Lugging the engine in 3rd gear

[Colorado Greg]

At what speed is considered 'lugging' the engine in 3rd gear? I know that it's not good to lug these engines but, if I can't downshift into 2nd until 15 or 20 mph, is it ok to run in 3rd at 20mph? Just trying to keep the babbit in this thing.

I rode in someone elses car and they seemed to lug the engine ALOT more than I have been. It just seems that 25mph in 3rd is lugging to me.

Thanks, Greg

[Grapejuice]

Originally Posted by Colorado Greg

At what speed is considered 'lugging' the engine in 3rd gear? I know that it's not good to lug these engines but, if I can't downshift into 2nd until 15 or 20 mph, is it ok to run in 3rd at 20mph? Just trying to keep the babbit in this thing.

I rode in someone elses car and they seemed to lug the engine ALOT more than I have been. It just seems that 25mph in 3rd is lugging to me.

Thanks, Greg

I have pulled into a parking lot and kept it in 3rd, just idling along. I wouldn't have tried to accelerate hard from that speed though. I don't think it hurts it any, unless it starts jerking, or unless I lay into it.

[pooch]

I just retard 1/2 way if I am going to go slow at light throttle.

[MikeK]

Originally Posted by Colorado Greg

. . . Just trying to keep the babbit in this thing.
Thanks, Greg

This is actually a separate issue from "lugging", no matter which of the many definitions of lugging you choose to accept.

Damage to the babbitt (other than normal wear) occurs from loading beyond its compressive strength (squishing it!) and repetitive impact shock that will work-harden and fracture it along grain boundaries.

Let's deal with the impact shock first. Contrary to uninformed opinion, classic "lugging" does not present impact load, only cyclic compressive stress which does not hurt the babbitt unless the compressive limit is exceeded. Pre-ignition, knocking and dieseling will however fracture the babbitt in short order. Analogy: A 200 lb guy can stand on a 1" thick glass table top and bounce up and down (heavy cyclic load, don't try this at home!!) without breaking it. Whack it with a 1 lb hammer (impact) and it will fracture.

Compressive strength- how much load a bearing will take before it squishes out of shape. Torque on the crankshaft relates directly to the pressure load on the bearings. At some torque load the babbitt will be permanently damaged. Analogy: You can put a penny on a railroad track and step on it all day, but when the train comes. . .squish. Now here we have to look at what goes on with a load on the engine. Let's say you want to go up a hill at a constant 20 mph. The amount of horsepower required to sustain that work load has nothing to do with what gear you are in. If that number is, say, 25 horsepower the engine can be throttled to deliver it while spinning very fast (1st gear) or slow (3rd gear).
Horsepower = torque (ft-lb) X RPM divided by a constant (5252) Therefore that same 25 horsepower to the drive-shaft in 1st gear (about 3:1 in the tranny) will require only 1/3 the torque delivery, but three times the RPM you would need in 3rd gear (1:1 in the tranny). That means THREE TIMES as much compressive load on the bearings for the same horsepower load in 3rd gear. Four times as much if you have a 25% overdrive!!

So what does all this mean, and how does it relate to "lugging"? Traditional definitions of lugging usually involve some 'seat-of-the-pants' feeling. You can damage your babbitt with no noticeable feeling, or you can 'lug' all day with no damage if the compressive limit is not exceeded. HUH? Henry's original 40 hp engine did not produce sufficient torque to do compressive damage to the original diameter, thin babbitt. Adding torque (by upping the Hp. with mods) or lowering the bearing compressive limit (shims, journal regrinds, changing the contact profile with excessive clearance) will leave you with an engine that is susceptible to damaging itself. From a reliability standpoint Henry couldn't up the horsepower without increasing the bearing size (B engines) yet many, including me, get 90+ Hp from an A. Moral of this whole, absurdly long story: Learn to down-shift! (read paragraph above about Hp/RPM/torque/bearing load)

I could go into oil film strength, but the wife just opened a bottle of champagne and is acting unusually..

[TerryH]

I have always heard that lugging is one of the worst things you can do to babbit..........some folks go around corners in 3rd gear and pull away, and you can do it, but I doubt that it does the bearings any good. I always upshift about 30mph, and downshift to 2nd going around a corner. I remember reading a post from Marco that said the same thing....don't lug in 3rd gear!

