Tell me about the LZ9's VVT cam

[robertisaar]

i think i'd rather just re-purpose the EGR 3 circuit(since no VVT motor uses EGR, and it can handle about double the current compared to the other PWM circuits on a 7727 style ECM) and adjust duty cycle in .39% increments based on a couple of different variables, rather depend on a 555 and only base it on engine speed. it wouldn't be able to account for differences in where the cam is commanded to be, compared to where it is either.

what is D-Tec?

[Joseph Upson]

This is an interesting quest but I'm doubtful about its worth in fixed lobe form beyond a simplified version of advance and retard to straight up cam specs. From what I've read so far it's a complicated control system because something as simple as a change in oil weight and more importantly for our purposes valve spring pressure can change calibration and would need to be compensated for.

It's also possible the oil squirters are partly responsible for the low oil pressure ceiling as well as a better understanding of oil flow dynamics in that pressure is not so much a representative of lubrication as is oil flow past the bearings. Plug the oil filter and you get peak oil pressure and still destroy the engine.

My engine is running much much better since discovery and correction of the exhaust leak rob, thanks again for all the help and guidance. Lowering my spark table more seems to have helped some to. Can't wait to get the T67 back on for hammer time.

[SappySE107]

20 psi is where the computer will allow cam movement from the locked position. It only retards max at cruise, so low load and mostly in the 2000s for RPM range. It is also slow to make the changes compared to throttle input but you also have a drive by wire setup to slow everything down for transitions compared to the cable throttle setup a swapper will have. Like I said, I have all the values so there is no need to speculate what and when it does anything. I also have a setup with the DBW that improves throttle response and it feels like it has more power but fighting the throttle angle vs pedal angle was a major issue.

[robertisaar]

This is an interesting quest but I'm doubtful about its worth in fixed lobe form beyond a simplified version of advance and retard to straight up cam specs. From what I've read so far it's a complicated control system because something as simple as a change in oil weight and more importantly for our purposes valve spring pressure can change calibration and would need to be compensated for.

It's also possible the oil squirters are partly responsible for the low oil pressure ceiling as well.

if GM can pull relatively good fuel economy numbers out of a current gen Impala while maintaining 240/240, it's very much an interest to me. they used a fairly oddly set up cam, but when advanced/retarded the right amount, it allows for most of the engine's positive attributes. change in oil weight? maybe, that might upset the tune a little bit, but since i'm planning on accounting for both oil temp and pressure, i SHOULD be able to keep relatively good control of it. valve spring changes? probably need a tune at that point anyways if it's really a necessity.

as for the relatively narrow range of oil pressure that it operates in, i'd like to think GM was just trying to keep the system consistent, and more range would make it difficult.

20 psi is where the computer will allow cam movement from the locked position. It only retards max at cruise, so low load and mostly in the 2000s for RPM range. It is also slow to make the changes compared to throttle input but you also have a drive by wire setup to slow everything down for transitions compared to the cable throttle setup a swapper will have. Like I said, I have all the values so there is no need to speculate what and when it does anything. I also have a setup with the DBW that improves throttle response and it feels like it has more power but fighting the throttle angle vs pedal angle was a major issue.

okay, so it is a GM calibration spec... i may have to follow that same pattern for reasons mentioned above. max retard at cruise? i would think it would advance in those situations.... then again, i'm in no way used to a VVT cam.

[trotterlg]

I think you will find that the cam advance/retard system just bleeds off any excess oil pressure, probably regulates it to something like 25 PSI in the cam system. Small aircraft use oil pressure to control constant speed props and they do not change pitch due to oil pressure or oil weight. The 20 psi thing is just the minimum to get it to work right, more will not affect the operation. It would not take any large volume of oil to activate the system, just the pressure to move a very small mechanical system. Larry

[Joseph Upson]

I'm just sharing my experience with the engine and knowledge that I've read on the subject including some from articles regarding performance modifications and the things that need to be taken into consideration when it is present. The Impala 3900 had VVT and DOD to help with it's fuel economy 20/29, 2007.

Fact at this point, another 3900 swapper was not able to get his stock engine to start with the camshaft locked in the full retard position, I had a very difficult time getting mine started in this situation and probably only succeeded because my camshaft had been reground. When I compared Gtech runs with the cam advanced and full retard, the power loss was noticeable and in excess of 20 points, the fuel economy was horrible also. That's why I suggested that full retard must be for EGR purposes as it's inefficient condition posibly helps to cool the exhaust. I just can't see it being useful as a performance enhancement benefit for those reasons, particularly given the nature of valve events lagging behind optimum crank angle such as in a motor with a stretched timing chain.

Since this is a fixed cam lobe VVT function you're held to the same limitation of a non VVT camshaft installation and from the literature I've read on that the chances of a positive outcome on advancing a cam are far more likely than when you install it with a retarded angle relative to ideal.

Does anyone know if the system is designed to modify spark timing in association with the cam angle as well since cylinder filling time will be affected by the camshaft variation?

I can't fight the GM technical info, but there maybe some important information being withheld or overlooked that is probably very important in this process. I just know the difference between full advance and full retard was night and day in performance and fuel economy and can only see it being different in operation with several other control factors that were missing in my test, but present in the stock vehicle the engine came from. It would be great if you could get it to work though.


Meanwhile, since the turbo is off the car for repair and I'm driving naturally aspirated, I closed the variable intake valve which has been open until now. If I notice anything different I'll let you know.

"At low engine speeds, the engine control module (ECM) closes the valve to create a longer intake path, improving combustion efficiency and increasing torque. At higher engine speeds, it opens the valve, shortening the path for free breathing and maximum horsepower. "

We'll see.

[robertisaar]

if i had to guess...... i'd say that when the cam is retarded, SA is added due to self-EGR effect(something i didn't think about until now, but i'll have to integrate that into plans as well), or since the MAF will show less airflow due to EGR gases being in place of combustible air, it will simply move to a different point in the SA map. cylinder fill time? if the changes are gradual enough, the MAF sensor should take care of that entirely. a VE modifier/multiplier table based on amount of advance/retard), should speed-density be used in the situation, but i'm sure it's manageable.