6.8 MPG ok?

[Ronw435]

Im driving a 65 C-10 which is basically stock. Has a 283 with a RV cam. It had the original 2jet Rochester which I believe was just wore out. The power valve piston was so bad that you could rock it back and forth in the bore. With that carb running around 60 I got 8.6 mpg. The exhaust was always black with carbon and would actually spit carbon out on the concrete just idling. I removed the 2 barrel carb and manifold and bought a Qjet manifold and carb off of a 67 327. Rebuilt the Carb and now I am getting 14 mpg on the highway running 65mph. It is a 3 on the tree with 3.07 gears and dual exhaust. I am running around 2400 at 65 which I believe is right in the power range. Now I am 63 and have all the hot rodding out of me .............well almost.....but for the test I drove it as reasonable as I could. That mileage was on a 300 mile trip so from what I have read is probably about the best I can expect. I hope to tow a 2800 lb travel trailer with it without the mileage tanking. If I can get 9-10 I will be happy. I have the total timing at about 34 degrees with the vacuum advance hooked up to ported vacuum.

[Ronw435]

Wanted to add that the mileage was done with a GPS as my speedo is not working at present.

[rich weyand]

Couple things:

1) You bought an off the shelf generic Qjet to replace a Qjet that was factory-tuned to your engine setup. The Qjet, in addition to the standard tuning parameters, has a lot of little teeaks, like idle air jets drilled to size. So, first thing, unless you are a very clever tuner working with an A/FR meter, your tune is off. And it is off to the rich side: you can't go very lean without losing the fire.

2) Drag increases as the square of the velocity; since work is drag x velocity, work goes as the cube of the velocity. So you are using eight times the horsepower at 100 mph as you are at 50 mph. Now, you get there in half the time, but you are still using a ton of gas to go those speeds. And drag on these trucks, which have the aerodynamics of a thrown brick, is much more than even a car of that period, much less a modern truck or car.

[68post]

Qjets were only built in two size castings. 750 max cfm and 800 max cfm. The difference was in the primary bore diameter.
Some had and a throttle opening restrictor that limited its opening to restrict CFM.

Believe it or not - the 800 casting was used on some 305's with a restricted opening amount.

TRUE ! Buy a good qjet book if you question this.

[rich weyand]

Originally Posted by 68post

Qjets were only built in two size castings. 750 max cfm and 800 max cfm. The difference was in the primary bore diameter.
Some had and a throttle opening restrictor that limited its opening to restrict CFM.

Believe it or not - the 800 casting was used on some 305's with a restricted opening amount.

TRUE ! Buy a good qjet book if you question this.

True. All Qjets were OK for the smaller engines because of the small primaries, which is where you spend 99% of your driving. For 100 mph sustained runs, however, your mileage will depend somewhat on how well suited the size of the secondaries is to the size of the engine.

[Dead Parrot]

To Ronw435: If it still has the points distributor, might consider swapping in a basic HEI unit and regapping or replacing the plugs. Check firewall clearance first. Don't worry about the GPS reading, probably more accurate then the stock speedo anyway. GPS doesn't care about tire sizes or gear changes.

[Ronw435]

Dead Parrot, it does have an Acell HEI Distributor. I have made sure the power wire to the distributor is providing battery voltage which it is (12.6vlts). I have a total timing of 36 degrees at about 3000rpm. I had it hooked up to ported vacuum for the trip and got 14mpg. I am going to try it with 14 degrees vacuum advance (it has an adjustable vacuum can) with the total being 36 and hooked up to manifold vacuum to see if it makes any difference. I have the timing set at 10 degrees at idle with the vacuum closed off and idle at 550. It does seem to idle a little better and was allowed to lean the idle mixture up a little bit. I read where someone had said that they got 14-16 mpgs at 60mph but for every mph over that he lost 1mpg per 1mph. That seems a little extreme doesnt it? I know they are just big boxes riding through the air but I cannot believe I would get 19 mpg at 60 mph???
One thing I did notice is that the mechanical advance is all in at 3000rpms.

