SixCylinders

This is mainly addressed to Loren but anyone with anyone is welcome to contribute (keep in mind that this is my 37mpg record holding DD which is rated for 19/24 and got 17mpg when I used to race stoplights)

I finally replaced my bad O2 sensor. Car is running full rich 100% of the time according to my gauge, no surprise there but I had kinda hoped I could get it down simply by plugging the O2 sensor in. I'm now thinking that my thermostat which I've been speculating has been stuck open for as long as I've owned the car is preventing my CTS from reading a temperature high enough to get the car out of warm up enrichment or whatever. I fully intend to replace the thermostat the next time I have to crack open the coolant system for any reason (gas ain't cheap but neither is antifreeze). In the mean time I'm planning on creating a controlled vacuum leak to lean it out at least at idle.

I began to question my transmission gearing a little bit ago, I've always heard that Corvettes and similar cars get their fuel economy from how high their 6th gear is, I've also heard that you don't want to run an engine too slow or (obviously) too fast for FE I know on a Saturn 1.9L I read up on BSFC and decided that I should keep a Saturn between 2000-3000rpm. Using that logic and popular opinion over at the Nissan forums I figured keeping it between 2000-2500rpm was the best thing for it. Now with overdrive on cruising 55-60 miles an hour I'm under 1800rpm so I'm thinking gearing might be too high for best fuel economy? In theory if I rewire my overdrive lockout switch (removed it last year because I've NEVER used it, nor did my parents) I can get torque converter lockup in 3rd gear and my RPMs should be around 30% higher which would put me at around 2400rpms. Do you think I could possibly see any gains from that? Especially seeing as 3rd is the 1:1 ratio and I've always heard that's where the least losses occur in transmissions (or at least some of them)

I've also been planning to get some cams for a while now because even though I love my gas mileage I'm a performance guy at heart. Engine is a SOHC 3L V6 with stock like 250ish duration, I'm looking to move up to a 266 or at the most a ~270. The most expensive option is JWT Billet S2 cams with a 266 duration ($460!!!) these have the biggest @.050 duration of the ones I'm looking at so I REALLY want these since in theory they should make similar power with less overlap than the other guy's 280s (and because of the lower overlap should retain if not improve my bottom end). Now the 270s I can get for $180, they are reground so a little bit more headache with shipping and core charge and all that. You think that I can keep getting the same gas mileage I've been getting with these moderate cams?

Loren

Lots and lots of variables as to what RPM an engine is going to be most efficient at, but so far I've seen that lower is better... down to maybe 600-800 rpm above idle. My cars don't seem to appreciate "cruising" that low. And how low you can go depends on how much torque is available vs. the weight of the car vs. the speed vs. hills, etc. But, the Yaris would happily putter around at 1500 rpm in 5th at 35 mph, and the BMW will do the same on down to around 1200 rpm in 6th. (I can literally drive the BMW and use 1500 as a shift point)

The BMW will get 31 mpg at 80 mph all day long without giving it a thought. That's a little above 3,000 rpm. Maybe 3500ish. That's "good", but it invariably gets better as the speed goes down. I haven't experimented with it enough to say exactly how much. The improvement in mpg as speed decreases is partly due to less wind drag. For this engine, I think it adapts (variable cam timing, variable intake and variable valve lift!) to be very efficient regardless of engine speed.

But, I know I've made some fairly short trips (5 miles) from a semi-cold start and managed to recover up to about 32 mpg for that trip keeping the speed between 35 and 45... but that's using a lot of DFCO coasting. What will the car do just cruising at that speed? I just don't know.

So, all that just to say that I'm not sure what rpm you should gear your car to be at for optimum economy. My gut still says you should keep the RPM as low as possible. If you can highway cruise (60 mph) at 1800 rpm, do it!

Regarding the cam, more lift is good for overall efficiency. More air in... but then more fuel... but ultimately more power per cycle. I'd think overlap would be bad for efficiency. A lot of unburnt mixture goes right out the tailpipe with high-overlap.

