Rebuild Engine= Terrible Gas Mileage! HELP

[CountryBoy1630]

Rebuild 5.9 Magnum, bored .030 over, keg intake with the reduction plates welded in along with the plenum plate welded, UTP 450 Cam, stock injectors, new heads, shorty headers, true dual exhaust.

Custom Comp Cam from a guy named Marty Fletcher. Cam specs-

Advertised Duration 273 intake/273 exhaust single pattern high torque

Duration @0.050 215 intake/215 exhaust

Lobe Separation 108 LSA

Lift Intake and Exhaust 0.284" (0.455" with 1.6 roller ratio)

54 Degrees overlap


I am getting like 7 MPG and I can not figure out why. I used to get around 14 when it was bone stock with a leaking plenum and cracked heads. Now that I rebuilt it and made it all “right” (I hope) it gets terrible gas mileage. Is anyone else running this cam? If so what kind of fuel mileage are you getting? The truck is still pretty stock, I mean the only real big changes I made were the cam and the intake and bored it out .030 over. I can’t afford to drive this if I’m having to fill up ever 100 miles.‍ Any help or suggestions will be greatly appreciated.

 

[Nick_rp]

Those are some serious mods, I'd expect a drop in mpg.

Could also be that your loving the new performance so much your foot is a little heavier

 

[Mike Murphy]

If the information is correct I see that factory intake duration is 249 and the exhaust duration is 269. Yours looks to be open quite a bit longer. More air = more fuel. There may be other influences but over all like Nick_rp stated, those are some serious mods. Fuel MPG's are going to decrease.

 

[Yeret]

Have you had the computer tuned? The factory tuning was "updated" in a half-assed attempt to fix spark knock, and it trashes performance with a stock engine, let alone a modified one.

Are the heads OEM spec? Aftermarket cams typically don't add much to our engines when you use such heads and the kegger. They're set up for lots of low-end torque rather than high-end horsepower.

That cam would work great for a street pounder with Eddy Performer heads and Air Gap intake manifold, both of which flow MUCH more volume than our factory stuff.

In my opinion, you've got a conflicting setup. You're running a brute low-end torque intake manifold with a "hot rod" camshaft. Also, there are a few veterans around here who have run dual-pipes and regretted it due to the loss in low-end torque.

How to proceed really depends on what you want. Since you're obviously looking for more mid-range power, I'd recommend looking at a fuel-injected Hughes Air Gap intake manifold and then sort out a custom tune to match it to your cam.

 

[JohnnyMac]

My 2000 5.9 (4x4 lift/35's) got 10 all day long, up hill, down hill, tail wind, rain or shine. Towed my boat up the AlCan to Alaska and got 5 the whole way. All those mods and I'd be happy as a bird with a french fry at 7mpgs....