If throttle setting hasn't changed and engine speed hasn't changed, what would cause manifold vacuum to change? I think at any given throttle setting and engine speed, manifold vacuum should be pretty constant.

It's true that if you want to be very scientifically precise about things, there are other factors that need to be considered, like atmospheric pressure, ambient temperature, and engine temperature. Probably also the chemical composition of the fuel. But all those things are likely to not change much, especially within any given brief interval. So I think it's OK to simplify things and more-or-less ignore them. But maybe you're thinking of other factors that I've overlooked.

You seem to be saying that even if throttle setting, engine speed and manifold vacuum all remain constant, that after "a short period of time" there is something else that is going to cause fuel consumption to change. I can't imagine what that would be, so I hope you'll explain what it is.

If your requirement is to maintain a 20% throttle position and a 2000 RPM engine speed in any gear, the only way to do that would be to have exactly the grade necessary to maintain that tps and rpm.

Imagine you have the magic ability to create a hill with the perfect grade that would be very steep in 1st gear, and less steep as you went to a higher gear. You would adjust the grade percentage so that at your 20% tps and 2000 rpm your vehicle maintained it speed exactly.

Then you would have a consistent vacuum reading regardless of the gear chosen, because you would be using more power for horizontal motion in higher gears, but less power for vertical climbing. in lower gears the opposite would be true. The load and fuel consumption would be exactly the same regardless of the gear you were in.

In reality such a magic mountain does not exist, so at a specific tps and rpm one or the other would not be sustainable, if that makes any sense.

If you were at 20% and 2000 rpm in top gear and happened to be climbing a grade that coincidentally was just the perfect amount to allow you to maintain 2000 rpm and 20% throttle, your vacuum would be lowest under those circumstances and your BFSC would be highest.

Your load would be matched to your power, and would be the maximum available at 20% tps position.

Otherwise you would have to go to a higher rpm or a lower tps to maintain one or the other (tps or rpm)

Hope that makes some sense.

regards
gary

I understand you, and I agree with you, and I don't think there's any fundamental difference between what I've said and what you're saying.

Let me start by pointing out that there are other ways to look at it, other than a magic hill (although that's a perfectly reasonable way to look at it).

Instead of a magic hill, imagine that you're dragging a giant adjustable parachute, and the drag it creates can be made to vary. Or imagine that you're fighting a headwind, and the weather keeps changing. Or imagine that there is a change in the pavement surface, which creates greater drag. These are all the equivalent of your magic hill, and any of these ideas can be used to help understand what we're talking about.

You're basically saying that a given throttle setting, combined with a given engine speed, will not remain constant except under exactly the right conditions. Of course you're right. They will remain constant only if the forces are in equilibrium (the power coming out of the engine, as compared with the power required to maintain the vehicle at that certain speed).

But the basic statement that's been made is still true: fuel rate per unit of time will remain constant if throttle and rpm are held constant, regardless of gear. You're basically saying this: 'well, you won't be able to hold those things constant, unless you're climbing just the right grade.' That's true, but it doesn't negate the statement.

Another way to think about this is to let go of the idea of holding vehicle speed constant, and just think about fuel rate at a particular instant. In the absence of the magic hill, a given state (e.g. 20% throttle and 2000 rpm) will only last for an instant. But in that instant, the fuel rate is the same as it would be if you were passing through 20% throttle and 2000 rpm in any other gear.

I think that may or may not be true, depending on what you mean by "highest." This moment might not represent the best BSFC your engine can produce. It just represents the best BSFC that is possible, assuming certain conditions: you want to climb this hill, and you want to do it at this exact speed. For example, it could easily be the case that you could enhance BSFC by choosing, say, 80% throttle and 3000 rpm (i.e., a 50% increase in vehicle speed).

We agree completely Monroe.

The reason I said only for a moment in the pervious post was because you could not maintain 2000 RPM and 20% throttle position in any gear. You would have to change tps to maintain rpm or vice versa. It might be possible, by pure coincidence that in high gear at 20% throttle you managed to maintain 2000 rpm, but it would be purely coincidental.

The adjustable parachute analogy is great. You are adjusting the load to make the 20% tps and 2000 rpm remain constant. When you did that the vacuum would be constant, as well as coincidentally the same regardless of which gear you chose.

That would probably be close to you maximum BSFC, as long as the 20% 2000rpm limitation was applied.

Under normal circumstances the point at which you reached 20% and 2000 rpm, the power created would be used to maintain your speed, while any additional power created would be utilized for additional acceleration, which is why I originally used the only for a moment caveat.

Great discussion.

regards
gary

Yes, this is a very helpful discussion, and we're basically seeing things the same way.

This is just where I want to add a clarification for the sake of readers who could become confused.

When I say 'load,' I basically mean throttle position. When you say 'load' (at least in this instance), you are referring to what I would describe as the various forces that the engine is working to overcome (like drag, friction, and inertia). "Load" seems like a natural way to describe those forces, but things get confusing because the word "load" also has a specific technical meaning: the volume of gases moving through the engine, as a % of the engine's total volumetric capacity.

The confusion is amplified because these two kinds of measurements tend to be in balance. They are in balance, by definition, when vehicle speed is constant. When we increase throttle opening (what I call load), the engine is producing more power and can overcome more drag. That could mean maintaining a higher speed, or climbing a steeper hill, or dragging along a larger trailer (or parachute).

I know this got us into trouble in another discussion, but hopefully at this point we can just accept the fact that we are using that one word differently, even though we agree on the underlying dynamics.

Monroe:

Just different perspectives of two intelligent people my friend.

I think of load like a tugboat pulling a barge. Without the barge all loads are related to the tugboat moving itself through the water. They can be internal loads (mechanical and friction losses) that occur when power is transmitted to the propeller, and hydrodynamic loads related to prop losses and hull friction.

When you add the barge it increases the sum of the loads, and when the total weight of the barge is increased the sum of the loads increases even more.

As long as the sum of all load factors is the same the amount of fuel required to move the barge at the same speed will be identical.

regards
gary

Since we're talking about ways to vary the amount of drag placed on an engine, it probably makes sense to mention tractor pulls, which use a special weight-transfer sled, for that purpose.

See here:

https://members.aol.com/pullingtractor/a1sled.htm
https://members.aol.com/pullingtractor/sledinfo.htm

My Elm327 finally arrived. I haven't tested it yet. I hope that tomorrow I'll be able to spend time on it.