Ok so now i came over to the bad side of chevy perfomance im all about efi but i dont have the money to get a tune for my hsr lt-4 vortec block i was gonna mate to my wc t5 but for now i decided to put a carb setup on my 355 sbc an i noticed gas mileage is terrible compared to the efi tpi i had on it how can i get better mpg out of this setup. With the tpi setup i was around 242 miles to a full tank with the carb setup im about 150 to a full tank pretty bad i think

 

did you also swap out to a mechanical advance distributor?
where is the timing set?
which carb?
did you get the mixture screws set on the carb for max vacuum at the lowest possible idle?

 

Yes i did switch out to a mechanical dizzy an im using a edelbrock 600cfm with electric choke timing is set 8 degrees before tdc i think thats correct an what mixture screws are u talking about are u talking about the two on the front on the sides of the pcv valve hook up

 

Did you unhook the vacuum advance when you set it to 8 degrees advance? If not, that could be your problem. If you don't have vacuum advance on the distributor, that is a big problem and would explain the poor MPG's. Also, 8 degrees advance is a little low. I'd like to see 12 (or more depending on your engine specs).

 

Yes i unhook the vac advance when setting timing it ran perfect with it off then hooked it up an ran smooth while driving it as well i looking to get better mpg out of it. I do have a vac guage just wondering how do i adjust the mixture screws to get it at full vac at the lowest idle point ???? An when u talk about lowest idle point u mean the idle screw turned all the way down right ??

 

Define "Terrible" compared to EFI. On the best day, a carbed 355, with a Qjet and 3.08 gears, might..MIGHT get 20mpg on the highway at 55mph.

My 355, 3.73 gears and 700R4 gets 17-18 highway at 65, roughly 14 combined.

My 67 camaro has a 350 with Holley 650 and 4-speed gets 15 highway at 60mph.

 

Yea see that is terrible doing 60 mph an u only get 15 mpg i use to do 75 mph an max out 252 miles to a full tank

 

i like to run as close to 20 degrees of initial timing as i can get away with, and also hook the vacuum advance up to full manifold vacuum for somewhere between 45 and 50 degrees at idle.. your starter and the ability of your engine to avoid detonation will be the limiting factors here. you need to get this set before playing with the carb.

regarding the screws on the carb- there are 4 of them you need to worry about.. the idle speed screw is on the driver's side of the carb, in front of the throttle arm. the fast idle is under or behind the choke coil on the passenger side of the carb. the idle mixture screws are on the front of the carb- there is one of them for each primary butter fly.

start with the idle speed screw, with the choke open and fast idle disengaged. back off the idle speed screw until the throttle is all the way closed, and turn it in about 1/2 turn or so, just to push the throttle arm open a little bit. set the idle mixture screws on the front of the carb 1.5 turns out from being gently bottomed out.

start it up- it should run pretty well.. if the idle is too low, just crank the idle speed screw open enough to get the idle steady.. go for a short drive to get it up to temp and get the choke open.. with the trans in park (if it's an auto) start turning each mixture screw on the front in 1/4 turn at a time until you get max vacuum.. this will probably result in a much higher idle speed- just back out the idle speed screw until the speed comes down, then go thru the mixture screws again.. keep repeating this process until you reach the point where you don't get it any better by turning the screws. once you get it as good as it's gonna get with the trans in park, find someone you trust to hold it in drive with their foot on the brake while you adjust the carb to get that last little bit of tweaking done.

you might start to get frustrated if things don't work better, but eventually it will reach a point where it starts with a tap of the key, idles nice and steady in gear, and has awesome throttle response from idle and you will feel like a tuning god. you will also get much better fuel economy and just generally enjoy driving the car with everything properly adjusted

 

If the MPG is dropping when traveling at 60 to 75MPH, you are no longer on the idle circuit of the carb. The main jets are probably too big...I assume this is a Holley carb? What size carb and what jets are in it now?

 

He's got a Qjet, and the MPG will drop a little, as the secondaries open up the faster you go, if you're cruising. However, unless you're doing WOT or 100mph everywhere, it shouldn't be that bad.

Right now, I get 240 miles per tank combined driving. On strait highway, I can get almost 310 miles per tank at 70. (2600RPM on highway).

In my Camaro, I get 240 miles to a tank on the highway (3000 RPM at 70).

