Change 1-2 gears to lower ration gears 1967 Dart Automatic (904?)
OK, Imagine the graph is accurate, and that when you floor it off the line, with the current combo, you unleash all 125 ftlbs of torque, right there at zero mph. (BTW that's 42hp.) Now your transmission is gonna multiply that by the low gear ratio, of 2.45, and your rear end is gonna further multiply that by 2.94.That's called
TM; short for
Torque
Multiplication. So that looks like ; 125x2.45x2.94 =
900ftlbs available. if you take of gently, you might use 30 to 50% of that, but if you floor it, that's all you got at 1750 rpm. Ok now remember that 900@1750.
Now you are thinking of installing the 2.74 low gear set., so let's do the math on that. Your engine still has the same 125Ft lbs and the rear is the same so; 125x2.74x2.94=
1007ftlbs.
Now it takes somewhere around 1400 to break two rear tires loose, skinny ones maybe less.
So, you are still 400 short. You spent a lotta money and did a lotta work, and you got 106 ftlbs for all your skinned knuckles, and you still can't spin the tires! Dad gummit!
So lets work the numbers backwards, this time.
Say you wanted to break the tires loose with less than full throttle. So lets target 1476 ftlbs. And we'll use the current trans with the 2.45 low and the current 2.94 rear gear. The question is ;how much torque will the 273 need to be making? Answer; 1476/(2.45x2.94)=204 ftlbs.(BTW that's now 93hp@2400rpm; more than double than at 1750)So now we go to the graph and find 204ftlbs, and I see it at 2400rpm;the point where 408 and 4800 cross.
So you install that 2400TC and you are all set to spin tires off the line. I can't say they will spin very far, cuz that chart shows you peak at 437/2=214ftlbs at about 5900/2=2950, and your engine has not got a lot of reserve.
But before you get all giggly, this graph is no-where near accurate for showing the torque of a 2bbl 273. I just used it for illustrative purposes.
But I have run a 2800 behind a 1973 low-compression 318, and I can tell you it easily broke 255/50-15s BFG T/As loose.....both of them,lol. But to be fair, I had 3.55s and the 2.74 low. So I needed just 1476/(3.55x2.74)=151 engine ftpounds. A 318 has no trouble making that at 2800,even at PartThrottle.
>And I can tell you that 340 A-bodies came with 3.23s and 2.45 lows, that's just 3.23x2.45=7.91TM, and they easily broke loose the skinny E70-14s that the factory put on them. So that would be 1476/7.91=186ftlbs required.
Your 273 can make that number. The 340 cars had 2250 stall TCs ,IIRC.
ok so, I hope that makes it a little easier to understand.
Happy HotRodding