Gas Mileage Improvements

[crawdad]

My truck (85 Silverado in sig) is currently getting around 11 MPG in town driving, back and forth to work, less than 10 miles each way. The quadrajet carb currently on the engine is just about shot, It's seeping gas around the gaskets and I can't seem to get it leaned out, it's really running rich. The smell in the engine compartment just reeks of gasoline. Rather than rebuild it, I'm thinking of going with the Edelbrock carb and Performer manifold. I'm just wondering if I'll see any significant improvment in my gas mileage. Of course any improvment would help, that 11 MPG is killing me! Also, would you recommend I go with the EGR type manifold or non-EGR? We currently don't have inspections in my part of the state, so if it would improvement mileage I might take it off. Then again, if it doesn't have that much of an effect on mileage, I wouldn't mind leaving it on. Higher performance is not much of an issue with me. I'm quite content to just poke alone these days. Your suggestions and recommendations would be much appreciated. Thanks.

[DrMaserati]

It sounds to me like your main goal is saving money and gas. I think the most cost effective choice would be another Q-jet from one of these two places.

http://www.jetchip.com/
http://www.smicarburetor.com/

A Q-jet has the potential for both better fuel economy and performance than the Edelbrock 1400 series. And you won't need a new manifold or an adapter. It will be a straight bolt-on replacement. Whether or not a Performer manifold would give you better economy is debatable, but it probably wouldn't hurt economy. It's just an added expense and a lot more hassle than bolting on a new carb.

But ultimately it going to take much more than just a carb/manifold change to see a significant increase in fuel economy. Think EFI, gear change, tire change, and maybe most of all a change in driving habits.

JP

[Slonaker]

I just bought the stock rebuild q-jet for trucks from Jet, 36001 I think. It looks like a real quality item. The hardware and everything looks brand new. I just put it on Sunday night, and have not had time to make adjustments (I need to learn how ). It already runs much better.

I was considering the performer until I realized how much it would cost me for an intake or adapter, the bracket for my kickdown, bracket for my cruise, bracket for my throttle, and the cost of the solenoid for the A/C. It was no longer a cheap alternative. If you have don't have any of that, it might be a good option.

BTW, the EGR helps stop detonation, so it can be a very good thing. I blocked mine off with the engine swap, and have no problems with pinging. I'm going to leave it that way as long as emissions checks don't start around here.

Slonaker

[crawdad]

JET sells their Stage 1 (#36001) for $510.00! Ouch!!! Summit, on the other hand sells a Jet Stage 1 (#35001) for $350.00. Better. Are these the same carb? If not what's the difference. I'm pretty sure I could get a Performer intake and Edelbrock 1400 series carb for less than $510.00. I'm also pretty sure it's the carb that is eating up my mileage. It's running extremely rich and I can't lean it out any more. Gear ratio is 2.73, so that doesn't need to change. My driving habits are fine. The years (and gas prices) have made me more docile, i.e., I drive like I have a raw egg between my foot and the gas pedal. I will, however, be changing out the tires that are on it now. They are quite a bit smaller in diameter than stock (PO installed), so I'm sure that has an effect on the mileage somewhat. EFI is out of the question. Main reason I'm asking this is because I keep reading where a lot of you guys are getting over 20 MPG, and I wondering how the heck you're doing it?

[Russell]

I already swapped to a 700r4, and got significant gains since most of my driving is at or above 50mph.

My carb is much the same as yours, underhood reeks of gas, and all the fuel from the fuel bowl leaks into the intake while it sits. Starts right up every morning with one pump, but if you try to start it less than 2 hours after you've had it running, its tough to start, and belches a thick black cloud of dollars (gas, lol) out each of the pipes.

So, I'm already about half done switching to TBI. Been a far simpler swap than my TPI was (maybe its becuase I know what I am doing now..) and should be running on the EFI in about a week or so.

[Slonaker]

I bought my carb from summit. It was, and still is, $340 and it comes with a $20 gift card (summit bucks). They don't list it in their catalog, but it is available on the website. Search for 36001 and it is the only result that comes up.

