Cause And Effect - Lugging The Flat Head Engine

[keithb7]

I am interested in digging in and learning the facts behind lugging a flathead engine, and the results of too doing it too much. A group discussion with your input, experience and knowledge is welcome.

 

Lugging, I believe is the act of putting the engine under heavy load while at lower rpm. Maybe your throttle is ¾ open or more while travelling at a lower road speed, under a heavy load. An example might be climbing a steep hill in a high gear. Say 3rd gear in an old 3 speed manual transmission. 1:1 output ratio on the tranny. Depending on your rear-end gear set, tire size, and how much weight you are trying to move. The number of passengers. Luggage. Tools, water jug in the trunk, amount of fuel in the tank, etc, all have quite an effect on the old flathead engines.

 

What actually is going on in your engine when you lug it? Some thoughts I have include:  Little to no vacuum. With the throttle is wide open the cylinders are sucking in all the air possible through the carb intake. If the engine is generating little to no low vacuum, I assume we have little to no vacuum advance. Is the timing retarded in these conditions? The rpm's are low, so centrifugal advance is maybe not having much of an effect either? If so, then I assume timing is retarded under these conditions? Is it? It seems to me the engine produces more heat when lugging. I can think of maybe a couple of reasons. The car is moving slower, the engine RPM's are lower so the fan speed is slower. There is less air moving across the rad. So it's not able to cool very well for the amount of fuel being burned. The water pump is turning slower, less volume moving through it. I can't be sure, does the combustion flame burn and travel in a way that is further damaging? Inefficient? Does it contribute to carbon build up?

 

Lugging under heavy load must have an effect on the crank rod and main bearings I assume. The weight of the car, going up a steep hill is bearing down heavily on the crankshaft. The reciprocal stresses I suspect are great. The results of internal combustion drives the piston down, turning the crank around in a circle by-way of the rods.  The throws of the crank 90 degrees to the piston travel, manipulate torque. The weight of the car and the load up the hill are fighting against this rotating mass. The stresses immense I presume. A weaker crankshaft might twist or break under such stress. High quality clean oil in that 0.0025" bearing to crank clearance is really, really doing its job at this point. Engine bearings are prone to accelerated wear I assume?

 

For what reason do people lug their engines? To try and maintain a faster rate of speed while navigating a steep hill? Pushing the engine to its limits, proudly thinking, "I can climb that hill without dropping a gear? If you do drop a gear, the engine rpm can be maintained at a steady reasonable rate.  You will indeed climb the hill slower. You'll have your throttle partially open, versus wide open in top gear.  The cooling fan turns faster.  You're probably putting less fuel in each cylinder with each intake stroke. More fuel means more heat. The engine stays cooler. Is there less stress on the crank and bearings? I suspect so.

 

Lugging the engine, the silent engine killer?

Edited April 3, 2022 by keithb7

[Bryan]

Things not to do, lugging the engine, revving before letting the engine warm up, over-revving the engine in general, letting it idle with the fluid drive engaged and foot off the clutch, riding the clutch, short trips constantly.   

[Sniper]

The closest I get to lugging an engine is when I am in a neighborhood driving something that has a loud exhaust.  I will generally get into high gear ASAP.  Not really putting a major load on the engine, but if I want to accelerate I am in the wrong gear for that.  Now if I am going up a hill with the pedal matted and the car isn't accelerating, or worse yet losing speed, I need to downshift.  That is lugging to me.

 

Now as to some of your questions.  Timing, loss of vacuum will remove the vacuum advance, which will retard timing.  Mechanical advance may, or may not be lost.  Only way to know for sure is to plot your mechanical advance curve then know what RPM your are lugging the engine at, then you ought to be able to look at your plot and see. 

 

Retarded timing helps with knock prevention, which lugging can cause, this is why vacuum advance goes away at WOT.  It will also tend to increase the heat load into the cooling system, which can cause knock if the engine gets too hot.  At a normal WOT the air thru the radiator increases as the car accelerates, when you are lugging it doesn't. 

