The Stovebolt Site Forums The Shop Area Engine Shop Does Lugging Down Harm an Engine?

(I have the SM420 4-speed. Let's forget the granny gear and call the next three gears 1st, 2nd and 3rd because those are the ones I use.)

When I come to a stop sign with no one around, I'll sometimes shift down to 2nd and not stop completely. Shame on me. When I step on the gas, because I'm actually going too slow for 2nd, the engine and truck will shutter a bit. The same happens, of course, when I don't shift down to 2nd and give it some gas while going "too slow" in 3rd.

Does this lugging down and shutter hurt the engine when it's in one gear too high for the speed?

Thanks!!

Yes lugging and engine will greatly shorten an engine's life. Eventually a connecting rod breaks and then the loose end knocks a hole in the block.

Think about what that piston is doing when you do that. It's trying to pick back up and not having a good time of it. Now as an example I'll give you my experience with lugging an engine. Please bear with me as this is an off brand, read Ferd. It was back about 20 years ago I bought a used ranger with a 2.3 liter engine with 110,000 on it. The previous owner got it new and I rode with him couple of times while he owned it. The 4 speed was in top gear buy the time it was at 35 mph. The lugging over a long period of time made those little pistons wobble on the wrist pin and about 20,000 after I bought it one went sideways. It egg shaped the cylinder wall enough that the piston lost the battle.
Now the old 2.3 pistons really have a short skirt on the and are really sussepitable to this lugging effect. The skirt on our old 235/216s are a little longer and probable not as apt to wobble in there bore.

Now in my opinion you should never let your engine work that hard to take off. You probably drive your truck enough to know when it's lugging so going back one more gear, to 1st, would be kinder to your old truck. I believe from experience that a few rpms are your friend, not your enemy.Larry

To put these old 235's in perspective:

If this were 1970 and we were talking about the exact same time scale (65 year old engine), the comparable year engine you would be working with would be a 1905 motor. (-65 years). I hope you get what I mean, anyway, being gentle with these engines is probably more prudent now than ever. (based on a 1955 engine)

Horsefeathers! Inline sixes are made to lug. The only time it does any damage is if there is an excessive carbon buildup or incorrect ignition timing that would cause preignition, which leads to detonation. As long as the bearings don't have excessive clearance and tie oil pressure is good, go ahead and lug it down if necessary.
Jerry

Wally,
Just to be clear, since you have re-numbered the gears from the standard designation, you are doing a rolling stop from what you call 2nd gear (what most people would call 3rd). Since the SM420 is synchronized in second gear (which you are calling 1st) why don't you use that gear? It should slip into gear easily, the engine is in a range that will allow fairly rapid acceleration (unless loaded going uphill) and avoid shuttering, and avoid the whole question of whether it is harmful to the engine. Unless we did a randomized blinded controlled study you will only get opinions, which few of us can back up with relevant facts.

Let's see, to do this study we would take 20 random brand new 1948 3/4 ton trucks, drive them for two years as you describe and then take 20 similar trucks, drive them for two years as I suggest, and then have Hotrod tear down all 40 engines (in a blinded fashion, meaning he doesn't know which truck belongs in which category) and then report back to us his findings.

Just kidding, of course but I am curious about why not drop into the gear just above the granny gear, unless your synchro has worn out.
Regards,
Kent

50, 51, and 52 powerglide model passenger cars start off in high gear when you are in the Drive (D) position. Now, granted, you have a torque converter in the picture, but still, with a 3.55 final drive gear ratio that 235 pulls it with no problem.

Thanks, Guys,

I lug only now and then, mostly from laziness.

I probably should actually stop at stop signs and start in "1st." And if slowing down, I should shift down to "2nd" but I won't worry too much if I stay in 3rd.

I also agree w/Hotrod. GM kept these Stovebolts in production way beyond Ferd's introduction of the V-8 because they were so dang good and tough. Thus, the Stovebolt Page!!

In 1970 my dad had a 194 ci cyl in Chevy that tow bared my 1935 Buick home from near Watertown, NY to Long Island. About 400 miles.

About a week after we were home that 6 cyl had a connecting rod blow a hole in the side of the block.

Wally, do what I do: double clutch into 2nd (what you're calling first). I've got the previous 4 speed w/o any synchros. Problem solved.

Question on this - I thought the old long stroke engines were designed to be capable of lugging? To prevent the need to downshift so often? Looking at the dimensions though, the Stovebolt is almost equal bore & stroke, so it really isn't a long stroke engine.

No engine is made to be lugged. Lugging is operating the engine at too low of an rpm for the gear it is in. The only way to not lug an engine is to be in the correct gear based on the engine rpm.

A long stoke engine will develop more torque at lower rpms then a short stroke engine. Though a short stroke will rev faster and accelerate faster.

That being said it still does not matter if an engine is an under square engine, meaning the stroke dimension greater then the cylinder bore. For if that engine's rpm is not high enough for the gear it is in you are placing to much strain on the engine.

Lugging is putting to much strain on the engine. Which is different then an engine was designed to have the torque and HP to move heavy loads.

And, even those engines that were designed to move heavy loads can be lugged.

A long stoke engine will develop more torque at lower rpms then a short stroke engine. Though a short stroke will rev faster and accelerate faster.

Or not.

"Lugging" is applying power (opening the throttle) at a combination of load & speed which causes the crankshaft rotation to dramatically slow down nearing TDC. Since the local oil pressure (at the journal) is developed by the speed of the journal passing across the bearing surface, it drops as the crank slows.
Obviously, light throttle at very low RPM is harmless - how else can you leave a stop?
This is also greatly affected by flywheel weight; a 60 lb. wheel allows much more violent acceleration from low speed than a 30 lb. wheel because the crankshaft speed is more nearly regular.

We have magnafluxed and zyglo'd thousands of engines through the years and just about never find main bearing web damage in engines that came from vehicles with anywhere near stock engines and tires. The pulled main webs are usually from sprint cars that were running on a very sticky and rough track---getting air born and coming down from 7-8k rpm to a near stop instantly will do the trick. Pull truck/tractor engines that run on a max horsepower track where the weight shut them down, and believe it or not boat engines. I guess the right prop is like a big tire on a sticky track and they do get a lot of air time. This is usually hand in hand with some kind of boost although turbo engines seem less prone than roots type blown engines.