 

the secondaries on a quadrajet will never open while just cruising down the road- the air door at the top of the carb will only barely open on a 300-ish hp engine when driven in anger.. they are a non issue when talking about fuel economy.

concentrate on getting the primary throttle blades as closed as possible while going down the road and leaning things out as much as it will allow.

 

If gas mileage was important, why didn't you just keep the tuned port.

 

a properly adjusted quadrajet is every bit as good as a factory TPI setup in every way- power, throttle response, fuel efficiency, and even emissions- the fun part is learning how to "properly adjust" them for the rest of the car and keep it there thru varying weather conditions.

 

That may be true, but the beautiful part was he already had Tuneport with good gas mileauge. So switching fuel systems seems pointless.

 

simplicity has it's value.. you can adjust a carb and mechanical advance distributor with a few basic hand tools and there are a lot less sensors to go bad and wires to short out.
if you want a little faster mechanical advance curve with an HEI, you swap in a spring under the rotor.. if you want a little faster advance curve with the TPI you need to find someone to burn you a new chip. same with playing with the carb- worst case you are pulling the top of the carb off, swapping a jet or metering rod, and putting it back together over a 10 minute period- the TPI requires the ability to burn a chip.
don't get me wrong, i don't have a problem with fuel injection, but the pre-OBD2 oem setups are more work than a good old fashioned carb and distributor.

 

I find that as inaccurate but each their own. Tuned my Firebird with a chip from a member here and ain't touched it since I did my motor swap. I have no clue what kinda gas mileauge I get because honestly, why pay attention in a hot rod? Not shabby I don't guess, once a month fill up driven four days a week.

Best part? I own a very fast car, a very drivable car, it always starts instantly when I turn the key and I never have to adjust any thing.

If I wanted gas mileauge I as a primary goal I would def stay EFI but if screwdriver simplicity and power was my goal, them I guess I'd slap a carb on it.

 

you don't know what your mileage is and that you haven't bothered to try to optimize the car beyond plugging in a chip and calling it "good enough"? you might not only be burning extra fuel, but you might be leaving power on the table in your "hot rod". with the efi and no ability to burn a chip, your tuning is limited to initial timing and fuel pressure- if you even have an adjustable fuel pressure regulator.

 

I'm not burning extra fuel. I don't have any care for burning chips and I dont plan to change anything else so what is my need? I am faster than 9/10ths of everything around here, no need to be faster?

 

You don't know if you're burning extra fuel or not, you said yourself you haven't checked your gas mileage, so I'm pretty sure you haven't datalogged either. This thread isn't about being faster or not, it's about getting better gas mileage, contribute something useful or shutup.

 

Dakota, ultimately if his thread is about gas mileauge, then why the hell did he try and fix what wasn't broken? Who swaps on older fuel injection on for gas mileauge? I don't know anybody that does.

 

Andrew, the answer to that question is in the first post, reading really isn't that hard to do.

 

I ignored that because that is not a strong valid reason.

 

I went carb because of the costing for the tune i had to do i swapped over to standard transmissio an u know that gm didnt make a program for the 350 tpi t5 since no 350 came with the t5 in the f body platform cars. I called places for a straight week trying tp find a place to get it custom tuned but most places wanted 500-600 bucks so why pay 500-600 dollars when i had the whole carb set up sitting wrapped up in totes down cellar, buy a couple of gaskets an call it a day. I just decided to take the car off the road an wait an save up for my 355 vortec lt-4 hsr, t5 swap an then go get it all tuned out with big hp/tq numbers an still get around 24 highway mpg if possible

 

Wow, puncutuation would really clear this up.

Anyway, properly tuned, the TPI will perform better in all aspects of driving than a properly tuned Qjet. Better power, better gas mileage, better part throttle better...etc. etc.

355 Vortec, big cam, and big HP/TQ numbers won't get 24 MPG highway with a carb. Unless by big HP numbersyou mean 250 hp, then you can probably get 20-21. I don't even think fuel injection woudl get you there unless your running a stock LS1 with 350hp and a manual, those guys would get 26mpg or so on the highway.

My Qjet (professionally tuned by Autopontiac in London), was tuned on my car, and the best I've gotten out of it is 18 highway. This is wth 3.73 gears and an auto transmission. I'm also putting out an estimated 300hp from the motor (240 at the wheels on the chassis dyno).

with a lot of tweaking, swappiong out to 3.08 gears and possibly a manual transmission install, I could eek out 20mpg on the highway.