As you're building more power, the car should be even more efficient at really low throttle and lower rpm. If your car is "struggling" to keep up at 1800 rpm at 60 mph now, adding more cam lift should give it more torque and make it happier. So, if you're going to do the cam, I'd do that first and see how it goes. Letting the engine rev higher than it is now AND letting more air into it (which is going to draw more fuel), can't result in better economy.

In general, I'd say that lower revs are good for economy. Higher low-end torque lets you get away with lower revs.

Empire

With regard to the gearing, yeah, lower rpms is better. But there is a point where you have gone too low. There is a lot of air that you have to push our of your way at 60 mph. So it is going to depend on the aerodynamics of the car.
Is there anyway for you to find out what your drag coefficients are? Might be able to find that information on-line.
From there, you would need to know how much horse power (well, torque) you are going to need to keep your car moving at that certain speed. Then you'll need to add at least 10 hp to that to compensate for other variables, like tire pressure, wind direction, things like that. Some would probably even say to add 15-20 to your minimum so you can compensate for extra things like, rain. rain is not just heavy, but it also adds extra pressure you have to push through.
Then, once you have that minimum number and you are happy you've covered all of your bases, you'll want to compare that number to your dyno chart and see at what RPM the engine makes that power at.
I think we broke it down a couple of years ago to figure out that a lowered 240sx needs 17 whp to maintain 65 mph assuming proper tire pressure, and flat surface etc etc. So to compensate for the variables of daily life, I would safely say that you need a minimum of 25 whp to maintain speed.
So for optimum fuel economy, you would have to figure out at what RPM the engine can make 25 whp at a minimal amount of throttle. Sure, 25 whp is like 900 rpms, but that is at WOT. I'm sure it would need to be more like 1600-1800 at 10-15% throttle.
You could get really crazy and add in the duty cycle for the injectors and all of that to determine at what throttle position itself would be the most efficient.
I have no doubt there is a way to calculate all of that. There is a way to calculate how much power you need to push through the air, so the rest of it is just further calculation. But, most of us don't know how to do it, so we would resort to tangible data. Make a few runs on a dyno. One at WOT (cause it is fun and gives you a base to work from) then do a run at 50% throttle. You'll need to rig something up to that it is a true 50%, not a "best guess" Then do another run at 20% throttle. record all of your data and see how different the HP curves are with different throttle positions. The injectors and duty cycles will determine what is better, lower RPMS with more throttle or less throttle at higher RPMS.
Break it down to see how much fuel you'll use over one hour. IE each time the injector sprays, it is this much fuel. times how many injector sprays there are over a second. how many seconds over an hour gives you X amount of fuel used.
Find the point on the hp curve where you make the minimal HP you need to maintain speed. If you set your ratios to that you are cruising at that RPM, you'll need that much throttle to maintain speed.

The actual ratio itself would be of little concern at this point because you are dealing with actual Wheel horse power, and not the power lost through the drive train. A 1:1 ratio has the least amount of power lost, but at that point, you're engine speed would be higher than it needed to be.
But perhaps you could work it out so that you were making the optimum WHP, while maintaining a 1:1 ratio with a minimal amount of throttle. Again, you'd have to break down the math to see how much fuel you would be using then to see if it is any different/better than another option.
Personally, I think you would actually be using more fuel because the engine would be spinning faster, but that is just speculation.

Sorry that was so rambling. might have to go back and clean it up if it doesn't make sense.

In regard to adding that cam to the mix, you'll be hurting fuel economy for the reasons that Loren said. The "better" overlap, just sucks some extra fuel through the cylinder so that you have an entirely fresh mix, instead of the half fresh, half burnt mix like a stock cam provides.

