nite crawler

By lugging, do you mean taking the car from say, 30 mph to 60 mph in 6th gear alone? Or do you mean just holding 6th gear at a very low rpm (say 1500)? OR BOTH? I will honestly say that I downshift to bring the car up to a higher speed, but the lowest rpm I keep 6th gear in is about 1800. I do not consider it lugging, and I do believe the mileage is a good as I can get it (for now).

Nate

wakeech

lugging would be when you start up with too little rpm (can happen even in 1st) and the car jerks forward. having the motor just cruise at low rpm, even under load, is not the same.

i've never heard of someone pinging an engine because it can't make enough power to move the car and keep itself running, but i do know that it stalls...
really, cruising at 1500 for 30 mph probably isn't something everyone will do, or even want to do, or even think of doing, but 2000 for 40 if you're just cruisin' home through a whole sea of green lights would be a-ok, it even acts like a natural speed control. if this car had a 7k redline, everyone would already be cruising there anyway.

well in theory, yeah, it shouldn't matter but it does because systems aren't perfect. genereally the less injectors you have running, the less you're going to be injecting too much fuel at any given time, and we've seen this profusely especially with the older ECU flashes.

with the ports *shrug* you have to be trying to get poor fuel economy to worry about other ports opening up.

RX8_Buckeye

Actually, lugging occurs anytime the output torque of the engine isn't sufficient (or is barely enough) to overcome the load on the engine. This condition will definitely occur under certain cruising conditions, most notably during high gear, low speed cruising. From experience, I know that one of the first steps in creating a shift schedule for an automatic transmission is to determine the lugging limit for each gear, i.e. the lowest engine speed that the PCM will allow in a given gear before downshifting. PoorCollegeKid did a good job of describing why lugging is very bad for your vehicle. I'll add one other comment: the excessive heat generated during prolonged lugging could potentially damage the catalyst as well. In an old thread, one of the rotary experts on this forum pointed out that lugging the engine before it reaches its normal operating temperature is especially harmful, much more so than keeping engine speed high during the warm-up period.

Let me point out that I don't know the lugging limits of the RX-8. I just know that I never cruise at less than 2000 rpm in any gear, and I'm not approaching lugging at those speeds. If I had to guess, I'd say that anything less than 1500 rpm in 6th gear would be very close to a lugging condition. Lugging can also occur when you try to accelerate in high load conditions. Generally when you're cruising along, if you punch the accelerator and nothing happens (response is very sluggish) or there is excessive powertrain noise and vibration during acceleration, you are lugging the engine and you should downshift (as common sense would tell you).

PoorCollegeKid

Cruising at a low rpm is fine; that is not what "lugging" refers to. I've never heard running your engine at too low an rpm level causing jerking and stalling called lugging, I guess you really do hear something new every day. Lugging your engine is when you are trying to accelerate hard in a high gear while your engine is running at low rpm. For example, you're cruising around at 30mph in 6th gear trying to conserve gas. You then take a turn on to a highway that your '8 can only handle at 30mph, but since you're already in 6th at 30mph you don't bother downshifting. Then, you floor it trying to merge with 70mph traffic but still don't downshift, causing a high load/low rpm situation. That is what I consider lugging your engine. Sorry for the confusion there.

nite crawler

That is why I asked about the lugging thing. Like I said, I never accelerate from low speed to high speed in a tall gear.

I know many, if not all, of us do not worry about "lugging" because we drive our 6MTs with some enthusiasm. However, what could this possibly do (damage-wise) to any vehicle in particular? Or to the rotary?

I just thought of something. Why did I never hear about half the things one should not do to vehicles? I took all kinds of drivers ed, and went through some traffic schools too. NEVER did anyone discuss things that will hurt a vehicle. Weird. Maybe it's just me.

Juxagent

without reading all post ill say if you are getting worse mileage in 6th @ 2000 rpm's its cause your bogging down the engine; dont go 45 in 6th, its too low thus a waste of gas.

ProtoConVert

Someone brought up a legitimate point about the manual's recommendations as being emissions friendly, hence good for regulates > good for mazda, so goes in the manual.

And why do you think this discussion of bogging is BS? If bogging or lugging at low rpm causes higher temperatures, then this should imply richer A/F at these rpm's, which we know happens, and so might explain why 3krpm in fifth might be more fuel efficient than 2 or 2.5krpm in 5th.

This on top of the fact that I think people are assuming constant speeds in this analysis, even though someone also pointed out earlier that normal driving or cruising might include low accelerations. A suggestion for 3krpm cruising is indicated here as well.


