Basic VVT Tuning info

[viceroykarl]

I believe that advancing the timing would cause your valve to open sooner (and close sooner) than normal and retarding it opens it later (closes later). Not really sure why there would be overlap once the turbo is spinning. I would have thought that for FI you'd want the intake valve closing as soon as the cylinder was at the bottom. as your suspecting i too think that overlap would just let the forced air right back out of the cylinder when under FI. Im following slightly different rules being NA so i may be wrong about the intake timing but im pretty sure FI you wouldnt want overlap.
Like on the first post saying, you can open the exhaust valve a little before the cylinder reaches the bottom without losing any appreciable power from the stroke and get you hotter exhaust gas pushing through the turbo. That'll get your exhaust valve closing sooner so it should make it easier to avoid overlap with the intake cam

Not to sure how the ecu in the lnf or lsj handles cam timings because on the e67 in the le5 i cant use negative numbers for cam actuation values, only from 0 to 25 in degrees where 0 is the most retarded and 25 is most advanced. At least thats what i gathered when i unplugged the solenoid for the cams and scanned the position sensor for where they were at. That and it felt down on power up in the higher range.

I'd give it a shot to see how it changes airflow and coolant temp and IAT but I don't have a turbo charger (yet )

I attached a pdf of the cam specs for the stock cams, though you probably already have a copy.
i think the natural overlap of the cams is like 6 for the lnf, none by 5 for the lsj, 10 for the le5, and 8 for the l61. Please correct me if im wrong as i very well may be

[cobaltssoverbooster]

so overlap with a turbo is still there because you want all the expent gasses to leave the cylinder but at a certain point you dont help anything meaning no gains. but compared to a n.a. this overlap is way less because like you said that turbo spinning forces the air through the cylinder decreasing the amount of time it takes to get a fresh charge thus less overlap. turbo= less time to clear the burnt gasses out and less time to fill the cyl with a complete fresh charge= less overlap and more time during the compression cycle to fully compress and also i could see the exhaust being a little bit retarded to allow a full burn of said charge, and back around the process she goes.

[Leafy]

Also with a turbo you can retard (well really have more total duration) the intake cam more than you could with na because the positive pressure in the intake manifold will continue filling the cylinder even after the piston has started moving back up.

[cobaltssoverbooster]

yes true

[silverbullet08]

So the best thing would be to find where the LNF starts overlap as far as GM techs method. He said anything greater than 0 is widenind the overlap and anything more negative is shortening or no overlap. so if we are getting around -6 up top would that mean we are getting no overlap or is it just bearly overlaping...? We need a starting point.

[viceroykarl]

Does the LNF allow you to retard the cam timings or only advance? In the e67 in the LE5 you can only advance the cams from 0 degrees of advance. Though you can reduce the overlap by advancing the exhaust cam more than the intake cam. Like at low rpm with not much cylinder air (less than .56g/cyl) the intake cam only is advanced at most by 10 degrees or if less than .56 g/cyl its between 3 and 5. While the advance on the exhaust is anywhere between 21 and 18. The overlap is much increased though once more then .56 g/cyl since to get higher than that at that low rpm would most likely be a high air velocity condition.

[Leafy]

I thought with the LE5 that 0 degrees of advance was some random number on the table like 17 or something and any lower number was retard and any higher was advance.

[silverbullet08]

Does the LNF allow you to retard the cam timings or only advance? In the e67 in the LE5 you can only advance the cams from 0 degrees of advance. Though you can reduce the overlap by advancing the exhaust cam more than the intake cam. Like at low rpm with not much cylinder air (less than .56g/cyl) the intake cam only is advanced at most by 10 degrees or if less than .56 g/cyl its between 3 and 5. While the advance on the exhaust is anywhere between 21 and 18. The overlap is much increased though once more then .56 g/cyl since to get higher than that at that low rpm would most likely be a high air velocity condition.

Allows you to advance and retard... not 100% on if that is actually whats happening. A log should provide that info.
Heres a copy of the overlap im getting. Im also looking at my cruize areas.. Is the goal to get more overlap or less during cruize for MPGs? mine seems to have none then a lot as rpms go up and load goes down... These tables could be made to be MUCh smoother after looking at them this way. Im going to work on making smooth transition points for overlap as well smooth individual cam tables. I bet it would make a huge difference i nthe way this car drives...

[viceroykarl]

Allows you to advance and retard... not 100% on if that is actually whats happening. A log should provide that info.
Heres a copy of the overlap im getting. Im also looking at my cruize areas.. Is the goal to get more overlap or less during cruize for MPGs? mine seems to have none then a lot as rpms go up and load goes down... These tables could be made to be MUCh smoother after looking at them this way. Im going to work on making smooth transition points for overlap as well smooth individual cam tables. I bet it would make a huge difference i nthe way this car drives...