[Ronw435]

Rich do you think I could lean the carb up a little with jetting since it was originally for a 327 Carb # 7037213 DY? Or would I have to change out the metering rods as well? Or do you really think it would make much difference?

I swapped the plugs out today since they had been run with the 2jet and they really looked good. No carbon most had a slight beige color to them and they only have about 1500 miles on them as well as the engine since rebuild. But thought I would give it some new ones since they had been burning very black.

[Firebirdjones]

The only way to really know where you're at with the carb is to dial it in with a wide band air fuel ratio meter.

If I'm reading this right, you have a mild 350, with a 700 overdrive and 3.42's or 3.73's??? Qjets are very forgiving carbs, and they are excellent at metering fuel (ie: fairly precise) And this is a short bed half ton truck right??

When I tune combo's like this I generally see 17-18 mpg highway fairly easy if you keep cruise about 70 mph. If the engine has a mild cam or near stock and a compression ratio that works well with it, I can usually shoot for a lean idle and lean part throttle cruise to help MPG without any drivability issues. This is providing the distributor is setup properly first.

My 79 long bed 1 ton SRW with a 454/400 turbo and 3.73's will tickle 14 mpg highway if I keep cruise around 65-70-ish. This engine now rebuilt with a mild 218 @ .050 cam, headers, exhaust, and mildly ported original peanut port heads, and some tricks inside the original intake manifold. Original Q-jet that I've done my own mods to, changing the power piston circuit around a bit, main jet changes and worked on the back half quite a bit to get the WOT AFR perfect and safe to coincide with the lean cruise front primaries. It makes a ton of grunt and actually economical to drive for what it is. Actually my wife has been daily driving it to work for the last couple months. A/F meter is your best friend if you're serious about tuning carbs.

[rich weyand]

Originally Posted by Ronw435

Dead Parrot, it does have an Acell HEI Distributor. I have made sure the power wire to the distributor is providing battery voltage which it is (12.6vlts). I have a total timing of 36 degrees at about 3000rpm. I had it hooked up to ported vacuum for the trip and got 14mpg. I am going to try it with 14 degrees vacuum advance (it has an adjustable vacuum can) with the total being 36 and hooked up to manifold vacuum to see if it makes any difference. I have the timing set at 10 degrees at idle with the vacuum closed off and idle at 550. It does seem to idle a little better and was allowed to lean the idle mixture up a little bit. I read where someone had said that they got 14-16 mpgs at 60mph but for every mph over that he lost 1mpg per 1mph. That seems a little extreme doesnt it? I know they are just big boxes riding through the air but I cannot believe I would get 19 mpg at 60 mph???
One thing I did notice is that the mechanical advance is all in at 3000rpms.

10 degrees at idle and 36 degrees total timing? You're running 26 degrees of mechanical advance?

14 degrees of vacuum advance? I hope you mean crankshaft degrees. Careful there, the distributor vacuum cans are normally in *camshaft* degrees; 14 camshaft degrees is 28 crankshaft degrees.

Mechanical advance *should* be all in by 3000, or lower. Usually more like 2500.

[rich weyand]

Originally Posted by Ronw435

Rich do you think I could lean the carb up a little with jetting since it was originally for a 327 Carb # 7037213 DY? Or would I have to change out the metering rods as well? Or do you really think it would make much difference?

I swapped the plugs out today since they had been run with the 2jet and they really looked good. No carbon most had a slight beige color to them and they only have about 1500 miles on them as well as the engine since rebuild. But thought I would give it some new ones since they had been burning very black.

At 100 mph, you are into the secondaries. No primaries or idle tuning will fix that. Be interesting to know what your A/FR was, though. Considered getting a meter?

[Firebirdjones]

Originally Posted by rich weyand

At 100 mph, you are into the secondaries. No primaries or idle tuning will fix that. Be interesting to know what your A/FR was, though. Considered getting a meter?

