Love my new engine/trans;hate the MPG!

[Scott Liggett]

I have a 383 that I built with BluePrint's 195cc heads. it has a far bigger camshaft than the one BluePrint uses in their 430 HP 383. It's in a 67 GMC with a A833OD trans, with near identical gear ratios to the 700R4, 3.73 geared 12 bolt. I get 16-18 mpg on the freeway doing about 2500 rpm, 75 mph.

I work at BluePrint and talk to customers all day, and I can tell you that our customers with the 815 HP 632 with a 3 circuit 1250 Dominator are getting better than 8 mpg. So, you have something wrong. Start by looking at the AFR's on your FAST EFI's system. You should be able to see that number on the handheld. Cruising AFR's should be in the 14's, WOT should be in mid 12's. Anything below that and your EFI system needs tuning and that is killing your mpg. Also, if you have any kind of exhaust leak between the engine and the O2, it will give a false lean signal to the ECM and the EFI system will over richen the fuel mixture.

A simple test is to pull the spark plugs, if they are black and sooty, it is running rich. Also, if you oil smells like gas, then it is running WAY rich and you need to fix the EFI settings ASAP before you fuel wash the cylinders..