Truck MPG/towing question.

HiTempguy wrote:

rslifkin wrote: In theory, you're right.

No, in theory, you are right. In reality, you are wrong. I really do appreciate the response, and the very accurate technical information you have provided, but it simply makes no difference. I've said before, there are some cases where higher rpm may return better mpg, but they are few and far between. I feel like I am arguing with engineers right now... "This works in the lab!" "Yep, but this isn't the lab, this is reality". You can do all the math you want, at the end of the day, results speak for themselves.

So now youre saying the 6.0 with less rpms will be the better option?

Bobzilla wrote: So now youre saying the 6.0 with less rpms will be the better option?

Well, if you can find a BSFC graph for the 6.0 and the 4.8, we might be able to find out

rslifkin wrote:

Bobzilla wrote: So now youre saying the 6.0 with less rpms will be the better option?

Well, if you can find a BSFC graph for the 6.0 and the 4.8, we might be able to find out

Been looking off and on for 2 days and haven't found squat.

alfadriver wrote: You do realize that engines run less than 12:1 at WOT even below 2000rpm, don't you? Not all, but many. Worse than that, most also have to retard spark, as knock becomes a big problem. It's pretty easy to find examples were making the same power at 2200rpm is more efficent than at 1500 rpm. Happens a lot.

Well let's see. I happen to have the 4.8l shop truck's tune right here in the laptop. GM has an air/fuel ratio load map (actually an equivalence ratio (inverse Lambda) map) so I can look up right here what they are calling for.

It goes into "PE enable" mode at over 55kpa manifold pressure. At 1600 the equivalence ratio is 1.167 (12.5:1) and at 3200 it is 1.186 (12.3:1) with a gradual slope in between. Goes as high as 1.235 (11.9:1) at 4400 and up, probably more to keep the cats cool than anything.

Also interesting, the IAT and ECT modifiers are zeroed out until coolant is below 14f (!) but overtemp protection shoots it up to a 1.4 equivalence, which I interpret as "cool the engine by dumping fuel to the limits of misfire".

I'd be curious to try towing again just so I have VCM Scanner logging so I could see what is going on with respect to when it is going in and out of power enrichment. There's a significant hysteresis too, it delays entering PE for emissions reasons. Transients won't do it, PE is for keeping everything cool and transient loads won't hurt anything thermally.

So many words, not enough pictures.

Now that I think more, the thing to do would be simply to log mass airflow Hz and MAP. MAF frequency IS airflow, and MAP would let me know if it is was in PE or not.

Related to that picture is this article: http://www.superchevy.com/how-to/engines-drivetrain/1306-4-8l-vs-5-3l-engine-tech-little-ls-slugfest/

Shows the 4.8 really isn't a torque beast but a potential screamer.

There's also that build where they did heads and cam on a junkyard 4.8 with re-gapped used rings, started adding boost and made over 1000hp without it blowing up

rslifkin wrote: There's also that build where they did heads and cam on a junkyard 4.8 with re-gapped used rings, started adding boost and made over 1000hp without it blowing up

i LOVE that article. I remember pre-boost they were making right close to 400hp out that little bugger andspinning over 7k. And it loved it.

that is one thing about these engines. Theyre compact, powerful and have plenty of options.

Knurled wrote:

alfadriver wrote: You do realize that engines run less than 12:1 at WOT even below 2000rpm, don't you? Not all, but many. Worse than that, most also have to retard spark, as knock becomes a big problem. It's pretty easy to find examples were making the same power at 2200rpm is more efficent than at 1500 rpm. Happens a lot.

Well let's see. I happen to have the 4.8l shop truck's tune right here in the laptop. GM has an air/fuel ratio load map (actually an equivalence ratio (inverse Lambda) map) so I can look up right here what they are calling for. It goes into "PE enable" mode at over 55kpa manifold pressure. At 1600 the equivalence ratio is 1.167 (12.5:1) and at 3200 it is 1.186 (12.3:1) with a gradual slope in between. Goes as high as 1.235 (11.9:1) at 4400 and up, probably more to keep the cats cool than anything. Also interesting, the IAT and ECT modifiers are zeroed out until coolant is below 14f (!) but overtemp protection shoots it up to a 1.4 equivalence, which I interpret as "cool the engine by dumping fuel to the limits of misfire". I'd be curious to try towing again just so I have VCM Scanner logging so I could see what is going on with respect to when it is going in and out of power enrichment. There's a significant hysteresis too, it delays entering PE for emissions reasons. Transients won't do it, PE is for keeping everything cool and transient loads won't hurt anything thermally.

One thing to note about your numbers- you mention Power Enrichment- which is basically WOT. At peak torque- 1600rpm, to make the same power at 3200rpm, that's half the amount of the same torque. It's reasonable to expect roughly the same peak torque at 1600 as at 3200rpm.

And lets think this is a really good N/A motor, so at WOT/1600, it's a load of .95. Give or take, the load for the same power at 3200 will be .45-.5. At that load, no way one needs to run rich, and probably not even terribly retarded for spark as well. The only differences will be the pumping losses for the lower MAP, and the friction losses at the higher speed. Will it be worse? Dunno. Not for certain.

