this is an age old dichotomy.
> a dragrace combo, is a different build than a street car combo, is different than a dual-purpose combo.
> The smaller the engine, the more specific the combo becomes.
> what makes the engines specific is the powerband, which relates directly to the type of trans you want to/have to run, and it's gear ratios.
a street car is a two-gear car.
a drag race car with a small hi-rpm engine needs more gears to keep the engine on the pipe/cam.
the problem is the Power-Band; and I will limit this discussion to a stick-shift car, because that is what I have.
That A833 comes with 4 gears only, and they are already too far apart for a small hi-rpm engine. The ratios are
2.66-1.92-1.40-1.00 with splits right around 72%. This means that when you shift it, the rpm will fall to 72% of what it was.
Your engine will have a useable powerband directly related to it's design;
so that as you strive for power, the higher the operating rpm needs to be, and then, the more narrow the power-band will end up being.
Thus if your rpm of peak power is at 6400, the torque peak might be around 5000, for a powerband of maybe 1400. So then, shifting at 6400, the rpm will drop to, 72% = 4600, fully 400 rpm below the torque peak. To get around this, and to increase average power during the run, you might think to shift at 7000. If you do, then with the A833, the rpm will drop to 72%=5040, and now you are sorta in business.
Well except for First gear.
Such a 273 combo might trap at say 106mph, thus requiring 4.88 gears to trap well/ ET best. Then the starter gear , being 4.88 x 2.66= 12.98, is just about right for the dragrace combo.
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The problem is that for the street, with a clutch car, this is all wrong, because;
1) first gear is screaming the engine, just a couple of car lengths out. but
2) when you shift into Second, the low-rpm power is so small and the engine is so far off the pipe, that it struggles to climb out of the hole. I say struggles as compared to a bigger engine, with a smaller cam, for a similar absolute dyno output.
3) by the time she gets into Third at say 32mph =2800, from Second at ~3700, she is so far off the power as to be very frustrating. So then, what ends up happening, is that you carry the rpm out to something like 5000 all the time, cuz .72 x 5000=3600, so you can have at least some torque after the shift. This might be fun for a day, but after that, not so much.
4) more gears is not the answer, cuz they are still so far apart. The answer, for this engine, is tighter splits.
Say your splits were 80%. Then, .80 x 6400=5120 , and so your powerband is now 1280 rpm. Carrying the rpm out to 6800 now, your best average power for this gear might be from 6800 to 80%=5440 , =1360rpm; then;
5) back on the street, this new trans has ratios of 1.00-1.25-1.56-1.95-2.44, yes, it's a 5 speed! and the starter is 4.88x2.44=11.91.. so then
6) first gear is screaming a lil worse now; but 32mph is 3800 in Second, 3030 in Third, 2430 in Fourth; so now you have a decent selection of gears. Well except for 65mph =3950..... she needs another gear! Say overdrive = 71%; which will bring the Rs down to 2800, about right for that crazy 273.
7) So now, to make the 273 work on the street, she needs a very close ratio 6-speed, with a deep overdrive, cuz honestly, as the miles and time go by, 65@2800 is also gonna get old. and
8) finally; , have you shopped for this theoretical transmission, with 80% splits? I did back in the day, (around 1998) and found something sortof suitable, but at the time, IIRC, it was over $4000 plus a very expensive shifter. All this just to run a 273 dual-purpose car? I said no thanks. So then;
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9) I built a lazy-azz, 223* cammed 367 with 3.23s; problem solved. and she got into the 12s no problem. About 4 years later, I installed the GVOD, and that was that. You see; that 72% split A833 is perfect for a hi-compression 223*-cammed 367 with 3.91s to 4.30s, and the GVod brings 4.10s down to a more-manageable 3.20 final drive.and 65=2590
10) I eventually found a Commando gearset with a 3.09 low, that allowed the rear gear to be 3.23s, for a starter of 3.23 x 3.09=9.98. But it trapped wrong in Third gear, so I installed 3.55s. This traps at 93=5760 in third or 6160 in Second over. and 65=2240 Badaboom! ....and 32 mph is 1980rpm in third, 2716 in Second, 4370 in First, so; around town I always have a decent gear for a torquey 367. I usually leave it in Third, cuz if I floor it in Second, it just lights up the 295s....... Good luck doing that with a 273 and street gears.
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I hope this presentation makes as much sense to you as it did/still does, to me, cuz;
it only took me 4 years, 5 freshenings, 3 cams, 3 transmissions, 2 clutches, MANY rear gears, and the GVod; to finally have ironed out the bugs, and
I started out this journey with a hi-compression 367! I don't even want to think about what it would have taken , had I dusted off my 273.
And if you ask me, a 340 would have been nearly as good a choice, cuz honestly, the 367 even with a now 230* cam, easily overpowers the street suspension, even with up to 325s on the back. and no, a 318 would not do...... for a dual-purpose car, for all the same reasons as the 273, namely, powerband. But;
the 367 was cheap to slam together; the KB 107s came in at .012 below deck, .028 gaskets got me .040 quench with closed chamber heads, and 78cc total chamber volume got me 10.6 Scr.; the 223* cam got me an Ica of 59*, so the Dcr came in at 8.84@181psi@930ftelevation. I mean this is how it bolted together! The only maching was for the pistons. With OOTB alloy heads she is happy to burn 87E10 all day every day. On subsequent freshenings , I dinked around a lil.
BTW,
during those early years, the 68 Barracuda was a 4 seasons Daily Driver. Every spring and fall, the 367 got swapped for a stock smog 318 with a TQ on it, and it got the TTIs that were in the car. And yes, I experimented with transmissions and gears with it as well. Not a chance did I ever consider to leave it in there come spring-time. . Not even when it ran A999 ratios with a 2800TC , and 2.76s for a starter of 7.56, and 65=2300 or so, and the magnificent Thermquad roar. Not even when it ran 4.30s and a double overdrive, for 65=1930.. I mean I loved that winter motor dearly, but it was just that.
Happy HotRodding whatever you choose.
If I was to run a 273, I would optimize it for the transmission of choice, and call it done. That's gonna point you toward Magnum small-chamber alloy heads, a small solid-lifter cam, and a minimum 185psi cylinder pressure. And I would run an A500 with deep gears, a 2800ish TC, and and let the ET be whatever it will be.
Why the A500? Because with a hi-stall, it will act like a 5 speed under power, and like a 6speed around town, and allow you to run those 4.88s if you insist, lol. For 65=2700 in lock-up
Why the solid lifter cam? Cuz it will give you one bigger size @.050 without the late-closing ICA. This will allow you to run one or maybe two sizes less rear gear on the hiway. With say 4.30s, 65=2400, fairly ideal for a modest cam.
Why the Magnums? cuz cramming air into those tiny bores is gonna take some porting effort so you might as well start with something decent.
Why alloy? cuz you can pump the pressure up to even 195psi on pump gas, with the domed tight-quench pistons that yur gonna need, and they are so much less of a PITA to port.
That's what I would do
BTW; the A518 has slightly tighter splits than the A500. But with 4.88s this is not a deal-breaker.
