New member, old hand

[omgwtfbyobbq]

Nice video! One word of caution, the more efficient you make your truck, the more inefficient you make the engine. Eventually, you'll hit a wall in terms of aero/rolling resistance modifications. However, because you've been making the truck so inefficient in terms of engine operation, once you hit that wall, you can generally pick up what you've already picked up w/ an appropriate OD gear. On the highway at least. For instance, lets say you make a canvas bed cover, get some LRR tires, etc... and get up 26mpg hwy from 20mpg hwy. With a gearing change, you should easily be able to get up to 30+mpg hwy, if not more depending on what level of acceleration compared to mileage you're willing to have. For a truck like that, I figure optimum is somewhere around 40+mpg cruising at 50-60mph, but you may not like being at half throttle on the highway at those speeds. My take is, everything my truck can do now, it could do w/ an extra OD gear, and I'll be able to get into the 40mpg highway range. If I want to accelerate, I'll drop it into fourth. But, you may not find this acceptable. YMMV.

[kitcar]

Glad you liked the first gas mileage video. The second one is on the way. What I'm also going to do is driving techniques - with a manual transmission you really have a lot more options for saving gas; clutch in downhill, short shifting, skipping gears, etc.. Should be pretty entertaining. Might work in a burnout or two.

Originally Posted by omgwtfbyobbq

Nice video! One word of caution, the more efficient you make your truck, the more inefficient you make the engine.

I try to do both hand-in-hand. Synthetic lube throughout the entire drive train, the cold air intake, IAT adjuster, colder/hotter plugs (depending on the season) and doing the aero mods. Remember, this particular engine (the 4.7) was designed with our German friends and is designed to be very efficient. With the manual and the 3.55 posi in it, I turn about 1,500 at 70. First gear is a creeper for towing. I generally go: 2nd, 3rd, 5th when shifting. Conversely, at 40 mph (which seems to be the "sweet spot") I'm getting anywhere from 32 to 36 mpg, steady state on a flat surface. It remains to be seen whether this changes with the newer aero additions.

[omgwtfbyobbq]

Hmmm... Ime, I figure that 1500rpm for a 3L engine seems to be the sweet spot for a normal car, or even an efficient Phil Knox kinda truck. For an engine that big, at ~50-60mph peak efficiency could be had at ~1000rpm, if not lower depending on what the mods do. Provided of course it's a dohc gasser with a relatively flat torque curve. I'm guessing that right now, BSFC is in the 400-500g/kwh range, while ideally, it could be around the 300g/kwh. When you're cruising at 55mph in OD, assuming flat ground and no wind, about how much throttle (&#37 would you say you're at?

[kitcar]

Originally Posted by omgwtfbyobbq

I'm guessing that right now, BSFC is in the 400-500g/kwh range, while ideally, it could be around the 300g/kwh. When you're cruising at 55mph in OD, assuming flat ground and no wind, about how much throttle (%) would you say you're at?

Darned if I know. I go so light on the gas pedal I'm surprised. I'm going to test the cab spoilers Sunday morning (5-20-07) so I'll pay close attention to that. I'll make sure I remember to shoot video of it. I can tell you this; gas mileage at 50-60 sucks. I get great at 40-45 and then it picks up again at 65 to 70 or so. 55 is (or has been) a joke. I was getting 24-25 at 65 mph and 20-21 at 55. Go figure. Depending on the wind (hopefully a dead issue with the side skirts) I get the same mileage at 70 as I do at 65. But I can get in the wake of a semi at 65 and stay there for a long time, so I drive 65. The secret is to find the spot where the buffeting is the greatest from the semi (usually about 10 car lengths back) and stay there.

I'm going to start another thread with the cab spoiler video.

[omgwtfbyobbq]

If you really are that light at a 55mph flat cruise, engine eff is probably about half of what it could be. Have you checked the mileage at different speeds by using bi-directional runs within minutes of each other at whatever average speed in the same gear? I haven't seen an engine that will decrease in efficiency compared to speed faster than power levels climb compared to speed, so I'm thinking there may be some confounding factors influencing your speed versus mpg results.

[kitcar]

Originally Posted by omgwtfbyobbq

If you really are that light at a 55mph flat cruise, engine eff is probably about half of what it could be. Have you checked the mileage at different speeds by using bi-directional runs within minutes of each other at whatever average speed in the same gear? I haven't seen an engine that will decrease in efficiency compared to speed faster than power levels climb compared to speed, so I'm thinking there may be some confounding factors influencing your speed versus mpg results.

