Large RPM and Vacuum drop when shifting from park to gear
Ported vac advance has no effect on mileage, at least not mileage at any sort of mph where mileage is measured
As opposed to what?
With a
[email protected] cam, my combo got best fuel mileage at ~56* @85mph=1900rpm; but I don't always give her what she wants.
With my
[email protected] cam, she likes 60* @65=2240
So,
You can't make
a blanket statement like;
Ported vac advance has no effect on mileage
Without a qualifier; As stated, that statement is false.
But if you say;
were the V-can is hooked to, makes no difference,
well them, that could be correct.
__________________________
The fact remains that;
the smaller the cam, and
the higher the cylinder pressure in that small-cam combo,
the less able the engine is do deal with tons of Idle-Timing; due to the butterfly to Transfer slot relationship having to fall into a very small window, before you run into tip-in issues, that call for early pump-shot to drive thru. As soon as the throttles are tipped in, the timing can be ramped up; and that is exactly how the sparkport works.
At idle, your automatic-equipped engine only needs enough power to idle on, and to not stall when put into gear, pretty much any old timing will do that., sometimes even down to 5*Advance.
And manual transmission cars are a special case.
As regards power and driveability, all our timing controls are designed to do one thing, to deliver the maximum amount of energy available in the fuel, to the piston, when it is in the best possible position, to transfer that pressure to the crank; for as many conditions as possible, as regarding load and rpm. This is most critical from stall to about 3600 rpm. After that the burn changes very little.
From stall,down to idle, there is very little advantage to perfection here. Furthermore, too much pressure is harder to drive, so you can tune this area very nicely with timing. It does not have to be super-snappy, because most of the time when you are in this zone, you are either just passing thru to stall-rpm, or cruising, or at Part Throttle. I mean it doesn't have to be lazy, but it also doesn't need to be perfect. And the more timing you have in this zone, that does not drop out with load (read mechanical), the easier it is to hit the detonation wall. This is why I run a 22* Vcan, and just 14 idleTiming, and I adjust the Vcan to to be fully proportional thu-out it's range of applied vacuum; so it's always ready and willing to drop out, and to slowly ramp back in. Cuz that's how I drive; namely, lots of cruising around, interspersed with periods of brisk acceleration, and since I live in the country, it's always a minimum 30 to 40 mile round-trip to go anywhere. So to me, bringing it home, cruse timing is very important.
Oh and since my combo is a manual trans, I am almost never in the zone below where the Vcan is coming in, which, with no load, on my 750DP is about 1600 rpm which is ~12 mph. My mechanical timing does not start until ~1100rpm, which is 8 mph., So below 8 mph, the engine is a nice stable low-powered platform. From 8 to 12 mph, the timing only changes 4*, so while the engine is picking up some power , it is still pretty stable. After 12 mph at PT, the Vcan quickly brings it in so that by 2000, even 1800, the engine is now snappy.Lemmee tel ya, snappy is an understatement; 1800 is 13 mph.
What this laziness does, is ; it allows me to blip the throttle just a bit, and more or less dump the clutch, then drive away like it was an automatic. None of this 1983Mustangchit. Where you had to rev that poor 5.0 to 3000, and slip that clutch for a half a city block, every time you took off. Slight exaggeration but you get the drift.