Full Throttle at low RPM, or High RPM and low throttle
 

so I just got in from a night of driving my mom's 1997 Subaru legacy (not outback) wagon around with the fuel needle just above empty. It's a 5 speed, and has god knows how manny censors and such dead in the engine, but at just under 200k miles, I'm lucky it's running.

Anyways, I know the IDEAL thing to do is to barely tap the gas pedal and shift as low as you can without making the engine knock/ping or stall. However, because this car has lost alot of horsepower over the years, to the point where in 5th gear it takes about 30 seconds to go from 60 to 65 and it tops out around 75, and the fact that around here you kind of have to be able to zip out into traffic, I'm wondering, which is the lesser of two evils? Shifting at a higher RPM but not putting your foot all the way down, or Jamming it to the floor and shifting at around 2,000 rpm?

I wouldn't think that 200k was that old for a subaru. don't get me wrong, it's no spring chicken. mine has 188k and does just fine (not a subaru though)

I did an experiment once and started a thread. the title was "gunning it for FE" or something similar. I found that it is better to get up to speed faster and thus heating up your engine faster and getting into higher gear. mine is an auto trans so I don't have the luxury of changing the gears myself.

I would say to shift as low in the power band as you can. if you shift too early, you will get the sluggish reaction that you are describing.

If it takes 30 seconds to accelerate 5 mph then there is something seriously wrong with that car.

Does it use a lot of oil meaning the engine is on its last leg?

If it does not use a lot of oil a tune up, new air filter, and an oil change would probably help.

Is the check engine light on?

Have the codes read for free at Advance Auto Parts and correct anything the codes indicate is not working properly.

regards
gary

It is old. There are numerous problems with it, none the least of which are several engine sensors are dead. But yeah, that' what I had figured, but I wasn't 100% sure.

I can't say for sure about a car that's got lots of stuff wrong with it, but my 2008 VW gets great fuel economy when I floor it and shift early. I shift around 1200-1500RPM, entering the next gear at 1000-1200RPM (or occasionally right at idle) and keep up with traffic.

For most vehicles it is probably better to only step on it to 75% or so; when you go wide-open, most cars will go to an excessively rich air/fuel mixture and ignore the O2 sensors ("open loop"). That Subaru with several dead sensors is probably in open loop all the time anyway, so WOT (Wide Open Throttle) might work out ok.

Why doesn't your mom get some of the sensors fixed? Is she planning to junk it soon?

Basically, we are tight on cash, so any repair only impacts economy/performance is not going to happen.

I'm also pretty sure it's not the O2 sensor. I think it's the "knock" or "noc" or something sensor (never saw what it was written down, so I can only guess as to what the actual word is). The mechanic said it was some wierd thing that only was in older subarus. Anyways there is engine knocking at 1,200 RPM, so I'll just shift as low as possible while avoiding that.

Knock sensors are in every modern car.

You definitely want to avoid knocking.

Yup, because we sure as hell can't afford a new engine.

Take it to Autozone and they'll hook up a diagnostic scanner to the OBD-II port for free. That will tell you what the cel codes are, and you can sort out what to repair/replace from that information.

subarus get the subaru tick at 150k
(aka old ones are kind of junky)

had a friend with two of em who told me that

At low rpm it doesn't take much throttle to drop vacuum to near 0 on the intake since even a small throttle opening allows more air to flow into the engine than it can pump so you really are not doing much by pushing the pedal further other than feeding the ECU with throttle position signals that may richen the mixture even further and push yourself into open loop.

In my Old Geo 1 Liter I got great mileage by gently reving the engine with small throttle - no tach so I don't know how high I was reving but it definately liked high rev very light throttle maybe because it was throttle body injection and vaporized the fuel better that way.

I knew quite a few people that had 300k+ on subarus and wouldn't own anything else. that was the family 4X4 where I used to live.

I never got one because they were (and still are) kind of expensive. when snow and ice are something that you have to deal with every winter (along with terrain), you have to have something to get you back and forth to work.

even the police force had a subaru outback sport. saw it around a few times and had a few friends that had the grand experience to see it up close (traffic tickets)

BTW, how did this old thread get brought up???

spot, it seems that all old cars (unless they are saturns) are junk. what's up with that?

That is a good thing. Making all that vacuum wastes energy.

The ECU won't enrich the mixture just because of increased throttle position, unless you're talking about WOT.

