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Old 02-26-2010, 03:45 PM   #1
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Best gear ratio for the quarter mile dragstrip - what is your opinion?

I have a 72 with a GM ZZ454 engine, a TKO600 transmission, and am currently running a 3.36 rear gear ratio.

I busted my original one, and have a rebuilt 3.36 in the car now, but I have not had a chance to run the car on the dragstrip.

Looking at the numbers, I can't decide whether goign to a 3.73 rear is the right thing to do.

The car has about 500 HP and 525 TQ at the crank with the cam and the fuel injection system I am puting on it. I run stock rallys with 245/60/15 BFG Radial T/A tires.

The motor redlines at 6,000 RPM, but I would probably shift by 5,500 RPM.

Based on some calculations, the car 's top speed in 4th gear is 141 with the 3.36 gear and 127 with the 3.73 gear.

It looks like I will have to shift into 4th gear with whichever rear gear I choose. That is, if I can assume the car will be travelling at about 120 MPH at the end of the quarter mile. There is a chance with the 3.36 rear gear, that I can finish the quarter mile in 3rd gear and save that last shift. There is also a chance of blowing the motor up like a grenade!

The 3rd gear top speed is 110 at 6,000 RPM with the 3.36 and 99 with the 3.73

If I shift from 3rd to 4th at 5500 RPM with the 3.36 rear gear, the car will be travelling at approximately 100 MPH, which would drop the RPM's to 4,200 in 4th gear. With the 3.73 the speed will be about 91 MPH, which would drop the car about the same to around 4300 RPM in 4th gear.

On the other hand, when putting around on the interstate and back roads, the car cruises best at aboug 2200 RPM which put me at either 51 or 79 with the 3.36 in 4th or 5th. If I go to the 3.73 rear gears, these number s drop to 47 and 73 or I have to bump the RPM's up to 2400 which is no big deal, really.


I say all of this to spark some discussion from the drag racers out there that may have a similar powered Vette. I would like to know what your rear gears are, and whether you like them or what gear you would go to if you were changing it.

I take this baby on the Hot Rod Power tour as well, so interstate and back road cruising is important as well.
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Old 02-26-2010, 04:15 PM   #2
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Old 02-26-2010, 04:23 PM   #3
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Do you take the C3 to the dragstrip? If so, what speed are you hitting at the end of the 1/4 mile? I assume you are shifting into 4th. Is that right?
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Old 02-26-2010, 06:05 PM   #4
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Old 02-26-2010, 07:39 PM   #5
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http://www.secondstrike.com/technical/GearCalc.asp

I just pulled out my 4.11 and went to 3.55

You need to look at bigger rear tires.

I use 335/35/17 and get pretty good traction

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Old 02-26-2010, 08:25 PM   #6
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Whatever lets you go thru the traps at or just under red-line in 3rd gear if an auto and 4th gear when manual. You need to do the math. Usually 4:11's will work for most applications.
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Old 02-26-2010, 08:56 PM   #7
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I would just keep the 3.36's. Your car is mainly a street car, not a drag car. The steeper gears won't do any good without a substantially bigger tire. If the car was heavier, maybe.

For comparison, I am building a 540 cid motor with only 3.08 rearend gears. I plan on running 275 tires in the back. Big blocks put out so much torque u don't need big gears, unless it is a race car with huge tires. It also depends on the camshaft and your "overall" gear ratio when combined with the transmission. My richmond transmission has a 2.89 first gear, which when combined with a 3.08 equals about 9:1 overall. That is equivalent to 4.11's with a muncie spd 2.20 first gear ratio.

Hope this helps
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Old 02-26-2010, 10:27 PM   #8
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If you currently have 3.36s in it and the rear end is working well/quiet etc, I would not change them....BUT if you are planning to go into the rear end to repair something else or you are just swapping center sections, I would definetly go with the 3.73s since you have a 5 spd.... My LS1 Trans Am with 4.10s and the 6spd would still run 80 mph at 2000.....That car really needed 4.30s to make 6th useful....

Just gear your run to top out in 4th at the end of the 1/4. Ofcourse this depends on your power/weight etc..... I would say a 3.73 would get you pretty close. But if your 3.36s are still working well, don't change em just for the sake of gaining ET because you won't gain much....
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Old 02-27-2010, 07:25 AM   #9
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The 3.36 that came with the car busted.





