Just to reiterate A FD of 2.50 is about as low as the 340 will handle fuel-economy wise, and second gear with the 3.91s is perfect for street work. I know I would love that combo.........................................with a 360 especially.
I think it's also worth mentioning, that for your stated useage, I/personally, would swap out the factory cam in a heartbeat, for one or two sizes smaller with a tighter LSA, and go with one or two sizes smaller rear gear. My actual thinking is a 262/270/108 in at 106....and 3.55s
And here's why;
The smaller intake duration will normally reduce power, but in this case the 108LSA will tend to maintain the original 268* absolute power. The tighter LSA will reduce the powerband some, but the new trans doesn't care, the ratios are much closer together. So; so far, it's a breakeven thing.Then first consider that the earlier closing intake will increase the Dcr about .4, so making additional low-rpm power over the 268. Now, consider the exhaust cycle the 262 has plus 11* of power extraction. This will very significantly decrease your fuel consumption. Those 11* represent 10.6% additional time to extract energy from the expanding gases. So now we're winning. Then consider the overlap; the 262 has 6 extra degrees so it's almost a wash. Without headers it won't make a significant power difference. So at least we're not losing.
But now it gets interesting; this new cam, say it has the same rates of lift as the old one; The VP will climb from about 126 with the 268 to about 142 with the new one.But don't look at the absolute numbers, but rather at the percentage difference, which is PLUS 12.7% for the 262. It won't matter what the actual numbers work out to, but the percentage is what you're interested in. This VP number is an indicator of low-rpm performance. And I can tell you that 12.7% is huge at just off idle. The VP ratio tapers off so that by somewhere between 3000 and 4000 the percentage difference may no longer exist. But considering that every time you leave the stoplight, that extra torque is making your 340 feel like it's 12.7% bigger. A high VP is a good thing. You can read about VP here;
Cam Timing vs. Compression Analysis
The other thing about the VP is that you can reduce your rear gearing by a like amount and feel no low-rpm change as compared to with the 268. So that lets you run a 3.91 less 12.7% which rounds to 3.55s. And now you can see where I'm going. 3.55 x .68= 2.41 final drive, bringing your
hiway Rs down to 65=1946 and 80=2395 (both with 27s). And now that power extraction is working together with the gears to increase your fuel economy. The difference in Rs is the difference in the rear gears or about 10%. Generally this translates as 5% more mpgs. But with the extra power extraction, I predict an even greater gain. Now if you have a TQ,or a carb with metering rods, you will be able to easily lean out that low-speed circuit and add a bunch of timing with the Vcan, and before you know it, you'll be passing every gas-station in sight.
So let's recap;
A close ratio trans with a deep enough first gear,a smaller cam, and less gear.
Compared to the current combo,this may result in : no loss of take-off power, no loss of acceleration to 60mph;Still in second at the 60mph mark but now right on the power-peak (so actually might be quicker),, less NVH at cruising speed (any cruising speed),and better fuel economy. Finally perhaps a slightly lopier idle, but that will depend on the tune.
And now for the icing; With a faster rate of lift,(The OEM was pretty good, but faster exists), you could keep the 262 but it can make another 10 to perhaps as much as 20 horsepower at peak, and with the 108LSA that would feel really fine,lol, and it will blast thru the gears quite a bit better, with the close-ratio trans.
There is only one fly in the ointment;the current compression ratio. If your Scr is currently more than a true 10/1 then it may be that you have to run a top-grade fuel when standing on it. Probably only when the secondaries are full open, at too low an rpm. This can be mitigated with a careful tune. But I'm willing to bet your Scr is not over 10/1, or at least not by much. A careful compression test with a good quality gauge will tell the tale.
And then there's 3.73s,lol. 5% more everything, including about 100rpm at 65mph. Hmmmmmmmmmmmmmmm that could be about 20ft lbs at peak, and maybe 10 at take-off. Hmmmmmmm that's about one cam size at thru the powerband; kindof the best of both worlds. That's also about equal to the fast rate cam. But it will steal a bit of mpgs. Cruise rpm now up about 100rpm at 65..........naw I think it could be a wash.
That's my thinking
