TBI mileage and power question

[DaveB]

On my maiden voyage to Moab in the Commando I got lousy mileage (10 MPG) and the power from the TBI setup was anemic. What kind of gas mileage are those of you running a TBI on a 350 Chev engine getting ? and is your power good ? The power I am getting is not much better than what I get from my 4.0 powered Jeeps that get 2x the mileage. I have been in contact with the guy at TBIChips.com and he thinks my exhaust is too restrictive. I used two inch pipes from the manifolds and collected them into a three inch collector that then goes into a 2.5 inch system. The outlet of the manifolds is 2 or less so I felt it would do little good to go bigger. The cross-sectional area of a two inch pipe is 3.14 square inches and the 3 inch collector has an area of 7.06 square inches so that isn't restricting the flow of the two pipes. I am going to look into some block hugger headers and change the connection to the muffler to 3 inch (it has a 3" flowmaster) to get better flow. I did use a stock TBI manifold which I know doesn't flow that great but I didn't want to go with a custom chip that would be required if I used an adapter plate on the original 4 barrel manifold. it seems that if I improve the exhaust flow that the intake needs to improve too or it doesn't help that much.
What do you think ?

 

[mbryson]

That seems like it's quite low to me. I've never calves mileage in my Jeep, but in the Suburban donor , I'd get an around town worst of about 12-13 and get about 15-17 on the highway.

Russ Fullmer had a Howell TBI swapped on his Jeep 4.2l that got horrible mileage. It seemed to just dump fuel into the TBI throat always. I don't know that ever got resolved.

 

[DaveB]

The commando passed emissions with ease and the plugs look like the engine is getting a clean burn so I am perplexed as to why it is so low. I would be pleased with 15-17 on the highway. I did make the commando a bit heavy with the skid plates, roll cage, bumpers, and such but I can't imagine it is close to the weight of a suburban.