6.8 MPG ok?

[ozkiwi]

My 86 c10 is running basically a cast iron head ZZ4 rebuilt 750 quadrajet elderbrock performer inlet with ceramic long tube headers and a stainless 2.5 inch mandrel bent dual exhaust/700R4/ford 9 inch with 3.50 gears.
Sitting between 70-100mph(GPS NOT SPEEDO) on a recent 60 mile run with hard 3rd gear acceleration between those speeds on flat freeway I am getting 6.8mpg does that sound about right?

[Caleb]

what RPMs where you revving at? around town in 3rd gear i avg 10 in my lifted k10 w/ 35's. so depending on what your RPMs where that doesnt sound right.

[kalbert]

Hard acceleration and freeway speeds and faster in 3rd gear? 6.8mpg sounds about right to me. You're running it pretty hard, that's the mileage you can expect. I used to get between 5 and 10, same truck, same motor, same driver, now I get 12 consistent. The difference? I'm 15 years older

[Skunksmash]

What wheels are those? Also it does seem like you should be able to pull off 12-14mpg.

[Bigstevex4]

Sounds about right there are rpm calculators your probably at those speeds between 3 and 4 grand not ideal for fuel mileage. The diff between running 65 and 75 is huge in fuel mileage at 80 plus it really tanks.

[Tom]

Sounds horrible. You don't say which heads, but it sounds like a 350 with small cam, performer intake w/ Qjet carb and exahust. With overdrive and 3.42 gears on the freeway you should be a minimum 10mpg. My 78 has always gotten 10mpg even with different engines, 3.73's and no overdrive and my 79 would hit 15mpg with a vortec 350 and 200-4R

[kalbert]

10-12 on the highway travelling 55-65 no problem. 70-100mph with full throttle 3rd gear acceleration, 6 is totally possible, and expected, maybe worse.

[ozkiwi]

The motor has vortec heads it's a GM crate motor 330hp HO but for some reason the Australian version comes with a roller cam which sort of makes it a iron head ZZ4.
Wheels are 17 inch rally replica but no idea who makes them.
I was pretty suprised how quickly the fuel needle was dropping but for short bursts I may have been hitting 110 as I was trying to keep a friend in sight who was not slowing as much in the corners.
6.8 seemed high to me as I usually get 9-10 on this run sitting on an even 85mph.I guess those extra 15mph and WOT burst's take there toll.
60 mph is 2100 rpm with this diff and wheel combination and it gets 14mpg-15mpg but rarely drive at that speed once I am out of the city limits.

[Bigstevex4]

Originally Posted by ozkiwi

The motor has vortec heads it's a GM crate motor 330hp HO but for some reason the Australian version comes with a roller cam which sort of makes it a iron head ZZ4.
Wheels are 17 inch rally replica but no idea who makes them.
I was pretty suprised how quickly the fuel needle was dropping but for short bursts I may have been hitting 110 as I was trying to keep a friend in sight who was not slowing as much in the corners.
6.8 seemed high to me as I usually get 9-10 on this run sitting on an even 85mph.I guess those extra 15mph and WOT burst's take there toll.
60 mph is 2100 rpm with this diff and wheel combination and it gets 14mpg-15mpg but rarely drive at that speed once I am out of the city limits.

Are you talking KPH or MPH?

[ozkiwi]

Originally Posted by bigstevex4

are you talking kph or mph?

mph

[dieseldawg142]

Originally Posted by ozkiwi

mph

yeah, i guess when you got 900 miles of wilderness between towns, you dont want no kmh's
i wish we would revert back to the "proper" way (mph)
at them speeds, i think your mileage is pretty good. i cant even get that with a egg under the throttle, & when i'm on it-half what your getting-glug, glug, glug!

[Bowed]

Originally Posted by ozkiwi

on a recent 60 mile run

To much margin for error in that short of a test . Even using the same pump to refill is no guarantee the figures are accurate .

[Dcheat]

For your engine, your WAY over carbureting. For the displacement of your engine, even if its spinning all day at 7000 rpm you dont need more than a 600 cfm carburetor. Period. At 100% volumetric efficiency (formula 1 engines are around 96% VE) You would only need a 700 cfm carb. Thats why you have crummy mileage. I know bigger seems better, but you engine will only use so much fuel and pull so much air. Things quickly turn to crap when your carb is not matched to your engine. Trust me here, you need a smaller carb. You will even see an increase in performance and mileage.