But, since we are rambling on anyway, perhaps there is a way to take advantage of that without giving too much of the power gain.
See, on a stock car, there is an EGR valve that is supposed to suck exhaust fumes back into the intake manifold to be burnt again, reducing the amount of harmful crap that goes out the tail pipe. Usually, by the time someone adds a cam, they have already gotten rid of the EGR valve by installing aftermarket headers. When it comes to making power, particularly NA power, the sum of the parts is greater than the individual totals. IE, an intake will get you 6 hp and an exhaust will get you 6hp, but together, they give you 15 hp. So lets assume we have an intake set up. And an exhaust set up with a nice header the whole bit. And now we want to add in the cam. We know we are already running rich because we are relying on a very basic O2 sensor to compensate for the added flow from the intake and exhaust and we all know how well that works. So going a step further, we add the cam. Notable power gains. Everything is great, but because of the overlap, that is making all of this extra power, our gas mileage is dropped. So we take it to the shop to get tuned. That helps a lot. Now we aren't spraying so much fuel, the timing is a bit better, we have better duty cycle on the injectors, and our air to fuel ratio is closer to the ideal 14.whatever to 1 Awesome.
But, do we think we can get even more MPG?
Going back to the EGR valve, on a stock engine, they designed the exhaust to have a fair amount of back pressure to optimize the EGR function. We've gotten rid of that back pressure so we could make more power. Even though we are tuned, there is still a fair amount of un burnt fuel that is being passed through the exhaust. We could lean the tune out even further, but then we are running the risk of blowing up. But what about pulling that exhaust back into the intake to burn it again, much like the stock EGR system worked?
We would need a strong vacuum source. We would want a good place to pull the exhaust from. We would want to keep the unburnt gas consistent between all the cylinders. Three key points the stock EGR set up just didn't do.
So, a strong vacuum source, that is also going to keep it decently consistent. What about a custom bung just behind the throttle plate? Just needs to be somewhere that is going to feed all of the cylinders evenly. We don't want to feed just one cylinder the crap exhaust, that would cause power balance problems, a tuning nightmare, and a major hot spot. Make it a pretty good size. Maybe the size of the brake booster line. Remember, we don't have any back pressure to assist with this, so the only air we are going to get is whatever we can suck in. (but what about idle? we'll talk about that in a minute) On the exhaust, it is going to have to be after to the collector on the header. The collector part of the header is where a lot of the magic happens as far as to why the header makes power, so we don't want to mess that up, so get at least a few inches behind that to tap into. this should let us get as much as we can from all of the cylinders, and not screw up the flow enough to kill off too much HP.
So now we are capturing that flammable exhaust gas, and burning it again, but because we are tuned, and running well, it isn't like the stock system where we are just picking up soot.
The stock system typically doesn't open until high way cruising speeds. The idea is that it will hurt HP a little bit, and it would kill the idle at low speeds. So maybe install a rpm switch that opens the tube at an acceptable RPM, which you would have to determine on the dyno and it would need to be "tuned for" The hot air, and the un burnt fumes would throw off your whole mix as far as the "perfect" tune goes.
You might be able to get away with keeping it open all of the timing and just compensating for the idle with the idle adjuster, but I doubt there would be enough exhaust fumes in the pipe to warrant it at idle, unless you have an amazing tuner that can compensate for equivalent of a rather large vacuum leak.
Again, it would come back to the tune if it would be worth having it open under acceleration. It might kill off enough hp to negate and of the gains. It might not. Under load, you're spraying more fuel through each cycle, so technically, that is when the exhaust is the worst. If you can get it tuned correctly, then you could have it be just a simple RPM switch to open the valve anytime the rpms are over 1500 and then close again under 1500. You would be able to simply unplug it when you had the "go fast" bug which would keep the valve closed and should let the engine make more power as the bolt ons allow.

again, all speculation based solely on my understanding of the way things work.
I'd like to temper all of this with saying that I have had a long few days so my brain could be a bit fried and I could have forgotten a mucho importante detail
that would alter all of that.

Empire

oh, and what car are we talking about? just so I can ponder the correct things.

SixCylinders

Working backwards through responses.