Very confused by this.

Any engine is an air pump. It was designed to make vacuum.

I thought it was better to see more vacuum if you're maintaining speed. That would mean you're using less pedal. :confused:

Anyway, however tight money is, you can't just neglect the car because you're broke but, at the same time say you hope the engine doesn't blow up... because you're broke. You've got to make a choice here. Spend some loot on repairs now, or be S.O.L. when your neglect pushes the engine over it's limits and it fails and the repair bills come in at 20x what you could've spent to save it. What is $200-$300 in sensors/tune up if you get another 200k miles out of the car vs. $2,500 in expenses for a new engine swap in 6 months.

Vehicle maintenance is mandatory to ensure longevity. Stop making excuses. You pay $ now, of you pay $$$$$$$$$$$$$$ later.

Yes, vacuum means you're using less pedal, but that's not the most efficient way to run. Vacuum in the intake doesn't make the car go forward, it's just energy wasted on making a vacuum.

The engine is an air pump, and a vacuum on the supply side (intake) is a waste. You want atmospheric pressure there and then you want it pumped out the other end (exhaust) easily. You want pressure is in the center of the system where the work gets done (cylinders).

One of the reasons that diesels are more efficient is that they don't waste energy pulling a vacuum by throttling (strangling) the air supply. The new direct injection gas engines try to emulate this. To a lesser extent, normal indirect injection gas engines with drive-by-wire throttle can (and some do) do it too, the same way I do it.

I always floor the gas pedal and shift low to take advantage of not wasting energy making vacuum.

The reasons that using less pedal and seeing more vacuum can be more efficient are:
- Most cars will go rich if you floor it (my car is an exception); but you could use 75% of the pedal to avoid that.
- In an automatic, the pedal also controls the transmission, which will shift higher if you lay on it.
- If you're not controlling your acceleration and speed by shifting low, then using more pedal means you're going to go faster, which uses more energy (which you've just produced by flooring it up to 3000RPM instead of 1500).

What about torque curves? Powerbands?

Why not shift where the engine is making optimal forward momentum at it's designed rate, at the torque peak?

I think we should clear the air on this subject really. How do your techniques related to highway driving? I assume you use light throttle pressure the maintain a speed, like most people, or do you not? Do you get on the gas and then coast (in gear) until you've slowed down to a point you must throttle up again?

I get what you're saying, it's clicking. I drive like that to an extent in my automatic... sometimes. When I'm feeling frisky I think it may be more beneficial to "gun-it" up to 35mph where my transmission shifts into OD then ease on off the pedal and maintain speed.

More often than not though, I just give moderate gas (15-20 TPS) and let off a little (maybe 5 or 6 TPS) to let the car shift into the next gear a little earlier.

I don't have graphs of torque curves, powerband, etc for my car. Besides, those aren't what matter for FE; a BSFC graph would matter though, if you want to do it that way. Dyno charts don't have enough dimensions; they only tell you what happens at WOT. That's fine if you're only driving at WOT but we're talking about partial throttle now too.

Anyway, I do it by experimenting and seeing what works for me.

For highway driving, if I really want to work hard to save gas I'll P&G. WOT, then neutral, then back into gear and WOT again. It's a lot of work, though. Usually I don't want to put so much effort into it so I mostly set my cruise control and forget about it. For decent descents (hah!), I will cancel the cruise and coast in neutral (engine on or off per my mood).

I tried "get on the gas and then coast (in gear)", a technique which I named "Pulse & DFCO". It was less efficient than steady cruising...while being a whole lot of trouble. It was really a terrible way to drive and I couldn't tolerate it even if I got 100MPG doing it. Maybe it wouldn't be so awful in a different car with taller gears or an automatic with an unlocked torque converter.

My driving is 40% highway. The rest is light stop-and-go driving through towns and rural areas. In that kind of context, you really get to use the WOT+shortshifting combination as well as P&G and EOC. In dense, hyper city driving you can't EOC and you can't P&G as much but the rest applies well.

gunning a tad right before you're about to shift a manual, gives your car a little momentum boost that helps you get it into the next gear more quickly hence saving a little of gas instead of floundering the wading pool of your clutch and gears at low rpms

did anyone ever figure out which one is more efficient?

