Ja. I don't shift until the redline when I am running away from.... ie monsters or dinosaurs. But for normal driving, I shift whenever... about 3000 rpm I guess.

 

And to answer the OP's second question, there's no way to determine the best gear for optimum fuel economy from HP/torque charts. You either have to do it experimentally, or put the car on a dyno and measure the "specific fuel consumption". This is a measurement of how much fuel you have to burn per hour per horsepower. The SFC for an engine varies with both engine rpm *and* throttle position, so it can take a lot of dyno runs to map it.

Then, armed with those numbers, you would record the engine rpm and throttle position for whatever road speed you're interested in for 6th, 5th, and 4th gears. (you can get the throttle position from a ScanGauge or similar tool). Then look up in your charts which of the three rpm/throttle position combinations gives you the lowest SFC, and that's the best gear for that road speed, efficiency-wise.

 

What this non-engineer thinks he understands from the foregoing is that to win a race, I run it to redline and try to catch the next gear around 4k.

To get best fuel economy (without dyno testing), I should shift around 2k and drive gently at 1.5k shifting only if I will risk lugging the engine.


Right?

 

Well, if you shift at redline (which you should), then you shouldn't worry about where the rpm will be in the next gear, since that's determined entirely by the gear ratios in the transmission.

For best fuel economy during steady-speed cruising without doing any experimentation or detailed measurements, then I would go with the lowest-possible rpm without lugging the engine. Just realize that you'll need to get in the habit of downshift one or two gears any time you need to accelerate, or you likely *will* lug the engine.

 

I see, CMT was talking about shift points during acceleration. I thought he was responding to the discussion of what rpm to cruise at. The thread had wandered into that question after someone said that the best way to save gas was to keep the rpm low.

 

This is what I think is in error. My impression is that 1.5K rpm isn't going to produce enough power to maintain efficient running on anything faster than one's driveway. Even 2K rpm isn't going to be enough at 50+ mph.

Set your tach to read instant mpg (not average) and try cruising a particular speed in different gears. So how the numbers look. Or, set your trip computer and run a particular stretch of highway and back at one rpm range. Note the average mpg, reset the trip computer and run it again at a different rpm range.

My impression is that for real-life driving at freeway speeds you need to be around 3K rpm for best efficiency.

 

What's the cruising rpm for an R56 manual in 6th gear at 70 MPH? On my R52, it's about 3100. I'd like to find out if a lower rpm would do better at that speed, but without a 7th gear, I can't find out.

I think your idea of using the "instant MPG" readout is excellent, though. Since that readout is driven by the duty cycle of the fuel injectors, and the injectors use a fixed amount of gas per second when they're open, that readout is directly-correlated to the specific fuel consumption I was talking about in my earlier post.

The only problem I've had when using it is that unless the road is perfectly flat, the instantaneous MPG readout can fluctuate pretty wildly, even with the cruise control set. Still, it's worth trying, since if one gear is significantly better or worse than another, you should be able to see that through the fluctuations.

 

Robin,
I'll try the tests you note above. I was careless in listing 1.5k as a constant. I find that my MC doesn't lug at 1.5 to 2k so long as I am gentle and it's reasonably flat in the city. On the hwy, sixth gear runs at what it runs--usu. around 3 to 3.2 in my neck of the woods.

In 70/30 hwy/city driving, I'm consistently pulling 39mpg. Really want to break 40 just to say I did.

 

With the torque curve being so flat on the MINI (and also looking at cmt's graph), is there any difference, in terms of max acceleration, between shifting at redline and shifting at 4000rpm?

 

Yes, that's what the HP and work discussion was all about a few posts back.

 

So how do I figure out where the optimum shift point is for each gear?

 

I'll have to check next time I'm out. What I've found is that on country roads with 50 to 60 mph speeds, 6th is too high.

It is a little hard to judge, but it helps give an idea of what's happening. The repeated round trip average would probably be more definitive, but takes more time and effort.

I'm driving an MCS. Not sure whether the gear ratios are the same between the MCS and MC.

My average mpg is hurt by my driveway. It is a switchback that climbs about 400 ft. in altitude. It is very steep and requires 1st gear. I get about 8 mpg going up it. I make up for it by commuting on the internet.

 

Another version of thrust curve that I find more intuitive would be Thrust (lbs force) on the Y-axis, and Vehicle Speed (mph) on the X-axis.

Charlie's curve is close, but would only need some minor mathimatics to convert wheel torque to Thrust (ie, tire radius), and engine rpm to Vehicle Speed (ie, gear ratios).

Any volunteers?

 

So I'm kinda curious (maybe a little nervious) because sometimes when I'm driving through town between 22-30 I like to keep it in 3rd on my R56 MC...when the traffic does slow down to 22 but then accelerates slowly, I keep it in 3rd to get up to 30...but this puts the engine around 1300 rpm and sounds all dieselly...is this lugging? Am I hurting the car more than just driving in 2nd? I thought it was safe to drive as long as above 1000??? Also I figured the Variable Valve Timing helped fix all this?

Please let me know cuz I do this basically daily and i hope its not hurting my engine

 

If it sounds "dieselly" when you accelerate at 1300 rpm, then you're lugging the engine, which is not good for it. You can safely *cruise* at a lower engine rpm than you can *accelerate* from.

So, a steady 1300 rpm might be fine, but if you need to accelerate from 1300 rpm, you should probably downshift.

