Augie

""Lugging a motor... I.E. low rpm with low gearing, puts stress on the crank and
connecting rod bearings.""

Tell me exactly how it puts stress on the bearings. In fact, tell me what you think "lugging" means.

""If you ask any engine builder they will tell you the most important part of the crank/connecting rod bearings is to never lose the film of oil that they depend on. By lugging a motor -1000-1400 rpm, full load, 6th gear, you are creating a situation of a slow moving crank, and high stress that can push that film of oil out of the way. This causes pre-mature bearing failure over time.""

I partially agree, except for the "bearing failure over time" part. If you get a condition wherein you've pushed the oil film out of the way under high load, your bearings will fry in less than ten seconds. My position would be that this simply doesn't happen in street cars, although it may very well be the case with race engines which have much narrower design parameters.

My partial agreement is based on operating the engine at "full load, 6th gear" at 1000 rpm. This may result in increased wear and tear over time, mostly on rings and cylinder walls. 1400 rpm? No problem.

""Also, if you look at how the engine's powerband is developed, max hp is at 5800 rpm, redline at 6800 rpm. This is where the engine designer does his duty to make sure there is generous oiling of all bearings, under maximum hp. He will have a hard time making an engine "Bulletproof" from 800rpm to 6800 rpm. The manual warns against running the motor at less than 1400 rpm in any gear but 1st. Why do they allow 6800 rpm?""

As mentioned, the issue is not the bearings, but the cylinder walls. At low rpm and high loads (generating significant side loads against the cylinder wall), the ring may begin pushing through the hydrodynamic wave somewhat further away from TDC and BDC, because it's moving more slowly. This will generate more ring and cylinder wear over time on production motors.

I'd like to find out why Audi cautions against continuous running at less than 1400 rpm. As I've mentioned (three times now), the one area of concern around low-rpm, high-load operation is premature cylinder bore and ring wear (I.E. - not "instant" problems, but increased wear over time. Might Audi be concerned about, say, emissions?

Augie

""This isn't my view alone, but what has been common knowledge in the engine building world for a long time..."

Mike, ever since I read an anecdote from a Chevrolet engineer in Car & Driver several years back, I've been interested in the lugging "issue". This engineer claimed that lugging wasn't detrimental to production engines any more, and challenged the C & D scribes (and the readership, presumably) to find anything to contradict his position. He basically said that, for production engines, the horror of lugging was simply an old wives' tale. Since then, I've looked for articles in the SAE archives that address this issue in depth, and haven't found anything. I've also discussed the issue with a number of SAE engineers (every time I find one, in fact), and the best information I've gotten is exactly what I'm telling you. The only issue in production engines will be increased cylinder and ring wear over time. Period. It's a bunch different now than it used to be, by the way. When we all ran carburetors and had no knock sensors, lugging could be a big problem.

""I agree that running a car at 6500 rpm CAN harm an engine, and can casue more wear, should lubrication fail, but you have to take into consideration the amount of time spent at 6500rpm in a one, two, three blast, or with 99.999% of the S4's out there. Most S4's, even hard driven cars, will spend less than one second per gear above 4500rpm, and multipy this by even the 20-60 hgih rev shifts a driver might make per drive, maybe 2-4 times per week. Your talking one to two minutes per week, which is unbelievably minimal.""

""Comapre this to a race boats motor, where it runs at 90-100% of full throttle, for hours on end. My best friend Brian Seiller of Race Rigged, Inc, in Lake Blff, IL, is one of the leading and best marine race engine builders in this country (builds CHock Norris's and many other race motors). His opinion was exactly that, reving the motor up and down is so minimal, it's not even worth considering.""

Your point about little time being spent at full throttle at red line is well taken (sort of), but it presupposes constant full-throttle lugging against transient full-throttle red line operation, which is apples and oranges. Out here on the mean streets :-), either condition is likely to be very transient, indeed.

Mike, I want more agreement from you than you're giving me. I want your agreement that operating these engines at full throttle at 1800 rpm (I think that was the original statement) is no more taxing on an engine than operating at full throttle at 6500 rpm, and, in fact, is less stressfull at 1800 rpm because you aren't flexing everything so much because of centifugal force and simple inertia.

""On the other hand, the milage example is provable, and has been the case on my S4, and any other I've tested. it was also the case with my past cars. Try running your sons 2.7T at 50mph in sixth (roughly 1800 rpm), compared to 50mph in fifth (roughly 2500 rpm). You will get better MPG at 2500 in fifth, than at 1800 in 6th. At least I and a few friends in our S4's due.""

I'll take your word for it, but would you mind equating this line of thought to your assertion that lugging is bad for your engine?

""It's akin to bike racers. they try to keep a cadance of around 80rpm, and select their gears to meet that cadance. Any faster, and their buring more energy to speed. Any slower and they are not attaining the highest speed per energy expended. Under you logic, they should pedal in as high a gear as possible, to save as much energy as possible, but this does not account for the extreme pedal pressure (energy) needed to move the bike at the same speed.""

This is completely beside the point. I am not advocating lugging. I am asking you how lugging specifically hurts an engine. Forget about efficiencies, etc. They're beside the point. I personally hate lugging, and wince every time I'm a passenger witnessing it. Who cares? Tell me specifically how lugging hurts your engine. Less verbal hand waving. More facts.

