Easy (?) OD Auto Install
Just went from a 7.25 with 2.76s to an 8.25 with 3.92s this winter. Drove 140 miles today and I came on here looking for overdrive ideas lol. Thanks for the info guys. Maybe get the gears in my head churning.
Hyup that's a pretty big 42% increase in rpm at any particular roadspeed.
Most overdrives are in the range of 72% to 78%, so that will get you a NET change back to close to what you started with, in other words your 3.92s will final drive at say 3.92 x.72 =2.82
The Mopar OD ratio is .69 so; .69 x3.92=2.70 WINNER. lol.
IIRC the chevy is .62 so; .62 x 3.92=2.43, so your engine better be able to pull down there; as 65=2000 or less. And it is sometimes extremely difficult to give the engine the ignition timing it wants down there.
IMO 2400 is about the lowest rpm you can cruise at and still make a decent timing curve, with a decent cruise rpm.
But here's the deal;
I bet you swapped those 2.76s out because your engine combo was a lil lacking in punch, am I right? Well the great thing about a streeter is it doesn't have to go very fast, as the speed-limits get in the way. So because of that we have a much greater flexibility in gearing.
We have in just the Mopar Line; two automatic ratio sets, and many rear gears, and several different engine sizes, and numerous other ways of punching it up.
For example; Say you had a slanty that made 80 hp at a stall of 1800. and say you had a 2.45 low gear and those 2.76s. So to see what the launch is gonna look like, first we have to convert that 80hp to footpounds and I get 233 of them. At the axles this would be 233 x 2.45 x2.76=1576 not to peppy.
Now say you swapped out the 2.76s for 3.92s, that would get you 233 x2.45x 3.92=2238, OH lookit that, that's nice. Lets say you were happy with that .
The next question is this; how can we make 2238 ftlbs and get the rpm down without going to the hassle/expense of the overdrive.
Well, your first cheapest and easiest option is the 2.74 first gear. With that;
2238/(233 x2.74)= 3.55 gears and 65 =2900, down from 3200. Better but not enough.
Say you were willing to run 3.23s for 65=
2640, and you had that 2.74 low gear, How much torque would the slanty need, at 1800 to make that magic 2238 ftlbs?
I get; 2238/ (2.74 x 3.23)=253@1800. That's only plus 20 ftlbs/plus 6.7 hp more than I started with. I'm willing to bet you can easily get that with a lil more stall.......
So, while an overdrive will really knock your rpm down, the 2.74low and 3.23s with a higher stall will get you pretty close.
What about those 2.76s that you still have? Can we use those?
Well, lets look at the numbers;
2238/ (2.74 x2.76)=296 ftlbs required to match what you might now have, or at least the combo I am exercising here.
OK no, it will not be easy nor cheap to get 296ftlbs out of the slanty at a reasonable stall. With a 2800 stall, this would take 158 hp
But a 318 can...................
So I hope you can see the progression
Now I realize that in Columbia, your resources might be somewhat limited...... but I just wanted to throw some ideas out at you.