Which table is that in the attachment?
I would think that for max economy you'd want to have little overlap since less air in the cylinder would mean less fuel would be needed. at least on an NA engine.
although on the first post there is a mention that overlap cools the leaving exhaust gasses which will push with lower velocity on the exhaust turbine of the turbo.
EDIT: I downloaded a copy of a stock tune for the lnf and im wondering if the wategate is open when desired boost is 0%. Im looking at the duty cycles and see 0% and am wondering if this 0% is referring to %closed. If there is no desired boost then little or no overlap should yield lowest cylinder airmass which would give better economy. but at low cruising rpm overlap wont give benefit anyway since the airflow isnt moving fast enough. I guess you could get lower cyl airmass from opening the intake valve much sooner so that its closing before the cylinder is at the bottom.
In the stock intake cam times in the le5 there seems to be a massive advance in the range where crusings values seem to be (between 2000 to 3k rpm and between 0.24 and 0.36 g/cyl )

I thought with the LE5 that 0 degrees of advance was some random number on the table like 17 or something and any lower number was retard and any higher was advance.

Im not 100% sure myself. When im logging output from the camshaft position sensors they report back 0 degress when 0 degrees is commanded from the table or will report back something close to the commanded value from the table as their actual reading. For instance if at 3000rpm and 0.60 g/cyl the camshaft on the intake side is set in the table to be 19*. I believe that this 19* is degrees of advance from its non advanced position

[Leafy]

Allows you to advance and retard... not 100% on if that is actually whats happening. A log should provide that info.
Heres a copy of the overlap im getting. Im also looking at my cruize areas.. Is the goal to get more overlap or less during cruize for MPGs? mine seems to have none then a lot as rpms go up and load goes down... These tables could be made to be MUCh smoother after looking at them this way. Im going to work on making smooth transition points for overlap as well smooth individual cam tables. I bet it would make a huge difference i nthe way this car drives...

I believe that weird hole is the "egr valve" and getting rid of it should theoretically increase mileage, if that is the exhaust table that is.

[gmtech16450yz]

Wow, you guys are working hard on this! Cam timing is not easy, especially when you have two cams to adjust, direct injection and a turbo thrown in also. Engineers theorize on this stuff constantly, there's just so much to all of it it's beyond me most of the time. Anyone that says they have cam timing all figured out, especially on these motors, is full of cr#p! (not that anyone made that claim, just sayin')

Here's a couple really short, grossly simplified comments...

What the engineers are trying to do with the stock timing is this, get the highest mpg's at freeway cruise while keeping the emissions within acceptable numbers. On most VVT motors, they do it by retarding the cam timing. What this does is it lowers dynamic compression, which reduces pumping losses, which gives you better mileage. Almost all GM tunes, 4, 6 or V8's, has a big "hump" in the freeway cruise area to take some of the pumping losses off the motor for mileage. If you reduce that "hump", you will gain throttle response and power in those areas, but mileage will suffer. On the LNF, it all depends on what you want, power or mpg's. There is a somewhat happy medium, you can gain throttle response and power without hurting mileage too much. If all you want is mileage, you can actually gain some more making the stock tables more aggressive on the retard at freeway cruise.

Turbo's don't like overlap. On an NA motor, it helps pull in the intake charge. On a boosted motor with pressure in the intake charge, it tends to let some blow out the exhaust if there's too much overlap. The problem with making any kind of blanket statements on how to tune the cams on a boosted motor is it's not always boosted. On the LNF you kind of have to think of it like an NA motor below 2k rpm, and a boosted motor above 2k.

BTW, GM supposedly optimized the cam timing to get the turbo spooling up faster, I personally don't think the drawbacks were worth it. Basically you're giving up dynamic compression to try to get the turbo spinning sooner. Same idea with ign timing, retarding ign timing "in theory" will help with spool speeds, but in the meantime you're loosing gobs of power because you've retarded the ign timing too far from MBT. I say don't worry about spool speed, it's fast enough on the stock turbo, tune the cam timing like it's an NA motor down low, meaning lots of advance, not a lot of overlap for lots of dynamic compression.

Short answers... for mpg's, keep the cam tables stock or even retard them more around where your particular car cruises. For max power, reduce or eliminate all together the EGR "hump" in the center of both tables, make the tables a flat change all the way across. (It also helps to massage the injector timing tables if you flatten the cam tables.)

Hope that helps. There is a lot of good info in some of the older LNF cam timing threads in here, go back and re-read them and you'll probably pick up a few more answers.

[gmtech16450yz]

Oh and guys looking at Silverbullet's screen shots, that table is neither intake OR exhaust, it's the sum of both which shows overlap.

Leafy, you got that one backwards if I understand you right, reducing the "EGR" hole or hump will hurt gas mileage, not help it.

[cobaltssoverbooster]

ok so the sum of both cams is overlap im learning again...yes..yes.. more please... haha

to bad we are getting into this hard core and in a few months instead of going turbo i may be just getting a new 5.0 stang with a 2.8 h20 cooled kenne bell system 8( still awesome stuff to know since direct injection should be increasingly popular in the future.

