[71gtx]

I would possibly like to add 4.11 gears in the rear end of my 71 gtx. Does this make it a posi, or does that have nothing to do with it? Also, how much power increase would the 4.11 give? And are there any other modifications needed to allow the 4.11 to work, such as in the trans or engine? And will gas mileage go down or work the same?

Thanks a bunch

[willys36@aol.com]

4:11 gears just means the drive shaft will turn 4.11 times to spin the wheels 1 time. This compares to what you have now, assuming a 3:54 which spins the tire once with 3.54 turns of the drive shaft. Gear ratio has nothing to do with positraction. Positrack refers to devices that are installed in a rear end (regardless of the gear ratio) that locks the wheels when going straight ahead to prevent one tire losing traction and spinning uncontrolled.

Changing ratios will not in itself gain you any more horse power. It may result in a little more power by putting your engine RPM more in its power zone, but the gears themselves don't affect power. They will give you more torque at a given engine speed though so the effect is easier quicker early acceleration. What you lose is top end speed, unless you turn your engine faster at a given mph.

Using the 4.11/3.54 example, starting line torque will increase 4.11/3.54 = 1.16 or 16% increase. On the other end, assuming you red line the engine with both gear sets, top speed will decrease by 3.54/4.11 = 0.86. If your top speed was 100mph based on engine red line with the higher gears, it will reduce to 86mph with the taller set.

Higher RPM capable drag racing engines benefit from taller gears. It allows the engine to rev higher all through the car velocity - better takeoff, and mid range power and hopefully, you reach the 1/4 mile stripe B4 you red line. Roundy-round racers want high gears so they can mainly go fast - they aren't as worried about raw acceleration. If you have a radical engine that has been hampered by high gears, the lower set will likely get a little better mileage. However, any warm street engine will suffer in gas mileage with taller gears. But you can smoke the hides easier at stop lights so it is probably worth the extra cost!!

[71gtx]

thanks for your help. so you're saying that i can possibly add up to 48 horses or torque off the line if i change to a 4.10 or 4.11? because usually i do not even go past 60 mph on this vehicle. however, will the new rear end put more wear on the engine due to the constant higher rpm's?

[gt2betubbed]

No offense 71, but you didn't read willy's post well enough.

Changing ratios will not in itself gain you any more horse power

Taller gears let the engine rev faster which makes take off faster. It's like taking off in second from a stop from the get go, the one day you use first to take off (swithing to taller gears). It takes off faster, but doesn't add power or torque.

I wouldn't worry about more wear on the motor, unless you're cruising @ rpm's over 3000. 3.73's are more of a "safe" street/strip gear. 4.11's on the street is doable, but puts the rpms up there on the highway.

[willys36@aol.com]

Originally posted by 71gtx
thanks for your help. so you're saying that i can possibly add up to 48 horses or torque off the line if i change to a 4.10 or 4.11? because usually i do not even go past 60 mph on this vehicle. however, will the new rear end put more wear on the engine due to the constant higher rpm's?

You will get more torque, not more horsepower, but at a slower speed. The equation for horsepower is,

HP = Torque (ft-lb) X speed (RPM) / 5252.

Your engine puts out a fixed amount of horsepower so if you increase torque, you must decrease speed which is exactly what installing taller gears does.

[DragTour69XL]

Like gt2betubbed said, go with the 3.75's and a posi rear. Up here we pay 600+ for a 9" diff like this.

[71gtx]

if i decided to add a 3.91 to the rear, that would give me more torque and such right at the line. however, that would also make the torque at higher speeds go down, so if i put an excellent cam made for roughly 3,000 to 6,500 rpm would they balance out and create excellent power from idle to 6,500 rpm???

would it help to add a torque converter too?

what type of cam would be needed to balance that out???

[willys36@aol.com]

As we have discussed many times on this board, setting up a car is a series of compromises. Set one up for easy cross country cruising and it will be a relative dog on the drag strip and vice-versa. The first thing you need to define is where will this car be used? Drag strip? Street/strip? Friday night Sonic Drive-In cruiser? Cross country cruiser? Daily driver/ family car? Each type of car has a different OPTIMAL setup. There is no way your questions can be answered until you say specifically what you want the car to do. Your 3.91 gear/3000-6000rpm cam/hi-stall torque converter car could do well at the drag strip with quite a few more modifications but would not be a very nice car to drive on the street.

[71gtx]

what would you suggest for the street? i am not looking so much for complete drag, but more-so a street/strip cruiser. able to have amazing torque off the line, and able to just cruize in whenever i feel, so i can drive it on certain nights, and if i see a car i'd like to smoke, i can do so.

[willys36@aol.com]

Totally depends on your engine as I said above. Need to consider the entire system. Assuming you have a +300hp 340 or 383, You could start with a 2000-2200 stall converter & 3.54 gears and positraction. Headers, Performer Plus manifold, 650cfm carb, 260-280 deg cam would all be things to consider for a tire burner that would cruise like a kitten.

[65 Imp SS]

If you get out on the hiway at all I would avoid the 4.11's