Full Throttle at low RPM, vs High RPM and low throttle

I think it depends mainly on the vehicle. Where the engine goes into open loop and starts reading the pre-set fuel tables for heavy load. Once it does go into open loop usually the fuel tables will be richer then stoich. The reason is to keep the engine from killing it self due to low air velocity and all the heat that's created from a high load low rpm condition.

In the beginning I was confused on why people were claiming better FE by using a higher load, low rpm shifting technique. Because anytime you push the gas pedal down the engine will produce more HP and with more Hp means more fuel that needs to burned.

So I crunched some BSFC numbers with different load values into my spreadsheet to do some comparisons. The first thing I found out was that even by running a much better BSFC number as seen in the BSFC maps of certain engines, the data would show more fuel was being used for the mere fact of a higher HP number from high load.

Then it hit me while doing some RTP testing on my Honda. The one thing I forgot all about was time.

As a example:

A engine running a BSFC of 273 g/kwh will use 0.0277 lbs/sec. and make 10HP with a load of -15.7 inch/hg of vacuum

A engine running a BSFC of 213 g/kwh will use 0.0549 lbs/sec. and make 20HP with a load of -2.04 inch/hg of vacuum

So from these two examples it looks like the one with a the lighter load (-15.7inch/hg) will consume less fuel at 0.0277 lbs/sec even with a poor BSFC of 273 g/kwh.

But what about the amount of time the first engine takes to get to the say 50mph or the next shift point? It could take as much as four times the amount of time verses the second engine. The second engine is running 10 more Hp so it will accelerate faster to 50mph or the next shift point then the first engine.

So now we have 0.0277 lbs/second x 4 = .1108 lbs/4 seconds verses 0.0549 lbs/1 second to reach the same 50mph or next shift point.

BSFC map from a Saturn
https://www.fuelly.com/attachments/fo...a944949129.jpg

You could also skip figuring time and go straight to figuring distance, except that there's no data for that.

Making more power would mean that you've used more fuel and gone further. However, if you're making more power then you're doing it wrong. You're supposed to shift low to limit the amount of power you make.

So now you're making the same power and the only differences are:
- lower RPM with less reciprocating loss, less engine friction loss, more work done per power stroke (and fewer powerless intake/compression/exhaust strokes)
- less energy wasted pulling a vacuum ("throttling" "pumping" loss)

You're making the same energy from the fuel but wasting less of that energy, putting more of it to the road.

Of course, if you get to open loop enrichment then that changes things. Therefore it is wise to know how much you can open the throttle before you hit enrichment.

Too bad that BSFC map isn't plotted for throttle position instead of torque, that would be really great for us.

theholycow

Whats the vacuum at when you do this?

So, walk me through reading and understanding this BSFC map for the Saturn if you don't mind. I really want to know and understand more about this.

Would the BSFC map change for automatic vs. manual? Slightly I'd guess?

I'll have to check, it's been a long time since I've looked at that...but vacuum is significantly reduced, as I remember.

No, it's for the engine, not for the vehicle as a whole. The wiki explains it pretty decently:
https://en.wikipedia.org/wiki/Brake_s...el_consumption
Just consider it a matter of how much fuel is used to do a given amount of work.

Unless the ECU for manual vs auto had different pre programmed fuel maps, then no.

Here's my interpretation of that graph:

The 250 grams of fuel per Kw/hr "island" is the lowest fuel consumption seen on that engine per unit of power it produces. This occurs between 1500-3500 rpms at the higher torque values (above 93 Newton meters).

This suggests that using heavy throttle application in that rpm range would be best for fuel economy. The lower torque values (toward the bottom of the graph) would represent a light throttle approach (less load= higher vacuum) and the lines show that the fuel consumption per amount of power produced by the engine is greater (more fuel consumption per mile if you drive this way).

https://www.fuelly.com/attachments/fo...bfcccaac3d.jpg

I will try to explain myself a little better using the map above.

In the above map can someone explain why the blue mark @ a BSFC of 275g/kwhr uses 0.436lbs/min of fuel based on the 122ft/lbs of torque and the red mark @ a BSFC of 325g/kwhr uses 0.144lbs/min of fuel based on the 34ft/lbs of torque????

Both marks are at a A/F ratio of 14.7 @ 2500rpm with a IAT of 60*F The blue mark is at 100% TPS with a vacuum reading of 0 inch/hg and 100VE. The red mark is at 35% TPS with a vaccum reading of -10.18inch/hg and 50%VE.

