Contour Enthusiasts Group Archives Forums Modifications Zetec Maintenance Removing the exhaust vct for more power

VCT = Variable Cam Timing. It's a heavy hydraulic mechanism inside the timing gear that with solenoid actuation and oil pressure, can advance the exhaust cam timing as much as 20 degrees.
This is done as a replacement for the egr valve, that by changing the intake/exhaust valve timing overlap, they can suck exhaust gas back into the motor to mix with the new air and fuel coming in during the intake stroke.
This takes away room for more good air and fuel, and creates a lean burn condition. Good for emissions, bad for power.

For those who'd like to discard the exhaust vct, I have adjustable cam gears and adapters so you can take off and chuck the vct exhaust gear, and dial in the right timing for more power.



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ZXtuner Store
1999 ZX2 S/R
128.5 wheel hp @ 6250 rpm
9.62 sec @ 72.11mph (1/8)
2340 lbs.

I'm definitely interested.

John

Here's some specs:

Valve lift: .405"
Duration: 258 degrees
Duration at .050": 227 degrees
Int Opens at 3.5 deg BTDC
Int Closes at 43.5 deg ABDC



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ZXtuner Store
1999 ZX2 S/R
128.5 wheel hp @ 6250 rpm
9.62 sec @ 72.11mph (1/8)
2340 lbs.

How much is that for?

How much? How are to install? How much power are we talking about gaining?

What kind of power increase could be expected? What would it do to emissions and would it cause a CEL? Starting next year where I am they are going to check OBD systems for codes on inspections

If you have a VCT engine, I would recommend keeping the VCT operative unless the car is a pure drag car and not street driven. I also think if you wanted to disable the VCT while using a VCT head you would need some kind of solid insert to cover the oil pressure port in the head. Finally the dreaded check engine light could probably be fixed by replacing the PCM with one from a non-VCT engine, but you will have to make sure you have the same type of MAF, 4-wire or 6-wire.

The VCT is somewhat misunderstood. Yes, it does advance the exhaust cam 30 deg, allowing exhaust to remain in the cylinder and this does dilute the intake charge similar to an EGR. However, the VCT rotates the exhaust cam to the normal position at high rpm full throttle operation. The VCT also reduces cam overlap at low rpm by 30 deg. This allows the use of a longer duration (on the ZX2 anyway) intake cam (214 deg @ 0.050 vs. 206 deg @ 0.050 for non-VCT) while getting good low end torque and a more stable idle.

I don't have data for the VCT and non-VCT exhaust cam profiles. ZXTuner or someone else might have this data. If they can share this data it would be appreciated.

If you want a drag car I agree with ZXTuner, disable the VCT. You are probably only using the engine from 3000 rpm and up. the Honda guys do that with VTEC running only on the high rpm cam lobes.

On the other hand if you are going to street drive the car, I recommend keeping the VCT. By reducing overlap, it will improve low rpm torque and stability. (Don't be fooled by the EGR efect, overlap of long duration cams at lower rpm will create more drivability and emissions problems than dilution of the intake charge.)

My own analysis suggests that you can get away with 220 deg @ 0.050 and 0.390" lift on the inatke cam and still meet emissions if you keep the VCT. Maybe even further. Up to now this was done by theoretical flow modeling (I work with a bunch of fluid dynamics guys) but I plan to test this myself in a year or so. The cam suggested by ZXTuner would also probably work and is a good choice for power. This is a good choice, but I would not go beyond 0.405" lift without changing the valve gear and beyond 227 deg duration on a street engine.

One thing that would benefit us all would be if someone modified the PCM code to change the VCT operation. Maybe make it rpm but not throttle dependent, so you always get rotation between 3000-4000 rpm.

Anyway I intended this note as food for thought. Comments ? (zxtuner are you the same person as Zonker on another board?)

zonker = zxtuner... true.

The cam I sell at the store is installed in my ZX2. 10hp is a common gain at upper rpms, and I recommend this for MTX guys only, not ATX's since the power band below 5000 rpm is not assisted with this cam. If you modify the ATX with higher shift points (7000-7200) then this cam will be a worthwhile mod for you as well. I have dynoed before and after cam, and here's the resulting graph:



The cam sells for $275, lash caps (required) not included. The lash caps sell for $65 (I'll take $10 off the store's computer price).

Rob