ZX2 Mods

Disabling the Speed Governor

The factory ZX2 speed governor limits the maximum speed of the ZX2 to around 106 mph. The reason for this is that the stock tires both 14s and 15s are rated for a maximum speed of 120 mph. It would be dangerous to disable the speed governor without having the appropriate speed rated tires.

You must first remove the front cup holders by firmly pulling them up. Also remove the molding holding them and the left right inner kick panel. The computer is now exposed, with many wires coming from it. Find the white wire with a black stripe and cut it. Your “check engine” light on the dash will turn on. Connecting the wires will turn the “check engine” light off after or about 50 miles. Lengthen the wires and add a lighted switch the inline with the wire to disable the speed governor only when the need arises.
This modification may not work with automatic transmissions and cruise control.

Lower Intake Resonator Removal

Like the above upper air tube resonator, the lower intake resonator is a sound dampening device. The stock intake draws air from inside the driver’s side fender through a 1.5" hole. Removing the resonator makes it easier for the engine to draw in air, since it will now have two openings once the resonator is removed. Removing this restriction gives a noticeable horsepower gain.

First, unscrew the hose clamp holding the air hose coming from the air filter box. Squeeze and separate the wiring harness that attaches to the air filter box. Remove the long, skinny tube from the side of the air filter box by squeezing its hose clamp with pliers. Unlatch the metal band that holds the two halves of the air filter box together. Separate the two air filter box halves. Remove the air filter box half closest to the engine bay. Unscrew the three, shiny 12 mm bolts holding the other half of the air filter box. Remove the other half of air filter box by slightly twisting and pulling the air tube from its tight rubber molding.

You should be able to see the lower intake resonator more easily now. It has the appearance of a black milk carton. You will need to remove the rubber molding that held the air tube. It is held into place by three, 10 mm screws: one holding the A/C tube bracket on, and the other two below. Slightly twist the lower intake resonator to remove it. Screw the rubber molding back into place. Assemble the air filter box’s first half by bolting it down again. Attach air filter and the second half of the air filter box by reaching the clamp. Using pliers, attach the long, skinny tube with the hose clamp. Tightly screw the air tube to the air filter box with the hose clamp. Remember to plug the wiring harness back onto the air filter box.

Location of lower air resonator.

The location of the lower air resonator is beneath the air filter housing.
You must first remove the clamped hose (large red circle) and 3 bolts (smaller red circles).

Photo of lower air resonator.

This is a photo of the air resonator removed from the car. It’s about the size of a 1 gallon milk carton.

Upper Intake Resonator Removal

This modification will add some extra horsepower to your vehicle and is an easy modification. It involves removing the upper air tube resonator and swapping a new piece of tubing. The original resonator is designed to reduce the sound of your engine, at the expense of horsepower.

You can obtain the PVC tubing at most hardware, plumbing, or electrical stores. The tubing will come in 10’ to 20’ lengths with a flared end. It is important that you purchase the PVC with the flared end. Some stores require you to purchase the whole length of tubing; others will sell you the 7.5" flared end. Make sure you buy 2.5" outside diameter (sometimes listed as 2" interior diameter), otherwise it will not fit. There are two different thicknesses of PVC as well. Buy the schedule 40 if you can find it. There have been some reports that the thin wall grade tends to warp over time due to engine heat.

Take the 7.5" long tubing and file, then sand the openings smooth to remove any burrs. Clean the tube well to remove any plastic particles and dirt. You may, optionally, want to paint your new tubing to match the black rubber tubing of the engine. Unscrew the hose clamp connected to the throttle body and the 90 degree tubing. This should easily slide off. Next, unscrew the hose clamp on the S-shaped rubber tubing and the upper air tube resonator. You will need to unscrew it almost all the way off. It will take a lot of wiggling to take these two pieces apart. You will probably invent a couple of great swear words while doing this. After removing, you should have your resonator and 90 degree tube still connected together. Unscrew the hose clamp and wiggle the parts apart. Connect the smaller diameter side of the PVC tubing to the 90 degree tubing and tighten the hose clamp. Insert flanged end of PVC tubing to S-shaped tubing and tighten the hose clamp. Place the 90 degree tubing over throttle body and tighten the hose clamp.

upper resonator

The upper air resonator (circled in red) can be found just before the throttle body.


The original upper resonator removed from 90 degree elbow and the S-shaped tubing.
Note:Â when looking at the installed air resonator in the engine, the “box” will be above the tube.


This is the 7.5 inch long schedule 40 PVC tubing that has been painted black.
The flared end should be on the right side.
If you look closely, you will notice some warping on the ends due to the clamps and heat from the engine.

Air Temperature Sensor

This modification will void your warranty. You will need to use premium gas, as the modification relies on advanced timing. The ZX2 is affected by local air temperatures. Hotter weather means less dense air, requiring the engine to use less fuel, at the expense of power. When the ambient air temperature is warmer the resistance value is less resistance. When the ambient air temperature is cooler the resistance value is more. By fooling the air temperature sensor into thinking that the air is cooler and more dense, the engine will receive more fuel, thus increasing horsepower.

This is done by splicing inline a 5K ohm resistor into the sensor wire. To do this, you will need to remove the air temp sensor from the cone-shaped air filter housing. To make this modification adjustable, attach a two position terminal block in-line in the wiring harness instead of the resistor. This will allow you to easily remove the resistors and replace them with custom values if you want to tune the resistance to your specific vehicle application. .

Photo of the air temperature sensor.

The location of the air temperature sensor is connected to the air intake tube leading from the air filter housing (circled in red).
Note: It does not matter which wire you splice.

Nice write up.

thanks ill put the pics in later as soon as i take them…

Air Temperature Sensor is not good to splice under the hood unless you can make it water tite. Better to splice at the PCM.

thats what they make heat shrink tubing for, the type with the adhesive in them that seeps out the ends when heated and forms a water tight seal…

Splicing a resistor into the IAT is a bad idea anyway unless you put a shorting switch on it. The PCM is not dumb, it will figure out it isn’t getting the right readings and recalibrate the sensor.

Instead of PVC which I found nearly impossible to find (I literally searched 9 different stores in my area, maybe we Oregonians don’t like it i don’t know), I just went to a muffler shop, got a piece of 2.5" muffler tubing and had the end flared to 2 7/8" and it fit PERFECTLY into the intake, sounds wonderful too, and you do not have to worry about the pvc warping as its muffler tubing. Cost me $10.