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Thread: Changed rearend and gears, lost E.T./MPH

  1. #1
    Junior Member APPRENTICE
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    Mar 2006
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    Baton Rouge, La.
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    Changed rearend and gears, lost E.T./MPH

    Swapped out the stock S10 (7.5") rear end with 3.73 gears for a Ford 9" with 4.30 gears and slowed down bad. The truck was running 11.40's @118 mph and is now running 11.90's at @111 mph.

    The combo:
    11:1 383 flat top shortblock
    Patriot 185cc "vortec" heads 2.02/1.60 valves Mild pocket porting
    Comp XE294H hydraulic flat tappet cam w/ 125 seat pressure, 310 open pressure springs
    Hurricane Single plane intake, Holley 750 (4779-7)
    Th350, ATI 8" Treemaster MRT 5000 stall
    27 X 10.50 hoosiers
    2950# Race weight

    The truck is shifted at 6500 and with the 3.73's, it was going through the traps at 5800-6000. Went with the extra gear hoping to get the trap RPM's 6500-6800. Would make a change to 28" tires if run out of gear with the 7K chip.

    Since the rear-end swap, the engine doesn't pull nearly as hard as with the 3.73's. Kinda feels like it starving for fuel.

    The fuel system is 3/8 line to a Holley blue pump, then stock 3/8 line to regulator. One side of regulator to dual feed line on the carb. That's being upgraded now to 1/2" line from the cell to the pump, 1/2 hardline to the regulator, and 3/8 line out both sides of the regulator--each going to a bowl.

    Any other ideas why it could have slowed down so hard?
    '78 Chevy Luv--383
    '68 Camaro-- 383 Twin 60-1 turbos
    '69 Chevelle--385 N2O

  2. #2
    Senior Member RACING JUNKIE
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    Sep 2007
    Posts
    4,143
    The ford rear I'd guess is heavier but I don't know how much.How did it 60' with the old rearend vs the Ford Rearend?.

  3. #3
    Senior Member RACING JUNKIE
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    May 2006
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    2,044

    Re: Changed rearend and gears, lost E.T./MPH

    Quote Originally Posted by poboyracin
    Swapped out the stock S10 (7.5") rear end with 3.73 gears for a Ford 9" with 4.30 gears and slowed down bad. The truck was running 11.40's @118 mph and is now running 11.90's at @111 mph.

    The combo:
    11:1 383 flat top shortblock
    Patriot 185cc "vortec" heads 2.02/1.60 valves Mild pocket porting
    Comp XE294H hydraulic flat tappet cam w/ 125 seat pressure, 310 open pressure springs
    Hurricane Single plane intake, Holley 750 (4779-7)
    Th350, ATI 8" Treemaster MRT 5000 stall
    27 X 10.50 hoosiers
    2950# Race weight

    The truck is shifted at 6500 and with the 3.73's, it was going through the traps at 5800-6000. Went with the extra gear hoping to get the trap RPM's 6500-6800. Would make a change to 28" tires if run out of gear with the 7K chip.

    Since the rear-end swap, the engine doesn't pull nearly as hard as with the 3.73's. Kinda feels like it starving for fuel.

    The fuel system is 3/8 line to a Holley blue pump, then stock 3/8 line to regulator. One side of regulator to dual feed line on the carb. That's being upgraded now to 1/2" line from the cell to the pump, 1/2 hardline to the regulator, and 3/8 line out both sides of the regulator--each going to a bowl.

    Any other ideas why it could have slowed down so hard?
    You didnt mention what rpm's you are trapping now, could be fuel starvation, as the motor should rev quicker.
    I seriously doubt the 3/8 is the culpriut but still not a bad idea, the dual lines coming out the regualtor should been from the get go however, all good upgrades.
    Now, back to the problem.
    Do you have jet extensions?
    Was traction a issue at the start?
    What are your 60 foot time compared to the 373 gears?
    The 9 inch gears are heavier and will absorb some HP/TQ which could of killed some ET.

