my question is what torque converter suggestion do you guys have to make this a good street car?
That cam makes peak power at around 5200, and will need to be kept in first gear to about 5600, or more, to drop into second at 3300. Depending on the Scr of that Duster, 3300 could be real soft, so you might have to keep it in first a lil longer.
You didn't mention your tire size so I'll just use 27".
So 5600 with 2.94s and 27s make 60 mph.
Your starter gear is 2.45x2.94=7.2
That 224/230/110 cam is a pretty good cam, but in your combo, the power peak of 5200 comes at 56 mph. If it doesn't spin the tires, both of them; it's gonna be a looooong haul to where the power is.
With a starter gear of just 7.2, you'd have to have pretty skinny tires to spin them both.
So your first go to is the stall rpm. With a power peak near 5200, the torque peak is likely to be near 3700, so that would be a good starting point for the 2.94s.
But as you can see, with 2.94s you are only gonna hit the power peak once on the way to 60mph. So you can't expect too much performance, even if a 3700TC gets you to peak torque, cuz the engine is still stuck on that long hard pull.
Furthermore, with 60 being 5600, then 30 will be 2800 @zero slip, but your 3700 is gonna spool it up to 3700.
If your engine makes 300ftlbs a 3700, that will be multiplied by the 7.2 starter gear to 2160, and that will be multiplied by the TC ratio, say plus 5%=2268 .. Ok that is not a lot of torque.
Lets redo that with 3.91s. Ok 300x2.45x3.91x1.05=3018ftlbs Now yer talking. That would be a great jumping off point.
But this also allows you to dial back stall, cuz you now have more take-off torque than you need. Lets see where you are at;
300x2.45x3.91=2874 at zero slip. But at zero mph, the TC can easily multiply that by an additional 80%, so that would be 5173 ftlbs, which will absolutely roast the tires for a good long way. But that 80% diminishes rapidly as the car begins to accelerate. Perhaps it diminishes to plus 10%, at about the torque peak thru first gear, and continues to diminish to plus 5% at shift-rpm. So at the torque peak of perhaps 3700, your combo will still be at 300x2.45x3.91x1.1=3161, and so, now at 28mph with the new gears, still frying the tires. Great! but we can give a lot of that up, on account of spinning the tires to past 28 mph, at the expense of cruising comfort and MPGs, does not exactly fit the picture of " good street car."
So lets take some stall out, AND back away from the 3.91s.
Lets try it with a 2800 and 3.73s.
Suppose at 2800 your teener, on account of it only has 125psi cranking cylinder pressure ( I'm guessing cuz you didn't say, so bear with me, this is just a study), only has 200ftlbs. Ok
200x2.45x3.73x1.8=3290 ftlbs at zero mph. This is enough to initiate a two-tire spin, any size street tires. But lets say 3-car lengths out, you are tached up to 3700, where your engine has 300ftlbs. Ok then at 3700, you now have
300x2.45x3.73x1.1=3016, still enough to sustain the spin..... When you lift,and the tires stop spinning, you will be doing 31 mph. Waitaminute, I said when you lift. Yes by 3700 your secondaries have been open for a lil bit, and they brought a few additional ftlbs to the party. Lemmee guess at 3700, maybe 10 or 15 hp, so the actual number could be
317x2.45x3.73x1.1=3186 ftlbs.
What this means to you is that ; you could be cruising along at 31 mph in "drive" at 1436rpm @zero-slip, and if you suddenly get the urge and floor it; then the trans will drop into First, the engine will tach up to 3700, the secondaries will open, and suddenly 3186 ftlbs are gonna hit the tires, and they, if BFGs lol, won't stand a chance...
Well if that's true, could you go less than 3.73s and less than 2800.
Well I think that you could, but not with 125psi cylinder pressure, the number I guessed at. The 2800 is about as low as you can go with that pressure. And you need the 3.73s for three reasons
1) to get the TM(TorqueMultiplication),to blast off the line with screaming tires
2) to be able to downshift at 31 mph, and make the tires scream
3) 65mph will be 3010 rpm at zero-slip, so your 2800TC is a good match for it.
You could trade a lil gear for a lil stall, but only one size , say 3000TC with 3.55s, or 2600 with 3.91s
But if you have 155psi, then NONE of the above numbers are valid, zero, nyet. But the science is still the same.
Furthermore, I guessed at 300ftlb starting point.
It's just a study
to help you understand the why of,why you are being ushered to the numbers you have been given.
xxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxx
But I just gotta mention; there is no one stopping you from slamming together what you got, and let the finished combo tell YOU, what about it, YOU don't like. After a few hundred miles, you will be able to figure out exactly what is right for YOU; because your definition of "good street machine" may not be the same as mine, or someone else's here on FABO.
Ok, a lil more opinion
I already ran the 2800 with gears of; 2.76/2.94/3.23/3.55/3.91/4.30, with a 904, behind a Smogger-teen. Which is a low-compression mid to late 70s 318. Mine was all stock except for the ThermoQuad and headers. It made 145psi.
The 2.76s were OK as it was my first winter combo; but I never ran them again,lol.
The 3.23s were a big improvement, and more than adequate for a winter car.
The 3.55s were my favorite, but without an overdrive, they were already too high for comfort on the hiway.
The 3.91s were actually very satisfying because they hit 65mph at 4800, about where the stock cam topped out. This combo was so much fun, I kept it installed well into the summer before I swapped my 367 summer engine back in.
The 4.30s were an absolute blast , the shifts coming fast and furious.But I was down to cruising at 50mph.
The 2800 was a blast to be sure, but my engine was making 145psi with the stock cam.
I doubt you can make much more than ~125psi in a smogger-teen with that 268 cam. So I think it was RRR who first called 3500, and I'm gonna agree that a 3500 and 125psi are a fair match. That 700rpm will more than make up for the loss of pressure.
My 2800 is from the early 80s and I bought it with the name of
DirtJerker. And that is exactly what it feels like when I hit it. At zero mph, when I hit the gas, nothing happens until it hits it's stall, then all the engine's available torque whams thru the driveline and hits the tires which instantly wind up the springs and punch the back of the car up.
It's like when you yank a car out of the ditch with one of those snatch straps. Your truck accelerates forward takes up the slack, runs ahead until the strap says stop! then the strap unloads all that energy into the car behind you which then leaps forward aiming for your back bumper.That's about the best way I can describe my DJ2800. That Tc has been with me since about 1980, going from car to car to car.
I'm guessing modern TCs will be yet better.
Happy HotRodding