I have a question, though... can a brake fuel consumption curve be derived from a dyno? rollers or engine dyno?

wakeech

no, higher EGT's suggests a leaner a/f ratio. there isn't a problem with higher combustion tempuratures considering the range we're talking about here, and i've never heard of an engine pinging on good gas in idealish conditions at low rpm, high load or not.

ok, so that's what you mean by "lugging" and yeah i suppose bogging (like a beginner trying to start up without giving enough gas) is what i meant before. yeah, i agree that trying to accelerate hard in a higher gear puts a higher load on the engine than at a higher rpm in a lower gear, but i don't really know if it's considerably (life shorteningly) worse for the engine.

i'm quite sure that you can measure fuel flow extremely accurately on the dyno (how else do you think a car maker would tune theirs?), and you could plot something out with a little thought and planning.

TRZ750

Common sence and don't abuse your engine are usually the best guides.

Common sence tells me to shift up when ever it is possible and the engine will no be straining.

I get 18~19mpg comuting back and forth from work on city streets. Because of trafic I usually shift slowly from 1 to 2 to 4th to 6th. I use 6th most of the time and rarely, when comuting, go over 4K. I even use 6th in a 25mph zone that is often radar covered by our friends. 6th works as I am just lightly on the throttle and have no need to accelerate much as this parts ends with stop signs. The 35mph sections are usually backed up enough that I can only run 35~40, so again no acceleration. My RX8 just purrs with delight through all this, then is ready to jump when I down shift from 6th to 2nd for the only part where I can get on the throttle. So what if when running slow in 6th some car ahead of me starts going faster and the gap beteen us widens up. When he slows down I won't have to. In fact there is a section where a stop light causes a back up and it usually takes 2~3 changes before I get through so I just walk along at 5~8mph in 2nd with the gap sometimes getting real large. When my turn come up I will be right there, why push so hard to be right on the but of the person in front of you?

nite crawler

Do you mind sharing how you think its a waste? Elaborate!

You should have read the posts. That is exactly what is getting refuted. Go back and read.

I was at about 1600 RPM yesterday in 6th (little above 30), and I put Cruise Control into effect. Smooth like nipple butter..... smooth. No strain on the engine from my standpoint. Five minutes, and I almost forgot I was moving.

And about the manual. Did I just see shift at 45 when accelerating and 46 when cruising? Should it not at least be the other way around? Either way, when I accelerate, I am not in 4th at 45, and when I cruise, I am not in 4th when I am doing 46. My car is just fine. Yours will be too. Prove me wrong! Please!

RX8_Buckeye

I did some experimenting today cruising at very low engine speeds. Cruising at 40 mph in 6th gear seemed fine to me--no lugging. Very little throttle was required to maintain speed. However, as I dropped speed down to 30 mph in 6th gear, I definitely felt the engine start to bog. More throttle was required to maintain cruising speed. It is my opinion that you should not cruise in 6th gear at speeds at or less than 30 mph. There is definitely strain on the engine in these conditions. Maybe it is better for fuel economy, but it is definitely not a condition you would want to run for prolonged periods of time if you value the life of your engine. If you still insist on running in these conditions, at the very least use cruise control, because if your speed drops and you need to accelerate moderately, you will put severe strain on the engine. This is opinion is shared by my peers who are also mechanical engineers in the automotive industry.

nite crawler: be catious of what you perceive to be "smooth like nipple butter". Just because you don't hear or feel anything abnormal doesn't mean your engine isn't being strained. Also, the effects of lugging your engine may not be evident in the short term, rather they could show up in the form of shortened engine life. No one will be able to prove anything conclusive one way or the other, I'm just giving you my opinion.

PS: For those with automatic transmissions, the numbers being thrown about in this discussion do not apply. The torque converter provides significant torque multiplication at low engine speeds--thus the lugging limits occur at lower engine speeds.

nite crawler

Being at less than 2000 RPM in 6th is EXTREMELY rare for me. Thanks for the heads up on the whole placebo thing, Buckeye.

r0tor

iTrader: (1)

Well its actually easiest to do on an engine dyno since you need to get the mass of fuel and the hp (remember you want to get (lb of fuel)/hp )... however you need to be planning on measuring it before you start the dyno because on the dyno you usually don't look for mass fuel flow. To get the fuel flow you can either measure the amount of air entering the engine and deviding by the a/f ratio OR simply putting the gas tank on a scale and weighing it while doing the dyno...


if i get some time this weekend I'm going to take out my CANScan and try to answer this question once and for all. I believe i can get some reasonable approximations of the true bsfc curves by doing both of the following...

1) driving a constant 50mph and go through 2nd, 3rd, 4th, 5th, and 6th gears holding the speed constant and trying to figure out the instantaneous fuel consumption from the a/f ratio and maf sensor ... also i can try just logging the mpg reading the CANScan itself calculates. Theoretically, while not taking into account drivetrain losses, the amount of hp the engine is producing should remain constant if the speed is constant - and going through the 5 gears will give 5 descrete rpm points where you can get the fuel data. This should show the shape of the curve, the real numbers wouldn't have much meaning though... but its really just the shape that counts for finding the optimal rpm.