[Safari bob]

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Greg,
When you are driving slower in 3rd, remember to retard the spark somewhat. I never have the spark all the way advanced. When I slow down, I just automatically retard the spark, just as a centrifical distributor would do. As I pick up speed, I advance it. Lugging in third gear is not good.

Bob

[dave in australia]

Well said Mike, the lugging doesn't cause damage but detonation does. My dad always gets his A into top as soon as possible, but he adjusts the adv/ret lever to suit engine speed and load. The bottom end of his engine is still in very good condition and it was o/hauled in 1986.

[Special Coupe Frank]

Originally Posted by TerryH

...I always upshift about 30mph, and downshift to 2nd going around a corner.

Wow... wish I could get my A up to 30mph in 2nd...

If my speed drops below 25 mph when pulling a hill, I will drop into second and stay there.


On level ground, I'm usually in high around 25 mph.

If traffic / lights slows me down to 20 or so (level ground again), I'll stay in high, but accelerate gently. If I need speedy pick-up ( heh-heh...), I'll down-shift.

I keep my spark-lever about 1/2-way down around town (running at 35mph or less), I use full advance only for "high-speed" running ( 40 mph & above ).

If the car can't pick-up speed in the gear it's in at the moment (engine not revved-out), I consider that "lugging territory".

My $.02...

[Louis]

I've only had my 31 for a year and a half, but when i go around a corner on level grd. and still going 10 mph or more i leave it in 3rd, it acc. just fine. if i get below 10mph or going up a slight grade i shift into 2nd. i keep my SLA about 1/2.... 90% of the time.
seem's like i read in the oper. manual that 1st and 2nd gear is used only to get car in motion or under a heavy load [hill] that 3rd gear should be used most of the time.
i think just common sense, and the sound of the engine. JMHO.

[Colorado Greg]

Thanks for all the replies! I'll get the hang of this ole girl yet.

[Special Coupe Frank]

With regard to suggestions in the operator's manual, I would also keep in mind Mike K's point:

Henry's original 40 hp engine did not produce sufficient torque to do compressive damage to the original diameter, thin babbitt. Adding torque (by upping the Hp. with mods) or lowering the bearing compressive limit (shims, journal regrinds, changing the contact profile with excessive clearance) will leave you with an engine that is susceptible to damaging itself.

The original recommendations in the original operators manual were probably "Safe" when the cars were new, the engines were at factory tolerances, and if you did trash the bearings, replacement rods or a quality babbit job were a lot cheaper and easier to find than they are nowadays.

10 MPH sounds a bit slow for high-gear.

The speedo in my '28 reads about 3-5 mph "fast" at 50 mph, so I imagine we should make some allowances for speedometer error between individual cars.

On level ground, I usually make the 1-2 shift around 10 MPH, and the 2-3 shift around 20 MPH.

When the car speed drops below 20 mph ( 25 when pulling a hill ), I will down shift (Or at least think about it ). I almost never drop below 15 MPH in high and try to accelerate from there.

The heavy flywheel and low compression of a stock A are somewhat deceptive, in that they will allow the operator to lug the engine way down, well below the point where a more modern engine would buck and probably stall... that doesn't mean it's a good idea to do it frequently...

Your results may vary...

[Commander Toasty]

Driving to or from my house I have to go down a 'swell' in the road as well as take a sweeping turn, and this is on a road with 45mph traffic. To handle the turn which I enter in 3rd gear I have to drop to 25 and then climb out. That's too fast a speed to downshift - my car is not at all happy about downshifting above 15. This makes me have to lug the engine regularly. I retard the spark way down. Sure hate doing this regularly but it is just the circumstance of the road and traffic.

[Louis]

I guess no two "A"s drive alike, but if im at 25 mph and on level grd. i have no problem accl.... but under a load i do advance the spark, not retard it. and i have never had to shift to 2nd as long as i am on level grd. and above 10 mph. it just chug's along. and accl. with ease. no lugging.
25 mph is also to fast to downshift to 2nd on mine, downshifts to 2nd better under 10mph.