Not sure what to do

[70CHEVYBB]

I have changed jets from 72 to 67 way to lean and running hot went to 68's the same and had to settle for 70's not a big difference but did help with mileage. I am at 34 degrees total timing. I have a 15 mile drive one way 10 of them are 60 mph the rest is in town but only two lights. I have thought of a cam swap but I really think I might put an edelbrock 650 on it. I really do not like the holley for everyday driving anyway.

[gvw5400]

You walk a fine line with the 10:1 comp and going to a "RV" style cam. The shorter duration can cause your dynamic compression ratio to go way up. This can end up being an engine that you have to pull timing out of. Something to ponder is all RVs and heavy duty truck gas engines came with 8:1 to 8.5:1 comp when built. I have a 408 BBC with 9.375:1 CR and a stock truck cam. I have to run prem. fuel to make it run right.
All the best Dirk

[WIDESIDE72]

Originally Posted by 70CHEVYBB

I have changed jets from 72 to 67 way to lean and running hot went to 68's the same and had to settle for 70's not a big difference but did help with mileage. I am at 34 degrees total timing. I have a 15 mile drive one way 10 of them are 60 mph the rest is in town but only two lights. I have thought of a cam swap but I really think I might put an edelbrock 650 on it. I really do not like the holley for everyday driving anyway.

A 750 holley is a lot of fun but about the worst carb you can run for mileage. The reason i am perplexed is back in the day my dad had an 80 Gmc one ton welding truck. It had a stock 350 when he bought it, sm465 4 speed and 4.10 rear. He built a work bed from 3/16" steel. Hauling a lincoln sa200 welding machine, full size oxy acetylene rig and all tools it pulled down 8mpg but was gutless. After a while we swapped in a 454. Rv torque cam and headers full dual exhaust with glass packs. I forget what heads but nothing fancy oval ports, late 60's oem intake with square flange 700 carter carb. It got 10 mpg and actually had power! I still remember hearing him fire it up at 5am every day to go to work!mluckily we didnt live in town or wod have had pissed of neighbors!

On a side note, my brother wanted the intake for his 69 camaro 396 so we swapped it for an oval port edelbrock performer. Not sure if it made any difference meage wise but it definitley affected the torque of the engine vs the cast iron oem ( and not in a good way!)

[gvw5400]

On my 408 (396+.060) I ran both a 750 Edelbrock and then tried a 600 Edelbrock.
The 600 worked well in a 1980 K-20 4x4 (465,3.73,33inche tires) The low end torque was stout! The truck would avg. 13.5 MPG in the mountains. My commute was 16 miles with a 3100 ft elevation change. (6000 to 9100). Give the 600 a try. What is your idle vacuum like with your present cam?

[WIDESIDE72]

Originally Posted by gvw5400

You walk a fine line with the 10:1 comp and going to a "RV" style cam. The shorter duration can cause your dynamic compression ratio to go way up. This can end up being an engine that you have to pull timing out of. Something to ponder is all RVs and heavy duty truck gas engines came with 8:1 to 8.5:1 comp when built. I have a 408 BBC with 9.375:1 CR and a stock truck cam. I have to run prem. fuel to make it run right.
All the best Dirk

True dat. I have only had low com pratio bbc's

[bbcmudtruck]

Originally Posted by gvw5400

You walk a fine line with the 10:1 comp and going to a "RV" style cam. The shorter duration can cause your dynamic compression ratio to go way up. This can end up being an engine that you have to pull timing out of. Something to ponder is all RVs and heavy duty truck gas engines came with 8:1 to 8.5:1 comp when built. I have a 408 BBC with 9.375:1 CR and a stock truck cam. I have to run prem. fuel to make it run right.
All the best Dirk

Completely agree! I would call Clay Smith/Straub cams and get a hydraulic cam built for you heads and needs. That will fix a couple of things as well as increase your motor value.
1. You need a cam that will perform good for your heads, at the same time, you will need a cam that will bleed off compression. With iron heads, 10:1 on 91 is pushing it.
2. While myself and a few other friends run the hyd big muthr thumper, they are by far not the best cam for mileage. Nor are they the best cam for performance. Don't get me wrong, they will put some power to the wheels, however a custom grind hyd roller will put more power to the wheels as well as increase value of motor and increase longevity.
3. You could sell the bbc for half of what it realistically cost you to build it and put a gm crate motor back in its place. However when you do that, you will end up with marginal gains in mpgs over the bbc.
4. I don't know how a newer ls engine will do as far as mpgs in an older truck, but in my 2013 Silverado 4wd, I avg 13.5.
5. If you really want to be money ahead, buy a daily driver for 2000 bucks and park the hot rod during the week.