Since gear gaps are not that wide, well...

(for anyone keeping score- this is all a power thing- force x speed = power- so 65mph x whatever force it takes to maintain it...)

Tuning OE computers still makes my brain hurt to a degree because, while tuning ignition timing based on cylinder airmass does make a lot more sense than using manifold pressure, it is still not something that is easy to interpret generally. So at X rpm and .5g/cycle timing is Y... how much load is that really? And I really only need to know that for trivia reasons anyway so do I even need to think about that?

So I think it's best to can all that thinking business and go with empirical data, use driving down the road as a standard load, drive it in OD and 3rd to see which one has lower airflow. If it is using less air, it is using less fuel, barring the crossover point if it is in PE but using less air in one gear and out of PE but using just slightly more air in the other.

The takeaway is that PE is used not for "more power" but "engine/catalyst safety". If it has to dump fuel in order to protect the engine, that isn't a very good thing longevity-wise, as opposed to running in a condition where you can just run stoich indefinitely.

There are also benefits to cruising above torque peak. If load increases, torque goes up if you slow down. That said, looking up the specs, peak torque is at 4600 and even I'm not going to suggest cruising that high on anything but a rotary or a motorcycle engine...

Knurled wrote: The takeaway is that PE is used not for "more power" but "engine/catalyst safety". If it has to dump fuel in order to protect the engine, that isn't a very good thing longevity-wise, as opposed to running in a condition where you can just run stoich indefinitely.

That depends on the ECU and its logic. In the case of my Jeep, PE is what determines when to exit closed loop at stop running at stoich (or in my case, leaner than stoich). Stoich at WOT is a bad thing, both for power output and durability, so mine goes into PE right away (stock had a short delay at lower rpm, but I tuned it out for better throttle response).

Some other ECUs have other ways of knowing when to exit closed loop and will start to richen up a bit for WOT. PE can then be used to bring in a little extra fuel beyond the basic needs for power / engine safety to ensure that things stay cool enough and don't blow up if you remain at WOT for more than x number of seconds.

Keep in mind, when I was talking about PE in my earlier posts, I don't necessarily mean the actual PE mode in the ECU / tune, but just the point where it goes to open loop and starts to run richer than stoich.

"Open loop" does not mean "rich", it just means that it ignores the O2 sensor.

Interestingly there are maps for cruise mode lean-of-stoich running that are zeroed out (or rather, don't engage until over 254mph) for "non US market vehicles". I thought GM quite doing that lean mode stuff in the 80s after the EPA told them to knock it off

They're not worried about more power so much as engine and catalyst protection. Keep the engine cool so it makes warranty, keep the catalysts cool so they make warranty. That is why PE engagement is delayed, and there are separate maps regarding catalyst temperature. And did I mention what they do to fueling when the engine gets hot? They want the engine to live. If they could run stoich at WOT and keep everything happy, they would, but I have a feeling that on a long pull you could take bets on whether the exhaust valves or catalysts would slag first. So they run rich, far richer than best power (chemically) would require, in order to keep everything cool and happy.

There IS a separate open-loop equivalency map. It's boring, it is all 1:1 until you get to Really Cold coolant temps and then it richens slightly. Which is as it should be, IMO, power enrichment is a separate concept from ideal fueling.

Incidentally, the "stoichiometric" figure is 14.68:1, which is kind of odd as I would have assumed it would be in the 14.2:1 range. The newer GM offerings have stoich calibrated as 14.2:1 as E10 is assumed, apparently in 2003, GM was betting that more gas stations than not sold E0. I should probably fix that... or I could just leave well enough alone, and that if it ignores LTFTs at WOT, then the incorrect stoich factor is merely giving me a better WOT fuel mixture for power than the "damn the economy, pour fuel into the chambers" strategy in place.

Max power on an NA engine comes around 12.5 - 13:1 (with AFR scaled assuming E0, so we can say .85 - .89 lambda to make the conversation more fuel-independent), typically. Going leaner than that will cost power, but improve fuel economy.

I'm betting the newer stuff that's set for 14.2 AFR has the lambda values scaled to AFR for E10, rather than E0. 1.0 Lambda is stoich, regardless of the fuel being used. If you're looking at it as AFR, then it's scaled based on a specific fuel (if scaled for E0, 14.7 AFR = 1.0 Lambda, for E10, 14.2 AFR = 1.0 Lambda). So both vehicles are tuned around stoich, they'll just show different AFR numbers to represent it.

As far as fuel trims go, it should apply the LTFTs when in open loop (including WOT). E0 and E10 are close enough together that it'll have no trouble adjusting from one to the other without getting anywhere near the max fuel trim adjustment. E85 would typically be out of reach without a dedicated tune (and probably bigger injectors) though.

My TJ when I had 33s nd 3.73 gears got better mileage in 4th than in 5th at 65-70mph(IIRC it was 15-16mpg vs 14 or so?). This was duplicated on many long trips over and over again.