Slonaker

[piecesparts]

You say the gear ratio is a 2.73. What tranny do you have? That is a pretty high ratio for anything that would have an overdrive tranny. If you are running the motor at 1900 RPM or less while going down the highway at 70 miles per hour, you MAY be working your engine to hard to make up for the gears in the rear end. The more you have to push the pedal to get the truck up to speed and then return it to speed after it slows down a little, then the more fuel you are wasting. My son's truck had a set of 3.08 gears in it and when we put a 700 R4 in place of the 350C tranny, we lost mileage. The motor was working to make the cruise work, as well as just driving around the city. There is such a thing as too much of a good thing. I finally got tired of that mess and we installed a 3.73 posi rear end and actually gained a couple MPG. The motor likes to run around 2100 to 2400 RPM cruising speed and with the truck at 70 MPH it runs at 2500 and it accelrates easily back to speed and the cruise is not always kicking down to make up for speed loss on hills. You do have hills in Georgia, don't you? I was down there two years ago for a visit to Atlanta and I basically remember some.

You could play with your timing some, to get it to work at it's optimal point. If you change the distributor weights, then it could advance a little quicker and get you into the RPMs easier. I set my carb and timing with a vacuum gauge and a timing light that has a build in advance feature to make sure that I get the best settings for my engine. I actually run my timing where it is set at a maximum of 35 degrees TOTAL advance, rather than setting it at the initial timing. I get about 14.5 MPG on my 84 GMC with a 383 stroker motor and a 700R4 tranny. It is the one in my avatar.

[crawdad]

Tranny is a TH-350C and I've complained about it in other posts. Dang thing can't never seem to find where it wants to fun. Constantly cutting in and out, in and out. I'm sure I'm wasting gas with this tranny. I think I will play with the timing a little bit and see if that helps any, but it seems like there would have to be something major causing this kind poor fuel economy, and this crappy carb seems to be the most logical choice right now. I'm still open to any other suggestions. I'll try them all. Oh, and yes, we do have a few hills here in Georgia, but I don't notice them much unless I'm pedaling my bicycle.

[snowbuzzard]

This is something I had done to a truck I used to have that had a 305 in it, a 350 THM and a 3.08 gear.

I scrounged an old 650 dp Holley and put a kit in it. I then slid a 350/350 chevy cam into it (I can find ya the part # if ya like) and put headers on it. Blocked the EGR and ran the stock intake.

With 15 degrees of timing I was getting 22-25 mpg with lots of run. Now, if you kicked it down to the 4 bbls, it would eat gas, but that Holley was super efficient on the front barrels.

I would also suggest that you check your kickdown/vacuum valve on your tranny, as it should shift nice and smooth.

I know a lot of guys don't like Holleys, but with the newer gasket material these things are pretty bulletproof, and are quite easy to build and maintain, IMHO.

edit: BTW, with the small runner intake and the small valves of the 305, this thing loves to cruise and sip gas

[piecesparts]

The tranny lockup cpnverter may be a large part of your problem. if it is jumping around on the lockup, it may be causing you to get a major loss on economy. The carb will be another major hit, but it all is a combination of the setup. I would look at the replacement of the carb, timing, plugs, and wires for getting a good spark. Then look at what you can do for the 350C torque converter issues. That 2.73 is a defininte strike on you in my book.

[crawdad]

Originally Posted by piecesparts

The tranny lockup cpnverter may be a large part of your problem. if it is jumping around on the lockup, it may be causing you to get a major loss on economy. That 2.73 is a defininte strike on you in my book.

Yea, there is no definite speed at which it will lock up. It seems to do it at random, even at real low speeds. Sometimes while driving slow, it will lock up and you have to floor it to get the speed up, or sometimes I'll just hit the brake to unlock it. As far as the rear gears, since the tranny is not an overdrive, I would have thought the 2.73 would be a good thing. What ratio should I be looking at?

[piecesparts]

First of all those older converters locked up from a vacuum switch on the engine (if I remember right) The TCC switch has a vacuum line with a two wire plug on it, mine was on the intake manifold. I think my son's was on the firewall. That might be bad and causing your tranny to go haywire.

