6 mpg... please help

[qu1cks1lver56]

Originally Posted by 3r!c84

with what tranny? th350
i got that on mine and the same gears,at 65 its 3100

th350. like i said, i dont have a speedo right now, but judging by keeping up with traffic i should be around 70.
i have stock size tires too.

[Hoods69BadBowTie]

I would go with some 3.73s if you can find one. I would just swap the whole rear end out! I forgot to mention I don't think your motor combo is the best either. I don't think the 305 heads are the way to go I would also look into a intake better suited for your driving just my .02 cents.

[arkracing]

Sounds like an overall bad combination of parts. I'm not trying to be a jerk, but the motor combo is probably hurting you quite a bit. The 305 heads will give you more compression, but the reduced flow and valve shrouding kills any performance gains.

I believe that the edelbrock intake manifold that you have is an older "Single Plane" model, that really only works well in a racing application that sees RPM ranges over 3k all the time. The intake will give you poor performance from idle - 3000k. That might be why it feels "slugish"

The 2.73 gears aren't helping either - so I agree with everyone on that.

Again, I speak from experience on the motor end of it. I had a 355 with small chamber heads like that, and I thought that the motor should have had more.

See if you can find a used edelbrock performer intake ($25-$60 ish)
Make sure your vacumm advance on the distributor is working properly as well.

Keep us updated on what you do, and if it helps or not.

[sav300]

See if you can find 3.08-3.42 gears IF you are going to keep the TH350 for a quite a while. However, if you think you can get ahold of a 700R4 or another over drive tranny I would go with the 3.73 gears.

I have a 355 with Edelbrock performer intake, matching cam, 10.86:1 compression ratio with my Fast Burn aluminum heads(58cc and yes they are off of a corvette), 650CFM Edelbrock carb with carb spacer and custom jets and needles, going through a TH350 into 3.73 gears. I get 10-14mpg in town depending on my foot and 14-18mpg on the highway depending on what speed I decide on. 55mph for me is 2100RPMS and I think 75mph is somewhere around 2800-3000RPMS.

Where everything mates up will dictate alot! A 3000-6500RPM intake manifold wont do as well intown as a 1500-5500RPM manifold.

Also you mentioned new, how fresh of a rebuild do you have??

Thomas

[MylilBowTie]

If your speedometer is off you wont get and accurate mpg reading or odometer reading.

[midnite]

i get around 10.5mpg in town with mine
350/th350 4x4 4:10 gearing and 31" tires in 2wd with front hubs locked

[Manbearpig]

Originally Posted by *****in85

Not sure about compression ratio but its gotta be up there

Probably around low 8.3-8.7:1
Depending on what pistons you used.

[ironroad9c1]

Originally Posted by Manbearpig

Probably around low 8.3-8.7:1
Depending on what pistons you used.

Figuring if he kept the dished pistons, 305 heads usually spec out to 56 cc not 58 as advertised, typical .041 felpro head gasket stock bore, and a conservative guesstimate of .015 piston to deck height hes looking at about 9.3 to 1 CR not bad, but that edelbrock carb sucks find a good qjet and don't forget to figure out how far off your odometer is between stock tire size and the tires you are running now, and add that to the miles traveled . If you are running a single plane intake, rip it off and get a dual plane, Single plane intakes have no place on a street driven vehicle.

http://www.powertvonline.com/forum/showthread.php?t=964

[sav300]

Originally Posted by MylilBowTie

If your speedometer is off you wont get and accurate mpg reading or odometer reading.

MylilBowTie is correct. If your speedometer is off it can give you a completely different reading depending on your tire size. Some speedometers reguardless of tire size are slow to begin with. Just a thought for you to consider.

[midnite]

if you have a gps, fill up and take it with you on a ride they all have odometers on them, then fill up when you get back... thats how i did it

[asphaltburner]

Yo! Fill,...drive a know distance and back, and fill again. I've had to do this with several vehicles. Working odometers or not,...and/or tire size changes.
Always works for mpg calculations.
Todd.

[asphaltburner]

Yeah, I run 4.11:1 rear gears and 33" tires,...FRANKENTRUCK is a 4x4 3/4 ton TH400/NP205 with a 2.02/1.96 67cc chamber heads, Crane cam, and 600 Holley. I get 12mpg regularly.
Ignition timing should be 2 degrees below predetonation at full throttle, and tune your carb to about 1/3 turn less than highest idle (Holley should be about 7/8 of adjustable range).
Todd.

[piecesparts]

With the gear ratio that you have and the size of tires, I would stay away from the OD tranny for now. You will drive your gas mileage into the trashcan, by making the eninge work harder to do the basics. We put a 700 tranny in with my son's truck with 3.08 gears and we lost a lot of mileage numbers---We dropped to 10 MPG, with the engine running at 1700 RPMs at 65 MPH. Too much weight to keep rolling and the eninge has to make up for it. I changed that over to a 3.73 gear rear end and the numbers came back to 14 MPG. You need to get your cruising RPMs into the power curve of the engine. Usually that is around 2000 to 2300 on these engines.