 

But what I run into is it is only a slow acceleration from 22 to 25 and sometimes 25-30...it just seems weird to down shift into 2nd for a few miles...how bad is the lugging? I have been downing this ALOT when im in the slow parts of town so I really hope I havent been doing damage to my engine D:

 

When you try to accelerate from too low of an rpm, you're dumping lots of fuel into the cylinders at an engine speed where the torque is too low to effectively accelerate the engine. The net result is that the cylinder pressure inside the engine jumps *way* up. If you use gas with too-low of an octane rating, this is also why you're more likely to have knocking/detonation while lugging the engine.

When you feed the engine extra gas at an rpm where it doesn't want to speed up quickly, it's almost like you're feeding gas into a locked engine. All the extra gas ignites, but the extra pressure can't speed up the piston quickly enough, and that's what causes the pressure spike.

Hopefully the engine management system will limit the amount of gas you can add at low rpm, and the knock-sensor will prevent the worst of the damage if you get knocking/detonation, but one way or the other, lugging the engine to the point where you're getting the "dieselly" sound is hard on it.

 

This is true, for your constant rate of motion. What about when you accelerate from a red-light / stop sign? Or are running a 1/4 mile track... ? In those cases you are accelerating, not in a constant state. The additional Torque supplied, and thus HP @ a particular RPM, results in acceleration above a constant rate of motion.

The formula is:

HP = (Torque * RPM) / 5252

Greatly simplifying, Torque is just the instantaneous rotational force, and what we care about in this context is how much force to the tire's contact patch regardless of rotational speed... HP is force in motion over time, i.e. the application of force to the tire's contact patch to produce the rate of change of the rotational speed; as RPM goes up, so does therate of the work the engine performs...

 

Thank you. Yeah its just hard cuz the speed varies soo much throughout town...most of the time 30 so perfect for 3rd, but second would be an unnecessary 3000+ rpm...but the 22 is too low for 3rd i never hear knocking or pinging or anything like that, just the diesally grunt which i was hoping was only a side affect of the variable valves changing at the low rpms this has happened A LOT for the 6700 miles that are on this so i really hope i havent taken a bunch of life off my motor

 

You're correct - torque is what actually changes the rotational speed of the wheel. Power is just an artificial term derived from torque and engine speed. When you put your car on a dyno, it's the torque and engine speed that are actually measured - horsepower is a mathematically-derived quantity.

When you're cruising along at a steady 60 mph, you technically don't need *any* torque, since you're not accelerating. In reality, you have to overcome the forces of friction in the engine/drivetrain, the friction between the tires and the ground, and the wind resistance. You'd be surprised how little horsepower you actually need to counteract all those forces - for our cars, it's probably less than 15 horsepower.

 

Well, I don't have the variable valve timing or the direct fuel injection in my first-gen car, so it's possible that the sound you're describing isn't "lugging" at all. But, 1300 rpm is very low on the torque curve, even for an R56, so it's not a good idea to try to accelerate from that low of an rpm.

If you keep doing it, just remember that if you're accelerating away from 1300 rpm, add gas *very slowly* so that the engine has a chance to speed up. That way, you won't be dumping a large amount of gas into an engine that can't speed up quickly enough in response.

(It would still be better if you avoiding lugging the engine altogether, though.) If it makes you feel better, you'll probably use less gas downshifting to second to accelerate than you would lugging the engine in 3rd to accelerate.

 

true... but most cars now'days don't have vacuum gauges like my 20 yr old 560SL so bringing that into it doesn't mean much to too many people anymore... lowest revs @ lowest intake manifold vacuum is the more accurate way to look @ it. You cannot however compare the same vacuum rate in different gears; the same vaccum in a lower gear than another will yield worse fuel economy because the vehicle in the lower gear is traveling less distance per revolution in the lower gear even when manifold vacuum is the same.

 

for speed? get your car dyno'ed, doing a run in each gear, go to http://www.cartestsoftware.com/ and get CarTest2000 (it does this, I'm c55m8o there), buy it, set up a customized simulation for a mini, enter torque values @ the best resolution in each gear it allows you to, and run a simulation... or, just listen to the discussion @ the end of page one and shift @ redline because as was proven by one who already did the dyno runs, no one gear yields a lower torque at any RPM to the contact patch in a gear than in the next higher gear... i.e. upshifting at any [edit] rpm before redline always yields lower torque to the wheels than the previous gear, so hold each gear to redline for max thrust.

 

A six speed transmission is meant for shifting!
That said, you aren't helping your gas mileage by running at 1,300 rpm in a variable speed situation. You would probably get better mileage if you let it go up to 3,000 occasionally. It can be annoying when traffic flow is at a point between two gears. I think you do more harm by reving too low, than going somewhat over the ideal. 3,000 rpm isn't a big deal for a high performance European car. We just have a tradition of big V8s detuned to tractor like rpms. Your MINI isn't like that.

 

This discussion of shift points and lugging has been very useful. I tend to let the rpm fall to 15oo and then accelerate very gently--egg under my foot gently--but I don't think so any more.

I must admit, after beginning my driving career in a 56 Chevy (Aquamarine) 3-speed on the column and driving ever more powerful standards, it is hard to get used to the high rpms. But at this point, if Robin says it's OK, then it's OK.

I will run around 2k for in-city gentle accel from now on.