As for fuel-economy ;
There are several modes of operation that affect your fuel economy; from cruise rpm, to time spent at lower rpms, to accelerations in getting up to speed, and the tune.
Street-Engines suck gas as;
1) the power-stroke gets shorter.
2) the intake valves close later
3) the overlap period gets longer,
4) the cylinder loses efficiency
5) the cam designs lose intensity
Street cars suck gas;
1) as the engine transitions from being a low-rpm power-plant, to a higher rpm unit
2) as cruising rpm increases
3) as the stall is increased
4) as tires get wider, or
5) tire pressures are reduced
6) overdrives, are NOT always the answer.
My street-experiences;
1) power strokes of less than 100 degrees suck gas no matter what you do.
2) Power-strokes greater than 120* are ridiculousdly long.
3a) long overlap periods without headers are not worth a nickel
3b) overlap periods of less than 44* are not worth having
3c) overlap periods over 76* are not very streetable
4) cylinder pressure by itself hardly affects steady-state economy. But, a low-compression/no squish, chamber sure as hell doesn't help. Furthermore, getting up to cruising speed with that low-compression chamber is gunna cost you.
5) advancing a cam to get pressure, is a lousy thing to do, cuz you are stealing power-stroke to get that pressure. Sure, it works at the race-track, cuz you replace that lost powerstroke with rpm. and you can do that on the street too, but who wants to cruise at over 3000rpm?
6) low-intensity/low-lift cams are are just not worth it. I'd rather wear out a cam every two or three years, and enjoy my car, than install a 10 year cam, and live with the lazy pos low-intensity design.
7) if fuel economy is important to you, the stinking tune is everything. You can't just bolt a carb on and go. Sure it may be working just fine, but until you mess with it, you'll never know what's costing you several mpgs.
8) for power WITH economy, yur walking a tight rope, and every aspect of the engine design has to be scrutinized. You gotta start with the Economy aspect, and work backwards looking for power.
9) a cruizing rpm of much less than 2400, in a hot-performer, is NOT realistic with a conventional distributor; because it simply cannot supply the cruise-timing required. Your combo might want cruise timing in the window of 50 to 60 degrees. Whereas, your power-timing might stall at 26*@2400; so you are or could be say 30 degrees short. On a Mopar, I can get 22 to 24 out of the Vcan, which is still at least 6*to 10* short. and if you run it that way, the VA may give you detonation at Part Throttle. So, you gotta take this into consideration during the design stage. There's no sense in cruising any slower than 2400, as the economy will go down faster by loss of efficiency, than can be gained by a reduction of rpm.
Thus, your final-drive is limited to a 2.94.
But, taking off with 2.94s and low-pressure engine, is a sure fire ticket to sadness........ so now, you GOTTA get another lower gear. If yur thinking of running a TorqueFlite od with a .69 ratio, now you can run a 4.30 in the back, which equates to 2.97s to cruise with, and the problem is solved. Or is it?
Now you gotta build an engine that suffers the indignity of cruising at 2400, but you want it to still boil the tires in at least two gears ........ right?
Well with 4.30s , a first-gear burn is assured. but that's a very short burn until yur out of pavement and Second gear is called for. In this situation, there is absolutely no reason to have a big cam in it, especially if the Dynamic cylinder pressure falls into the basement.
So, in lieu of camshafting, where is the power gonna come from?
Well, yur gonna need a lotta cylinder pressure, which you cannot get away with by using iron heads..... so now yur buying expensive alloy heads to run near 200psi. and yur gonna want to get them ported for a modest-duration cam. Speaking of which, you can make that cam work like a bigger one using a higher-ratio rocker. and of course, your automatic should have a matching stall to tie everything together. and because the cam is by now at least two sizes smaller than you may have originally thought to use, now you can surrender intensity for a longer-life cam-install.
And then
10) comes the choice of carburetor and intake ..........
11) if I told you that I once got 32mpgs out of a 360 with a 223/230/110 cam, in a straight, point to point, all-day roadtrip, would you believe it?
and same combo but with a bigger carb, still got into the 12s at 106mph, on 245 street tires. Thas ok, nobody else believes it either. Like I said earlier; once the parts are matched, the rest is in the tune.