The 355 engine with 305 heads can be part of your problem, #1--did you do anything to open up the flow capabilities of the valves when installing the 305 heads? The streetmaster intake and the 500 CFM carb????? The build that you did, sounds like you pushed your compression up and then squeezed off the air and fuel into the engine. Which makes the driver work the engine harder to get the RPMs up and the truck rolling.

I am running a 383 stroker engine with a Performer intake and a 650 AVS Edelbrock carb and I get 14.5 to 15 MPG. All of this along with a custom roller cam and Dart II heads and 3.73 gears. The trick is to match what you have and what you do for most of your driving. (Highway or city)

I agree with the others, also look at your timing for this engine. If it is retarded too much, it will cost you fuel mileage and run the engine too hot. What do you have your timing set at? EXAMPLE: I run mine at TOTAL advance from the dizzy and that has pushed my initial timing quite high. In your case, try setting your timing advanced from the factory timing to 12 or 15 degrees. Using a vacuum gauge to set your idle air ixture is a great way to get it optimized. If you buy a timing light---I found out that Sears sells a great internal advance timing light for a great price. I have had mine for a long time.

What do you have for an exhaust system? What size of piping and what mufflers do you run?

[ironroad9c1]

See my post above to solve your problems, the 305 heads have nothing to do with it.

[bitchin85]

So I'm gettin about ten now. I bought a brand new gastank because mine started leaking real bad.. And I put a mallory distributor in it. That helped a lot. Also I bought a 3.42 rear end out of a suburban today for 75 bux.. Its a ten bolt. I asked the guy and he had no clue, is there a chance it came stock with posi? I should be putting the rear end in this weekendsome time after I clean it all up and paint it.

[Chlsnk]

Originally Posted by sav300

I have a 355 with Edelbrock performer intake, matching cam, 10.86:1 compression ratio with my Fast Burn aluminum heads(58cc and yes they are off of a corvette), 650CFM Edelbrock carb with carb spacer and custom jets and needles, going through a TH350 into 3.73 gears. I get 10-14mpg in town depending on my foot and 14-18mpg on the highway depending on what speed I decide on. 55mph for me is 2100RPMS and I think 75mph is somewhere around 2800-3000RPMS.



Thomas

Tell me about these fast burn heads. What years are they found on CC's? You know basic specs. Sorry not to hijack this thread.


Those 305 heads and the rear are definately killing you but then again I think thats already been covered.

[bitchin85]

Did an 84 burban ever come factory with a ten bolt 3.42 posi? If so, is it rare?

[3r!c84]

my naber got a 95 1500 rear end with a 3.42 posi

[gchemist]

Did you tune the carb? Everything else is not helping until you tune the carb properly.

[jitteryguy]

its the 305 head's , they work good on a 305 , but on that 355 there kill'n it . I got a stock 305 with a edlebrock carb in an S10 , it get's almost 20mpg with 373 gears and 275/60 tire's .

[MR .T]

your carb might be jetted a bit fat.

[bitchin85]

Paid 75 bucks for a complete 3.42 3/4 ton rear end. Got it all finished last nightwithnew brake lines fluid and axel seals. It takes a lotless to getup to speed. The only problem ihave now is my foot likes the gas pedal a little more nowand iwas testdrivingit on a bumpy country roadand now I needa new windsheild. If it aint one thing, its another.. Lol

[bitchin85]

Well I was right back to 6 mpg.. Still.. Like clockwork.. I tried tuning my 1406 eddy and did the thing where u back the idle mix screws out till you have the best vaccuum and I got a timing light and set the timing to 6 degrees. Is the timing off? Its sluggish again and the mileage blows. It seems like ill tune it up and it runs awesome for a week or so then right back to bad. Also it has the stock distributor so the weights and springs are stock. What should I set my timing at? Any other suggestions?

[piecesparts]

May I ask first---what type of distributor are you running on your engine? If it is the stock computer controlled one, then there is NOT much you can do here. If you have changed it out for an older HEI dizzy, then you can change the weights and springs inside it for better acceleration curves. I do suggest that route.

Once again set the carb idle mixture and then set your timing at 12 degrees to 15 degrees BTDC and let it go from there. The lower settings makes the engine work harder at times of heavy use.

[bitchin85]

Its got a stock 83 hei distributor. And what's the curve timing? Is that the timing at a higher rpm? I'm new at this but I know the timing at idle is 6 degrees. And with the weights and springs, where do I start? How do I know what ones to put where? Sorry if this all sounds dumb.I have never played with distributors yet