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Old 07-28-13, 06:20PM   #1
T/A 1975
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Default Gear swap

Hey guys,

Not sure if I am posting this in the right place, but I have a gear ratio question. I currently have 3:73's in my 3,900 lb T/A. 3,000 stall and 420-50ish HP 400. Cam is 2,500 to 6,200. TH-350 with manual valve-body. I am strongly considering 4:11's for snappier response. This is a street car, and it doesn't see much highway driving. I can live at 55-60 mph on the highway for the small amount of time I actually drive on the highway.

Thoughts?
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Old 07-28-13, 06:36PM   #2
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400 likes more gear because it makes peak torque at a higher rpm, sure you already know that. A 4:10 isn't that bad on the high way don't know what the rpm difference would be cruising at 60 mph vs 3.73. Don't have that formula in front of me. I think you'll like the trade off tho.
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Old 07-28-13, 06:45PM   #3
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This is what I came up with dont know how accurate it is

Say you are traveling at 60 mph. The RPM with the 3.73 gears should read around 2200, With the 4.10 gears you will see the RPM around 2500. Of course your gas mileage will suffer with the 4.10 gear. But as you can see not a big jump.
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Old 07-28-13, 06:54PM   #4
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Agreed. Right now we are talking (with 28 inch tires and 3:73's 2,700 rpm @60 mph) So 3,000rpm at 60mph not a real big deal. I can "feel" that the car wants more gear sometimes... I believe the trade off will be to my satisfaction.
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Old 07-28-13, 07:15PM   #5
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I think you'll like it.
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Old 07-31-13, 01:14PM   #6
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Hi Gach,

Engine builder is not happy about the gear swap. Says 4:10's are a race gear for a Pontiac. Thought a 100 shot of juice would be a better idea.

Truth is I'm having trouble with traction from 1st - part of second gear already with the 3:73. Says I should get the car to hook, then worry about gearing. I agree , but as we stated a 300 RPM jump is not that drastic.

Thoughts?
Trevor
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Old 07-31-13, 04:06PM   #7
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LOL. 70 Ram Air 1V GTO came through with 3.90 gears. If anything the gear will help. Yes you need a good tires and shocks on the back to hook good. Nitrous, sure, but definitely wouldn't be using it off the starting line.

If your having trouble hooking the car up definitely work on that first, some good tires shocks and traction bars and maybe losen up the front end with some 90/10 stocks get some front end travel to help transfer the weight to the back.
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Old 07-31-13, 04:45PM   #8
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Yeah. I have trac bars and some of the other goodies, but I do not have drag radials yet. I am getting the cart before the horse I think. The 3:73's actually roast the tires through 1st and part of 2nd from a standstill. Even at a 25-30mph roll I can light em up. I just feel like the car should pull harder in upper 2nd and third. I thought gearing was the answer. I'm confused now. I guess HP is my issue.
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Old 07-31-13, 05:21PM   #9
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No it will definitely poll better with the 4:10 gears, that the point I was trying to make, why Ram 1V cars came through with 3.90's. Some good tires and 4:10's will wake that baby right up.
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Old 07-31-13, 05:40PM   #10
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The gear Multiplies the torque and make it pull harder.
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Old 07-31-13, 07:00PM   #11
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Understood. 3;90, 4;10, even 4:33 were offered from Pontiac correct? I think a set of sticky tires and some suspension tweaking 1st. Then study the numbers. Then look into gearing and n2o after the 1-2 shift. I think I know where this is heading....bigger , badder motor... Thank you Lou.
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Old 08-01-13, 12:21AM   #12
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Oh yeah, it's a sickness. You think, I just want a street Cruiser, then that get boring and you want more. Oh yeah the stickies will make all the difference. Nitrous your hooked. LOL We all started out that way.
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Old 08-02-13, 02:57AM   #13
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Turbo Hydro 350 has a lower 1st & 2nd ratio than a stock TH400 so it may already have enough gear to launch.Put the better rear tires under it and
get it to hook first.Then decide if it needs more gear.
Shorter tire may work out for you instead.
That car can fit a 28 x11.50" wide MT slick in the wheel well with no issues.
Weld Draglite wheel 15x9 w/ 4.5" backspacing works really well. Tire will clear the traction bars & leafsprings OK.
I use a shorter 26" tire and I do not have anywhere near the gears you are using (2.73 LOL) and still have traction problems .Will be upping to 3.10 after I get it to hook reasonably well for a "street car/ daily driver".
Drive shaft safety loop required on cars with slicks ,dot slicks or drag radials.I would run one anyway.
Get a 3" dia drive shaft made for it with performance U joints.

Original 10 bolt rear end?
open or posi?
Stock 28 spline axles will be at their limit especially if it really hooks good.
Differential side gears definitely will take a beating.
Wheel studs should be replaced w/ bigger dia.
Below a certain ET C-clip eliminators are required.
More axle splines and better alloy.
You may have to upgrade soon.

Gear Venors overdrive unit would solve the highway problem to a degree by saving wear & tear on the engine but you say the convertor is 3000ndish stall. At 2500 or lower the convertor will be slipping a lot and making tons of heat if you are not into it all the time.
Forced draft monster trans cooler required for getting around in stop & go traffic.
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Old 08-03-13, 12:08AM   #14
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Have 28" tires looking at 28" - 29" drag radials. Have drive-shaft safety loop already, have custom aluminum balanced drive-shaft with hd u joints. have a Moser 12 bolt with c-clip eliminator and 30 spline shafts. Eaton posi and 3:73 currently.Have deep trans pan with extra qt capacity (no cooler yet good call though) gear vendor overdrive is an excellent idea. Maybe for Christmas... lol
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Old 08-03-13, 02:04AM   #15
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drag radials will do the trick. Nice set up you have.
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