[Dan Hiebert]

I learned to drive on a manual transmission in a hot mountainous region.  My teacher (Dear ol' Dad) made me learn on hills and preached not to lug the engine - ever - because it overheats the engine.  No in-depth explanation how at the time (I figured that out once I decided to learn how automobiles tick), just always keep the RPMs up or the engine will be ruined from overheating.  There is no scenario where lugging the engine is beneficial, I have a practiced befuddled expression that I give folks who insist it needs to be done.  The explanation why always comes back to some nonsensical reason.   Most of the time when I would encounter someone lugging the engine up a hill, it was because they were afraid to downshift because they didn't know they could, should, or how.  (A foreign concept to most of us, but there it is.)  

[Plymouthy Adams]

never underestimate the Word of Dad 

[Sniper]

52 minutes ago, Dan Hiebert said:

No in-depth explanation how at the time

 

I really try not to do this.  I usually explain the fundamentals to my son.

[Matt Wilson]

Lugging can indeed damage the crankshaft bearings.  They become fatigued and the surface material starts flaking off.  Exactly why this happens, I don't know, but the comment mentioned earlier about knocking (pinging or detonation, I believe was meant) may be correct.  With detonation, the loads go up and I suspect they can cause damage to the bearings, amongst other things.

[Sniper]

Matt is on to something I think.  knocking hammers the top of the piston and that is transferred to the rod and the bearings. 

[TravisL17]

Lower engine RPM also means lower oil pump speed which means less oil volume. If your engine is already tired, the low volume equates to lower oil pressure at low RPM. Combine that with the high load on bearings from lugging an engine and it's a recipe for increased bearing wear.

[harmony]

22 hours ago, Bryan said:

letting it idle with the fluid drive engaged and foot off the clutch

Unless I'm showing off to a first time passenger, I always shift into neutral and let out the clutch at a set of light.  I have an understanding what's going on in the fluid drive, when you don't do that.  But I'm curious what harm that it might cause?

[plymouthcranbrook]

Although lack of maintenance was a factor my Father had flathead Plymouths and Dodges all his life.  He drove by starting out in first and speeding up to five mph or so.   Then into second and up to ten or twelve. Then into third and lugged it up to twenty or so. Every engine he ever had either threw a rod or spun a bearing.  Heavy lugging even at lighter throttle is bad.

 

[Bryan]

4 hours ago, harmony said:

Unless I'm showing off to a first time passenger, I always shift into neutral and let out the clutch at a set of light.  I have an understanding what's going on in the fluid drive, when you don't do that.  But I'm curious what harm that it might cause?

 

[Bryan G]

With a previous car I owned, I was often guilty of lugging it when making the turn onto my street. Mostly it was that small joy, knowing I could chug along at 10-15mph in high gear. Until maybe a year ago, I never realized there was anything wrong with it. (My father was beyond stick shifts by the time I was learning to drive! And when he did have a stick he really abused it.) Of course, now with Fluid Drive I no longer have to worry about it.

[Dan Hiebert]

22 hours ago, Sniper said:

 

I really try not to do this.  I usually explain the fundamentals to my son.

I was a quasi-typical teenager, in DoD's defense, he may have explained it to some degree, but I was more interested in the driving aspect and didn't pay enough attention to remember that he told me anything. 

[Dodge City]

The diesel trucks I drove in the 70's came with explicit instructions from the bosses I drove for that said "never lug the engine, it will score the cylinders and is hard on the bottom end" "you'll ruin the engine you dumb kid" The operating  RPM on a cummings was 1800-2300 then. Todays redesigned diesels are 1000 0r 1100 on the bottom of the range to 1450RPM on the high side  they want you to upshift then for better fuel economy. We are upshifting to achieve lower RPM now at a  point where we would have ruined engines back then.

The old Volkswagen engines I have were very explicit as to the harmful effects of lugging the engine.  

Interesting how they managed over the years to redesign the harmful effects of lugging out of the engines to improve miles per gallon.

Good Topic Keith!

[John Fleming]

Back in the late sixty's truck driver who learned his trade in the army made this statement .  An engine often lugged  will consume oil. run hot . and have head gasket failure .  The man drove a tanker truck and delivered Exon gas to service stations.  

