I have a '94 NA manual transmission 850 sedan. I tend to shift at around 2000 rpms unless I need to accelerate more quickly. If I'm not accelerating and just cruising at say 40 mph I definitely drive in 5th gear with the engine running at lower than 2k rpms. After just reading an ancient thread about gas octane in which someone mentioned that running at higher rpms will keep the valve train cleaner I'm wondering if I should change my shifting habits?

I do a lot of highway driving so it's not like my car doesn't get driven at 2500+ rpm ever, it frequently does, but for around town driving I keep it pretty low in an effort to improve fuel economy.

My car averages 27-28mpg with mixed, but more highway than city, driving.

Any other consequences of my shifting technique? Thanks.

Yeah, some folks will say, shift at lower rpm as long as the engine is not lugging; and they'll add that lower rpm means lower piston (etc.) speeds and this equates to less friction (which increases with the square of the velocity) and therefore less engine wear.

That's all fine, but there's the other side of the coin. I'm not an engineer, but referring to an article (one of the several car mags I've read for the past 45 years) several (or many -- at my age there's little difference) years past, it recommended moderate to high rpms often (like everytime you shift). It seems that if you don't get the piston rings and/or cylinder walls nice and hot, "varnish" (the article's words) forms on the surfaces -- this substance tends to repel oil (it doesn't have the adhesion that metal has), and so oil won't stay on the surface. This, in turn, will lead to greater cylinder-wall or piston ring wear. Moral: you're encouraged to use higher (although not excessive) rpms for greater cylinder longevity.

And here's another tale. A decade or more ago, I had a nice conversation with my Mercedes (when I had a few cars of that make, along with my Volvos) dealership's service manager on his lunch break -- we were just sitting and talking about everything. But the conversation went to making the engines last. He told me that the biggest mistake owners make is in babying the engines, and he explained that you have to get the pistons nice and hot to get them to expand to their full "hot" dimensions once in a while to wear down the "lands" of unworn cylinder on the top and bottom of the bores -- if you don't, they'll build up (or rather, won't wear down) and then, when you really need the engine's full power, you have the pistons smashing into the ridges, causing all sorts of stresses on the pistons and cylinders.

And, personally, I use high rpms a lot! But I also use a good (AmsOil) synthetic with phenomenal film strength to negate much of the high speed friction shortcomings of driving like this. In fact, my daughter once had a boyfriend (who was taught by his father) who let her use his car once in a while, but he instructed her to always shift early. Then I once rode with her, and she emulated her boyfriend -- I told her flat out that while he wants her to drive his car like that, she should never, ever drive any of my cars like that! But that's me. Anyway, my engines last (without using oil) 250 to 350K miles, so I must be doing something right.

So, now you're confused. Welcome to the club -- go with your gut!

Regards,

I'm running Mobil 1 synthetic, but don't know what the car had in it for it's first 115k miles. It was dealer maintained if that means anything.

I've also heard it's good to "open it up" every once in a while too to help clean off the deposits you speak of. I would imagine that my daily commute which involves 15 minutes of 60-65 mph (~2500rpm) driving is enough to have this effect, or do I need to go past 3k rpm? I'm no expert, but the lower rpm=lower friction=less engine wear argument makes senseto me. So maybe if I do that for my city driving and acceleration (unless I need to go higher) and let my majority of highway miles clean off the deposits I'll be fine? I'm sure it'll be fine anyway, but just want to make sure I'm not doing something seriously wrong.

Thanks for your input.

"Cleaner" is a relative term. Most of the gas you buy today allready has some sort of additive to help keep valves clean. No need to do any "Italian tunups". As long as your engine isn't lugging you shouldn't have any problems. It's hard to say what's best to do all the time but after all that's part of the fun of driving a stick. You can vary your driving style and pretty much drive it like you want and not be locked into the way the automatic's computer is programmed. One on the many little things we all can do to help keep our minds busy and keep the Alzheimers demon away.

Yes, I do love having a stick. Both for the control and the huge improvement in gas mileage. Although driving in stop and go traffic or while trying to talk on the cell phone or both is very annoying.

I use really cheap 87 octane Arco gas. Should I pay a few cents more per gallon for a different brand? My last '95 auto 850 didn't seem to benefit from higher octane gas (well maybe a little more power, but it actually decreased my mpg). I've sort of assumed my newer to me, but older to this world '94 manual 850 would behave the same way.

What's an "Italian tuneup"?

Your overall efficiency is usually best if you shift near the max of the torque curve for your engine which is probably near 2000-2500 rpm. That's where an automatic is usually programmed to shift during moderate driving. And it's why experienced bicyclists use their gears freely to help maintain the cadence they are most comfortable with. The issue is not critical unless you are revving so low that you lug or lose power or so high as to risk premature wear or mechanical damage. When driving a manual I try for smooth acceleration at the rate needed - that is likely to be more or less the most efficient.

No reason to believe shifting habits would affect engine condition. Your engine's control system automatically compensates for a wide range of gasoline octane variations.

If you are driving under heavy loads or doing extremely severe work with the car I would be more concerned about the clutch and driveshafts than the engine, unless you are overheating.

Can you educate me about lugging? I know what lugging sounds and feels like, but what is actually happening when the engine lugs? I do my best to avoid it, but occasionally a steep hill and not enough gas pedal screws me up and it happens.

From what you've said I think I'm shifting fine when accelerating. Around town I shift around 2000 rpm, when I'm accelerating on an onramp or something like that more toward the 2500 mark. But is there any harm in shifting into 5th at 40 mph if I'm not really trying to accelerate but just maintain a constant speed? My tach stays steady somewhere around 1500(+?) I think.

I'm primarily interested in fuel efficiency and engine longevity. I'm just thrilled that with my mixed driving habits my '94 manual gets such good mileage (27-28mpg) compared to my last '95 auto which averaged 22-23mpg with the same driving.

I don't know the answer but I think it occurs because of inconsistent fuel and air signals - if your engine is at idle or low speed, the intake air flow is slow and not much fuel is injected (in proportion controlled by the fuel system). When you step on the gas the air flow increases and the fuel system injects more fuel (helped by the throttle position sensor detecting how rapidly you stepped on the pedal). Again the response should be proportional. If the engine is under some extra load, it will not rev as much when you hit the gas pedal. I think the fuel system detects this too and adds some extra fuel. But this adjustment can only go so far - with a really heavy load at low rpms, there is no combination of fuel and air that will produce enough torque. I suspect lugging is the result.

Think of the old cars where you could adjust the mixture and ignition timing. A good test of the state of the engine was to try to take off on a level road in top gear. A torquey engine in very good tune can do this but a weaker, damaged or out of tune engine cannot - it will lug or even stall, no matter how careful you are with the clutch.

I doubt that several seconds of lugging will hurt your engine. If the fuel system is not operating correctly that would make things worse but you would then see your check engine light.