1994 Maxima GXE. I believe drag coefficient is around .31 and I think the CdA (drag coefficient x area) is 7.09...I forget what unit I used for area but whatever it was it puts my car slightly below average in aerodynamics (at least in the list of cars I've bothered trying to compile info for. Honda Insight being a 5.10, Mazda RX-7 (FD, 93-95) being 5.61, etc. This is of course stock and I am an inch or so lowered

I have already removed EGR. I have a 3" aluminum intake and 2.5" exhaust. I was also thinking some newer cars use cam phasing to increase overlap at low RPM to serve as basically EGR, so maybe some extra overlap wouldn't be that bad of thing? After all in Europe factory cams are 264/262 anyways.

example post from a 350Z forum
"It's not advisable to use aftermarket cams with the Rev-Up motor if you plan on going forced induction. Reason being that Nissan didn't install an EGR valve on the Rev-Up motor and instead uses the variable exhaust cam as the EGR valve. This means that there is alot of overlap anytime the cam is in EGR mode."

Now for Loren's post... My car doesn't give a rats ass how low it's revved. It has no AC compressor, no power steering pump, etc. Only things sapping my low end are the water pump, alternator and the torque converter (which I do want to remedy and soon). It spins tires on accident on the street...I chirped them just the other day in a Walmart parking lot at probably 1200rpm making a right turn under maybe 10% throttle. As you well know my car weighs about 2800lbs with me in it versus probably 3400 stock, so my torque to weight ratio is sky-high compared to stock. Though it doesn't really wake up till the 2Ks. My car doesn't struggle to do 60 @ 1800....my throttle plate is sooo close to closed that the intake manifold vacuum is at 19psi which I think is low for what my engine is capable of. I think if I could get it to 21 or so if it was tuned right. My idle is 19psi as well, I usually have around 12psi when I'm accelerating at a decent rate, 9psi when I'm accelerating decently but faster than anything else on the road.

Loren

This will affect your top end HP if you care, but shouldn't affect part-throttle low-to-mid range torque, and will make it easier to maintain and modulate smaller throttle openings for better fuel economy: Fit a smaller diameter throttle body.

If you still want that top end, you could do something creative to get that full-throttle intake opening. Dual throttle bodies with progressive linkage (Some early FI VW's used such an arrangement), or even a simple secondary intake tube that bypasses the TB with some sort of solenoid valve that pops open at WOT. (which, I guess would be a sort of progressive dual throttle body of sorts)

You could also futz with the TB linkage. Instead of a round cable pull on the throttle shaft, some sort of egg-shaped or off-center arrangement that would make the first half of the throttle-pull turn the throttle plate less, and the second half turn it more. Progressive throttle. I know there are aftermarket kits for some cars that make the throttle more responsive, this would be the opposite of that.

Anything you can do that will keep you from opening the throttle too far and "accidentally" using excessive fuel would help to improve fuel economy.

SixCylinders

Actually my factory throttle body assembly is fairly small. I haven't measured nor is my memory great but I'd say it has dual 1.5" throttle plates...no crazy TB here.

Not really looking forward to messing around with different throttle designs. Only thing I even want to consider is individual throttle bodies off a sport bike connected to a shared plenum (think BMW S65 V8 ) and even then I have about 20 different things that will take an equal number of Benjamins that I want done first. Honestly half the reason isn't bragging rights or power, it would just be a lot easier to get to injectors, valve covers, etc. than the current plenum design

Only time throttle opening is a problem is from an absolute standstill (my grandfather could light the tires up on accident before I modified it) even then it's nothing major...I drive straight while torque steering in the rain...though I used to purposely turn on AC to limit that but I haven't had a serious problem with that in years....lack of ABS on the other hand has gotten my attention twice in the past year or two and again I have no plans to retrofit that either, too much added complexity.

