Stock 340 cam? For mpg? not gonna happen
Mileage in overdrive is likely to be worse than in direct; that cam has to be reving to slam the door on reversion, and for fuel economy, should maybe possibly, not have long-tubes, depending on the cruise rpm.
268/276/114
in at 110 (4* advanced) the Ica is 64* which reduces your effective stroke to 2.79 inches. This is all you are left with to make cylinder pressure with. But that means your piston is already .79 inch up from the bottom (with the intake valve still at ~.012 off the seat)
This is all fine with the engine over 3600rpm.
but cruising under 2400, two things will be happening, neither of them any good for fuel economy.
Firstly, the piston, rising on the compression stroke, will be trying to send some of it's just-inducted A/F mixture back up into the intake. The slower it revs the easier this gets for her. This, at idle, messes up the plenum-action and reduces the vacuum signal, so you gotta mess with the carb, and then the cruise mixture takes a dive.
Secondly, that cam has a modest 44* of overlap., which is the time when both valves are slightly open between the end of the exhaust stroke and the beginning of the intake. Long tube headers are designed to work with this period by helping the A/F mixture move towards and into the combustion chamber.... before the piston actually starts working at that same job. That's all fine after 3600 or so.
But at lower rpms like cruise rpm, those headers are gonna pull on the plenum and suck a portion of that mixture right across the top of the piston and out the tailpipe it goes.
But there is a third problem with a low cruise rpm;
Under 2200 rpm it is nearly impossible to give the engine enough cruise-timing, and still keep the power-timing in check, with the factory type distributor. At 2200rpm your engine might want 56 or more degrees of timing to get all the mixture burned before the exhaust valve opens. This is as good as impossible to make. Even if you lock the distributor at 36*, you will still need 20* more from the vacuum advance system, which can be done..... but you better have 1) the right combination of dynamic compression and knock-resistance for that locked distributor, and about a 3500TC so as to get the engine up over the hump of being prone to detonation.
How much is it worth, in terms of fuel mileage, to get the timing up there?
IDK, but I can guess at least 2mpg, maybeeven up to 4, it depends on how bad your current timing curve is. Typically, from the factory, the timing mightabin 28* at cruise rpm of 65=3100 with 3.23s and typically short tires. That's barely half of what she needs. You can probably get away with power-timing of 1* per 100rpm , and starting at 16*@1200, that will get you 26* at 2200 for power timing, and you can mod your Vcan to about 22*, so a total of 48*. And so that's how that goes. So; the lower the rpm the worse it gets. You will lose economy faster with not enough timing than what you will lose with increasing rpm. The sweetspot is around 2200 to 2400.
Your Iron-headed 360 with the factory 340cam, will not like 2.76s I can almost guarantee it; and 2.94s are only 6.5% better. However with 2.94s the rpm is already around 65=2500 with 27" tires and 3%TC slip, so you are good there. For fuel-economy, you want the convertor to not be slipping too badly at cruise rpm, I mean unless you have a lock-up, you are always gonna have some slip.
But there is a Fourth problem with this cam and this is likely the big economy killer. In at 110* the power cycle ends at is 76*BBDC with the exhaust valve already ~.012 off the seat. This cycle is only 104* long. So there will be a lot of pressure going right out the exhaust when that valve pops open. And even worse, because you cannot start the fire early enough, this exhaust will not be finished burning in the chamber. So then, all that energy that could have been used to propel your car, is instead, heating up your exhaust, on it's way out the tailpipe.
But hey, it could be worse; you mighta picked the 292 cam, lol.
Now, as far as that 340 cam and it's modest 44* of overlap is concerned, this is actually a step in the right direction. with the factory manifolds killing the overlap cycle, they did reasonably well on gas. If you switched to a 110 Lsa, it could be a lot worse.
However, the thing to remember is that this ICA spec of 64* is at .008 tappet-rise so, the intake valves by the math are still open at .012. It's gonna be a long time before the intake valves actually close, and the compression cycle begins; I'm gonna guess that the pistons will be close to half way up, and that really sucks in producing pressure, which makes heat, which builds the torque, which moves your car.
For your application as regards low-rpm fuel-economy, the SPECS of this cam are just wrong. I'm not saying the cam is no good, cuz they make power. But they also suck gas.
You need a cam that slams the intakes closed sooner, OR,
you need to cruise this one at a lil higher than 2400rpm.
With 2.94s hitting 65=2450 @3% slip, this is as good as it gets. Now you just gotta get the cruise-timing up to where it needs to be.
And finally, I'm not trying to rain on your parade. I already made all these mistakes, and know what NOT to do. So
Happy HotRodding
