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Thread: Basic VVT Tuning info

  1. #1
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    Basic VVT Tuning info

    I'm hoping this hasn't been posted before. I did some searches on the forum but its possible I've missed it.

    On four stroke engines, it is important to realize that the cam rotates once for every two rotations of the crankshaft.

    Volumetric efficiency is based on cylinder fill. If a 2.0L engine is filled with 2.0L of an air/fuel mixture, we say its volumetric efficiency is 100%. If a 2.0L engine fills with 3.0L of an air/fuel mixture, we say its volumetric efficiency is 150%. A forced induction engine will have a larger than 100% volumetric efficiency since the intake charge and combustion chamber are being pressurized. A naturally aspirated engine can also have a slightly larger than 100% volumetric efficiency, but it will only happen for a short duration, and is usually only in the peak of the powerband.

    The art of designing camshaft profiles is meant to increase the volumetric efficiency in the RPM range that the customer requires. Camshafts don’t make magical horsepower from nowhere, they simply move the powerband around by changing the volumetric efficiency to attain the desired results.

    The four strokes of the engine are:
    Exhaust
    Intake
    Compression
    Combustion
    **The “start” is not important because it’s a CYCLE, meaning it repeats**

    Looking at a camshaft, the sequence would be as follows:
    The exhaust lobe pushes open the exhaust valve and the piston comes up to push the exhaust out, then starts to close. The intake starts to open, just as the exhaust is closing, piston goes down, and the intake valve closes. Then both valves stay closed for the compression and combustion strokes. This means that the first lobe to come through the rotation will be the exhaust lobe, immediately followed by the intake lobe.

    Overlap is the point where the exhaust valve is closing, and the intake valve is just opening.

    To increase overlap, you have to RETARD the EXHAUST, and/or ADVANCE the INTAKE.
    To reduce overlap, you have to ADVANCE the EXHAUST, and/or RETARD the INTAKE.

    Simple cam tuning rules for NATURALLY ASPIRATED engines:
    Advancing both cams => more low-RPM power, less high-RPM power
    Retarding both cams => more high-RPM power, less low-RPM power
    Less overlap => more low-RPM power, less high-RPM power
    More overlap => more high-RPM power, less low-RPM power

    In a naturally aspirated engine, the extra overlap is called "scavenging". Scavenging is using the out-flowing exhaust to help draw in the next intake charge (partially causing lumpy idle).

    Simple cam tuning rules for BOOSTED engines:
    Advance intake and exhaust => more low-RPM power, less high-RPM power
    Retard intake and exhaust => more high-RPM power, less low-RPM power
    Less overlap => lower EGTs, faster turbo spool, less fuel
    More overlap => higher EGTs, slower turbo spool, more fuel

    Boosted engines don’t like overlap. The incoming cold air and fuel cools down the outgoing exhaust charge, condensing the exhaust gasses. This is VERY counter-productive in a turbo application since the engine needs no help from scavenging to fill the cylinder. I've heard this being called "turbo chill".

    Cool, condensed gasses in the same space push less hard on the turbo, causing lag. HOT gasses are better at spooling the turbo, thus the advanced exhaust timing to open the valve sooner in the power stroke. This steals some of those hot, expanding exhaust gasses to help spin the turbo a little faster. When the piston is near the bottom of the bore, hardly any energy is going into rotating the crank anyway, so stealing expanding gasses won’t hurt anything. The retarded intake just helps cut down the overlap further.

    Retarding overall cam timing:
    Retarding overall cam timing is better for high-RPM power. This is because the valves are closing later. The intake valve is closing AFTER the piston has started to travel back up the bore for the start of compression stroke. This is terrible at low RPM because the intake air velocity is low, and air that was once in the cylinder is now being pushed back into the intake manifold and causing turbulence.

    At high-RPM, the rules change. Air has weight, and thanks to Sir Issac Newton, we know that once it is moving, it doesn’t want to stop moving. This means that the air can continue to flow into and fill the cylinder, EVEN AFTER the piston has begun to travel UP the cylinder bore. This can allow an engine to exceed 100% volumetric efficiency, if even by a small amount.

