3.9L LZ9 engine internal upgrade & performance?

[robertisaar]

The ECM constantly monitors cam position relative to crank position and adjusts the actuator via PWM to achieve desired timing. Due to variations in oil pressure, internal clearances, valve train load and other variables a specific pulse will not provide a consistent timing location.

right, the factory PCM works like that, but if someone running OBD1 intercepted signals from either the 3X crank sensor or even the tach output from the ICM, wouldn't it be possible to advance/retard the cam to our liking using some homebrewed hardware?

[CNCguy]

Just as I am doing with the variable plenum, you can use raw voltage to toggle between max advance and max retard. But, as Joseph said, you need to limit the hydraulic actuator to usable travels since the stock actuator has around 52 degrees of travel IIRC.

[robertisaar]

right, thats what i was looking for.

say we did it PWM, would we be able to dial in more than just max advance and retard? seems easier and would have more adjustability than phsically preventing the cam from moving as much...

and in the "3900 Info" thread i saw 27* on the cam, so 54* on the crank.

[torq455]

why not use a stepper motor and proper circuitry for the intake?

[Joseph Upson]

It's to complicated to try and accomplish anything more than an optimal low rpm setting and optimal high rpm setting without the stock PCM. In my Dyno simulator I found the HP curve intersection between full advance and full retard to be around 3500 rpm depending on the cam specs. I decided on 4000-5000 rpm. My Zeitronix WBO2 controller has an assessory signal that can be programmed to energize based on a number of variables, MAP, RPM and some others I don't recall off hand. I plan to use it in this case should I decide to apply any VVT at all.

The temptation to do so is all a result of the cam dynamics between the stock cam and my regrind. The stock cam increases power all the way up to the max retard I can enter into the program -15 deg, the reground cam with the LSA change immediately reduces power under the curve with a slight shift in peak power to the right with anything past 0 deg negative. So as was mentioned by someone earlier, the OE cam specs are designed to perform linearly at least to a point with advance and retard, the regrind wants to be installed at card spec and left alone.

So I may send my second cam off and have it ground to about 220/220 with fatter lobes and .510 lift. I've calculated the LS1 pistons and chevy 2" big end rod combo along with the .125" offset grind to produce about 11.2:1 compression. I discussed this with Sappy and decided against the larger journal rod and .075" offset combo which would keep my stock compression ratio about the same but yield a quench area of about .090" not good.

The engine is very detonation resistant as it is at 8 psi barely applying 1 deg of retard at full boost in a very lean area of the tune (BLM ~163) so with the addition of the water/meth kit, and keeping oil and coolant temps near 180 I should still be able to run the same amount of boost and probably more with the current mid grade fuel. The increased cam duration will also work better with the higher compression.

I read that the oil squirters take about 100 deg out of the pistons and my current oil temps run about 240-250 since my oil cooler is not in an air stream however I lost power to the gauge during the test drive so I'm not sure what it runs at while the car is moving.

I linked to a BMW M3 supercharger system here some time ago that used an intercooler alone to make 8 psi work with stock 11.3:1 compression so I believe 10 psi is a reasonable aspiration with all of the extras I have to prevent detonation although I'm reaching for more without premium fuel.

[CNCguy]

why not use a stepper motor and proper circuitry for the intake?

The stock Intake Manifold Tuning Fan only has two positions, long runner or open plenum.

[robertisaar]

The stock Intake Manifold Tuning Fan only has two positions, long runner or open plenum.

exactly like the LT5 Vettes are...

i've been playing with all of it in desktop dyno, and with a stock cam, i retard it 15 degrees and i'll see ~240HP where it should be, with the peak around 5500. any advance and it drops like a rock, and at 15 advanced it manages 147HP at 4000-4500... wheeeee for a iron duke, i guess.

[Fieroghini]

For what it's worth my 3900 dyno'd 206hp at 5500rpm and drops off quick. This is with a fully advanced cam. I believe my numbers are optimistic because of a stock 3400 that dyno'd the same through a 5speed. Also retarding the cam fully will stall the engine at idle and cause a severe loss of power at any rpm. As pointed out already the cam needs to be degrees in and I would never recommend vvt after my experience. I will be converting to a 3400 cam as soon as I get the car back from paint.

[robertisaar]

this is from car craft:

"Advancing the camshaft means that you are starting the opening and closing process sooner in the cycle. It generally improves low-speed torque and mid-range power while sacrificing top-end hp. Conversely, retarding the cam detracts from low- and mid-range power in order to help top-end power. Generally, moving a camshaft a couple of degrees will not make dramatic changes to the engine's power curve."

this is how i understand it, but when you advance it in desktop dyno, EVERYTHING drops like a rock. you don't gain anything anywhere. maybe under 2000, but DD doesn't go down that far. i'm not seeing the trend above though either...

IDK.... i want a 3900 making the power(and fuel economy) of a small port 3100 until i really start getting on the throttle, then for it to turn into the beast it is... its what the LT5 did, i don't see why we can't do it.

it seems through DD i would be better off referencing MAP for the power switching... maybe at 70-75kPa it retards for full power. i''d be happy with that.

[torq455]

The stock Intake Manifold Tuning Fan only has two positions, long runner or open plenum.

the stepper motor can do that and something in between. if you have, say a MAP value to turn the control circuit on, have the stepper motor controlled by rpm, then you can have the fan slowly open throughout an rpm range. i know, a little complex and more of a wild thought, but could it work?

[Joseph Upson]

this is from car craft:

"Advancing the camshaft means that you are starting the opening and closing process sooner in the cycle. It generally improves low-speed torque and mid-range power while sacrificing top-end hp. Conversely, retarding the cam detracts from low- and mid-range power in order to help top-end power. Generally, moving a camshaft a couple of degrees will not make dramatic changes to the engine's power curve."

this is how i understand it, but when you advance it in desktop dyno, EVERYTHING drops like a rock. you don't gain anything anywhere. maybe under 2000, but DD doesn't go down that far. i'm not seeing the trend above though either...

IDK.... i want a 3900 making the power(and fuel economy) of a small port 3100 until i really start getting on the throttle, then for it to turn into the beast it is... its what the LT5 did, i don't see why we can't do it.

it seems through DD i would be better off referencing MAP for the power switching... maybe at 70-75kPa it retards for full power. i''d be happy with that.

VVT cams perform differently than non VVT cams. That's why my reground VVT cam to specs more similar to fixed camshafts, stopped showing the power increase that it did when retarded in stock specs.

[gectek]

all the tuning valve does is close when it is grounded. i did try a 6 volt battery from a lawn mower on it, but it still opened and closed it with nowhere in between. it is actually a fail intake IMO. the plenum neck up to the top chamber is way small and retarded, as well as the pocket at the front.

you should watch the vvt events with a scan tool and graph it. crazy stuff. it is never a constant at any speed. it moves all the time. during WOT for a while it will stay somewhat static. it could be a good idea for pwm the duration of a lobe too....but that can be a bad thing as well if your homemade circuitry doesnt make it work like it is supposed to and a piston hits a valve due to it hanging open

[robertisaar]

i imagine the constant movement is for EGR purposes... that would explain why it doesn't move much at WOT.... idle is probably fairly still as well...

[mrtohil]

Does anyone know what the stock rocker arm ratios for this engine is? Since I didn't measure them before I tore it apart...Doh!

[robertisaar]

IIRC its like 1.7 or 1.8:1...