[piecesparts]

Originally Posted by Dcheat

For your engine, your WAY over carbureting. For the displacement of your engine, even if its spinning all day at 7000 rpm you dont need more than a 600 cfm carburetor. Period. At 100% volumetric efficiency (formula 1 engines are around 96% VE) You would only need a 700 cfm carb. Thats why you have crummy mileage. I know bigger seems better, but you engine will only use so much fuel and pull so much air. Things quickly turn to crap when your carb is not matched to your engine. Trust me here, you need a smaller carb. You will even see an increase in performance and mileage.

Agree on the to large a carb, thought. To add to that, you have 3.50 gearing that would take more to get to the speeds you are asking for, so more fuel is involved in the acceleration for what response you are looking for. A set of 3.73 gears woud work well with your truck and the 700 R4 tranny. A 600 to 650 carb would be a lot better than the 750 carb. That truck weighs close to 4000 lbs and you arre not generating enough air signal with the 750 to get the proper fuel mixture unlesws you runn the engine at the higher RPMs. What size of tires are you running on the rear is another item that affects fuel consumption.

Just for instance, I have an 84 GMC SWB truck with a 383 stroker engine, 650 carb, 700R4 tranny, 3.73 gears and a 28" tall tire on my truck. I can average 15 MPG running down the highway at 75 MPH--running at 2700 RPM. This was with a tranny that did not have a lock-up converter. I just added a new converter and I now have the lock-up feature and I can move down the highway at a lot lower RPM and I will have to see what I get for mileage, this summer. The lock-up is worth 200 RPMs.

Get a tire size, gear, RPM calculator out of the computer and start working to bring all of your components together and match them up and your mileage will grow.

[ozkiwi]

Ok I am confused now. The quadrajet is the factory fitted carb that was fitted to it's original 305 motor?????
So that must have been wayyyy over carbed?
I have not changed the jets so seems odd that I am over carbed on a bigger motor making double the horsepower.
Not saying I won't try a smaller carb just don't understand the factory 305 with this set up.

[Titomars]

Not all Q-Jets are 750cfm. Q-Jets were made from approximately 635cfm to 760cfm. unless you have a Q-Jet from a performance big block or the biggest was only in the GMC motorhome. The smallest were used on 6 cylinder engines. Yours would be most likely in the 650cfm range for a 305.

When you are running down the highway with a Q-jet and you are tipping into to secondaries you can count on fuel economy to go out the window. Many years ago I had a 67 GTO and did a WOT run on the freeway. I was doing a 20 mile race WOT with AX secondary rods and you could literally watch the fuel gauge go down. I remember burning almost a half a tank that night just on that run.

[kalbert]

When you're running down the highway at 70mph, racing up to 100 you're getting into the secondaries on any carb, not just a Q-Jet. The Q-Jet is fine, it's not where your fuel mileage is going. No carb will give you the driveability of a Q-Jet on the street.

You're pushing a 5,000 pound square steel box to 100mph and running it hard to get there. That's where your fuel mileage is going. Your right foot. No amount of carb tuning or swapping is going to get you better than what you're getting in those conditions. They're all just garden hoses dumping fuel at that point.

[Dcheat]

Originally Posted by kalbert

When you're running down the highway at 70mph, racing up to 100 you're getting into the secondaries on any carb, not just a Q-Jet. The Q-Jet is fine, it's not where your fuel mileage is going. No carb will give you the driveability of a Q-Jet on the street.

You're pushing a 5,000 pound square steel box to 100mph and running it hard to get there. That's where your fuel mileage is going. Your right foot. No amount of carb tuning or swapping is going to get you better than what you're getting in those conditions. They're all just garden hoses dumping fuel at that point.

I beg to differ. A carb is Never just dumping fuel into your engine. Its a metered amount of fuel that is variable depending on the volume of air flowing past the jets and then the venturi. Per cfm of air flowing past, the carb will deliver roughly the same amount of fuel to maintain the stoichiometric ratio that its been setup to deliver. Opening the throttle just allows more air to enter, and thusly allowing the engine to rotate faster and pull more fuel to maintain its rotation and combustion.