    Advancing overall cam timing:
    Advancing overall cam timing is better for low-RPM power. This is because the valves are closing a little sooner. The intake valve is closing AT or NEAR when the piston is at the bottom of the bore for the start of the compression stroke. This is great at low RPM because the intake air velocity is low and easily affected by changes in the direction of piston movement in the engine. Almost as soon as the piston gets to the bottom of the bore on the intake stroke, the valve gets slammed shut so no air can escape as the piston begins to travel back up the cylinder on the compression cycle.

    At high-RPM, this may become a restriction since the air has inertia and responds a little slower to pressure changes, potentially choking the air flow to the engine a little.

    Conclusion:
    This information is aimed at allowing tuners to understand what happens when cam timing is altered. When a larger duration camshaft is being installed, unless the lobe centerlines have been changed, the overlap will be increased. If installing larger camshafts in a turbo application, advancing the exhaust and retarding the intake will reduce the inherent increase in overlap caused by upgrading to a larger profile. Most cam grinders, especially regrinders, put the new profile in the same position as the old profile because it is easier, or the only way possible. This has to be changed when the cams are installed in an engine to attain the desired result.

    A forced-induction engine should idle smooth when properly tuned, and a naturally aspirated engine should be “lumpy” and have a lope if it is tuned aggressively towards the high-RPM range. If a forced induction engine is loping at idle, fuel is being wasted, turbo spool time is being increased, and power is being lost.

    -Dave Atchison
    Source: http://www.supraforums.com/forum/sho...ticky-please-*

  2. #2
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    Sorry for the double post but I'm having a bit of trouble finding the maximum angle that can be phased in the LE5. I though it may be in the ecotec camshaft info sheet but it wasn't there.

  3. #3
    Advanced Tuner silverbullet08's Avatar
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    Very nice. i havnt seen this yet but i have been working on another set of cam tables for my car and this helps assure some of my guessing lol. I understand them its just a lot to grasp at one time making it harder to get right.
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  4. #4
    Senior Tuner cobaltssoverbooster's Avatar
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    very accurate knowlege been around for a long time now good job on posting it. should help alot of people understand the moves we make just a little bit more.
    2000 Ford Mustang - Top Sportsman

  5. #5
    Senior Tuner Iam Broke's Avatar
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    Saved for reference. thanks!
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    A question. The GM Volumetric Efficiency PID ([PID.2312]), should I just be reading that as 1000 is a VE of 100% and anything over 1000 is VE over 100%

    Just wondering since while I'm tweaking the vvt im looking for changes in this number.

  7. #7
    Senior Tuner cobaltssoverbooster's Avatar
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    i use PID.2311 VE Airflow lbs/min and the higher i got that the better it ran. anyone else got a better one to watch?
    2000 Ford Mustang - Top Sportsman

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    That gave me an idea. That is, compare mass air flow to ve airflow in a custom pid
    (([PID.2311]/[PID.16])*100) to get some sort of VE from the airflow.
    Is that right or am I going along the wrong way?

    I've been using this one that I got from ls1tech's forums i think.
    (([PID.16]*[PID.15])/([PID.11]*[PID.12]*2.39436))
    (([Mass Air Flow SAE]*[Intake Air Temp SAE])/[Manifold Absolute Pressure SAE]*[Engine RPM SAE]*Engine Displacement))

    EDIT:
    Found link http://www.ls1tech.com/forums/pcm-di...e-cracked.html

    EDIT2:
    Looking back at some of my scan logs after changing some of the VVT values I was seeing max hits around 130% ve using (([PID.2311]/[PID.16])*100)
    It seems to reflect the changes too because when I change the timing around for more advance and less overlap the % value is lower. A similar rpm range value would be lower % compared to better vvt timings
    Last edited by viceroykarl; 07-09-2011 at 12:37 AM.

  9. #9
    Senior Tuner cobaltssoverbooster's Avatar
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    if there is a way to make a custom pid that technically follows the math for true VE that would be awesome.

    VE=(3456 X CFM in cu.ft/min) / (C.I.D. X RPM)
    2000 Ford Mustang - Top Sportsman

  10. #10
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    it would probably be useful to see how the computer is calculating VE airflow. Something tells me its doing something in the background already involving the air density

    The problem is that I've seen so many different ways to calculate VE on a spread of different car and engine tech forums. I've just been trying them all out and comparing them.

