big bbl TPI setup will do much better than a well tuned carb., a factory style cam that favors low end torque over HP numbers that you can brag about, and then gears, tires or OD that will give you those low rpms at your cruising speed. That's a good chance.

Of course, you could just put the 4.3 vortec v6 in and be done, i had one in my beatup astro van that had a wonky tranny that lockup didn't work in, 220K miles and 22mpg all day everyday for several years. It's about the same level of (non)aerodynamics.

[tincan1966]

I honestly believe that the reason we get good mileage is the STOCK 305- it's lower compression and is well worn, too, with about 185K on it. it does have a new timing chain with a true "0" for TDC which helps the performance a bit. The only modification I have done is the short tube headers that are actually for an '88-98 full size truck, with 2 1/2" collectors and 2 1/4" exhaust. I am even running the EGR valve and solenoid, but no other emissions.

[theastronaut]

Originally Posted by jesseo

Thanks I will be getting one of these for sure. I know a little but am learning a lo on here and from listening to the "old timers" who have worked on cars for year. I know I can get close at least. Hmmm this makes me think a fine tuned four barrel might be better than my little two barrel pushing so hard.

You're right about the larger carb being able to work better. They stay on the low speed/idle circuit longer, so they stay leaner longer if its tuned right. A cylinder head temp gauge with a ring thermocouple mounted under the spark plug will also help determine when the engine is loaded enough to need a richer mixture under load.

This is in the VW, the gauge on the right is a Dakota Digital CHT, and beside it is an Autometer wideband. The CHT wasn't hooked up yet in this pic. Dakota Digital makes the only temperature compenstated automotive CHT gauge that I know of, and the response time is instant- no waiting on the needle to move and squinting to see about how hot it is. The digital readout makes it easy to read, and you can program it to flash above any temperature you set that's too hot for your engine. With the tuned carbs and vac/centrifugal advance distributor, it gets 28.5-30mpg mixed highway/city driving it HARD! It could easily get 33-34+ if I drove it like grandma... but that's no fun!

[R_W]

I've got an Innovate LM-2 setup in my Camaro right now, it data logs as well, so you can really tune in. Made all the difference getting throttle response on a Holley SA570. Gas milage is also up some, but I'm not done yet...
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[AcampoDave]

When I built my 283 I figured some hot rod parts were a given for mpg like roller rockers, HEI, and hot coil, but I wasn't sure about an rv cam and a Performer e.p.s. intake. I decided to go with them anyhow and I do enjoy the extra torque they provide. I also decided to use 305 heads based on the recommendation of knowlegeable people. My machinist also did a port and polish on them. But, like the cam and manifold, I'm not sure if the heads helped or hurt my mpgs. My thinking is all of the upgrades I made were oriented for off idle to 4000 rpm so they should boost mpgs right? And lets face it, a stock 283 lacks a bit in the power department anyhow and those parts sure did make it more fun.

[joedoh]

I get 21 combined half highway half city in my 65, but towing is not really the goal and there arent many guys who would repeat this swap. Before anyone throws rocks, it has almost the same hp and torque as the stock I6. Guys who say its too heavy for this engine are just wrong, I drive it and it does great, goes with traffic and cruises on the highway. It wont win a stoplight race, but it sounds cool and the modern, everything, is nice.

[Sixxer]

LS swap /thread

I just got 21 mpg on my last fill up, and it's awesome to see peoples faces when I pop the hood.

[jesseo]

Originally Posted by Sixxer

LS swap /thread

I just got 21 mpg on my last fill up, and it's awesome to see peoples faces when I pop the hood.

This was a thought for sure and still may go this way. I am really liking the old school engine with the 283 valve covers

[jesseo]

Like on this 283 I just broght home for $75 that has a spun rod bearing.

[66Submarine]

My personal idea would be a pretty much "stock" 195HP 283 rebuild (9.25-1 advertised comp). I think most all of your gains will come from tuning the carburetor and working with the timing curve. I'd probably put an HEI in it, and stay with small (283/305) heads and an intake with small runners. A Quadrajet would probably be the carb of choice.

Sharps40 on here was getting 18.5MPG mixed I think, with a 230/3 speed, 3.73's, and no overdrive, running 60-65MPH @ 2800-3100 IIRC. He has a small Weber carb he has tuned, can't remember the distributor, probably HEI. Someone posted an old magazine article here a while back where an overdrive three speed 230 I-6 truck got 20MPG average.

I think 20MPG with a well tuned 283 should be fairly easy. I'd focus on timing and fuel, like I said, and probably run a t-5 OD manual trans or something with 3.73's.

[jcramsey]

I picked up a holley economaster 450 cfm carb with thoughts of using it on my 6 cylinder, but went with a 390 cfm holley instead. The guy I bought it from had it on a mid 70's c10 and said he gained a few more mpg and was around 20 or so after he swapped. I don't know all the other factors of his truck though. I wouldn't mind having another truck with a 283 to put the economaster on...I wanna see how it will do.
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