Factory powerband is something like 182ft/lbs@2800rpm and 160hp@5500rpm. I really don't want to push too far on the autotragic transmission because it's a known failure point on even unmodified cars (seriously check craigslist, half of them are at least slipping). I think reasonably I can expect the tranny to survive at over 200hp (for a decent period of time, not forever) as long as I don't bump the low end up any more and focus on top end breathing rather than torque (and not downshift under throttle, which I'm better about than most normal drivers). I'd start working on the 5 speed swap today if I could find a damn junkyard with a loaded 5 speed car.

Traction is also in the process of being addressed. I've got wheel spacers coming in this week to allow me to go from my 15x6+35 stock wheels to 16x7.5+42 300ZX wheels which soon I'll be picking up some 225 width 300 treadwear tires (up from stock 205 @ 440TW). This does of course does bring up some concerns for my transmission but I've already cut some 2.0 60 foots (on 600TW tires!) so I should be okay taking it easy for a bit.

Also, found out my O2 gauge was wired wrong my friend INSISTED it was the green wire and I said we should check the FSM but he was determined that green was right. It turns out the green wire is for the O2 heater. I'm in the process of remedying that but my wire crimpers and all my good pliers have gone missing (it just figures). I was going to wire up a potentiometer to allow me to play with what the computer sees as far as coolant temps so I could richen the fuel mixture up as needed to race with but I'm to the point I'd rather just plug in a 30ohm resistor which would register as a 176F coolant temp and negate the warm up enrichment process and give me some more time to live with my questionable thermostat. Maybe if I really wanted to race rich I could just put a higher resistance resistor in, I still like the idea of adjustability because I've seen it done on a different car with the same engine but oh well.

Loren

Properly working O2 sensor should help you out.

I'd still consider SIMPLE ways to get better low-end throttle control. Could just be a simple matter of fitting a larger diameter pulley from another car onto your throttle shaft. That will give you less throttle plate movement from the same pedal movement. You're wasting fuel right now by not having smooth throttle control. Every time you over-shoot your target and get anywhere near wheelspin, you're wasting fuel. It takes a light foot, and if your throttle is too touchy, that's not easy.

SixCylinders

Pedal travel is pretty long as is, I usually have very smooth throttle control (I can keep it within .5psi of where I want it). I just have a hell of a lot of low end, an open diff, tires at 38psi (factory recommended is 27, which makes the sidewalls look flat), etc. If anything would help it would be a stiffer spring under the pedal but my foot already gets worn out on long interstate trips. If getting cams killed torque under 1600rpm though I would probably appreciate it though.

I've heard that the more throttle you use with the (reasonably) lower RPMs the better your gas mileage gets because throttling losses are reduced. Keeping it under 2500rpm I've always managed to get 26mpg (which is better than factory highway) in the city. Lean burn would be the IDEAL way to improve gas mileage since I could open the throttle way more on the same amount of fuel. That plus a fuel cut switch for coasting would rule.

SixCylinders

Nothing really good to report. I haven't been keeping track of FE numbers like I should, I made a trip recently that was probably over half city driving (with up to 400lbs worth of passengers and 200lbs worth of cargo) and half interstate and managed to get in the neighborhood of 30mpg if I recall correctly. Still working on getting everything I've mentioned in previous posts done and coming up with new ideas to shave weight, clean up the body and what not. One of my more recent setbacks has been my fuel pump appearing to have died and the relay testing good. I bought a Walbro 190LPH pump and came to find out that there are two fuel pump relays, one is a "safety relay" which is supposed to break in case of an accident....I think I hit the train tracks a little too hard. Anyways I have fixed that issue but not without further wasting money that could have gone to better uses but you live you learn.

Any idea if DFCO might be (at least partially) responsible for my exhaust popping on decel? It usually pops pretty loudly when downshifting, the higher the revs after downshifting the louder, a second downshift made before dropping below 1500rpm doesn't make it pop at all and when it does drop to 1500rpm there is a little burble to it. I am at this point assuming that this is due to unburned fuel in the exhaust getting mixed in with a sudden burst of atmospheric air. Honestly it's a bit embarrassing for me to be unsure about this so don't grill me too hard about it.