It was my first trip to the dragstrip, and my first run. I went about 2". Just enough to get a reaction time of 0.251

I have a rebuilt diff with 3.36 gears in there now that I bought with a restored chassis I got for my 73. A tech school in Tupelo MS rebuilt it. It has a little hum to it, but I only have 100 miles on it so far. When I get the heavy duty one, I will open it up to check it out.

I am building (having GTR1999 do it) a 10 bolt HD differential and decided on 3.73 gears, but I am second guessing myself (as I always do).
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Old 02-27-2010, 08:26 AM   #10
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My husband says the Big Block produces to much torque for a 3.36. It is sorta like torque to weigh ratio if you have to much torque and not enough push with your gear, the gear will give because of the weight of the car. A 4.10 gear would be your best bet to get because it will push the weight out of the hole and it would be easier to adjust your MPH with tire size.

He had a 3.36 on his BB and it ripped the ring gear bolts off 3 times. He then changed to a 4 series carrier and put a 4.10 GM gear in it. He then went to a 28/9 tire 11 1/4 inches wide and to get a tire like that in the car you have to cut the emergency brake cable bracket off and weld it to the bottom of the trailing arm that will allow another 1.5" wider tire to be put on at stock Vette suspension. The setup has been in his car for around 10 years now.

His motor produces about 650 hp. He has a turbo 400 with a short first gear and a long 2nd and 3rd gear. There is a kit that changes a turbo 400s first gear to act like a stick coming out of the hole with a 3800 stall speed and no trans brake.

Nothing has broke since. BTW...this is a 396 bored 90 over and a 13.5:1 piston. He has yet to lose to a 454. The only thing that has stomped him is a turbo car.

He isn't afraid to beat on his car and it is still street legal.

Good luck with your desicion.
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Old 02-27-2010, 09:05 AM   #11
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Quote:
Originally Posted by 73VetteGirl View Post
A 4.10 gear would be your best bet to get because it will push the weight out of the hole and it would be easier to adjust your MPH with tire size.
Great minds think alike...
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Old 02-28-2010, 12:27 AM   #12
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Quote:
Originally Posted by 73VetteGirl View Post

He had a 3.36 on his BB and it ripped the ring gear bolts off 3 times.
His motor produces about 650 hp.
This is one of the dummest things I have heard!

At work today I was rebuilding a Vette rearend. To install the ring gear had to heat the it to 300 degrees just to get it on the posi carrier and then align the bolts. The ring gear to posi housing is very mechanically strong.

Everything before and after the ring gear would break before you could shear all of those bolts.

Last edited by gkull; 02-28-2010 at 01:28 PM.
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Old 03-02-2010, 07:08 PM   #13
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Quote:
Originally Posted by gkull View Post
This is one of the dummest things I have heard!

At work today I was rebuilding a Vette rearend. To install the ring gear had to heat the it to 300 degrees just to get it on the posi carrier and then align the bolts. The ring gear to posi housing is very mechanically strong.

Everything before and after the ring gear would break before you could shear all of those bolts.
That is why there is a 4 series carrier in the car. The bolts are bigger than on a 3 series carrier.

Fixed the problem.
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Old 03-02-2010, 10:58 PM   #14
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Take a look at these pic's The carrier has the same bolt size to the ring gear.

http://forums.corvetteforum.com/c3-t...rier-diff.html

gear ratio does not determine what breaks. 3.36 according to your logic should have caused my Camero 2.73 4 speed to fail on it's first day out back in 1967. Or my old dragster with a two speed glide that could go 100 mph in first gear
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Old 03-03-2010, 08:33 AM   #15
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Corvette Ring Gear bolts are all the same,even the 12 bolt conversion- 3/8-24 RH thread.

With that said, it is not uncommon to find sheared ring gear bolts. This is usually as a result of the bolt backing out rather then shearing in place. The 76-79 are more common to find then the 63-75. The 76?- 79 used flanged head bolts less the hardened washers the earlier setup had. No loctite was found on them so they would back out and lock up the diff.
Now ARP RG bolts for the corvette also do not use washers but are set with #271.

As far as differential failure, given enough abuse they will break: stock, custom, super 10 bolt, 12 bolt. The 63-64 posi case were weak, the stock 65-68 Eaton was prone to cracking,the 69-79 posi case was better but some still cracked or the cross shaft hole opened. The current aftermarket options: Auburn cone units are weak, the new Loaded Eaton's are stronger but loaded with weak internals, I would not use either. The best is a custom built one with the proper parts and machining procedures.

PS the only difference between a 3 & 4 series is the thickness of the flange.

Last edited by gtr1999; 03-03-2010 at 10:44 AM.
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