Like it has been said, sometimes there is too much of a GOOD thing. The 2.73 gear is great for a really low geared tranny and the size of tires will change that ratio in either direction, depending on how it figures out. If your motor is running at a low RPM while driving, and you are working it (accelerating more) to keep the vehicle rolling, then it is hurting you. If the motor is running easily (I call it the "Sweet Spot") then you are where you shouold be. The vehicle should be able to go down the road and run without sounding like it is idling and having to be revved up to go up an incline. The cruise control (if you have one) should move the vehicle down the highway without having to get the tranny to kick down excessively (passing gear) to go up and down inclines. Do you have an idea of what the RPMs are on you r truck while you drive at 60 MPH, or 70 MPH? If you do then you can calculate the gear ratio that you would want to acheive the RPMs you want.

Use the formula below to calculate the different ways you have to make your vehicle better. The trans output ratio for a 350C is one (1). The output ratio for a 700R4 is (0.68). The tire diameter is the height of your tire to the top of the tread. The rear axle ratio is the (2.73). The number 336 is the constant in the formula. This formula can be amnipulated to calculate MPH and tire diameters also.

rear axle ratio X MPH X trans output ratio X 336 = RPM
tire diameter

[stepside454]

last year my truck got 10.5 MPG on the hiway with a mild 454 T400 & 3.73s.
This year its up to 12.5, only change was new paint.....oh yeah & 3" taller rear tires LOL

[GoobersBlueBurban]

Does the truck have stock exhaust manifolds and stock exhaust pipes? My suburban was getting about 11 mpg in town and i put headers and 2 1/2" dual exhaust through Flowmasters it put it up about 13-14 in town. If you dont have a free flowing exhaust you might check into that. Also a carb spacer may help you get better fuel economy as well.

[jimfulco]

My '79 Sub has the stock 350 w/dual exhaust and with a 3.08 gear it got 10mpg, highway, city, sitting in the driveway, whatever, 10mpg. I changed it to 2.73, and it now gets barely over 11 on the highway, still 10 in the city, but I drive around town in D2 quite a bit now.

I'd say get your Q-jet rebuilt if there's a decent carb place near you. One of the local auto electric shops here has a good carb guy, so that's where mine get fixed. A Q-jet rebuild is ~$125 plus any extra parts needed (kit is included in the $125). I'd expect the Q-jet to get better mileage than any other carb, plus yours is probably already calibrated for your vehicle, and all the linkages, etc., already fit.

I'd keep the stock manifold, because it's designed for lower-rpm operation than the aftermarket ones, and as such, I'd expect better mileage from it.

[piecesparts]

Originally Posted by jimfulco

My '79 Sub has the stock 350 w/dual exhaust and with a 3.08 gear it got 10mpg, highway, city, sitting in the driveway, whatever, 10mpg. I changed it to 2.73, and it now gets barely over 11 on the highway, still 10 in the city, but I drive around town in D2 quite a bit now.

I'd say get your Q-jet rebuilt if there's a decent carb place near you. One of the local auto electric shops here has a good carb guy, so that's where mine get fixed. A Q-jet rebuild is ~$125 plus any extra parts needed (kit is included in the $125). I'd expect the Q-jet to get better mileage than any other carb, plus yours is probably already calibrated for your vehicle, and all the linkages, etc., already fit.

I'd keep the stock manifold, because it's designed for lower-rpm operation than the aftermarket ones, and as such, I'd expect better mileage from it.

I am guessing that after you changed the gears in your Sub, the RPMs are really low when you are driving around on the streets. You stated that you are driving around in D-2, that might be the clue that you need to get the RPMs up a little to make the motor more efficient. Idling is not ALWAYS the answer. It is how hard your motor has to work to keep up speed and accelerate. The taller tires will hurt performance also.

The factory intake for any vehicle will work, however the "Volumetric Efficiency" of those intakes are questionable at the most. The ones that the car manufactureres build for performance cars (specialty vehicles) are designed to flow very much better than your "Run of the Mill" intakes. Keep in mind that it is all what can be made cheaply, so that they can make a good profit. Look at each aftermarket manufacturer's info and you will see that they HAVE designed flow patterns to improve across the power band. Different manifolds are designed for higher or lower profiles and you can decide where you want to drive the most.