    The motor overall may not be making that more power in the upper rpms like you suspected it is or would, might try swapping out the 430's with some 373's and see what happens.

    Cp
    ***IN GOD WE TRUST***

  4. #4
    Junior Member APPRENTICE
    Join Date
    Mar 2006
    Location
    Baton Rouge, La.
    Posts
    15
    I could see the 9" killing a tenth or so with the weight and efficiency, but that's a drastic loss.

    It lost some 60' also. Went fron 1.55 to 1.63. Don't have jet extensions yet, but I'll add them soon. Felt like it was breaking traction a little, trying to hop on the line.

    It was trapping right at 6500 with the 9"

    Engine was pulling to 7K easy in 1st & 2nd with the 3.73's. Now it don't seem to pull. I'm hoping the engine is reving faster and taxing the fuel setup. The new lines will tell.
    '78 Chevy Luv--383
    '68 Camaro-- 383 Twin 60-1 turbos
    '69 Chevelle--385 N2O

  5. #5
    Senior Member RACING JUNKIE
    Join Date
    May 2006
    Posts
    2,044
    Along with the fuel line upgrades etc, stick a set of jet extenions in for for the helluva it and holla back.

    Chow, Cp
    ***IN GOD WE TRUST***

  6. #6
    Junior Member APPRENTICE
    Join Date
    Mar 2006
    Location
    Baton Rouge, La.
    Posts
    15
    Changed out the fuel lines and went to a 28" slick. Truck ran an 11.55 @113 mph with a 1.53 60'


    Still missing some MPH/ET but the 11.40's/118 mph were in cold weather (40's) versus in the heat now (upper 80's) It's not laying down though so fuel was an issue. The 6500 rpm shift light is just coming on going through the traps.



    I'll get those jet extension this week
    '78 Chevy Luv--383
    '68 Camaro-- 383 Twin 60-1 turbos
    '69 Chevelle--385 N2O

  7. #7
    Senior Member RACING JUNKIE
    Join Date
    May 2006
    Posts
    2,044
    Heat and humidity play a huge factor in the ability of an engine to make HP and that in turn affects ET's.

    My car will lose a tenth in hot humid weather verses cool dry air.


    Chow, Cp
    ***IN GOD WE TRUST***

  8. #8

    Re: Changed rearend and gears, lost E.T./MPH

    Quote Originally Posted by poboyracin
    Swapped out the stock S10 (7.5") rear end with 3.73 gears for a Ford 9" with 4.30 gears and slowed down bad. The truck was running 11.40's @118 mph and is now running 11.90's at @111 mph.

    The combo:
    11:1 383 flat top shortblock
    Patriot 185cc "vortec" heads 2.02/1.60 valves Mild pocket porting
    Comp XE294H hydraulic flat tappet cam w/ 125 seat pressure, 310 open pressure springs
    Hurricane Single plane intake, Holley 750 (4779-7)
    Th350, ATI 8" Treemaster MRT 5000 stall
    27 X 10.50 hoosiers
    2950# Race weight

    The truck is shifted at 6500 and with the 3.73's, it was going through the traps at 5800-6000. Went with the extra gear hoping to get the trap RPM's 6500-6800. Would make a change to 28" tires if run out of gear with the 7K chip.

    Since the rear-end swap, the engine doesn't pull nearly as hard as with the 3.73's. Kinda feels like it starving for fuel.

    The fuel system is 3/8 line to a Holley blue pump, then stock 3/8 line to regulator. One side of regulator to dual feed line on the carb. That's being upgraded now to 1/2" line from the cell to the pump, 1/2 hardline to the regulator, and 3/8 line out both sides of the regulator--each going to a bowl.

    Any other ideas why it could have slowed down so hard?
    Tighten up the torque converter.... to 4000 or so...I think you'll have it fixed....Good luck...MileHighMan.

  9. Racing Junk


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