The thing is, your engine is only going to pull so much air/fuel at a given moment. A cylinder can only pull its volume in a single stroke *forced induction aside*
So, running too large of a carb is in no way beneficial. Even if a 750cfm q-jet has been re-jetted to run on a smaller displacement engine, then its still going to over fuel the engine as its designed to run on an engine pulling more cfm than a 350ci can provide.

This is why mass-air fuel injection systems give stellar power and fuel economy. Even with a 100mm throttle body, the fuel system reads how much air your engine is using, and adjusts the fuel to match.

Lastly, Q-jets are a great carb when they are running right. But when they are in need of a rebuild, mileage goes right down the crapper.

[Titomars]

Originally Posted by Dcheat

So, running too large of a carb is in no way beneficial. Even if a 750cfm q-jet has been re-jetted to run on a smaller displacement engine, then its still going to over fuel the engine as its designed to run on an engine pulling more cfm than a 350ci can provide.

A vacuum secondary carb is only going to allow as much air as the engine can use. A Q-jet's secondary fuel metering is keyed directly off the secondary air valve not the throttle plates. Assuming the carb is working as designed the air valve will only open as fast or as much as the engine can take. This is why the vacuum secondary carbs are more popular on the street. They by design are more forgiving to the matching of CFM to cyl. volume.
our squares are big concrete blocks on wheels neither aerodynamic or light you could stick a Rochester 2GC on top of a 350 and the gas mileage is not going to improve much at all. why? because you foot will be in more to keep up with the Q-jet version doing the same thing.

[ozkiwi]

The fact that my quadrajet is not a 750 makes much more sense.I cannot even remember why I thought it was.
How do I identify what size carb I have?
I bought a rebuilt carb from summit racing so the old one is just sitting in my workshop.I will have a good look at it tomorrow once I know what to look for
I am very happy with the performance of the motor and fuel mileage in general
I was just surprised at the difference between fuel use at steady 75mph and big increase when you give it a bit.
I have never owned a vehicle running a carbed V8 before this truck.
I have been driving modern small fuel injected pickups that I have just been A to B transport but I love driving this old truck and have developed a bad habit of driving it quiet hard since the new motor went in.

[Titomars]

Originally Posted by ozkiwi

The fact that my quadrajet is not a 750 makes much more sense.I cannot even remember why I thought it was.
How do I identify what size carb I have?
I bought a rebuilt carb from summit racing so the old one is just sitting in my workshop.I will have a good look at it tomorrow once I know what to look for
I am very happy with the performance of the motor and fuel mileage in general
I was just surprised at the difference between fuel use at steady 75mph and big increase when you give it a bit.
I have never owned a vehicle running a carbed V8 before this truck.
I have been driving modern small fuel injected pickups that I have just been A to B transport but I love driving this old truck and have developed a bad habit of driving it quiet hard since the new motor went in.

The reason you thought it was is because that's what most people think and preach. Outside of having a Rochester engineering manual for Q-Jets or a flow bench there is no way I know of to determine a particular CFM rating by carb number. In my years long ago racing and building these carbs. it was a typical standard that in you wanted a small one you looked to small engines for your builder if you wanted big you looked to big blocks. The rule of thumb was use a core that was on a like sized engine for a stock or mild build and go up for a wilder build. like a decent built 350 pushing the 350 to 375 hp range would love a Q-Jet out of a 400 engine. Like wise a built 454 would love a built GMC motorhome Q-Jet. I am sure there are sharper guys out there than myself for these carbs. My knowledge is over 30 years old also I have forgotten a lot. But back then if you wanted to race a Q-Jet it was more experimental than say going with a Holley.

Also you have to take into account our trucks were designed back in a time when fuel economy was not a factor in America. Honestly American vehicles of that era worked on blunt force. Meaning raw power gets it moving and keeps it moving. they did not have the same finesse like their foreign counterparts.
Yes ... that power is addicting hehehe

[kalbert]

Originally Posted by Dcheat

I beg to differ. A carb is Never just dumping fuel into your engine.

I think you took my statement to literally.

[Tom]

The size of the Qjet doesn't matter for cruising, they all have the same tiny primary bores. The secondaries are vaccum controlled and adjustable, hence not a problem. If it was a problem GM would not have produced tens of thousands of the combo. If we were talking edelbrocks or holleys then its slightly different, but Qjets work fine on small engines which a 350 is not.

[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.