OK, I'm up for talking TM (Torque Multiplication).
Say that you have a regular automatic,with ratios of 2.45-1.45-1.00 and a popular rear gear like a 3.23.
Your TM will thus be the trans gear selected times the rear gear.
and say you had a tired 318 or something.
Now say this engine has 120ftlbs of torque available at 2200rpm. This torque will be multiplied by the TC,briefly, then reduced somewhat by powerloss in the trans and rear gears, and the whatever is left over,can be multiplied by the TM ratios to come out as available torque for acceleration purposes.
So lets take that 120 and estimate 15% loss in the powertrain and the gross it up to road-torque.
I get 120x .85x2.45x3.23=807ftlbs available for take-off in first gear. No smoke and not at all impressive. The TC might deliver a 10% boost on the hit but it decays rapidly as the vehicle moves ahead, and so I will ignore it for this exercise.
Ok so this sucks for performance.
So the first thing we need to do is find more power. Out goes the 2200 and in comes a 2500. Let's say this engine can deliver 180ftlbs at 2500. Ok so the math is 180x.85x3.23x2.45=1210 ftlbs. Ok so now we have enough to chirp the tires, and on a good day maybe spin them for a short ways, but mostly still not impressive.
Now lets look at cruising. 3.23s will get you 80mph=3340 with 26" tires,at zero-slip, so more likely is 3400@2% slip.That's quite a bit of revs.But guess what, 2.94s will only bring it down to 3100, and 2.76s to 2900.
> So let's say you want the 2.76s. What's that gonna do to your take off? Well, still with the 2800 and 180 numbers, I get 180x.85x2.76x2.45=1035ftlbs. ouch, back into the doghouse we go.
Okso,we need more power, out goes the 2500 and in comes a 3200. Now that little engine is making say 220, and the new numbers are 220x.85x2.76x2.45=1265, and tire chirping is again a reality.
But you wanna smoke the tires
So you install an engine that makes 35% more torque at 2500 or 240ftlbs and away you go. Now you find out that the 2500 and 2.76s get you 80=2850, and 240x.85x2.76x2.45=1380......but it's not quite enough,to satisfy you; so you hop it up and get 280@2500. This then gets you 280 x.85x2.76x2.45=1610 and that is now plenty of torque to smoke any street tire you can fit in the stock tubs.
So this is how it works, trading TM and TCs around and playing the numbers game. I purposely chose an imaginary engine with imaginary numbers so I could control the results with math.
> But let's say you had an overdrive trans with ratios of 2.74-1.54-1.00-.69od ....................... With this you can bias your cruise rpm against first gear TM at take off
Let's go back to that 180flb@2800 engine and give this combo
180x.85x2.74x3.91=1639 ftlbs, sweet, and 80=2850@2% slip. That's pretty sweet. So................180@2800 is like a 318
But you have a 360, with a long stroke,and say you pumped her up a bit so lets try it with 3.73 gears, and say 280ftlbs at 2800. And the math is 280 x .85x2.74x3.73=2430 tire-frying ftlbs, and 80mph =2700@2% slip.
> But let's say you took a bit of cam out of her and geared her down one more time and went back to a 2500TC; It might look like 250x.85x2.74x3.55=2070ftlbs, still plenty, and now 80=2580.
By taking cam out of her it now works with the wide-ratio trans, but more importantly, this cam has a longer power-extraction period and less overlap, and so you picked up say 2 mpgs, and this affects not your cruise torque nor your take-off, and only slightly slows your zero to 60mph ET.
I love playing these games.
Now lets compare3.55s with and without the overdrive
without, take off is 1848, and 80=3750
and with .69od, take-off is 2070 while 80=2580
That's a pretty good balance....IMO.
Now I gotta remind you that all the numbers I used are fictitious,lol. I made 'em up for the exercise.
But I just gotta say;IMO, 250 ftlbs at 2500 to 2800rpm,is easy-peasy for a lo-po 360.
Oh, I see I'm late to the party ,lol
