selecting a rear gear for new bbc

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
redoing my rear. need advice for 540 gearing selection
Hey guys. I'm gonna post my dyno sheet. I'm going to beef up my rear in my 70. i'm currently running 4.10s with 28" tire. would like to know if i should keep gears or go numerically lower. running a nyes 540 and i love it. th400 with a 3800-4000 pat's converter in PA. he had my dyno sheet in hand when he built the converter. car is driven on the street with minimal track time. full weight car minus a/c. aluminum heads. i'm worried my car will red line through the traps or before them. peak torque is 724 at 5000 rpm. peak horsepower is 744 at 6100. this was during the break in when i bought the motor. it's supposed to make 775 i think. i know converter slippage is a factor. i'm guessing i will have a fair amount of slippage. the builder told me to run 3.90 - 4.10. my trans guy thinks i can do a 3.42. thoughts? thanks!!
heres the dyno sheet
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any time your putting together a performance car combo, you should stop and think through all your options and the likely requirements that the increased power and speed and related factors will induce.
if the cars engine makes significantly more power the car will require a larger radiator to dissipate the increased heat, it will require bigger brakes to control the cars increased speed and it will require you to match the cars new and higher torque levels with stronger than standard drive train components.
if your driving the car on the street, you will usually need semi reasonable mileage, if the cars used for transportation, and decent traction, with street tires.
there's a huge difference between ,maximizing a race cars performance, where your use slicks and modify the suspension, and putting together a decent street performance combo you'll occasionally use for transportation.
If the cars original components were designed for the original power level, and you've effectively added 50%-100% more power,and heat and speed, you can depend on the fact that your more likely to break original components.
I just a few months back I helped a local guy, built/ assemble a BBC engine in his shop,and install, an engine swap, a 540 BBC, with a crane 139791 roller cam,(installed retarded 4 degrees from split over lap,) 11.0:1 compression and some ported 320 cc trickflow heads and we installed it in a 64 LEMANS with a dana 60 with 3.54:1 gears, and a 4l80E and a 3200 rpm rated stall.
too be fair he purchased most of the engine at an estate sale at a damn good price and while I helped put it together, and offered several suggestions,as to how he could increase durability, he was not willing to do much about changing the combo, as he felt that just assembling the components as he purchased then would be faster and cheaper. granted its an o.d. trans but it easily destroys street tires, with near zero effort if you just give it a bit of throttle, a 4.11:1 would be a P.I.T.A. to drive on the street if it rained, with street tires.
yeah it needs high test and some race gas added to run without pinging, but its a grin to drive!
I did not design the combo it was all his idea , but over all it runs well.

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similar to this but his is navy blue

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related info

http://garage.grumpysperformance.co...hing-the-drive-train-to-the-engine-combo.741/

http://garage.grumpysperformance.co...electing-a-torque-converter-stall-speed.1715/

http://garage.grumpysperformance.co...late-gear-ratios-and-when-to-shift-calcs.555/

http://garage.grumpysperformance.com/index.php?threads/4l80e.11645/

http://garage.grumpysperformance.co...strongest-rears-differentials-available.1282/
 
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