So...

Blue mark equals 0.436lbs/min useage of fuel at a BSFC of 275g/kwhr making 122 ft/lbs.

Red mark equals 0.144lbs/min useage of fuel at a BSFC of 325g/kwhr making 34 ft/lbs.

conversion 1 Lb-Ft = 1.35582 Nm.

wow that last response totally confused me, I'm sure it means something..

I've been doing some experimenting lately with lower rpm shifts, and I'm finding that 1800-1900 works well and keeps fuel usage down, the only thing I am finding is i have to give it a little extra kick at the end of each gear like that so it shifts correctly into the next gear, this is accelerating though, I do not feel that on a saturn accelerating to 2400 rpms is efficient moreso at all, in fact its is risky business to shift as low as 1800 but it can be done and you make the most of the time, by giving the boost at the end of each gear so like I said there's no time wasted in low rpm clutch/gear land... But I also do have a 99 and the pistons/cams in my car were made better in 99+ versions for better low end torque, so I think that is why I am getting away with this.. and honestly I have the lightest saturn you can buy from what Ive gathered a 99 sl2 is lighter than sc2 by 40 lbs and ive stripped 80lbs + out of this car so im at 2270 I'm assuming.. I dont need those extra 200 rpms to get me going... But cruising at 58 mph is a 2400rpm cruise, so that's the distinction on what we're talking about here is that 2400 is the best cruising speed, that vs wot or not I guess is lost somewhere in the graph..

check the f ookfilter

What you are looking at is at light throttle and low burn rates (red) you burn more Grams of fuel to get a KWH of energy output 325 Grams per Kilowatthour. At wide open throttle burning fuel at a high rate (blue) you burn less Grams of fuel to get a KWH or energy output 275 Grams per Kilowatthour (50 grams per KWH less fuel). And the center of the graph is the "sweet spot" to operate the engine however it is also where the engine is producing a lot of torque - centered around 124 Nm of torque in the 1500-3500 RPM range which is probably when you are climbing a hill if you don't have proper gearing to travel at the speed of the road you are on. Keep in mind that this may be the power level needed to go 80 mph which although you end up operating the engine at peak efficiency it is also where there is really high air drag off setting this efficient engine operation with higher than normal speed air drag.

So, in the simplest terms, are you saying that it's generally most efficient to stay at about 75% throttle, shifting early, until @ cruising speed?
My car's powerband is from about 4K to 7,400K rpm, but I usually shift at about 2,500rpm.

That has worked best for me. Any time I try to use lighter throttle it ends up being less efficient.

how do you know what throttle percentage you're using exactly, scan gauge?

cow is that possibly only specific to your car?

oh dont put words in my mouth thats just what someone told me about subarus...

Thanks for your post. I find alot of people forget that you just don't look at the BSFC number itself but you also have to consider how much power or torque is being made to figure out actual fuel consumption.

People tend to get hung up on the good BSFC numbers and don't take in account that if you have great BSFC numbers with high load to medium load you will also have higher Torque or HP numbers in which require more fuel to be burned.

If you have better BSFC and have produced more power, the power hasn't necessarily been wasted; you use the extra work that was done to go further. If you just end up going faster and then braking then you've certainly wasted it.

This makes me very happy, because it's alot more fun!
That gives me a new tuning challenge; doing as lean of a burn as possible at 75% throttle, while keeping it at about 13.8:1 at 80%, and 13.7 at 80-100%.
Anyone in the Portland, OR area want to help with this?

It's no more fun. You don't go any faster.

Oh, I do ;) The difference between 25% and 75% is drastic on my car.
I'm concerned that I may get a ticket for "display of speed", or something. Sometimes I have to not give any gas coming from a stop, and always have to slip the clutch a bit, to not spin the tires.
At 75% throttle, my car would do the ? mile in about 17 seconds (at 90+%, it does high 13s).
I acknowledge that in normal cars, it wouldn't make much difference.
My car is far from normal, though.

If you're going faster, you're doing it wrong. You may not gain a fuel economy improvement; you may in fact lose fuel economy.

I often shift when I've only reached 1200RPM. You may need to go lower than that. The strategy may not be compatible with your car. Experimentation will answer all of these questions for sure.

Oh.
I get it.
I'll try it, and see how it works out.