Engine RPMs

[Mister Moose]

Originally Posted by The Master Cylinder

Not sure I agree with this statement, low manifold pressure would indicate lugging.

Isn't lugging a condition of too low an RPM for the charge being burned in the cylinder?

Isn't 28 inches of MP at 1,000 rpm worse for the engine than 28 inches MP at 2,000 rpm?

Perhaps you meant low vacuum indicates lugging.

[alexiskai]

I was told that lugging in a Model A context is defined as (a) low RPM + (b) high gear + (c) throttle and timing combination putting the engine near peak torque. Putting high torque on the pistons and crankshaft at low RPM for an extended time (because you're in high gear) can exceed the ability of the oil layer to keep the moving parts and the bearings separate, causing permanent damage. There's also the possibility of detonation, because the compression stroke is relatively slow for the amount of fuel-air mixture you're trying to combust in each stroke, which increases the chance that the FAM will detonate. That's what I was told anyway.

What this means is that you can avoid lugging despite being in a gear that's "too low" by backing off on throttle or spark advance. You won't get the power you would get if you were in a lower gear, but neither will you damage the engine.

Edit: I meant a gear that's "too high." Y'all know what I meant.

[Patrick L.]

Originally Posted by alexiskai

I was told that lugging in a Model A context is defined as (a) low RPM + (b) high gear + (c) throttle and timing combination putting the engine near peak torque. Putting high torque on the pistons and crankshaft at low RPM for an extended time (because you're in high gear) can exceed the ability of the oil layer to keep the moving parts and the bearings separate, causing permanent damage. There's also the possibility of detonation, because the compression stroke is relatively slow for the amount of fuel-air mixture you're trying to combust in each stroke, which increases the chance that the FAM will detonate. That's what I was told anyway.

What this means is that you can avoid lugging despite being in a gear that's "too low" by backing off on throttle or spark advance. You won't get the power you would get if you were in a lower gear, but neither will you damage the engine.

Someone that understands ' lugging ' ! .

I've always felt ' lugging ' is pretty much of an old wives tale.

If you have an oil film that is doing its job and the engine isn't detonating, no harm done.

I think 'lugging' bothers the driver more than the engine.

[Patrick L.]

Originally Posted by Mister Moose

Isn't lugging a condition of too low an RPM for the charge being burned in the cylinder?

Isn't 28 inches of MP at 1,000 rpm worse for the engine than 28 inches MP at 2,000 rpm?

Perhaps you meant low vacuum indicates lugging.

That tends to be general consensus.

You've never run an engine at 1000 rpm with 28" of MP ? Sure the 2000 rpm may/will be better for performance but then I prefer more MP than that.

[Billy 1931 Model A]

To all - your responses are super-helpful, especially the table from AlexisKai. Conclusion from the table: driving at 25 MPH in 2d is about 2000 RPM. Shifting into 3d will drop RPM to about 1100 RPM. Although that feels like low RPM, it's still in the proper RPM range - it's not lugging.

I want to drive now.

[The Master Cylinder]

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Originally Posted by Mister Moose

Isn't lugging a condition of too low an RPM for the charge being burned in the cylinder?

Isn't 28 inches of MP at 1,000 rpm worse for the engine than 28 inches MP at 2,000 rpm?

Perhaps you meant low vacuum indicates lugging.

Ah, my bad. I misunderstood you. Low vacuum, high suction (high manifold pressure). High manifold pressure is not something I would normally measure or have a way to measure in my car. Maybe if I was flying an airplane?? I do have a vacuum gauge tho.