When I went to 40s, I ran 5.86 gears(numerically identical to 33s with 4.88) and I got 11-12mpg which is really damn good considering it was a 4560lb Jeep as un-aerodynamic as one could get with 40x16-17 Swampers hanging out in the wind at 82in wide overall.

FWIW.

I'm a big believer in regearing it for power. People make such a big fuss over higher RPMs and the truth is it will pull at 3000rpms all day long if it really needs to. 2500 or whatever it will end up at with 3.73s will be fine. It will impact your unloaded mileage a bit, but it still won't be near 6.0 mileage bad. They drink gas like its going out of style.

Great, I don't check this thread for a bit, come back to get caught up and this page makes my brain hurt....

Back to your magical ECU Lambda Lambda Lambda / Alpha Beta / Pi Delta Pi discussions.

-Rob

In reply to Knurled:

FWIW- the Lean Cruise thing wasn't GM....

So in terms of fueling- the more modern cars have really good temperature estimates in them- so while the systems can run cool enough, they will run LBT fuel to make the best power. That only lasts for a few seconds, and is irrelevant to this trailer discussion.

Also, cars from the last 10 years or so are rarely open loop, as they all use WB sensors. They are only open loop when the exhaust is too cold to safely run the heaters for the O2 sensors.

For this discussion- when the engine has to run rich for safety is important, as many trailer drivers run at a fairly high power constantly. Which is one reason diesel is better for towing a trailer. One thing that hasn't been brought up- the idea of lugging- which is what most people call it. It's not harmful, but the combustion really turns to crap at low engine speeds- which really hurts overall efficiency. Low speed/high load just does not burn well in gas.

Kind of a public service post- one of the posters shot me a question about their towing issues, which to me, turned out to be a transmission calibration issue more than anything else- and then what to do...

Well, I'm not all that knowledgeable about places that do performance transmission re-flashes- but doing a quick search does find a few of them out there- for instance this company- http://jetchip.com/ offers a wide range of products.

Something like that could also help the OP find a way to get his truck to get better performance as well.

The core problem with recommending one company- I have NO idea how good they are.

Still- for towing, it's pretty important (as evidenced by some of the posts) that transmission performance needs to alter as well- so you need to find a place that will do both the engine and trans.

I talked to one of their tech folk ... and said exactly the same thing ... the added 20 - 30 hp would help keep me in OD/TQ lock up, on some of the "gentler" hills .. but the transmission reflash, along WITH the their module would get me the most bang for my buck ... of course if I could find a pair of the upgrade heads (much more hp and torque) I'd really be better off .. pricier but ....

I don't really want to thread jack, but theres lots of smart talk happening.

I've got a 94 chevy 2500 with a dying 305 and 4l60. I'm swapping in a 5.3 or 6.0. I'm not super concerned with fuel economy and the last few days I've been thinking of getting a gooseneck 2 car hauler.

GRM- talk me into a diesel swap.

Honestly, a 6.0 with the right gearing should pull almost as well as a diesel, it's just gonna burn a good bit more fuel (especially when un-loaded or lightly loaded, it'll get closer when you're really loaded up).

Honestly, for a gooseneck 2 car hauler, depending on what you want to put on it, you might want to consider upgrading from a 3/4 ton to a 1 ton or even a 1 ton dually. The 3/4 ton will likely be maxed out payload-wise carrying the pin weight of a trailer like that even if it can otherwise pull it fine.

The drive in this morning got me thinking.... this truck is geared to run 220mph (1k per 40mph, 5500rpm redline). We know this is impossible because the aerodynamic drag would be astronomical. To put it another way..... This truck has a 1-piece driveshaft, which means it has a 96mph top speed limiter to keep it from exploding. That means, a brick on the throttle, this thing hits the top speed at 2300rpms. 96mph at 2300 rpms.

Bobzilla wrote: Would a small engine turning more rpms be better for mpg than a larger engine turning lower rpms?

This thread is full of practical advice, but to answer the original question:

It depends on the BSFC of both motors. In general, the higher you can keep the throttle plate open, the better for mileage, until you get to very low vacuum as most OEM tunes will run rich there.

Here's the saturn BSFC (can't find one for an LS, but they are generally the same shape):

Note that the Y axis is torque (usually its BMEP). So for a given HP requirement i.e. whatever it takes to move the truck + trailer at 70mph on flat land, you could plot a point in each gear. In OD that point will always be higher on the Y axis and lower (lefter) on the X axis than that same point in D.... for the same engine. The question is how do the 6.0 and 4.8 BSFC #s compare. Given the same heads and whatnot, they are probably very close at higher loads but at lower loads will vary due to the added friction of the 6.0.

Bobzilla wrote: This truck has a 1-piece driveshaft, which means it has a 96mph top speed limiter to keep it from exploding. That means, a brick on the throttle, this thing hits the top speed at 2300rpms. 96mph at 2300 rpms.

But, does that also mean if you re-gear to a lower gear ratio ( higher numerically) that your max safe speed also decreases ( or should decrease) to maintain the same limit for driveshaft RPM?