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Old 12-07-2006, 03:02 PM
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bbc what gears to run.?

what would be the best gears to run with a bbc. i hear people putting in 4.56 , 4.11 etc gears with a bbc. but why , i know that people put them in there sb cars cause they dont have a torqe like a bbc. in my mind a bbc with really high gears is just a waste of energy. wouldnt you want like a 3.73 , 3.55 gear instead. i know with lower gears you multiply torqe to the tires, some one help me. i have a bbc came range of 3000-6800 ,, with 2500 stall and posi 3.26 gears th-400. i want to install a built 700r4. i know i need a higher stall but this is mostly street driven friday nights. what would you run with a bbc??
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Old 12-07-2006, 04:11 PM
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How tall of tires are you going to run? Rember tire size can play a HUGE role in overall gear ratio.

If you don't have one you might want to play around with a drag simulator, on my 383 sbc it was actually faster to run 3.08 gears because of my short tires.

Jordon
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Old 12-07-2006, 04:19 PM
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Leave the gears alone until the 700R4 is installed. That way you can see what rpm's you run at in OD.

And tire diameter makes as much difference as gears.

Example 3.42 with a 26 in tall tire is just about the same as 3.73 and a 28 in tire.
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Old 12-07-2006, 04:56 PM
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well my slicks are 29 in tall. my street tires are 26 in tall , i can smoke the crap out of my street tires and it does seem to have more accleration with the street tires when they hook at higher speeds. , but i was looking at the knowledge base about what your top speed will be with what gears and size tires your runing.its saying with my combo i will top out at 172 mph but i know that doenst count wind resistance etc... and i would never ever go that fast any ways, theres no need tooooooo. so i will put lower rear gearing to top out at 130 or 140 mph. to acheive that i would need 4.11 's so i guess i will go down that path.
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Old 12-07-2006, 05:35 PM
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The first question you may want to be asking is how well a 700R-4 is going to hold up behind a strong 454 and a set of slicks. You may have to invest quite a bit of money in the 700 to make it durable. Or you can dump a pile of cash on a 4L80E...

As far as what gears to run you can do the math but you still need to know what speed and RPM you're comfortable running your engine at forextended periods of time. I generally like to stay right at 3000-3200 RPM but I also have a three speed auto. The choice is yours.

I found a calculator and this is what I came up with. In 4th gear with a 26" tire cruising at 60 mph using a 3.73 gear you'd be right at 2200 RPM. Running the trans in 3rd (1:1) would put you just under 2900 RPM. A non-lockup converter will put you a bit higher than that but it gives you an idea.

You can check it out for yourself. For the transfer case ratio just put "1" since you obviously don't have a t-case. There has to be a value there or else the calculator won't work.
http://www.contractrobotics.com/4x4/Gears.html
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Old 12-07-2006, 05:47 PM
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One other thing to toss in the mix is the weight of the car or truck...? Sure a BBC will move a heavy car better because of the torque but also gear it to do what you want... If you want max RPM at the end of the 1/4 mile then gear it that way... But if your just out to cruise or do a little street racing or even just get the groceries then gear it more twards the highway...

A good trade off gear is the 3.73 and a BBC will pull good and hard even off the line... If you want the hole shot and have plenty of RPM range then either a 4.10 or 4.56 is it, but cruise will suffer...

Also I agree a 700R4 behind a BBC is money not well spent no matter who builds it... A solid turbo 400 can be built for about 1/2 the cost of the 700R4 and will live behind your BBC...

If you just have to have an OD trans then look more into a 4L80E with independant computer control but be prepared to take out a second loan on the house... (the trans and converter & computer will put you in the poor house)
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