[JBNeal]

Flywheel mechanical advantage decreases with engine speed, and the lugging can be considered the dithering of the mechanical advantage from flywheel over the driveline to driveline over the flywheel.  In the instant between cylinders firing, the driveline decelerates the flywheel during lugging so that with the next ignition, the piston transfers force into the lower end bearings resulting in higher pressure on the bearing surfaces, as well as a momentary spike in piston, head gasket, and head temperatures as the ignition wavefront increases pressure in the combustion chamber.  The longer the lugging occurs, the more that the cumulative effects of pressures and temperatures build, accelerating wear on critical components.

[47 dodge 1.5 ton]

After reading these post I guess the general idea that lugging an engine if when it’s flat and can’t pull any in that gear, pinging, running it hot. 
  I have always liked any inline 6 cylinder due to the low rpm torque. Mopars in the seventies were common to have high ratio rear gears(2.76-3.23’s) and overdrive to boot. That combo provides a real dog for power at 45mph. Remember, those slant 6 taxis ran forever with steep gears.

 I just wanted some clarification to what is actually considered lugging.

[keithb7]

I think if you are climbing a steep hill, in say 3rd gear with a 4:11 rear end , able to maintain minimum 40-45 mph, yet no power left to accelerate, that’s probably not lugging. You are likely maintaining a decent higher engine rpm.  As you climb , if ground speed drops, you are headed towards lugging. You could remain in 3rd gear, mash the throttle and maybe maintain 25-30 mph. To me, that’s lugging.  You are now at a low engine RPM. The engine torque will keep you going up the hill. Engine is under a lot of stress. Carrying-on like this the temp gauge creeps up. 190-200. Depending on length of hill climb. Ambient air temp has less affect than you’d expect. Low fan speed. Heavy load. Low RPM. 

Down shifting to 2nd, the engine rpm jumps up. You certainly quickly loose some travel speed. Then you can accelerate again. Say up to 25-30 mph comfortably. The engine is buzzing now but not severely. Lots of cooling fan speed. You can easily maintain 25-30 mph in 2nd and accelerate more if needed. I’ll hold second gear and climb the remainder of the hill at about 25 mph. The temp gauge stays steady at 170.  Sure it seems slow compared to modern cars. My car is 84 years old. I don’t expect it to perform like a modern car.
 

These old cars have no tach so its hard to tell you exact rpms in these scenarios.  The flathead engine makes decent torque but it has its design limits. I could pull the hill in 3rd. Sling shot it. Get a good run, reaching 50-55 mph before I hit the uphill grade. Pull the entire hill and not drop below 40 mph in 3rd. I’ve done so, many times. Yet its a residential area. There is only 1 car like mine in town. Irritated neighbours may take offence.  Plus the speed limit is 30 mph. County-mounties have been hanging out there lately. I often thought I’d share a good laugh with the police officer if he ticketed a 1938 car for speeding. 

Edited April 22, 2022 by keithb7

[greg g]

I have posted a video of me driving my car in third at 30 mph uphill, slowing to make a 90 degree right turn, then pulling the hill from the turn and accelerating back up to 30, still in third gear. No popping,no vibrating, no pinging, no bucking.  I also posted a video off a 7 mile uphill run from stop then pulling the entire hill at 60 indicated in high, probable at about half throttle.  Two years ago, I drove up mount Graylock in Massachusetts in third gear for all but about 300 yards.  Also been up Mt Cadillac in Arcadia National Park, only down shifting to second only to avoid running into the Toyota Camry in front of me that was struggling to make the climb.  This with a 230, 4:11 gears, and 27.5 inch diameter rear tires.

[keithb7]

I'm not saying the car or engine won't do it. I pondering whether or not it shortens engine life when we do it.

[greg g]

Doesn't seem to phase i

 

 

 

[greg g]

Catch that Tacoma.  I had just pulled over to setup start my camera.  As I finished a Toyota pickup passed doing the speed limit or more.  Started the record and set out to catch him...