    What about:
    VE= ((3456) X (([PID.16])/(([PID.11]*0.145037738)/(53.35 X [PID.15]))/(146.112812 X [PID.12]))
    (([PID.11]*0.145037738)/(53.35 X [PID.15])) this is to get the density of the air in the manifold (i think) using pressure/dry air gas constant (53.35 (ft·lbf)/(lbm·R))) multiplied by the IAT and I also converted PID.11 (map) from kpa to psi using 1kpa = 0.145037738 psi

    146.112812 is cubic inch from 2.39436 liters
    Should I be using data for the manifold to get calculate the air density or rely off the ambient air temp and ambient pressure sensors to get ambient air density?
    I didn't use the cut and dry air density for converting lb/min to cfm since that is a STP value and we're not always driving around at 760mmHg at 0 deg C.
    It may be completely bunk though since I kinda just did this now without really working it out. Its also very possible i have some mismatched ()'s
    When I get time to think about it I'll see whats what, make sure the units cancel and result in lb/ft^3, and how to work humidity into it. Busy writing papers for a summer class so whenever time is free.
    Last edited by viceroykarl; 07-09-2011 at 11:13 PM.

  11. #11
    Senior Tuner cobaltssoverbooster's Avatar
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    that may work, whats the 0.145037738 being multiplied to pid 15 for? a conversion non the less but to achieve what?
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  12. #12
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    Actually its pid.11 thats being multiplied by 0.145037738. pid.11 is manifold pressure measured in kPa and I believe it needs to be in psi for the air density calculation to have the right units. 1 kpa = 0.145037738 psi.
    Pid.15 is multiplied by 53.35. Thats the specific gas constant for dry air.

    Basically what im trying to achieve is that im dividing the lb/min to cfm and to do that I think you need to divide lb/min airflow by the density of air to get it in cubic feet/min using the d=m/v solving as v=m/d

    EDIT:
    Try this one out
    ((([PID.16]*7.559872833)*(([PID.15]-32)*(5/9)+273.15))/(([PID.11]*0.01)*[PID.12]*2.39436))
    I fixed up the one I got from ls1tech's forum. Turns out i was using the wrong units and forgot to convert some numbers. Needed to convert measurements into SI units.
    Last edited by viceroykarl; 07-10-2011 at 11:10 PM.

  13. #13
    Senior Tuner cobaltssoverbooster's Avatar
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    ok and i believe the air temp after turbo is more acurate for density on ve because its the actual density the engine as a pump is going to see.
    2000 Ford Mustang - Top Sportsman

  14. #14
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    Well now that i've tested the ve airflow/maf pid as well as the one from ls1tech's forums Im noticing that they both, for the most part, agree with each other in terms of value fluctuation at WOT. They both seem to stay steady with each other and will decrease or increase with each other as expected with changes in throttle and valve timing.

    I've attached a scan output that shows it pretty well. Event to watch is at 2:08:919 and also again at 3:22:895.
    It helps to plot the 2 in the chart display.
    While just driving they seem to do the opposite with each other.

    I agree on using values from data gathered at the manifold as well since thats the air the engine will be getting
    I'll scan another output with vvt times that are not as optimal to compare again the difference in the numbers these custom pids are generating.
    Last edited by viceroykarl; 07-11-2011 at 03:12 AM.

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  16. #16
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    Anybody know what the limits of the of the cam shaft timing is before u start hitting the piston or other valve?

  17. #17
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    I believe its beyond what you're able to adjust on hp tuners. If its the same as the v8 vvt guys. I can tell you that stock 2.2 litre eco's can jump 1 tooth (~7* retard) and not hit. No idea about other cams or engines or advancing the cam.
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  18. #18
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    Does that apply for 2.0 LNF?

  19. #19
    Senior Tuner cobaltssoverbooster's Avatar
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    no i replaced a head on a car with a misadjusted exhaust by one tooth and they hit after they shifted.
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  20. #20
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    Quote Originally Posted by viceroykarl View Post
    A question. The GM Volumetric Efficiency PID ([PID.2312]), should I just be reading that as 1000 is a VE of 100% and anything over 1000 is VE over 100%

    Just wondering since while I'm tweaking the vvt im looking for changes in this number.
    no, look at the units of GMVE, have nothing to do with % of volumetric flow. it's more of a normalized airmass measure, how how airmass you're gonna get at particular pressure and temp