Effect of RPMs on fuel economy

[iburgess]

Hi there!I don't know if any of you can help me out with this question:I find that my manual base Vibe lugs at best, and typically slows down, on long hills on the highway. I use cruise control, and the RPMs are pretty low at 110 kph, so it doesn't handle the hills so well because there's not much torque at these low RPMs, or so I assume. My pedal will be to the floor just to maintain speed, and on larger hills the car will slow down despite having the pedal to the floor.So, my question is: as far as fuel economy goes, am I better to leave the cruise on, leave it in 5th gear, and allow the pedal to stay to the floor to get up hills, or is it more efficient to gear down to fourth, or third when necessary, and not have the pedal to the floor, but of course, have the higher RPMs?Which is more efficient?CHEERS!

[joatmon]

I can't answer your question, but I always downshift because I hate that lugging feeling, and I don't want to slow down unless I have to.In http://forums.genvibe.com/zerothread?id=450&page=2, MadBill says "Generally low RPM lugging is best for economy"

[MadBill]

Yup. Lugging is the way to go, if all you care about is economy. The reduced friction at low RPM plus the lower pressure drop through the intake system at low RPM and large throttle openings means the crank has to do less work to drag the pistons down against vacuum on the intake stroke. (called "pumping losses") This is one of the resons that diesels are so efficient; they have no throttle, thus little pressure drop even at cruise.By way of confirmation, QUIKAG can probably tell you about the infamous CAGS or "skip shift" on his Corvette, which, to improve economy and keep the car from being classed a gas-guzzler, forces a manual to shift from 1st to 4th instead of 2nd during gentle driving, leaving you chugging along at about 600 RPM in 4th!

[iburgess]

Thanks, I was under the impression that higher RPMs and less throttle was the way to better economy, but it seems as though I was wrong. Question - I thought that lugging the engine at low RPMs and full throttle was bad for an engine, is there any truth to this? Or will this not affect an engines durability?CHEERS!

[MadBill]

Years ago lugging was a concern, because lubrication was pretty crude, and oil wasn't nearly as good as the modern stuff. At low RPM, oil pressure was low or nil, often just "splash lubrication", like a Briggs and Stratton lawnmower engine. The heavy loads from full throttle could break down the oil film and cause bearing failure, not to mention that the crude spark control and ragged carburetion was a recipe for engine knock.All that is different now, and unless carried to ridiculous extremes, there is no chance of damage, but there are still lots of folks who drive around in 2nd when 4th would work better. (I read a report about Corvette oil consumption, where it turned out the problem was that the GM Engineers hadn't done any testing for people cruising around in 1st gear at 4,000 RPM because they wanted everyone to hear their car!)

[dmitri]

its better to rev your engine up a little...flooring your car in high gear just puts more gas in the cylender that it doesnt need.

[goodvibe]

That's not lugging, just not enough juice to get you up the hill. Lugging occurs around idle. Just put your foot in it.

[QUIKAG]

quote:By way of confirmation, QUIKAG can probably tell you about the infamous CAGS or "skip shift" on his Corvette, which, to improve economy and keep the car from being classed a gas-guzzler, forces a manual to shift from 1st to 4th instead of 2nd during gentle driving, leaving you chugging along at about 600 RPM in 4th! All I can say about that is I put in the CAGS bypass a few days after I got it. Drove me nuts! At certain low throttle openings, rpm, etc. it would lock the gate for 2nd gear and the transmission would basically slide from 1st into 4th. At 20mph, it would lug the engine for a second or two when accelerating. Anyway, this bypass, basically a fuse that disables it, will let me go into 2nd gear whenever I want, as God intended. Anyway, lugging may be better for fuel economy, but I cannot do it to my motor's irrelevant of whether the lubrication systems nowadays are good enough at low rpm.For our 1ZZ and 2ZZ motors, revs is a good thing. I drive at a low enough rpm to accelerate smoothly. If you cannot accelerate smoothly or if you're having to go to larger throttle openings for an extended period of time to maintain speed, I would recommend downshifting.

[esjones]

quote:This is one of the resons that diesels are so efficient; they have no throttle, thus little pressure drop even at cruise. Then what does the Diesel use that functions as a throttle? Is it simply controlling fuel flow through the injectors? Forgive me, but I am old enough that I normally think of carburetors as the natural way fuel is delivered to engines, and all this new fuel injection stuff is smoke and mirrors! (I do like it a lot, though!)

[ragingfish]

I, on the other hand, don't even know what a carbeurator (sp?) is, does, or looks like......every car I ever had has been EFI...

[NovaResource]

[MadBill]

quote:This is one of the resons that diesels are so efficient; they have no throttle, thus little pressure drop even at cruise. Then what does the Diesel use that functions as a throttle? is it simply controlling fuel flow through the injectors? Forgive me, but I am old enough that I normally think of carburetors as the natural way fuel is delivered to engines, and all this new fuel injection stuff is smoke and mirrors! (I do like it a lot, though!)Short answer: YesPS: Great carb, Nova! Whose main body is that?

[NovaResource]

It's a Proform main body. A great piece. It flows very well. With only the main body change I had to jet up a few sizes to keep up with the added air flow.

[ragingfish]

Umm...ok...not quite sure what that does, but I'm assuming it's the elusive carbeurator we have been referring to...