Another thing to consider is a better tune and or carb. A 750 is just about spot on for that CI motor. Are you running vacuum advance? Are you running HEI or at least a hotter coil? A carb change maybe able to gain you at best 2-3 mpgs, however your still looking at spending upwards of 700 for a good carb. I personally don't like the eddys for performance/reliability, but each their own. Downsizing the carb completely takes away from the engine. The better the engine is tuned and built to run with the compatible components, ultimately end up in better performance and reliability. A by-product would be increased fuel mileage.

Very last note: When you were building this (very nice) truck, were your expectations 15-20 mpgs? I'm not a small block fan but I don't hate them either, but even a stout small block wont ever replace the torque on demand you have right now. MHO Keep the bbc, re cam it accordingly, make a carb swap if needed and enjoy the fruits of your labor. Buy a cheap go to work car and drive it till the wheels fall off. Either way, your still not going to recoup your gas money in a 5 year period.

[70CHEVYBB]

I guess I wasn't thinking clearly. I have a mechanical advance Mallory with 34 degrees total timing. I already have 2007 monte carlo for a everyday car. I sold my last car (69 nova 396/350hp) cause I never drove it. I said I didn't want to do that to this truck. I fell of track trying to relive my youth and building a big block. I guess I need to start with a card change, put in the 2000 stall I just bought and look in to a cam change this winter.

[bigmac73]

i would just keep it for and drive it occasional ,(every other day ) and get another truck with a 6 banger for a daily driver, there are still plenty 67-72 6cyl /3 on the tree out there available
as soon as some little douche bag in daddy's new challenger or camaro strokes you , you will be banging your head for getting rid of the big block

[WIDESIDE72]

Another option: do like i did and leave the truck as is and buy a used HHR SS for $12k. 265 hp and 29 mpg (if you keep your foot out of it, 25 mpgif you dont). I dont! Added bonus is I have room in back to haul all but the biggest parts I score for my truck(s).

[BigDaddyV]

I'm building my first BBC truck right now. Thx guys, you have convinced me. 454 stays.

[Spartan]

Dean'smeanmachine - I would drive the cars you mentions. I wasn't knocking classics just pointing out options.

However I had a Vega with a v8. Gas mileage was no where near the honda. More iron more weight. Had a pinto for awhile. Nobody tailgated me. He he. And my 53 split window bug had a 2275. No real gas savings there but I recommend bugs for learning on because of their light weight and unique driving characteristics.

Of course there's not much that compares to the grunt of a BBC.
2%

[luvbowties]

Originally Posted by 70CHEVYBB

I guess I wasn't thinking clearly. I have a mechanical advance Mallory with 34 degrees total timing. I already have 2007 monte carlo for a everyday car. I sold my last car (69 nova 396/350hp) cause I never drove it. I said I didn't want to do that to this truck. I fell of track trying to relive my youth and building a big block. I guess I need to start with a card change, put in the 2000 stall I just bought and look in to a cam change this winter.

Hey 70BB, I too have some opinions. Sounds like you have a mechanical-only advance dist. If so, replace it with a good, stock HEI with its vacuum advance properly functioning. In my ltd. experience, whenever I'd re-curve stock GM dist's(back in the 60's, and on sbc's) and mod. to all-mech. advance, it'd drop mpg noticeably. Just a re-curve, with vac. advance still functioning, would do better on mpg than the fully-modded one. Vac advance units deliver what they were designed to do--giving you a dynamic timing to match idle-thru wot demands/needs, and resulting in better mpg (in addition to several other improvements).

I also agree on using the 2k stall convertor to replace the present more-mpg-robbing unit--this idea based ALL on theory that I have studied, as I have no direct actual exper. with those "things". My understanding is they DO rob mpg's.

Lastly--and here probably come some disagree'rs--swap that carb for a stock, properly-tuned GM quadrajet. In my opinion AND actual experience, they perform close to a 2-bbl, with a light foot; and they "put out the flow" when your foot calls for it. Another opinion: they give about as close to fuel-injection performance and economy as a carb can get. They seem to just about deliver the best parameters for whatever is called for on what you put them on.

It shouldn't cost much at all, in the overall scheme of possibilities, to swap these 2 items--the carb and the dist. Plus, if u don't like them, you've still got your present ones to re-install--or sell them if u do like the 'new' ones.

Keep enjoying that bbc! Almost forgot: that's a very nice ride!!!
Sam

[Dean'smeanmachine]

Originally Posted by Spartan

Dean'smeanmachine - I would drive the cars you mentions. I wasn't knocking classics just pointing out options.

However I had a Vega with a v8. Gas mileage was no where near the honda. More iron more weight. Had a pinto for awhile. Nobody tailgated me. He he. And my 53 split window bug had a 2275. No real gas savings there but I recommend bugs for learning on because of their light weight and unique driving characteristics.

