MPG Question

[mholmes]

I've got an '82 Scottsdale 4X4 that I just dropped a new stock 350 engine from JEGS into. This one is rated at 260 HP. Also with the kit I got the Edlebrock 650 CFM carb. Truck has the 700R4 transmission in it also. I'm in process of replacing the gauges as I broke off 2 of the needles when I was putting in a new circuit board trying to determine why gauges don't work. Anyway, what kind of MPG should I expect with the new engine. I've gone through about 4 tanks of gas with the new engine and I'm guessing I'm getting about 8-10 MPG. I'm waiting on a new speedo to compliment the new engine so I can't be precise but I was thinking I could expect around 14-16???? Am I expecting too much?

[slugish]

Im guessing your carb. is too large for what you have . with a 700r4 you should be getting 14-16 mpg at least & if you have the stock rear gear ratio around 2:73 Ithink , you should get 18-20 depending on your driving habits of course. there is a calculater on holly.com & other web sites , that you can compute the correct carb for what you have based on a proven formula.
For instance I have a modified 408 ci engine in my truck with a holly 650 cfm . I did the calculation , plus I talked to a lot of engine builders & both said 650 cfm is where I shoulsd be . An over carberated engine can create sometimes more problems tha one that is under caberated.

hope this helps

[slugish]

Ididnt know carb came with the kit . Iwould call JEGS TECH HELP , . i May be off base here / maybe they can tell you how to retune or rejet it . for better miliage

[piecesparts]

Originally Posted by mholmes

I've got an '82 Scottsdale 4X4 that I just dropped a new stock 350 engine from JEGS into. This one is rated at 260 HP. Also with the kit I got the Edlebrock 650 CFM carb. Truck has the 700R4 transmission in it also. I'm in process of replacing the gauges as I broke off 2 of the needles when I was putting in a new circuit board trying to determine why gauges don't work. Anyway, what kind of MPG should I expect with the new engine. I've gone through about 4 tanks of gas with the new engine and I'm guessing I'm getting about 8-10 MPG. I'm waiting on a new speedo to compliment the new engine so I can't be precise but I was thinking I could expect around 14-16???? Am I expecting too much?

Let's start with your engine install---You put in a 350 stock rated engine that is about 260 HP. That is basically the same motor that GM sells as their replacement motor, so far you are good. Now you say that JEGS sent a 650 CFM Edelbrock carb with the kit. If it is a "Performer carb", and NOT a "Performer AVS" (Thunder carb) then it is a 625 CFM carburetor not a 650 CFM unit. That is a good size for a standard 350 motor. You are not over carbureted.

What size rear end gearing and what size tires are you running on your truck? You say the motor has gone through 4 tanks of gas--does it act like you are running sluggish? If so, what is your timing set at on your motor and what kind of vacuum signal are you running on the intake manifold? There is a great chance that you may need to advance your timing to get into the proper power band. I run my truck at 35 degrees TOTAL ADVANCE with my stroker motor and I run my son's truck with the GM replacement 350 motor and an Crane aftermarket cam upgrade at 31 degrees TOTAL ADVANCE. Our trucks have 3.73 gears and tire sizes that puts us in a range of 2500 RPM at 70 MPH. The son's truck actually got really bad gas mileage (11 MPG) with a 3.08 set of gears in it and a 700R4 tranny, so we changed the rear end gears to the 3.73s and GAINED MPG. He now gets that 14 MPG and I get 15 MPG with my 383 stroker, and I don't drive slow.

You don't need the speedo to find out what you are doing--although it helps. Use mile markers on the highway and use a tachometer to figure your speed. Take a watch or a stopwatch and figure how many seconds to do a mile and then work from there. EXAMPLE: at 60 MPH it will take 60 seconds to go a mile. If you do the mile in less than that, divide that number in seconds into 3600 and see how fast you are going. With this basic set of numbers you can calculate your mileage and speed.

[mholmes]

I'm running the stock tire size on the truck, 15" I believe. I will have to check on the gear ratio but it should be stock also. One other thing I will note is that I have been driving it very conservatively, no hard accelarations etc. I am not sure on the timing as my stepdaughter's boyfriend's dad set the timing on it during the install. Is it possible that I might have a vacuum line hooked up incorrectly that might be contributing to the poor MPG ? He hooked those up also and I have no clue on that part.

[piecesparts]

The stock size tire is OK, but what size are they? If you have the tire size or the overall diameter measurement, we could give you an idea what your RPMs should be at different speeds. Yes the vacuum lines could make some difference on the hook up. One port on the carb is direct to the manifold vacuum and the other is orficed to allow it to time the distributor advance in slowly. Did you get a book on the Edelbrock, in the kit? If so, it will tell you which port is timed and that would be the one you need for a stock setup. I have set my stuff up to advance early and I use the non-orficed port on mine, but my distributor is built for that function. I still su[p-port the checking of the timing, if it is to low in the setting, then you will have a couple problems---sluggish response, poor mileage, engine running hotter than it needs to, and bad mileage.

What type of distributor do you have on your engine? Is it still the GM computer controlle monster that wasin the truck or have you changed out to a HEI unit? If you haven't changed to HEI, the truck's computer may be confused on the fact that the carb is different than what used to be in there. My suggestion would be to put a good HEI distributor in and take the computer connections out of the circuit. Go back to the basics and eleminate the pain.

Things to be checked for efficiency improvement--Carb adjustments (idle mixture, hose connections, idle RPM); timing setting (try setting it at 12 degrees BTDC intial timing setting, WITHOUT the vacuum line connected to the distributor); sparkplug gaps (.035 to .040 your choice); what temp T/stat is installed--look at the idea of a 180 degree Superstat; what kind of air cleaner are you running--it could be restrictive in design--suggest and open style cleaner. Are you running the stock exhaust system, which is resrictive.