I was thinking the same thing. Driving 100mph with the 4 barrel open is not the preferred method for an accurate gas mileage reading either, unless you live in Germany and drive the autobahn

Need to pick a more reasonable cruise speed and run a tank through it to see where it's at.

[Firebirdjones]

Originally Posted by Ronw435

I swapped the plugs out today since they had been run with the 2jet and they really looked good. No carbon most had a slight beige color to them and they only have about 1500 miles on them as well as the engine since rebuild. But thought I would give it some new ones since they had been burning very black.

Plug reading nowadays isn't too accurate simply because we all run unleaded fuels now. The plugs just don't color like they did on the good stuff. They can be an indicator if something is wrong like detonation or a rich condition. For instance running really rich will show black as you mentioned, but once you lean it out enough to see a lighter color that's about as good as it gets, from there you can't tell where it's at. That's where a wideband comes in.

I can change the AFR a full point for instance 13:1 to 14:1 (which is a big change) and see absolutely no difference in how the plugs look. They both look light grey to white.

[Ronw435]

Rich I could be wrong on the vacuum as what I did was according to Accel I backed the vacuum can all the way out till it stopped and then turned it back in 5 turns which is supposed to give me 14 degrees according to their chart with the HEI ignition. I have a timing light that shows the total degrees of timing by setting everything running at 3000 rpms and dialing the mark back to 0 and it shows 36 degrees. Its seems to like where it is at as the engine accepts the throttle without any hesitation, sounds nice and crisp, and has no problem starting. I might go to a shop that does exhaust sniffing and see what it shows. They do inspections and have a dyno to run it on. After looking at the Accel instructions again you are right Rich 7 is 14 engine.

[rich weyand]

Originally Posted by Ronw435

Rich I could be wrong on the vacuum as what I did was according to Accel I backed the vacuum can all the way out till it stopped and then turned it back in 5 turns which is supposed to give me 14 degrees according to their chart with the HEI ignition. I have a timing light that shows the total degrees of timing by setting everything running at 3000 rpms and dialing the mark back to 0 and it shows 36 degrees. Its seems to like where it is at as the engine accepts the throttle without any hesitation, sounds nice and crisp, and has no problem starting. I might go to a shop that does exhaust sniffing and see what it shows. They do inspections and have a dyno to run it on. After looking at the Accel instructions again you are right Rich 7 is 14 engine.

I think you might get better low-rpm mileage and performance with 16 base timing and 20 mechanical advance, rather than 10 base timing and 26 mechanical advance.

The 14 engine degrees of vacuum advance at idle is right. Put that on manifold vacuum.

[Ronw435]

Thanks for the help Rich. Will I need to worry about pre-ignition with the base at 16? The engine is 9-1 compression from what I read as it does have pp heads. The engine does have the WA designation for a 65. I hope to pull a 2800 lb travel trailer with this be a good set up for that?

Thanks for the information.

[rich weyand]

Originally Posted by Ronw435

Thanks for the help Rich. Will I need to worry about pre-ignition with the base at 16? The engine is 9-1 compression from what I read as it does have pp heads. The engine does have the WA designation for a 65. I hope to pull a 2800 lb travel trailer with this be a good set up for that?

Thanks for the information.

I wouldn't think so. A street sbc shouldn't start to have pre-ignition problems until a base timing of 20 degrees or more. What you will get from more advance is more torque in the 1500-2500 rpm range, which is what you want to get that rig moving. You've got the tune spot on for the high rpms, but that isn't where you want to be doing most of your driving, dropping into a lower gear all the time for hills and such. You want to get the pressure wave to the piston face sooner in the lower half of the rpm range to get your best torque.

[tucsonjwt]

I have always used this method of checking mpg.

Fill up as close to the highway entrance as possible.
Drive about 30 miles, away from and back to the same highway gas station.
Refill the tank from the same pump, same nozzle shutoff technique.
Calculate gas mileage.
Of course, this is driving as conservatively as possible, with no hot rod style driving.

