95mysticta

iTrader: (4)

Just throwing the idea of converting a carbed 455 poncho to a FI LS 5.7 or 6.0 and keeping the four speed setup untill i can afford to properly convert to six speed. The 455 isnt the greatest on fuel and i know going to FI LS swap will help out on fuel economy but will retaining the 4spd and spinning the higher rpms on the highway make much difference? Just want to know if its wiser just to wait do a ls/6spd swap all at one time?

99Bluz28

iTrader: (35)

What's the rear gear size?
What rpm was it running with the 455 in 4th at 60MPH?

95mysticta

iTrader: (4)

Stock 3.42 rearend

I can't remember exactly but its above 2k that's for sure prob 25+- realisticly.

S10xGN

iTrader: (11)

With a fairly tall 275-60x15 tire, 60 mph is 2465 RPM's, a shorter tire will rev higher...

1994Z28Lt1

iTrader: (1)

With a carbed 455 its going to be more important to get the carb in perfect tune. Get yourself a wideband and a box full of jets, metering rods, springs etc for whatever carb you intend to use and get the air/fuel ratio correct at cruising speed. With the kind of rpm you'll be turning cruising around I'll be you could get away with a slightly lean mixture until the secondaries open.

murphinator

iTrader: (2)

any efi ls engine is going to be a vast improvement in fuel economy with the same drivetrain behind it as the carbed 455 poncho.

I have thought about it a lot , I would love to have an ls in one of my old ponchos , probably pull the 400 out of this one eventually , pontiacs can go good built right but they are heavy pigs when it comes to having to fuel them up

DaleTx

I ran a 4 speed Muncie after swapping from a small block to an LS engine. My car has 3.73:1 gears and a 25.6" diameter tire. At 70 mph the RPM was about 3,400. Your set up would be better with the 3.42:1 gears.

I ran the Muncie for a year before swapping to a T56 and it worked out fine.

garys 68

iTrader: (1)

5.3, T10, in a 72 cutlass with a 2.73 rear, about 22mpg as a daily driver. Figure about a 10% decrease with your rear end. The original 455 was about 10mpg.

95mysticta

iTrader: (4)

Unfortunatly its not cheap to convert these older cars to new school all in one shot. Thats why i was contemplating just doing the motor and running the 4spd behind it for a year or two. The gas mileage right now is bad and even tuned im sure its not going to make that big of an improvement. Thought about putting a T56 behind the 455 but its not cheap to buy all the conversion parts from Keisler and Mcloed that id be better off just waiting to do the motor and trans swap. I do not want to swap to 2.73 or 2.56 gears either as im sure it will take the fun factor out of the car as it will be a turd.

garys 68

iTrader: (1)

Go for the 4 speed, but also consider a Richmond 5/6 speed later on. It's a direct swap, reuses the driveshaft, bell, clutch, and clutch linkage. Tremec 5 speeds are another option, it will require a new driveshaft. Just use the 1" setback plates to keep everything close to stock location.
Keep in mind the T56 will probably require a hydraulic clutch, engine moved forward, modified shifter, new crossmember, clutch, and driveshaft.
Btw, my cutlass uses a 3.42 1st gear T10 with the 2.73 rear, almost an ideal 1st gear overall ratio. Great cost effective combo and gets good milage. Might consider a Richmond 6 speed later on.