 

This topic has been beat to death on these boards, but you're selling the carb short. Fuel injection or carbureted, they are both means of delivering fuel to the engine. Your engine doesn't care if it's getting fuel from computer controlled injectors or a mechanical venturi, all it sees is the AFR in the cylinder. If your carb is tuned properly, you'll get the same AFR as a fuel injected car, and thus the same mileage and power. Only difference being, you can now adjust it with hand tools instead of having to deal with a computer and the boat-load of sensors that fuel injection comes with.

The quadrajet is an extremely complex mechanical beast. In my experience, PROPERLY setting up a quadrajet takes more time and expertise even than properly tuning a EFI setup. The EFI makes it easy, because you can plug in and see exactly what ignition advance and fuel conditions you are getting vs specific loads, RPMs, throttle position, etc, etc. When tuning a carb and mechanical advance, you have to be able to understand enough about the processes in action to get it tuned right. Even so, you're still left to reading plugs, tailpipes, etc to determine whether you are lean, rich, or right on. I've found after tuning them for years that a wide-band O2 sensor makes this process incredibly easier.

The main advantage of EFI is its ability to adapt to changing environmental conditions, and its wealth of sensor information available, which contributes to ease of tuning IF you have the proper equipment and computer capability. It's disadvantage is high cost, complexity, and the heavy front-end-load of the equipment necessary for tuning. Having a mail-order "chip burned" for you never has been, and never will be proper tuning of your vehicle. Not even dyno-tuning will give you a complete tune, you've got to log while experiencing real driving conditions to really get it right.

The main advantages of carburetors are low cost, both in components and tools for tuning. The main disadvantages are its limitations in adapting to changing environments, and the extreme amount of mechanical knowledge and persistence necessary to get them tuned right, as well as the experience to know which way you're going with it.

My 305 (XR258 cam, 113 Heads, 1.6:1 RR, Weiand Street Warrior Intake)/T56/3.27 combo, running a mechanical advance HEI distributor and a quadrajet carb regularly knocked down 27-29 MPG at 75mph cruise. My 305 TPI/TH700/2.76 car got 26-28 mpg at a 75 mph cruise. Both cars had the exact same headers and exhaust, and both cars had free flowing intakes. The carb car had a bigger cam, bigger valvetrain, worse compression ratio, and got better gas mileage. Ditto on my 454 powered suburban. I'm getting 14-15 mpg at 70 mph in a 6500+lb 3/4 ton '86 Suburban, on 32 inch tires. It's also running a Quadrajet and mechanical advance dizzy. Try beating that with a fuel injection setup.

However, as the OP already posted, he's not using a Quadrajet. He has an edelbrock. I'd be willing to bet that if he switched to a good rebuilt quadrajet with factory spec jets and rods, he'd pick up about 2mpg right off the bat. The spread bore is simply a more efficient design. (albeit much harder to tune properly).

-cal30sniper

 

i have a 350 sbc bored .040 over can u tell me the best carb i can get?thanks a lot.

 

A 650 double pumper and some time spent tuning it (a wideband O2 sensor would be nice, but a narrow band will work too). This will give good power and good MPG.

 

I would not recommend a Holley, especially a double-pumper, for gas mileage. The Quadrajet is a much better setup for economy.

 

There is no difference in fuel mileage between a properly tuned double pumper and a properly tuned vacuum secondary. This is one of the biggest misconceptions there is with carburetors.

 

I swapped in a 355 TPI to replace my 305 TPI changed from MAF to speed density got a chip burned on the dyno by Kevin Lorentzen a couple of years ago and haven't touched it since. I get 15 to 17 mpg city and about 23 highway.

 

And I call . There IS a difference between a properly tuned Holley Double Pumper and a Quadrajet, it's called a spreadbore. Having a Quadrajet is like having a really small 2-barrel for cruising around, and a big honking 750 cfm (or choose the number less than that if your engine can't utilize all of it) 4bbl when you stomp on it. Not to mention the plethora of part throttle primary and full throttle metering circuit tuning abilities that the Holley can't even dream of. Just because someone does not understand the intricacies and tuning capabilities of a Quadrajet does not mean it's a bad carb. The Quadrajet gets a bad rep because it's smarter than your average mechanic. Hell, it's smarter than a lot of people out there who claim to be good mechanics.