For reference, I have always had mpg of 9-10 city and 13-15 highway at various times with the following squares:

73 C20 454/TH400 (rear axle ratio unknown) -this was the best at 11-12 city
76 C20 350/Th350 (rear axle ratio unknown) - this was the worst at 9 mpg city best
83 C20 454/SM465 granny (4:10 rear axle) -this was getting 11 city until I installed a National Carburetor rebuilt Qjet - now it gets around 8 city or less. (current truck)

Note that these are all 3/4 ton trucks, so they are dragging around an extra 1,000 lbs. or so compared to a 1/2 ton truck. My current 83 sits at 5600 lbs. empty, with my portly self inside.

One thing I noticed on these old GM V8s is that the idle speed is set way too high for my liking (I think some are up around 700-750 rpm). I know that is best for the engine, but it adversely affects gas mileage. Since I spend most of my truck driving time idling at city stop lights, I have dialed my idle speed back a little by turning the throttle adjustment screw out 1/2 turn from the proper setting. The fuel mixture screws are left where they should be. The BB still idles ok - does not lope or hesitate from a start - so I am leaving it set that way until it gets real hot in the summer and I have to use A/C all the time.

I have owned several new 1/2 ton Silverados and Sierras from 2000- 2006 and they all got 16-18 city with conservative driving. Those were 4.8 and 5.3 LS engines. My 2000 V6 auto work truck got 19-20 city. So, if you need a truck and want good gas mileage and can drive like the seniors, then you need a GMT 800 style truck. I even got 17 mpg city with a 4WD 5.3 Suburban I had for a week as a loaner vehicle when my 2004 Tahoe was at the dealer for warranty work (PCM replacement.)

[jjmorrse]

Sometimes fuel mileage problems happen as part of a system. The intake, exhaust, compression ratio, and the flame kernel quench combine with the effective gearing......

Based upon the description, it is a defective right foot assembly that is giving you a problem.

For a permanent fix, remove the right front valve stem, affix it to a replacement honda civic, and then fuel mileage should improve.

For what they are, which is a carburetor, stock quadrajets tuned to the vehicle are pretty efficient. Even when they aren't they generally aren't that bad, but they are like mullets, tidy and proper up front, but the back is all party. When you can hear the air horn sucking over the exhaust, because the secondaries kicked in, time to budget for more fuel.

[rich weyand]

^^^

[jjmorrse]

In August 1982, a Pickup Van and 4WD magazine article entitled "GMC's Marvelous Mastodon" said "equipped with the available gas 454 ci big block, mileage plummets to 5 or 6 miles to gallon" in reference to the carbed crew cab dually.

[Firebirdjones]

Originally Posted by jjmorrse

In August 1982, a Pickup Van and 4WD magazine article entitled "GMC's Marvelous Mastodon" said "equipped with the available gas 454 ci big block, mileage plummets to 5 or 6 miles to gallon" in reference to the carbed crew cab dually.

That thing was either just so out of tune or someone couldn't keep their foot out of it. Granted a dually and crew cab but still.....I'd expect 9-10 mpg minimum. I have over 700 HP 570+ cubed pump gas engines that get 5-6 mpg, lol.

My 79 1 ton 454 is SRW and standard cab, but still scales at 5250 lbs. without a driver so it's no featherweight, 3.73 gears and 400 turbo and when it was bone stock with smog pump intact it would still get 12 mpg highway pretty easily. I sharpened up the tune by dialing in the carb with a wideband and recurved the distributor, I can knock down 14 mpg highway, and 12 around town with the AC blasting.
There is actually quite a bit to be found (both mileage and performance) in these smog motors with some tuning.

[jjmorrse]

They might have measured during the blizzard of 82. The snowplow adds a lot of weight

[Firebirdjones]

Lol

[toolboxchev]

A Black Brick moving over 100 is kinda fun eh? That mileage is normal for the rate of speed and your gearing.

After I rebuilt the top end on a stock 350 in a 1970 rig I got it moving over 110. This was at 3/4 to 7/8 throttle with the speedo buried.

Keep a hold of that wheel my good man!