3.9L LZ9 engine internal upgrade & performance?

[ericjon262]

why? VVT is used to boost torque at all RPMs by advancing or retarding the cam to shift the powerband around. adding more lift and duration will favor high RPM power while sacrificing low RPM.

my understanding was that in the LSx engines, you run into piston to valve clearance issues if you go to far.

[robertisaar]

my understanding was that in the LSx engines, you run into piston to valve clearance issues if you go to far.

this is probably true. i was more interested in the "why VVT would be useless" portion.

[ericjon262]

if you have to limit it to >2-3*, I imagine the gains wouldn't be worth the hassle...

[robertisaar]

with that kind of range, it would be worthless. you can probably get that from chain stretch.

[paramount]

SO does increasing duration limit VVT or not. I would like to know and I emailed mast motorsports since they make cams for VVT engines but they haven't got me back yet.

[NateD4]

yes it does... sorta...

If you imagine the lobe centerline and on either side of it you have some degrees of valve duration then yes it will effect the max cam retard or advance angle you can use.

to exagerate if the valve is open 70 degree and you can run 20 degrees advance that means the cam is opens at 70/2 degrees + 20 degrees max advance. or 55 degrees. If you exagerate and go to 90 degrees total duration with the same 20 degrees of cam phase. Thats 90/2 + 20 = 65 degrees (relative to crank angle). As you can see the actual opening angle is greater with a greater duration. The closing angle is also greater (relative to crank angle). Of course in absolute terms that means the cam opens earlier and closes later with a longer duration....

So you have to verify the valves don't hit the pistons with the timing changes. If they do you have to limit the cam phase advance/retard from say 20 degrees to 16 degrees etc... If no piston to valve interference occurs you'll probably be ok.

[NateD4]

btw asking if it limits VVT isn't really the right question.

BUT to answer. yes duration limits VVT. More specifically it limits your advance/retard ability purely on geometry of the valves interfering with the pistons.

If you look at the valve timing in time. Then what you are gaining off course with higher duration is higher end HP. As a longer duration cam translates into more total time the valve is open at a given time. OR longer valve opening at higher RPM (technicallly at some RPM the total valve opening time is longer with more duration at a higher RPM than say a lower duation cam slightly lower RPMs.

The only gain I can see with duration is moving the power band up a bit.

Lift might be a different story.


My opinion....

[SappySE107]

You want more lift and less duration on a VVT cam compared to a non VVT cam, if you want to keep low end power and use the variable timing to extend that power to higher RPMs. VVT is a small factor to anything high performance, because the duration itself is needed to get more top end power. In terms of what the build is for, street/mpg, keep it small duration high lift with full VVT. Street/Strip add more duration and lift and limit the VVT travel. Race, bye bye VVT.

[NateD4]

You want more lift and less duration on a VVT cam compared to a non VVT cam, if you want to keep low end power and use the variable timing to extend that power to higher RPMs. VVT is a small factor to anything high performance, because the duration itself is needed to get more top end power. In terms of what the build is for, street/mpg, keep it small duration high lift with full VVT. Street/Strip add more duration and lift and limit the VVT travel. Race, bye bye VVT.

Why more lift and less duration on a VVT cam compared to a non VVT cam?

[SappySE107]

What kind of engineer are you?

[gectek]

There was really only one company that was focusing their time on the VVT custom cams as Comp Cams shelf grinds are retarded, but they have since stopped that venture as they ran into issues with it not delivering as promised and the cost of parts was more than just using non vvt stuff. Honestly I have NEVER seen a power increase on any LS VVT system with a custom VVT camshaft over a non VVT design and I have been in the business for quite some time. It is just like the LS truck intakes with the longer runner supposedly promoting more tq in the lower rpms than the car intakes, again that myth was busted also. The LS VVT system is used primarily in the truck applications, where the longer runner intake is supposed to help them, but again, it fails horribly.

Just put a reg camshaft in it and be done with it. Changing the ICL is not that huge of a deal on the fly to even worry about. The biggest factor would be to being able to change the amount of duration the lobe is lifting and setting down the valve, but with the weak ramp rates of the cams, it does not make that big of a difference anyway and that is not the angle that people approach it from. They only see it as advancing or retarding the ICL and ECL, which is does as well, but the events happen so fast, its really a lose situation honestly.

[NateD4]

What kind of engineer are you?

Aerospace.

[NateD4]

There was really only one company that was focusing their time on the VVT custom cams as Comp Cams shelf grinds are retarded, but they have since stopped that venture as they ran into issues with it not delivering as promised and the cost of parts was more than just using non vvt stuff. Honestly I have NEVER seen a power increase on any LS VVT system with a custom VVT camshaft over a non VVT design and I have been in the business for quite some time. It is just like the LS truck intakes with the longer runner supposedly promoting more tq in the lower rpms than the car intakes, again that myth was busted also. The LS VVT system is used primarily in the truck applications, where the longer runner intake is supposed to help them, but again, it fails horribly.