Of course there's not much that compares to the grunt of a BBC.
2%

I gotcha. I know someone who drives a 1973 vw beetle as a commute. he gets 30-35 on a gal. pretty impressive. have a nice day. DEAN'SMEANMACHINE

[70CHEVYBB]

I think my plan is pull the big block install a full roller thumper cam, put in the 2000 stall and a 600 cfm edelbrock carb this winter.

[70CHEVYBB]

Originally Posted by specialtyretail

I took the well built big block out of my Cheap Bastard and replaced it with a super mild small block. I hated the change from the first drive after the engine swap until the time the new owner loaded it on his trailer a VERY short time later. I felt I had ruined, shall I say, neutered, the truck. Just my thoughts. Thanks, Fred.

I did it, I put in a 260hp 350 from gm. I miss the big block. my wife 's not that happy with me but I may just put the big block back where it belongs.

[Grumpy old man]

Get a better job , or sell it , why complain about what horsepower costs ? Want gas mileage buy a Geo !.

[leftybass209]

You had a big block, didn't like the gas mileage so you spent more than you'd ever save in gas to buy a low horse sbc crate, and now you want to go back to the BB. Look, I don't mean any disrespect and it's nobody's business what you do with your truck but it sounds like your lack of satisfaction with anything is more serious than what changes you do to the truck. Decide what your priorities are and where your satisfaction resides.

[tlcrz1972]

Well I have proof that these old trucks can get great fuel mileage.
I'm running a 1995 350 throttle body FI with, 4L60E (running on modified GM wiring harness) with 3.08 posi rearend. It idles along at 1600 to 1700 RPM at 70 MPH.

When it was carbureted it was getting around 12 to 14 MPG. (350 with th400 with 3.08's.

Areodynamics really doesn't apply, and the so called extra weight really has no major affect on these trucks, it's getting the same MPG the doner truck was getting.
I doubt there is major difference in weight.

I'm getting easily 17+ mpg unloaded, and can easily pull my flatbed trailer (1500+LBS), with my land cruiser (4500+LBS) on it plus tools camping gear etc. (300 - 500 +LBS) in OD with no problems.

And yes, I don't get 17+ with a full load, but that is expected.

Plus if the truck isn't pulling a load on a daily basis there is no need in a BB.

Originally Posted by WIDESIDE72

From my experience even a sbc 350 will only return 12-15 if you are lucky! I have had everything from 250 6 cyls to 454's in these trucks and they are just too much weight and non aerodynamic to get much better. An LS type engine might do a little better. One day that will be my next adventure!

[tlcrz1972]

I-6 engines were designed to be torque motors not fuel economy motors.

Originally Posted by bigmac73

i would just keep it for and drive it occasional ,(every other day ) and get another truck with a 6 banger for a daily driver, there are still plenty 67-72 6cyl /3 on the tree out there available
as soon as some little douche bag in daddy's new challenger or camaro strokes you , you will be banging your head for getting rid of the big block

[70CHEVYBB]

I solved my gas mileage problem today. Had a guy make me a cash offer including a 2005 Harley.

[y5mgisi]

Youll sell it soon too. you suffer from a condition known as boredom.

[burnin oil]

Originally Posted by 70CHEVYBB

I think my plan is pull the big block install a full roller thumper cam, put in the 2000 stall and a 600 cfm edelbrock carb this winter.

Bad idea! No matter what you do quite thinking about those crap Thumper cams. If you want lope buy a performance cam and slam some lower gears in that truck and call it fun. If that is not the route you want, which it isn't, get a decent cam in that motor. Thumpers are about the absolute worst thing ever. They have the sound but it comes at the cost of efficiency. Now the bigger problem is that they make the sound but nowhere near the power of a real cam that lopes. Pick one and go with it as you will be way happier.

My K20 has a 406 sb in it and I get 8-10 MPG and never see the interstate. It is local trips to work only as I still get it tuned in. My motor is way more wicked than yours and I have 4.10 gears! You need to fix the mismatch of parts and tune it. I know plenty of guys that get 12 MPG with 454s and you should be no different.

[WIDESIDE72]

Originally Posted by tlcrz1972

I-6 engines were designed to be torque motors not fuel economy motors.

X2

I had a 71 GMC swab stepper with a stock 250 6 cylinder, 3 ott and a swapped in 3.07 rear. On a good day it would get 14 mpg highway.

[CST10]

I have a 454 bored 0.040 over with a mild comp cam .218/.224 @. 050 lift. With 0.510 lift total, RPM air gap intake, holley 770 avenger, HEI, timing 18 degrees initial and 40 degrees mechanical all in at 2800 RPM's no vacuum advance, 8.5:1 compression, turbo 350 tranny with 1800 stall, 3:08 rear gears. I get 10-12 city/hwy mpg. buddy with same teanny rear gear with a SBC 350 about same HP/TQ ratings get 14-16 mpg. I like my BB,,,don't change it out. You may regret it later.

[IwantaK5]

i would sell the big block and get a 6BT 12v.