3900 Info

What about the piston to valve clearance at TDC, that would be important for anyone considering a cam regrind, especially considering how shallow the intake valve lift is compared to the exhaust valve lift. Quite a bit of calculating would have to be performed to figure out how much closer the valves get to the piston when the VVT is doing its thing. I'm not sure but I'll have to check Desktop Dyno to see if simulates valve timing.

[Superdave]

try that on a 3400 block... i bet it would clear...

[CNCguy]

Did some measuring today and took some pics. At TDC the intake valve had .260 lift and the exhaust was .405. The measurements were taken with the headgasket on and the head bolts slightly snugged down. The intake and exhaust valves started to make contact with each other at .51 lift. For those interested, I took some pics of the piston, combustion chamber and a pic of the 3900 (99mm bore) head on a 3100 (89mm bore) block. As you can see in the last pic, there is no room on a 3100 for the 3900 head. The valves hit the top of the cylinder wall and the chamber overhangs the cylinder wall. Although the valves don't clear, the 3900 top end does mount to the older engines. I am curious if the 3900 top end will fit the 3500 (LX9) since it has a 94mm bore.

One other thing.... the reluctor wheel has changed since the 3500s. It now has a single row consisting of 60 (58 + 2 missing) tabs.

3900 Piston


Intake & Exhaust Valves


Combustion Chamber




Valve to Valve clearance


3900 head on a 3100 block

[RacerX11]

As an alternative, I was thinking about altering the VVT actuator to limit its travel to about 10~15 degrees.....snip.....It wouldn't be continuously variable but would give you low-end torque and top-end power.

That's not a bad idea.

[RacerX11]

I seem to recall someone mentioning in another thread about the closed loop control, plus, I have worked on embedded engine control systems for over 8 years, and I would be amzed if GM was not using closed loop control on the cam actuator. It is pretty fundamental.

The capability of the stamp has nothing to do with how many instructions per second it can handle. The stamp would need to be able to process both the crankshaft and camshaft position signals, and make a determination of a reference point for each signal. This is handled by a timer processor unit in most ECU's, something the very simple stamp does not have. The cam may only have one pulse per rev, but the crank has many (54? I don't recall the exact number). Can the stamp handle interrupts based on the input of each signal tooth? 54 teeth at 6000rpm = 5400 tooth samples required per second.

Open loop may seem good enough, but what happens when you apply a PWM you **think** will give you 10 degrees of retard, but the system actually ends up advancing 15 degrees due to one variable or another? Might not be so good.

Some of those basic stamp modules are capable of 12,000 instructions per second that sounds fast enough to handle the job.

[427v8]

The stock ECU uses closed loop feedback comparing the crank position sensor to the cam position sensor to determine the advance of the cam. You will need something more powerful than a basic stamp module to do this.

Without this closed loop control, a given PWM duty cycle will result in different cam phasing depending on oil temp, oil pressure, etc. Will you be able to move the cam? Sure. Will it be very accurate and controllable? I would say probably not.

Interesting, but where did you get this info?

Yes closed loop controll is always better, but if your just looking for more / less then open loop is fine. Definitly a good way to experiment.

Also, How many pulses per second are we talking about on these sensors? a PIC can measure pretty fast pulses. and at 6000 rpm thats only revs per sec on the cam sensor, not too scary, well unless there are multiple pulses per rev?

[CNCguy]

The stock ECU uses closed loop feedback comparing the crank position sensor to the cam position sensor to determine the advance of the cam. You will need something more powerful than a basic stamp module to do this.

Without this closed loop control, a given PWM duty cycle will result in different cam phasing depending on oil temp, oil pressure, etc. Will you be able to move the cam? Sure. Will it be very accurate and controllable? I would say probably not.

As an alternative, I was thinking about altering the VVT actuator to limit its travel to about 10~15 degrees. Then use a small controller or see if an older ECM could toggle it at a given RPM. The toggle RPM could be figured out with a few dyno pulls. It wouldn't be continuously variable but would give you low-end torque and top-end power.

I just need more time for all these projects and my regular work.

[RacerX11]

The stock ECU uses closed loop feedback comparing the crank position sensor to the cam position sensor to determine the advance of the cam. You will need something more powerful than a basic stamp module to do this.

Without this closed loop control, a given PWM duty cycle will result in different cam phasing depending on oil temp, oil pressure, etc. Will you be able to move the cam? Sure. Will it be very accurate and controllable? I would say probably not.

Better still, since pulses are the basis for it's operation, perhaps a rudimentary tie into the fuel injector pulse can be made since the pulse directly increases based on load.

Since oil pressure varies so much there must be a pressure signal fed into the PCM to make adjustments the way fuel injector pulse is corrected against voltage.

[427v8]

I'm sure it's 27 degrees total, +- 27 degrees is a LOT!

PWM's are easy, I do PICs all the time ( they are the basis of the stamp) not a problem to make a manual controller to vary the cam timing

[SappySE107]

Drool....27 degrees. Is that +- or like x + and y - together are 27 degrees?

So we can make something that you can program the cam timing yourself? Or perhaps there is am empty output in the ocmputer that can be programmed?

I speculated that it was controlled by a variable series of pulses. That means it can very easily be controlled with a basic stamp micro computer and two tables one with MAP vs. RPM and TPS vs. RPM and a formula. I have programmed one as far as pulsing a fuel injector. I stopped after I stumbled across software that allowed me to reprogram my epromm.

Check out their stuff, cheap and effective.

http://www.parallax.com/html_pages/products/basicstamps/basic_stamps.asp

Look at their manuals to which have hundreds of code examples for applications.

[smilinguy99]

I'm really curious about the operation of the VVT. It only uses two wires to control it. If anyone has any info on it's electrical operation, I would appreciate the input.

I had a chat with the supervising engineer of the VVT and he said the thing is controlled with pulse width modulation. Basically it uses oil pressure to actually move the cam back and forth, but the oil pressure is turned off and on by an electrical valve controlled by a stream of off and on pulses that vary in their ratio of OFF times to ON times. It is ultimately controlled by a map in the ECM. Kind of like a fuel injector is controlled.

The gizmo has a LOT of authority over cam timing. The cam timing can be varied by a total of 27 camshaft degrees in relation to the crank.

[Fiero11]

There are many of the old style 94mm 3.5L engines out. Can you put a head gasket from a 94mm 3.5L on the 3.9 head? I would like to see if the 3.9L valves would interfere with the 3.5L cylinder walls. Since the valves are angled the block could be knotched for small interferances if needed. 3.9L heads and intake on a 3.5L engine could be a real performance improvement. The intake/head intersection would need some machining due to the 1.5mm offset of the 3.9L block. I can send you the 3.5L gasket or money to buy one if you need one. Thanks.