Headers and gears. But with a caveat
Headers will for sure wake that 340 up. The Low-C version of the 340 was a little soft out of the gate, so TM and stall will be your friend.
But if you don't have the suspension to make it hook, then stoplight racing will not benefit from gears, or headers.
Then you say your 3.21s are spinning 3000 at hiway speeds now, and you're not interested in anything higher. So that puts even more limitations on stoplight racing.
Lemme throw some numbers at you. Suppose you're current TC flashes to 2500 off the line. And suppose your 340 makes 200ft lb at that 2500. And let's suppose you are adamant about keeping the 3.21s. OK.
So 200 x 3.21 x 2.45 x 1.7say in the TC=2728ftlbs. This means when you nail it, you will be putting down 2728ftlbs to the road. So those 235s will instantly light up. Then immediately the TC will start dropping back on the TM to perhaps 1.1 at the 60ft; so now your rpm has risen to say 3500@22 mph, and the engine torque has climbed to 270, so now this is how that looks;270 x 2.45x3.21x1.1in the TC =2336ftlbs. So the road torque has dropped to 2728/2336=85.6%. But the tires keep on spinning cuz their threshold was maybe 1600.
Lets take her up to 4500. By now the engine torque has climbed to well past the factory torque peak, and fallen down the other side of the curve, so let's guess to 300ftlbs. And the TC has fallen to it's lowest TM of maybe 1.05. So now at 44 mph, your engine is putting down; 300 x 2.45x3.21 x 1.05TC=2477ftlbs, and the tires are still spinning.
Now imagine installing 4.10s. All the numbers will jump by 4.10/3.21= plus27.7%. And all the mphs will drop that same 27.7%. As you can imagine,how much quicker will your car really be? Well since the tires are spinning, Ima thinking not very much.
In this exercise, all the numbers I used are guesswork, and it doesn't matter, cuz the tires are spinning!
So for stoplight work, the very first thing you have to take care of is the traction issue. More power, more gear, more TC,or more torque, more anything will NOT help you until the traction issue is solved.
Stated another way, if you are spinning now, it's no wonder Volvos are driving around you.
As to the big end, here gear ratio is almost critical. If you are stuck at the wrong rpm when the opportunity comes up, you are sunk. And with a 340 this is critical. The 268* cam will powerpeak at about 5000/5200 in that low-C engine, so you cannot stray too far from that rpm at the big end. With 3.21s this is about 82 mph at the top of second. By 5680/93.5mph, the power has fallen off very significantly. Any gear ratio bigger than 3.21s that forces you to shift into third will just make it worse. In fact, the next smaller ratio, 2.94s would give you about 10% more mph, but the road horsepower would also drop by the same amount. For example, say your 340 was a TQ engine with headers, and was putting out 310hp@5200. Then with 27"tires, and 3.21s she would be putting down 310 x 3.21 x 1.45 x 1.05TC= 1515 roadhorsepower @85.6mph. Now lets swap out the 3.21s for 2.94s . Now she will be putting out 310 x 2.94x1.45x1.05=1388road horsepower at 93.5mph.
And increasing the rpms of the 3.21 equipped combo to hit 93.5, we get 5680rpm. and by this rpm one might expect the power to have fallen off to 270say, and the numbers look like this 270 x 3.21x1.45x1.05=1320
So in this case, at 93.5mph, 2.94s have the edge.
A higher number rear gear, will let you blast thru the gears faster, but if you end up in the wrong place on the power curve, for what you are wanting to do, then they're no help at all.
Let's try the above exercise with 4.11s.
At the power peak of 5200 and in second, the rpm will be 67mph, so we are gonna have to shift. And that gets us about 97 mph @ 5200 in third. And the roadHP will be 310 x 4.11x1.00x1.05=1338.
So then,at 85.6mph again, the rpm will be just 4590, and the power might be 280; so then we get 280 x4.11x1x1.05=1208, or just 80% of what she was putting down earlier (shown in purple above) with the 3.21s . So in this case 4.11s are a disaster, posting just 1208/1515= 79.7%.
So, for a streeter,it all boils down to selecting the right gear for the job at hand. Or bait the comers to work in your rpm territory. That's how I did it back in highschool. My 1970 Swinger 340 4-gear 3.55s was not the quickest nor the fastest car around.It barely managed 98 mph in the qtr at 14.4 seconds (on street tires). But in second and occasionally in third she could put the hurt on a lot of the big boys. I just baited them into my zone. The zone where that little 340 really sang. But I stayed outta first gear, cuz it spun all the way through, and on the shift spun most of the way thru second as well. That was the first thing I fixed. Then in went the 4.11s,lol. After that,I was all but untouchable ......................................up to about 80.
So when one of the other boys wanted to race my stock engine Swinger, I made the rules. And the rules were always in my favor. Rolling start,and to 80mph; no exceptions. If I had to get into 4th I was pretty sure it was over for me. 80 in third, was already well past the engines happy place.
