Gas mileage compromise...
#46
07-24-2014, 01:22 AM
Sorry, I was on vacation and lost track of this thread, I wasn't intending to throw a stink-bomb and then hide. BruteFord has probably been making my case better than I could anyway, so it maybe was for the best. But now that I'm back...
Yes, when engine vacuum is highest and engine load is lowest an engine is least likely to knock. So under those conditions you can do other things, like lean the mixture and advance the timing, that improve your efficiency but that you can't do under higher load conditions where knock is more likely.
So I think your "why not if" question is a good one. In my understanding the "why" is that you do both when you can afford to, as opposed to your suggestion that you do one when you do the other to cancel it out.
By the way, a long time ago (mid '80s) I was an engineer in an engine development lab at GM (sorry, but I did drive a Ford pickup to work when I was at GM, true story!). I know I've forgotten most of what I learned there, so I'm not saying that to prop myself up as an expert. But what I remember us aiming for in engine tune was MBT/LBT. Doing a quick Google search now I find "MBT" meaning "Minimum timing for Best Torque" but what I remember was "Maximum advance for Best Torque". "LBT" was "Leanest mixture for Best Torque."
So what we'd do is go in that order, advance the timing until the torque dropped and then back it up to get the torque back. Then we'd lean out the mixture until the torque started to drop, and then richen it a tad to get it back. That apparently gave the best efficiency, generally with no knock (knock will cost torque).
So based on going in that order it would seem that a lean mixture doesn't really make an engine much more prone to knock. But my understanding is along the same lines as what BruteFord said earlier. A slightly rich mixture or a slightly lean mixture aren't much different in terms of knock. But as a mixture gets very rich it cools down the burn making the engine less likely to knock. And as it gets very lean it can create more heat and leave "hot spots" that can start a second flame front, leading to knock. But mixture is definitely secondary to timing when it comes to knock.
Yes, when engine vacuum is highest and engine load is lowest an engine is least likely to knock. So under those conditions you can do other things, like lean the mixture and advance the timing, that improve your efficiency but that you can't do under higher load conditions where knock is more likely.
So I think your "why not if" question is a good one. In my understanding the "why" is that you do both when you can afford to, as opposed to your suggestion that you do one when you do the other to cancel it out.
By the way, a long time ago (mid '80s) I was an engineer in an engine development lab at GM (sorry, but I did drive a Ford pickup to work when I was at GM, true story!). I know I've forgotten most of what I learned there, so I'm not saying that to prop myself up as an expert. But what I remember us aiming for in engine tune was MBT/LBT. Doing a quick Google search now I find "MBT" meaning "Minimum timing for Best Torque" but what I remember was "Maximum advance for Best Torque". "LBT" was "Leanest mixture for Best Torque."
So what we'd do is go in that order, advance the timing until the torque dropped and then back it up to get the torque back. Then we'd lean out the mixture until the torque started to drop, and then richen it a tad to get it back. That apparently gave the best efficiency, generally with no knock (knock will cost torque).
So based on going in that order it would seem that a lean mixture doesn't really make an engine much more prone to knock. But my understanding is along the same lines as what BruteFord said earlier. A slightly rich mixture or a slightly lean mixture aren't much different in terms of knock. But as a mixture gets very rich it cools down the burn making the engine less likely to knock. And as it gets very lean it can create more heat and leave "hot spots" that can start a second flame front, leading to knock. But mixture is definitely secondary to timing when it comes to knock.
For example during a condition like highway cruise you can absolutely get away with being lean cause it's a light load/high vacuum condition. You do this for the simple reason of saving fuel when you can. However timing is a linear function of cylinder pressure and rpm. This means for example if at say 10% throttle/30inHg and 2000RPM you might make best power at stoichiometric/14.7 but can run as lean as 12.5 for MPG gain with little/no power loss. While at the same time you make the best power at 52deg advance but going to 56deg won't net anymore mileage and will lose power.
On the flip side you have to look at an example like pulling a hill at WOT and 3000RPM with enough hill and load that you aren't accelerating. Here you need max power without knock. Likely stoichiometric or just slightly lean will make the most power but what about timing. It's a similar but flipped reasoning and where tuning to the knock isn't a way to tune. You might make the best power at 36 and knock at 40, this doesn't mean 38 is better. If you have to tune to the edge of knock for best power then your engine is too stressed in other ways. This brings me to your notion of too rich preventing knock, this may technically be true but is a very short term solution as a rich condition will lead to carbon buildup causing a knock condition that may be unsolvable while also costing power and fuel. A better solution is to cool, colder plug, colder thermostat, cold air intake, water injection, or higher octane.