[Leafy]

Oh and guys looking at Silverbullet's screen shots, that table is neither intake OR exhaust, it's the sum of both which shows overlap.

Leafy, you got that one backwards if I understand you right, reducing the "EGR" hole or hump will hurt gas mileage, not help it.

Really I thought getting rid of it would net you more volumetric efficiency there and therefore more torque and better mpg's because less airflow would be required to make the same power.

[viceroykarl]

Am I still right in thinking that if you advance your intake cam further at cruising conditions you'll lower the volume of air in the cylinder essentially filling the cylinder with less air than it can hold, like in my case less than 0.59859 Liters in order to sort of have the engine act smaller than it really is by not filling it as much as it can hold. i would think you'd have to advance the exhaust as well to avoid overlapping.

That seems to be what GM is trying to achieve in the stock cam times.
The first attached image is intake advance, the second is exhaust advance
Those conditions seem to be met when traveling around 55mph and upwards when in the overdrive gear

[gmtech16450yz]

You guys are all getting it and you're mostly all right.

Leafy, everything you wrote in that sentence was right except for the mpg part. And even that, is somewhat true. More efficiency obviously means better mileage, but there are other ways to get mpg's too. I'll explain after I comment on viceroykarl's post...

Everything you wrote was also correct, except for one word, ADVANCE. That part you got backwards. If you RETARD cam timing, it does everything you mentioned, and yes, you have to retard the exhaust cam also to keep overlap in check. Those numbers in the tables are retard, not advance. In general, you want to advance at low rpm's and retard up high. (In general) In most NA tunes you'll see an overall RETARD of the cam(s) as rpm's rise. Too much compression up high will make a motor hard to rev out.

So the way they get mpg's is making the motor run like a much smaller engine with a much lower compression ratio, by retarding the cam timing. Remember, compressing the charge in the cylinder more makes more power when it fires, right? The part that's easy to forget is it takes power to compress that charge. These are totally random and probably not accurate numbers, but I'll try to make some sense out of what I'm saying...
Say your motor puts out 300hp at 2500 rpm, but while cruising at a steady 2500 rpm (freeway cruise), it only takes 100hp to maintain that speed. That means 200hp is being made, and wasted (somewhat). So what if it takes 50hp to build enough cylinder pressure to be able to make that 300hp? You're only using 100hp, but another 50hp is being used just to overcome that high dynamic compression and pumping up all that pressure, even though it's not being used.
Now lower that dynamic compression by retarding the crap out of the cam timing, and that 300hp motor is only capable of 100hp, but that's fine because that's all you're using. But it also only takes 20 more hp to pump those cylinders up to make that 100hp, instead of 50hp. All the sudden they have a little motor and super low compression when they don't need a big one. Make sense? Sorry if it doesn't, it's late! And btw, I make mistakes just like everyone else, so if you read something I wrote and say to yourself, "wtf, that can't be right", it might not be! Like I said before, this stuff isn't easy to understand sometimes and there are SO many variables and compromises you could drive yourself crazy trying to figure out what to do or change.

CobaltSS, Mustangs suck! (Coming from a guy that bought, restored and sold dozens of Mustangs in the '70's and '80's, but those were '65's to '70's Mustangs, they were cool. New ones, not so cool.) Buy a Kappa, put an EFR on it and you'll be able to smoke any Mustang you come across! I actually went up against a 535whp 'stang a few weeks ago, we ran to about 130mph twice, walked on him both times. He was shocked and bummed! Oh yeah, and after the second run that thing was smoking like it was gulping oil! I felt kinda bad because it looked like he hurt that motor. Then again, most all Fords smell like they're gulping oil!

[Jar Jar]

Just a thought...but wouldn't pulling back intake and maximizing overlap give you the best mileage? (maximizing cold air going to turbo and closing intake valve before BDDC). At first the ECU would think you are getting more air but after you do your MAF tuning you would get max mileage?

[cobaltssoverbooster]

yea but i always wanted a v-8 haha and its 10 k cheaper than a camaro lol the only thing im not liking about my cobalt is its fwd and i want a highly reliable 400whp car and v-8 is the easy way out i just buy it and tune her and she pretty much makes my quota illbe happy haha plus im a car guy i can fix all the leaks and stuff before they get real bad 8D

[cobaltssoverbooster]

Just a thought...but wouldn't pulling back intake and maximizing overlap give you the best mileage? (maximizing cold air going to turbo and closing intake valve before BDDC). At first the ECU would think you are getting more air but after you do your MAF tuning you would get max mileage?

not necessarily, the intake cam has large effect on cylinder filling but by shortening the overlap by moving the exhaust cam it reduces the amount of gasses escaping the cylinder. so by letting the exhaust not clear out you get less fresh charge which makes the car run like its a smaller engine because you dont have all the fresh air/fuel charge to have a more powerful burn. its the same effect as an egr system pretty much just a different way of controlling it.

[Leafy]

But they you'd also be fighting the same dynamic compression ratio more or less but it would be exhaust + intake gasses instead of just intake gasses.