OR

2) keep the throttle position steady and take a single gear from 2000rpms to 9000rpms, figure out the fuel flow again from the a/f ratio and maf, and then use my own dyno program to find the instantaneous hp and find the real bsfc numbers... it should work


..just have to see how much time i get this weekend

Xyntax

^ Good luck pr0ber! Watch out for those "noises" that we get in our Canscan data. Let us know how it goes or what you find out.

Rosco

pr0ber - Let me know if you log the mpg from the CanScan. I've tried a couple times and got unrealistically high numbers.

Also, I've been working on the dyno programming quite a bit this week and running into one roadblock after another. Once I get the communication between my program and the CS solid, I think it would be fairly easy to add in a graph of the bfc curve.

Juxagent

nite crawler please dont read too much into my post; what i am reffering to with "waste" is a waste of gas. i dont know about engine abuse, only time will tell; but i do know i get better gas milage when i stay around 3 - 3.5 k. So if im cruising at 4.5 or 5 k in 5th and want to save gas ill pop it into 6th at around 3- 3.5 k and retain my current speed.

PoLaK

iTrader: (1)

Thats incorrect. See page 5-8 of your owners manual.

r0tor

iTrader: (1)

1) accomplished this part doing 35mph in 2nd, 3rd, 4th, 5th, and 6th gears... it turns out at a low speed like this internal losses compared to drag forces are quiet high and get worse with higher rpms (makes sense). So in other words, for constant low speed cruising you need to be in the highest gear possible and keep the revs as low as possible.

...need to try this again at a higher speed where drivetrain losses to aero losses won't be a ratio of 90%/10%... :o


2) tried doing the sfc dyno pulls at 35% throttle... i started both runs with the revs to high so I have to do this again as well. The area between 5-7k actually looks surprisingly good though...

nite crawler

This is the whole point I started this thread. Because I keep seeing comments like this.

You say you KNOW you get better gas mileage when you are at 3-3.5k. Better compared to what? Are you saying that in 5th gear, if you are doing 3300 RPM, you do not shift to 6th gear because you are getting better mileage. NO. Or I should say, I DEFINATELY THINK NOT! This is what the last few pages are discussing.

Another way to look at it:
1st Gear nets you the worst gas mileage. 6th gear nets you the best mileage.
It goes in order in between. Of course, like I said before, nobody should do 15 MPH in 6th gear to try and save gas. Obviously this would probably stall the car.

I will admit this. MAYBE 3000-3500 rpm will get you BETTER GAS MILEAGE, but only BETTER than a DIFFERENT RPM in that same gear. I think the answer to this "known unknown" is what Prober is trying to figure out.

Another question: Do you really cruise at 4000 rpm in 5th? And to follow that, do you always shift like that when cruising?

r0tor

iTrader: (1)

i'll attach what i got so far...

pic 1... shows running at a constant 35mph in 2nd gear, 3rd gear, 4th gear, 5th gear and 6th gear. The lowest calcuated fuel flow and best mpg come in 6th gear (the lowest rpm). Physically I needed almost 15% throttle to keep 35mph in 2nd gear and only 7% throttle to maintain 35mpg in 6th gear... so it makes sense

not shown -> The a/f ratio was pretty much a constant 14.7 in all gears.

Note: the "mpg" numbers are comming from CANScan... they seem wacky and I have nothing to do with them but probably the shape of that curve is trustable.


pic 2... the sfc chart at 35% throttle position and 40% throttle position (lower is better on a sfc curve) in 2nd gear. I want to redo this to capture lower rpm's when I get a chance because I believe there is a better spot to the left of where my charts show... I also threw in the a/f ratio for the hell of it. SFC is usually in units lbm/hp*hr but I didn't feel like converting them from the gm/hp*sec units i have. SO stay tuned for a version 2 of this guy...

Note: I wouldn't put too much confidence in the efficient looking spot at 7,000 rpms... looking at the dyno plots in pic 3 it looked like my beloved CANScan is taking a dump again while logging 6 parameters at once at high rpms even at 1 sec intervals (I physically don't feel my car taking a dump like it shows... so i'm pretty sure this is a software bug) :o

mdmaclean

Can you just take the displacement of the engine X RPM X a/f ratio to determine fuel usage per unit time? Then you could use the gear ratio's to turn it into miles/gallon. I think that the major assumption here is that the density of air remains constant over RPM.

nite crawler

PolaK:

Good looking on that page outta the book. When I said Mazda gives no good ranges, I meant this: I get 17 MPG when I cruise-shift, and when I tried Mazdas' shift points I got 15 MPG. I asked a veteran rotary tuner (lives literally 30 seconds away) if I can hold 6th at 2000 RPM. He said there is nothing wrong with that according to the health of the engine. He laughed and said that there is something wrong with that according to the fact that this car is meant to be DRIVEN. I told him if I drove like I wanted to, then the MPG would definately be less than 13.

Thats pathetic. All it takes is some tuning, and I think all of our MPG would shoot through the roof.