#47
07-24-2014, 11:49 AM
Logistics Pro
And as I implied in the last sentance in my last post, I would NOT use mixture as a way to address a knock problem. Getting the mixture right (as lean as you can go without losing power) is separate from knock. And I agree with you, that ideal will typically be right around stoichiometric (but not richer) at high load and will be leaner at lower loads.
As far as tuning to knock not being a way to tune, I'd modify that statement slightly to say that it's not the best way to tune,. But it is a way that is used often by shade-tree mechanics. And while I don't doubt that it leaves a little efficiency on the table (and in engine terms, efficiency means both power and mileage), it's not the worst someone can do either.
#48
07-25-2014, 09:33 PM
Posting Legend
David - You have an MSD ignition system. Right? I don't have a dial-back timing light and have been considering getting one in time for the GTG and testing your truck. But, they don't work with MSD. So, we will need to either put a timing tape on the harmonic balancer or mark it accurately. Could you measure your balancer so I can do some math and think about how to do this?
#50
07-25-2014, 09:44 PM
Posting Legend
#51
07-25-2014, 10:00 PM
I like these; http://image.highperformancepontiac....ming_light.jpg
The ones with the back to zero **** on the back. They are sold under many brand names but are basically all the same just with different stickers. I got my current one used of craigslist for $40. I've owned 3 of them, of the two previous ones one got stolen the other got crushed(left it on top of tire) but I've never had one go bad on me.
The ones with the back to zero **** on the back. They are sold under many brand names but are basically all the same just with different stickers. I got my current one used of craigslist for $40. I've owned 3 of them, of the two previous ones one got stolen the other got crushed(left it on top of tire) but I've never had one go bad on me.
For this reason I keep around a really old non inductive timing light, that goes in line with the plug wire, it will work on anything. Though that isn't really needed, just either a replacement cheap wire for #1 or a section of wire to put in line and clamp to.
The issue with the Innova I'd assume is it's digital, the MSD messes it up. I've never liked the digital ones, the dial is much easier, and faster. You might wonder about accuracy with the dial. The only accuracy issue is wearing off the sticker with the markings, the dial itself is pretty darn accurate. I've had them on enough vehicles with a timing tape that I could check using both to confirm I got the same reading.
#52
07-25-2014, 10:06 PM
Posting Legend
#53
07-25-2014, 10:22 PM
Not sure what you mean here, "do both" both of what? Cancel what out?
Anyhoo, Somebody said something about lean mixtures needing less advance (but later deleted it) and I had responded saying that lean mixtures need more advance, not less.
Somebody else responded "Nope." So, I pointed out the eleventy gazillion websites, manuals, Auto Repair 100 books and engineering treatises that explain vacuum advance, but then we are told not to pay any attention to that, they apparently don't have any idea what the fvck they are talking about, but you're a retired engineer so we should, or something.
My only point was that the manufacturers have the most advanced timing at cruise, when engine vacuum is highest, fuel mixtures are leanest, etc. I'm not making this stuff up. Sheesh.
Again, the maximum possible advance at cruise is for fuel economy. That about covers it for "why" doesn't it?
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#54
07-26-2014, 02:41 PM
Logistics Pro
In your post back on July 8 when this back-and-forth started you said "lean mixtures need more advance, not less." To me that sounds like you are saying that if you lean out the mixture, then you need to advance the timing to get things to work right with the lean mixture. Sorry, “cancel out” was probably a poor way to say that.
Anyhoo, Somebody said something about lean mixtures needing less advance (but later deleted it) and I had responded saying that lean mixtures need more advance, not less.
Somebody else responded "Nope." So, I pointed out the eleventy gazillion websites, manuals, Auto Repair 100 books and engineering treatises that explain vacuum advance, but then we are told not to pay any attention to that, they apparently don't have any idea what the fvck they are talking about, but you're a retired engineer so we should, or something.
My only point was that the manufacturers have the most advanced timing at cruise, when engine vacuum is highest, fuel mixtures are leanest, etc. I'm not making this stuff up. Sheesh.
Again, the maximum possible advance at cruise is for fuel economy. That about covers it for "why" doesn't it?
