What is the Best VTEC setting?

What is it?

The Honda VTEC system is a dynamic valve timing adjustment system mounted on the engine head. It dynamically adjusts the exhaust valves’ timing using oil pressure. The ECM controls this oil pressure through a solenoid valve. The ECM “decides” whether to activate it or not, based on parameters like oil pressure, oil temperature, vehicle speed, engine speed, and engine coolant temperature. Other car makes have developed similar systems under different names or “variations” of that name. For example, Toyota VVTi, BMW Vanos, Mitsubishi MiVEC, and some others.

This article covers the early system used to activate or deactivate VTEC. Unlike the new “i” systems, which are called VTEC-i or VTEC with “intelligence”. It is a more dynamic and sophisticated system that not only activates or deactivates VTEC but also controls the activation angle in degrees as needed. So instead of ON and OFF, intelligent VTEC also controls how much it is ON and how much it is OFF.

A more detailed explanation of what VTEC is can be seen in the following video:

Back to the early models

Once the solenoid opens the oil path to the VTEC mechanism, it changes the camshaft angle to match the engine’s operating conditions. Older systems have two valve settings: ON or OFF. When it is OFF, the camshaft is set to maximum engine power at lower engine speeds. When it is ON, it is set to maximum power at higher engine speeds by slightly opening the exhaust valves during the intake stroke to allow better cylinder filling at high engine speeds and reduce the pressure that opposes piston down movement, helping engine performance.

Facts

For an internal combustion engine at low RPMs, maximum power is obtained by opening the intake valves exactly when the piston starts to move downward on the intake stroke while the exhaust valves are closed. That is the typical working condition of an internal combustion engine. But this is not true at high engine speeds. At high RPMs, the short intake time leaves less time to fill the combustion chamber. In that case, it’s better to open the intake valves slightly earlier, before the piston starts to move down on the intake stroke, to fill the combustion chamber more completely in that short time.

Fixed Cams

Engines with a fixed cam can only be adjusted to low engine speeds, high engine speeds, or somewhere in the middle. If an engine’s cam is set to maximum power at low speeds, let’s say to have the higher power at launch, it will increasingly lag a lot of power as it approaches high engine speeds. It will be great at the beginning, but it will continuously lose power as it reaches top engine speeds.

If, on the other hand, cams are set to produce maximum power at high engine speeds, it will be fast at high vehicle speeds, but it will be painstakingly slow to reach that speed. It will start slow and get faster as engine speed increases. If it is set somewhere in the middle, it will be slow at launch and at high speeds, but better at mid-range. The only way to make it respond best across the full range is to use a dynamic cam system that adjusts automatically on demand. That’s where VTEC comes in.

The Best Setting

The idea is like having two different engines in one. One will take care of the lower range, while the other takes control anytime you go to the upper range. The VTEC system does this.

Each engine needs a different setting. Even the same models have slight differences, and the factory accounts for that. You should not need to fiddle with this. But if you modified your engine and need to readjust the VTEC setting in RPMs, you will need to hook your car up to a dynamometer and make some tweaks. Each car tested will need at least two dyno tests. One with VTEC ON and the other test with VTEC OFF. We’ll use the 1995 Honda model as an example. By default, many Honda engines of those models have a factory VTEC changeover at 5500 RPM or higher if the engine load is light.

For the first dyno run, disconnect the VTEC and connect a resistor with a similar value to the VTEC wiring that goes to the ECM to avoid the check engine and limp mode. The value of the resistor we refer to must be the value in ohms of the VTEC solenoid coil. After you do this and run your car on the dyno, save the graph so you can compare it with the next dyno run.

Now do the second dyno run with the VTEC energized by a wire supplying 12 volts to the solenoid to keep it active the whole time. Keep the other end of the connector that goes to the ECU as in the first dyno run, with the resistor. In both tests, run the car through the full acceleration range without VTEC and with VTEC externally powered, and plot the output chart.

Basically, test the engine in two conditions across the full RPM range with the stock chip (stock maps) to avoid incorrect readings. Afterward, you will have two different dyno plots. All you have to do is put one plot over the other, and the exact point where the horsepower line crosses over the other in both dyno plots is your best VTEC activation point in RPMs. The best point should be around 4000 and 6000 RPMs. You can then program the resulting value into a chip and install it in the ECM.

For the resistor in the VTEC connector to the ECM, a dummy inductive load might be better; if you have a spare VTEC solenoid coil for easier connection, or any 12V automotive coil with a similar ohms value, that would be best. The VTEC solenoid coil resistance should be 5-10 ohms.

This is a “basic” VTEC best activation point in RPM. Please remember that VTEC considers various factors, like oil pressure, engine load, etc. This point may vary if the factors change. So it is important that you at least do the tests under the engine load you expect it to run at.