""I thought the "towing a trailer" example was clear, but somehow you don't get it.""

I do get what you're saying, but it's not an apt analogy, and it's off the point.

""Let me put it this way. If it didn't take more energy to accelerate a car in sixth gear, vs in fourth gear, the car would accelerate just as fast in sixth. Also, why wouldn't the engineers just gear sixth gears twice as tall as it is, so cars could turn 1000 rpm at 60mph? Because the efficiency curve doesn't work that way.""

It doesn't take more energy to accelerate a car in sixth than in fourth. In fact, it takes *exactly* the same amount of energy to accelerate a vehicle at a given rate from a given speed, no matter the gear.

What it *does* take in sixth is a greater throttle opening than in fourth to attain the same rate of acceleration, because you have fewer power pulses for any given unit of time, so each power pulse has to be stronger. Operating at higher throttle at lower rpm will result in increased cylinder and ring wear as I've mentioned several times, but operating at higher rpm at lower throttle openings will result in increased cam and valve train wear.

With apologies to constant readers, I'll refer again to a warranty study that Chevrolet did back in the 60s, when any number of final-drive ratios were available by just checking the order blank. Thay found that cars with higher (lower numerically) final drives had more cylinder and ring wear, and engines with lower (higher numerically) final drives had more cam and valve train wear. From their perspective, it was pretty much a tossup.

My actual position regarding driving habits is "everything in moderation", which translates to the mid-range, rpm-wise - but the horror of lugging is ancient history.

The origianl issue is a guy asking whether gentle acceleration from 22 mph in third gear can be a problem, and you said it's marginal. I say there's simply no issue. He can do that forever and have no problem.

Mike S

I agree with you whoe heartedly that lugging along at 1800 rpms is not going to hurt a motor any more than reving the heck out of it, but my point was that people lug the engine duing acceleration, and that their is an efficiency point or range in any motor, where you are opperating at the greatest efficiency, and at this point, you are offering the least wear and tear. Your points are well taken, but as people who ask about down shifting vs braking, and are told by any knowledgable car nut, brakes cost less than transmisions. I would argue that replacing a block, sleeving a cylinder, or boring out a block and repalcing with larger pistons, all of which require engine removal and significant expense (blocks, pistons and sleeving cost a pretty penny) vs replacing cams and valves, which are far cheaper and can be done without pulling the motor, it is far better to wear and beat on the latter. Cams run $300-500 for the S4, and a complete valve job with parts is around $1500 ($400-500 on a chevy 350), about the same cost to just pull and reinstall an S4 or most high tech motors. Add in machine shop charges, pistons, and or a new block or sleeving, and you will spend a minimum of $4000 to do it half assed, and $5000-7000 to do it right.

Also, with self adjusting valve trains today and the ease of changing overhead cams, I think it is ill advised to base the S4 and other modern HP motors on a 1960's chevy study (I'd lend a hell of a lot more credence to a 1960's MD or Porsche study, just based on GM's lack of quality, engineering and general technology). Also, considering the already thin cylinder walls on todays 100 HP per liter and greater small displacement motors, there frankly isn't much room to bore or sleeve most of these engines. I think by todays warranty standards, that audi would vertainly see it the same way.


Thanks for the dialogue.

Mike S.

Augie

""I agree with you whoe heartedly that lugging along at 1800 rpms is not going to hurt a motor any more than reving the heck out of it, but my point was that people lug the engine duing acceleration, and that their is an efficiency point or range in any motor, where you are opperating at the greatest efficiency, and at this point, you are offering the least wear and tear.""

That's an intriguing and appealing viewpoint, but I'm scratching my head a bit...
Arguably, an internal combustion engine is most efficient (at full throttle) when it is operating at the torque peak. At that point, every power pulse is at its strongest, bsfc is at its most efficient, and the engine is happiest :-).

However, having the greatest thrust on pistons (plus piston pins, and rod and crank bearings), therefore providing the greatest side loads and resultant potential cylinder and ring wear may very well mean that you are far away from offering the least wear and tear.

Now, at *part* throttle, where would you find the least wear and tear? I dunno, but it's far from a simple calculation.

Re the top end vs cylinder boring argument: That's far from simple, either. The fact is, at 150,000 or 200,000 (or whatever) miles, you're likely to be doing a complete rebuild, regardless of whether the top end or cylinder block shows more wear than the "other" set of components. We're only talking a matter of degree, here. Furthermore, the penalties of high-rpm driving will also affect everything that is engine-speed dependent, including water pumps and other driven accessories, the transmission input and main gear cluster, etc.

Re the Chevrolet study: I hadn't thought about it until reading the article, but when you *do* think about it, it's simple physics, and I'd bet it isn't engine-dependent.

Re GM in general: There isn't anything they make that interests me (except perhaps the Z06 Corvette), but they've been building long-lived, well engineered powerplants for as long as I can remember. Those old small-blocks would run forever, and only problem they ever showed was that blue puff of smoke from a cold start-up that signalled loose valve guides leaking a bit overnight. Never affected the way they ran, though, and generally never got really bad enough to bother screwing with, either.