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Somebody else responded "Nope." So, I pointed out the eleventy gazillion websites, manuals, Auto Repair 100 books and engineering treatises that explain vacuum advance, but then we are told not to pay any attention to that, they apparently don't have any idea what the fvck they are talking about, but you're a retired engineer so we should, or something.
My only point was that the manufacturers have the most advanced timing at cruise, when engine vacuum is highest, fuel mixtures are leanest, etc. I'm not making this stuff up. Sheesh.
Again, the maximum possible advance at cruise is for fuel economy. That about covers it for "why" doesn't it?
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And sorry, I didn't mean to imply that I was a retired automotive engineer with a career's worth of experience. I am a mechanical engineer, and I did focus on power and propulsion in college, then I worked for GM for 2 years before being laid off. But automotive has just been a hobby since then.
#55
07-28-2014, 10:28 AM
David - You have an MSD ignition system. Right? I don't have a dial-back timing light and have been considering getting one in time for the GTG and testing your truck. But, they don't work with MSD. So, we will need to either put a timing tape on the harmonic balancer or mark it accurately. Could you measure your balancer so I can do some math and think about how to do this?
#56
07-29-2014, 01:43 PM
#57
07-29-2014, 02:14 PM
Posting Legend
Thanks, David. From that I can figure out what the measurements are for each degree or couple of degrees and we can mark the damper. One approach would be to mark a piece of masking tape and gently lay that on the rim of the damper to ensure it doesn't stretch. Or, we could just get a timing tape for that size damper.
Another approach would be to pull the damper and mark it using the mill. That's more work but wouldn't be too bad. We'd just have to loosen all of the belts and use my puller. And I could have already clamped a crank pulley on the rotary table and zeroed in the mill so we would just bolt your damper to the pulley, zero the setting on the TDC mark, and then mark every 2 degrees up to something like 50 degrees.
The masking tape wouldn't last long at all, but would be adequate for our purposes. A timing tape would be much more permanent, but making via a small slot on the mill and then filling those with paint would last a long time. Your call on the approach.
Another approach would be to pull the damper and mark it using the mill. That's more work but wouldn't be too bad. We'd just have to loosen all of the belts and use my puller. And I could have already clamped a crank pulley on the rotary table and zeroed in the mill so we would just bolt your damper to the pulley, zero the setting on the TDC mark, and then mark every 2 degrees up to something like 50 degrees.
The masking tape wouldn't last long at all, but would be adequate for our purposes. A timing tape would be much more permanent, but making via a small slot on the mill and then filling those with paint would last a long time. Your call on the approach.
#58
07-29-2014, 02:52 PM
Or?
Thanks, David. From that I can figure out what the measurements are for each degree or couple of degrees and we can mark the damper. One approach would be to mark a piece of masking tape and gently lay that on the rim of the damper to ensure it doesn't stretch. Or, we could just get a timing tape for that size damper.
Another approach would be to pull the damper and mark it using the mill. That's more work but wouldn't be too bad. We'd just have to loosen all of the belts and use my puller. And I could have already clamped a crank pulley on the rotary table and zeroed in the mill so we would just bolt your damper to the pulley, zero the setting on the TDC mark, and then mark every 2 degrees up to something like 50 degrees.
The masking tape wouldn't last long at all, but would be adequate for our purposes. A timing tape would be much more permanent, but making via a small slot on the mill and then filling those with paint would last a long time. Your call on the approach.
Another approach would be to pull the damper and mark it using the mill. That's more work but wouldn't be too bad. We'd just have to loosen all of the belts and use my puller. And I could have already clamped a crank pulley on the rotary table and zeroed in the mill so we would just bolt your damper to the pulley, zero the setting on the TDC mark, and then mark every 2 degrees up to something like 50 degrees.
The masking tape wouldn't last long at all, but would be adequate for our purposes. A timing tape would be much more permanent, but making via a small slot on the mill and then filling those with paint would last a long time. Your call on the approach.
#60
07-29-2014, 03:27 PM
Posting Legend
Or, since my harmonic balancer is original with 176K miles on it, what if you purchased a replacement balancer locally and I will reimburse you. If agreeable, that might lessen the time crunch. That would be more work on your part, though, and the other guys might not want to spend too much time on just one truck?
.
Yep, that was one of the options I mentioned and would be almost as permanent as grooving the damper on the mill. And, it would be faster as we just have to clean the outer diameter. But, if David thinks it is time for a new damper....