97 z28. lt1 rebuild with only 80k miles, lt4 swap, lt4 hot cam, 1.6 rr, no cats, no egr, cai, 6 speed.. 3.42s and im only barely squeezing out 20mpg. Why is this?

 

Who tuned if for the modifications?

 

was told it was custom pcm ordered from gm that was meant for lt4 swap... but thats what i was told.

 

Other things that might cause poor fuel mileage:
-faulty O2 sensors
-exhaust leaks before the sensors
-misfires
-fuel pressure regulator leaking into vacuum compensation line

 

what do you estimate what my mpg should be around or close to? i mean im getting 20mpg coasting, and using 6th gear when ever possible!

 

When my Formula was stock (M6, 3.42gears), it ran as high as 30MPG at a constant 80MPH on the CA freeways. I never really checked it on the highway after the stroker build, but just a normal mix of local and highway driving was producing 20MPG. That's with the M6 and 3.73's, with an engine making 500HP at the flywheel, and decent solid roller cam.

But it was very well tuned by the guys that built the engine.

 

my point exactly, my 79 trans am, 383 with 5speed can still work out 20mpg on highway. I need help finding someone near columbia sc or aiken south carolina who can check or mod my tune. I think thats whats causing my high idle (@ 1000rpms) as well.

 

get a real tune. either talk to a mail order tune guy of your choice or take it in for a dyno tune if you got the cash. who knows how your computer is tuned.

 

High idle= sucking air == more gas spent. You have a vacumm leak somewhere....Use the K.I.S.S principle when working on these cars....

 

Are you lugging it at lot?
The "custom GM pcm" smells like bull.
with the .50 OD you want at least 3.73s and even 4.10s should be fine by almost any driver.

 

Lol I get 8-10 highway. A4 with 4.10's and 3000 stall. And I run stoic when cruising

 

I get up around 19-20mpg on mixed highway and racing tanks of fuel. 3800/4.10s, over 4200lbs in "race" trim, probably pushing 44-4500 on the street with the slicks and tools and all in the trunk and the heavy street rubber on.
About 17-19mpg on my rural work commute 55mph with stop signs every few miles.

 

im running 93 (premium) and I have a 6speed with 3.42s...

What do you mean am I lugging it a lot?

 

He means are u putting around town at 45 in 6th gear

 

Negative. I only will put it in 6th gear if im atleast running 60, most of the time im just switching to 5th gear at about 45-50mph.