Just put a reg camshaft in it and be done with it. Changing the ICL is not that huge of a deal on the fly to even worry about. The biggest factor would be to being able to change the amount of duration the lobe is lifting and setting down the valve, but with the weak ramp rates of the cams, it does not make that big of a difference anyway and that is not the angle that people approach it from. They only see it as advancing or retarding the ICL and ECL, which is does as well, but the events happen so fast, its really a lose situation honestly.

When you say retarded, do you literally mean they are retarded compared to centerline of the crank. Or are you saying they aren't good?

Also I thought the whole reason for VVT on pushrod engines was to adjust the RPM range where peak torque happens (as much as you can with fixed duration). I know you can manually advance or retard most cams with a variety of aftermarket items (offset bushings, pulleys etc..). I've never heard that the effects aren't worth it before. I usually hear it provides some shifting of peak torque.


I also though the OEMs use VVT to help flatten the torque curve out and to also allow them to get better economy.

When you say the longer runner fails, do you mean it produces not appreciable torque gains throughout the RPM band? Just curious.

With that said are you saying runner length and plenum volume have no effect on power? Or just that the difference between the long and short runners are negligible?


Sorry had to ask this is contrary to everything I've ever read or heard about cam timing and VVT. I know on DOHC engines they are able to alter the lobe separation angles.

[gectek]

Answers in red.

When you say retarded, do you literally mean they are retarded compared to centerline of the crank. Or are you saying they aren't good? I meant they are horrible for any type of camshaft honestly.

Also I thought the whole reason for VVT on pushrod engines was to adjust the RPM range where peak torque happens (as much as you can with fixed duration). I know you can manually advance or retard most cams with a variety of aftermarket items (offset bushings, pulleys etc..). I've never heard that the effects aren't worth it before. I usually hear it provides some shifting of peak torque. The whole reason is to try and use advancing and retarding a camshaft while in motion to provide more torque down low when needed. Honestly the VVT system shuts off at like 3k or somewhere of the like unless otherwise tuned. So it is used to try and promote lower end power and possibly raise fuel economy by making more power available sooner without going to a larger camshaft for OE vehicles. On every VVT application that I have put a regular camshaft in, I have gained fuel economy. And I tuned the vehicles before with VVT but to less avail on the MPG than putting in a straight cam and tuning it. This is in LS of course, but you get the point. Those cams you are referencing and the idea behind them are normally race inspired and would have no actual application in a DD world. Normally now, the advance is ground in (or retard ground in) so there is no need to advance or retard the cam, and in some instances you cannnot. They are normally to help promote power pickup in the lower RPMS due to building cylinder pressure. only being able to advance a camshaft by (honestly) a small amount, or retard it the same way, will not make any more power in the upper RPM, because the engines are limited to air flow in those upper RPMS anyway. Since the duration is fixed, it would not make any more power up there anyway honestly. Changing the LSA is where most of that comes in. The only real application that a normal person would see with this is the Gen 4 STR-10 vipers. They have an intake lobe phaser that changes the LSA of the cam and is more beneficial than any fixed VVT cam or controller.


I also though the OEMs use VVT to help flatten the torque curve out and to also allow them to get better economy. They do, but it does a piss poor job of it, as well as active fuel management. Every truck I have tuned, I turn it off and they pick up at least 3-4 MPG everywhere and some alot more than that. I can tell you the torque curve does not change at all and is mostly helped due to the availability to lock in more spark advance in the lower RPMS without it building so much cyl pressure.

When you say the longer runner fails, do you mean it produces not appreciable torque gains throughout the RPM band? Just curious. I mean the longer runner does not do its job as theorized and help make more torque early on in the RPM range. I will link those graphs in just a moment.

With that said are you saying runner length and plenum volume have no effect on power? Or just that the difference between the long and short runners are negligible? I am not saying plenum volume hurts anything, but the long runners of both the truck intakes and the 3.5/3.9L engines will kill any power that would be ideally promoted in the upper RPMS by say that tuning valve. That thing is ridiculous. The 660s already have a good length runner on them, there is no need to make it 9 times longer. Plus the 3.9L already has built in torque by displacement alone. More attention should be spent on porting and correct cam selection, which Ben addresses, and less on theorizing what better way to use the VVT cam honestly. Get rid of that Dual plane UIM also and go with the reg 3.5l one. That is worth power alone.


Sorry had to ask this is contrary to everything I've ever read or heard about cam timing and VVT. I know on DOHC engines they are able to alter the lobe separation angles.

[paramount]

So go with a non VVT cam and tune out the VVT?