Fuel Economy, Vac Vs Rpm

[CozmoNz]

Ive always wondered, even though its a fairly pointless thing to wonder (but gives people something to read / reply to for the time, until we all die).

Your cruising along, in say 3rd gear, 50kph, 3300rpm, at around -15 vac....

holding just fine... whats better for fuel economy, the above, or changing to forth, say 2400rpm, -10 vac... .

Less throttle, more rpm... more throttle, less rpm....

Since vac basically means more airflow (less vac).... and more air = more fuel? yah? but more rpm = more air... so uhh, cozmo confused .

[Quint]

I've often thought about that.... Yea, i can't help you

[fangsport]

didn't the XD,XE falcons have an economy gauge, that was essentially a vacuum guage?

[Lloyd]

And some Commodores I though, and a heap of BMWs

[Dirtbag]

Lotsa mercs had them aswell, and yea, i have always thought about that question, i would think its less revs, but could be wrong

[bluemaumau]

i always thought -10 was good

[fuel]

Yes I've always thought about that too.

I would rather the engine to be under less load (ie -15 vac) and do higher revs rather than vice versa. Just because the engine is doing higher revs, doesn't mean more fuel is being injected. Think of a car engine braking or on overun at high rpm with throttle shut - it's under high vacuum and little to no fuel is being used.

[Dirtbag]

im damn sure ive read in a 16v manual or something that without the throttle on fuel is only returned once the revs get below 1400rpm. Eg engine braking there would be no fuel flow till the revs dropped right down...

[Akane]

I have seen a certain "modern" engine's "fuel efficiency" graph to maintain constant cruise speed, the sweet spot was around 2.5k~3.3k rpm, then it's a x^2 (well not that steep) graph as the RPM rises.

hth.

[Dirtbag]

so with most "sports" cars that would mean around 4th gear up to 70ish or even sometimes higher? I know im doing 2000rpm at about 65ish in my ae101 in 5th.

[slighty_sykotic]

I know the answer to this... I just have to remember....


Driving with my laptop hooked into the link so I can see the injector duty cycle. Now, i just can't remember which way around it was


In 3rd gear it was 4% duty cycle at 50, and dropping into 4th was 3% duty cycle at 50km.....

I think.... Pretty sure it was around that way (not the other).

in a 4agte in a ae101 with a c52

--Sykotic

[Stealer Of Souls]

Ive always wondered, even though its a fairly pointless thing to wonder (but gives people something to read / reply to for the time, until we all die).
Your cruising along, in say 3rd gear, 50kph, 3300rpm, at around -15 vac....
holding just fine... whats better for fuel economy, the above, or changing to forth, say 2400rpm, -10 vac... .
Less throttle, more rpm... more throttle, less rpm....
Since vac basically means more airflow (less vac).... and more air = more fuel? yah? but more rpm = more air... so uhh, cozmo confused .

I would have thought that fourth was better.
I think that in third or fourth at a constant speed the required power output of the engine should be the same (???), or at least pretty close to it. So if the power output is the same then the relative fuel consumption should be the same. Except that at a lower rpm each cylinder is filling more completely to produce the same output. The change in vac could be because of that couldn't it? Also at lower rpm there are probably lower losses (pumping, pressure drops) in the engine. So slightly more efficient, and therefore lower fuel use...
???

[ee904age]

I read in an AA mag a while ago that most engines are most economical at the point of highest torque. True or Not True....who knows?

[Stealer Of Souls]

I read in an AA mag a while ago that most engines are most economical at the point of highest torque. True or Not True....who knows?

I think I read that they are most efficient at peak torque. Not economical... Could be wrong though...

[strx7]

injector duty cycle x engine rpm will give you a number to go by for comparisons

[Akane]

injector duty cycle x engine rpm will give you a number to go by for comparisons

Is it safe to say that inj duty has NOTHING to do with engine RPM for such scenario?
Since injector duty reads how much time the injector is held open, and that's that.

:/

[slighty_sykotic]

yup, so higher injector duty cycle = more fuel. Simple.

At same speed, compare duty cycles (well, must be same fuel pressure aswell), and that will tell you how much fuel is being used.

--Sykotic

[neon_spork]

Wouldn't it just depend at which rpm the engine operated most efficiently?
As someone has mentioned, both cases should require the same power output, so it will come down to the design of the engine, ie which rmp it can most efficiently make that power at.

[strx7]

the injector duration is directly related to how much fuel it injects right?
that and the engine RPM will give you a number, which you can relate to YOUR OWN configuration. in regards to comparing it to someone elses set up its USELSS but it's fine to compare in your own set up.
Provided you know how your injection system is configured ie: 1 squirt per engine rpm, or 1 quirt every cam shaft rotation (laymans terms of course)

you can deduce a 'value' for comparison purposes.

[Stealer Of Souls]

the injector duration is directly related to how much fuel it injects right?
that and the engine RPM will give you a number, which you can relate to YOUR OWN configuration. in regards to comparing it to someone elses set up its USELSS but it's fine to compare in your own set up.
Provided you know how your injection system is configured ie: 1 squirt per engine rpm, or 1 quirt every cam shaft rotation (laymans terms of course)

you can deduce a 'value' for comparison purposes.

This makes good sense. If you can measure your Duty Cycle (DC) for your car you can figure out for yourself what will use the least gas. DC is a "complex signal", by this I mean that it can tell you engine load, air consumption and all sorts of other little things.
So yeah. More higher DC = more gas used.