The shining advantage of a Quadrajet, is it has so many circuits and tuning points that it's the closest thing in the carb world to EFI. It's biggest detraction is that it is so complicated that most people become frustrated trying to tune it before they figure out how it works.

Sorry, I'm not seeing how this relates to carburetors...

 

I never said a quadrajet isn't more efficient!!! There probably the most efficient and best driveability carbs ever made. Although the newer annular booster holley style carbs are really efficient, but they still don't have the tuneability of the power enrichment circuits of the quadrajets and edelbrocks.
I said a holley double pumper and vacuum secondary get the same millage because you said "especially a double pumper".

 

Ah, gotcha. I thought you were referring to the Quadrajet as a vacuum secondary, because it is. Shoulda read that a little closer.

That being said, the double pumper is still one of those things that people think they "really need to have", that they really don't. It's one of those things like "Corvette 350" that you see just get beat to death in the hot-rod community.

I would still argue that a vacuum secondary carb of any type is inherently more efficient in its design. It only uses the amount of air that an engine needs, not whatever your foot tells it to put in.

 

Yea but even with a double pumper, the engine is still only going to suck in the air it wants and no more. You may just loose a little velocity if the carb is to big.

And btw I love the "corvette 350" comment! Those are the same guys who keep saying "they don't make them like they used to"!

 

That's the main problem with a mechanical secondary carb. Fuel metering is highly dependent on velocity. The carbs are engineered so that they compensate for the average joe who puts way too much carb on his 300-350 c.i. engine. Therefore, they dump more fuel than is necessary through the secondaries to help combat the low secondary venturi velocity (which correlates to higher secondary venturi pressure, and less fuel draw). It doesn't hurt performance, engines can run quite rich and still make great power. It can often kill efficiency though. If your engine is properly matched to your carb, it's almost a moot point. Still, most street engines out there do not require a double shot of fuel from the accelerator pumps to function properly. Again, it doesn't hurt performance, but it definitely wastes fuel.

 

I have never had luck getting a vacuum secondary carb to run as good as a double pumper and I have had good luck getting decent mileage with double pumpers. I haven't messed with a quadrajet in a LONG time, so I can't speak to that. However, a Holley has an idle circuit, a transition circuit, a main circuit and a power circuit. There are also IFR's, IAB's and MAB's to fine tune each circuit. Most Holley's are more complex than the average person can handle. The problem is, guys think "double pumper"...it squirts in twice as much gas. Not true. It only squirts in twice as much gas when the pump arm is activated by the cam and even that can be adjusted.

I get 14MPG with my car in mostly around town driving. 350 (around 450HP), 4.10 gears, 3600 converter and a 750 Mighty Demon. I would think 18 on the highway is possible. A basic 350 should be able to do better.

 

Yes but you can still change the rear jetting and hsb's to get the fueling where you want it. Plus this is all basically at wot which really has nothing to do with fuel millage.
And I have to agree with pancherj, I've been tuning carburated cars on a chassis dyno for years and have switched cars from vacuum secondary holleys to the same size double pumpers. Max hp is usually very close (depending on the vacuum spring), but the double pumper always comes in much harder and faster making a lot more low end torque than the vacuum secondary.
I have actually used a vacuum secondary carb to help a car that was launching to hard and blowing away the tires to make the power come in slower.

 

Interesting stuff. I've had good luck with most of the vacuum secondary Holley's that I've worked with, but most of those were on pretty mild applications that weren't putting out much more than stock. The only bad experience I had was with one of the Holley Spreadbore vacuum secondaries on a 440 Dodge. It was awful, got bad mileage, and never would tune out right, even after the rebuild. I ended up replacing it with a quadrajet, and that was the end of the troubles with it.

I've run a double pumper on another built up 440, and it made awesome power, but drank gas like it was going out of style. Of course, that was also a lot more to do with the engine than the carb.

All that being said, the original suggestion to run a double pumper holley on an engine where economy is one of his major concerns was still bad. If you want an engine that performs great and gets optimum gas mileage, go with a Quadrajet. There's no replacement for a properly tuned Quadrajet on a daily driver. They are very insensitive to changes in temperature and altitude, and will fuel anything up to about 500 cubes without much of a problem with the right jets and rods. A squarebore will never be able to match the efficiency of a spreadbore for daily driving.