A nasty sounding cam with rear tires that are too large quite often has disappointing power below 3000 rpm.
some to most of that can be negated by boosting the cylinder pressure back to the pre-cam level.
Op said "built 340" but that means different things to different folks. A small cam 340 is a bit like a hi-compression 318. Choosing a hi-compression 360 allows a slightly bigger cam without the drastic loss of low-rpm power, associated with loss of cylinder pressure.
The common trait among these is the finished cylinder pressure.
When working withe three common SBMs;
At the same static Compression ratio, and same Intake closing angle, and same local elevation; the Dcr and cylinder pressure will come in very similar . However;
the
low rpm performance will differ by the same ratio as the displacement ratio. So
from a 318 to a 340 is plus 6.9%, and from a 340 to a 360 is plus 5.9%, and that is the difference you can really feel.
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The thing is with 3.23s as the go-to rear gears, this low-rpm business in first gear, does not end until about 3000rpm/30mph. But it is always there, anytime the rpm is under 3000 to 3300 or so. That would be
between about 30 and 50 in second gear, and between about 50 and 75 in top gear. So then any time you want Rapid Transit, you need to downshift.
All things being equal, the 360 has the strongest low rpm, the 318 has the weakest, and the 340 runs right up the center.
To get around this with an automatic, you can just up the stall......but that affects the cruise economy.
So then if you need the bottom end because of the modest rear gear, then the 360 becomes attractive.
But if you don't mind running 4.10s then the 318 is fine.
And of course the 340 runs up the middle .
Since the differences in the bottom-end equate so nicely to the differences in displacement, you can use those same percents to determine what rear gear to run, like this;
318 x 4.10= 1304 and 1304/360 =3.62, so the 318 with 4.10s will feel similar to the 360 with 3.62s....... below about 3000 rpm.
However this doesn't tell the whole story, because 3000 rpm in first gear with 4.10s is only ~22.5 mph, whereas with 3.62s it would be ~24.5..
Of course, the 340 wants 1304/340= 3.84 to have a similar sub 3000rpm bottom-end performance.
As you can see, there is a huge difference in the rear gear differences.
As to cruising speeds, at the same 65mph, the rpm maths out to;
2930 with 3.62s
3110 with 3.84s and
3320 with 4.10s ; all at zero-slip.
Some to all of these differences can be negated with stall. The goal is to net the same torque number at zero mph, with each engine.
Lets say the 360 makes 240ftlbs at a stall of 2000rpm. The 340 would then need to make that same number to offer the same take-off performance with the same rear gear; but that might not occur until 2400; and the 318 might not get there until 2800. These are arbitrary numbers I just picked at random to illustrate the point.
But of course each of these engines will make progressively more power using the same components..... so the 318 is still gonna need more rear gear to keep up with the 360, as the speed gets incrementally higher.
The bottomline is that if cruise rpm dictates a small-number rear gear, but you still want the blast-off power with a smaller engine, then the only option, with equal build parameters of Compression ratio/heads/cam/and elevation, as stated above; is the stall.
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What does it all mean?
well, if you have a low-pressure 340 and you stuff a big cam into it with no other changes, this guy, ir333 says;
Long duration cams move the power band up.You will have to rev quite high to make any power.On the street you want to make lots of power at lower rpm where most of your driving is done.
A nasty sounding cam with rear tires that are too large quite often has disappointing power below 3000 rpm.
On the street you want to make lots of power at lower rpm where most of your driving is done. and I agree. You can always back off the throttle if you get too much wheelspin. But if the engine won't even squawk the modest 295s back there, that would be such a disappointment.
Happy HotRodding
BTW
if you do not yet have a 340, the 360 is a cheaper build and falls together with closed-chamber heads, with enough compression ratio to make worrying about cylinder pressure, a thing of the past.