As a matter of fact, I have. I'm suspecting a torque curve issue. These engines respond readily to changes in the front as well as the back end. It has to do with the fuzzy logic the Dodge ECU uses and its' learning curve as well as the physical limitations of the stock components. I "probably" could do a temporary fix by resetting the computer to a learning cycle but hesitate to do that until I'm sure what the aero changes are doing. With the cold air intake (and) without decreasing the exhaust back pressure...well there you go. I doubt if it's a physical efficiency issue in the engine itself, rather a ancillary one related to intake/exhaust flow. I've seen that before on this family of engine. Until now, I haven't had the gumption to do anything about exhaust flow but it might be time to think about a more free flowing exhaust system. Once you free up the intake system (and these engines are pretty darn good flowing in the hydrodynamics department right out of the box) you need to pay attention to removing the combusted gasses - which I've neglected thus far. Headers wouldn't help much - Dodge has always had excellent stock manifolds. The intake is formed plastic, the TB design is top notch - darned thing is fly-by-wire. The exhaust however can be improved on a lot just by ditching the stock muffler and putting in a free flowing box. Some Dakota guys also delete the second cat which I hesitate to do. Which would bring us to the state of the cats themselves - 56k miles on them. Now you've got me thinking about the actual engine efficiency. Another point is this; my last Dakota had the 3.9 V6 Magnum, same transmission, same body style, 500 lbs less curb weight, same wheels, same tires right down to the brand. In fact, I used the same air filter from that truck on this one, even the same intake hose system I made - I took it off when I sold it. I got the EXACT same mileage (up to now) 22.5 on the highway with it as I do with the 4.7. That's one of the reasons I'm so secure in my empirical evidence - I have a fairly large body of experience to draw from with this vehicle. That's why I'm doubtful that the 40 mpg as a top isn't possible. I'd be surprised by 30 to 35 mpg. But what I've gotten so far has surprised me so anything is possible.

Well, I see everyone has that glazed look in their eyes so it's time for another beer. Beer is the perfect food. It's a fluid, has carbs, fiber, relaxes you and tastes good as well is an excellent substitute for common sense in many cases.

I started a new thread with my videos.

[omgwtfbyobbq]

It's just the nature of most gasoline and older diesel engines ime. Unless it's a newer diesel, or an Atkinson cycle gasser, BSFC will always be poor at lower loads. Peak efficiency is around 250g/kwh, but if what you're saying is true about throttle position, then you're in the 400-500g/kwh range. It's just something I've noticed that's pretty consistent across the years. Be it older diesels, or newer gassers, anything below ~half throttle will be less than ideal, and at a quarter or less throttle, horrible. The intake and exhaust will move these ovals around slightly, but it's just a matter of low load pumping losses in general. Ideally, we want a little more or less than half throttle and whatever our desired cruising speed. Any more, and we only gain a little bit of efficiency while losing a big chunk of available power. Any less, and efficiency goes in the ****ter at an increasing rate. Even economy cars these days are sometimes geared way high. For example, the Honda Accord and Element both have the same engine, and for the conditions on the EPA highway test, need nearly the same amount of power, with a little more required for the Element. But because the Element is geared so high in terms of OD, probably due to the market it's being sold to, it looses nearly 10mpg on the highway compared to the Accord. Naturally the downside to taller gearing is less available power, but I'd give up being able to go 75-85mph in OD for an extra ~7-15mpg.

[kitcar]

I'm playing devils advocate here with this response, so bear with me. I'm trying to simplify this as much as possible - I'm going to use it in my next full episode of "This Old Shack". Doing my reporter BS, I am.

But if that's the case why would I be seeing 35-36 mpg at 25-35 mph where my throttle is so much lighter? Even at 40 mph, I'm seeing mpg in the 30s. I understand the aero drag causes big losses in mpg but even in a tail wind I only see another 1-2 mpg at highway speeds.

If we take the inefficiencies of the engine to be a truism I wouldn't be seeing the mileage rate that I do at lower speeds but would see less. Now, with the drive train taken into account (I'm in 5th gear as low as 25 mph), I'm just loping along at 900 or 1,100 rpms at low speeds and I see higher mpg. That just seems to scream higher efficiency at lower speeds. If the efficiency is lower at lower throttle levels, one would expect to see lower mpg.

At a gut level here's what I'm saying: at low speeds, I get higher gas mileage. At higher speeds, it's lower. Logic dictates no matter what the efficiency of the engine, if I get higher mileage at "X" speed, then the drive train is more efficient at that particular speed, which is in the 25-35 mph range.

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At the one end, without the aero drag at low speeds, we see high mpg (even though the efficiency of the engine might be at its lowest), at the other end we see lower mpg with higher aero drag even though the engine might be operating at its peak efficiency. As we decrease the aero drag, mileage increases until we reach a break even point in engine efficiency at "X" rpms. The key point is to get your rpms, engine efficiency and aero drag to be as co-equal as possible at the highest speed possible given all of the parameters.

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As I said, I'm trying to simplify this for the average viewer so I can communicate it to others.

[SVOboy]

I think the most important think to note for the average person is that engine efficiency and fuel economy do not correlate exactly,

[kitcar]

Originally Posted by SVOboy

I think the most important think to note for the average person is that engine efficiency and fuel economy do not correlate exactly,

Earlier tonight I was enjoying a double Boca Burger (honest, I've got $1.00 off coupons) and thinking the exact same thing to myself. At this point in the gasoline pricing cycle it's going to be even more important to educate the uneducated. They've got to understand that you have to slow down and if you're not going to get a subcompact at least consider hanging some crap off the thing to streamline it, even a little bit would help.

The important point that I make to myself as I've learned after jumping with both feet into the aero deal after pissing around with it for a year is that getting the two as close as possible is the key.

The hard part for me is to actually get in front of the camera and explain this for the layperson. I keep asking myself if I should dumb it down or gradually ramp the theory up until the viewer understands the concept. I prefer the latter. But that's the hard part because I like to screw around. :sigh:

I need some PMs to discuss this. Once I get the theory together I'm going to need some people to check my point of view.