Bad gas mileage LS swap

[sixtoes1313]

Never really cared much about it before, but now that I have my speedo hooked up and calibrated and can calculate my mileage its looking terrible at best. I haven't been able to get above 10mpg usually 6-8 with fun throttle input. All the tanks calculated have been stop and go suburban driving (small town ish?) Just curious if this is normal or somthing to be concerned with!

99 vette ls1 with fuel injected magnacharger set up (8lbs around 480hp)
th400 trans with shift kit and 3200 stall converter
9" rear with 3.50 gears.

I'm thinking between the three speed and large converter I'm loosing alot of fuel economy. I have a smaller converter I could swap in if I was curious enough
Would the big 9" Effect mileage significantly vs a 12bolt or 8.5?

Another thing is the car hasn't been dyno tuned. been running on a mail order from Steve Williams (FROST) Which does great. Runs fantastic, plenty of power, doesn't smell rich or idle abnormally. I have no codes in the pcm, fuel trims are normal and o2's are switching normally. Any idea's? I usually daily drive this and I'm just curious as to what you guys experience for real world city mpg's
http://www.montecarloss.com/community/ubbthreads.php?ubb=showflat&Number=633338#Post633338

 

[StreetBu]

The 3 speed, stall converter, and gears are killing your mpg. The rear end may be making a SMALL difference but not enough to matter. You really need an overdrive with a lockup converter and that will make a huge difference. The blower doesnt help either BTW. I would guess with overdirve you should be able to get around 12-13 around town.

 

[MalibuRacing]

Yep, the converter, trans and gears are killing the MPG. You should look at getting a 4L80E, which is as tough as the TH400. You really don't need that much stall with a roots type blower, they make great torque down low.

I think the overdrive and a slightly tighter converter with lockup will dramatically improve your MPG's.

 

[vickysbu]

When you decided on the setup, what was your goal for the build? Did you build it to be a cruiser, or a bruiser? Whats the duration of the cam, that could be killing mpg also. Either way, a dynotune is definitely in order.

 

[vickysbu]

Check out this threadhttp://ls1tech.com/forums/general-lsx-automobile-discussion/573699-stalled-camd-ls1s-whats-your-mpg.html

 

[sixtoes1313]

I was originally building a bad@ss daily driver. Expecting around 10 city 18 hwy when it was all said and done. I was kinda pressured into the th400 trans and many times have regretted it. I actually have ALL the pieces to swap a 4l80e in including a stock 4l80e trans...just dont wanna tear it apart again anytime soon (Had the 400 out 7 times with issues) Maybe this winter I'll tackle the project. All I need is a different converter to match my flywheel. then theres getting the PCM reflashed, shortening the driveshaft and other problems that arise when changing out big components. I have a lokar shifter, Harness adapter, iceman Xmember, and trans...just no motivation to disable the car for a whole season AGAIN!

 

[vickysbu]

The other easier but more costly option is to install a GearVendors unit, then all you'd have to do is cut the ds.

 

[sixtoes1313]

I'll probly go t56 or 4l80e...I feel t56 would yield the best mileage and probly be more street fun...but also have a funny feeling that I'd end up doing alot of clutches! 480hp and not so friendly driving habits!
You guys think a tune would help that much? How much power gain would be expected? 10-15hp?

 

[Mean87SS]

Seems low no matter what...

Theres some real good clutch companies out there now...

 

[1320chevy]

i'm getting 21 on the interstate. LS N/A, t56 4.10 but the .5:1 6th gear is the big thing, your 3rd is 1:1, equivalent to my 4th. but if your thinking of swapping, never use a stock clutch, mine ate three before i got smart, and over 5-5.5k rpm it throws the alignment out and gets stuck in gear. overall i'd stick with what you have, and save up for a gear vendors overdrive, although expensive they are amazing.

my opinion is simply your stahl, with a low lock converter, your controller wont have to compensate for your s/c so much, at 3k your entering your powerband when it compensates the most where beyond 3k its fluent.

swapping the stahl converter is a bit of a pain everytime you want to go thrash about but its all on preference.