What is it and What is the best RPM setting?

What is it?

Honda VTEC system, is a dynamic system mounted on the engine head, that is adjusted by the car’s ecu after some parameters, like engine speed, vehicle speed, temperature and oil pressure, are met.

The adjustment or activation is done, using oil pressure from the engine, which its path is opened or closed by a solenoid. Such solenoid is electrically activated by the ecu.

Once the oil path is opened, the VTEC system is said to be activated and the mechanical valve timing (when valves open or close) and lift (how much time stays opened or closed) is changed to another preset setting, different from that when it is deactivated. So, at least two valve settings are present on VTEC engines; the low setting (VTEC Off or deactivated) and the high setting (VTEC On or activated).

Facts

It is known that for an internal combustion engine, at low RPMs, the best power is obtained by making the intake valves to open exactly when the piston starts to go downward in the intake stroke and closing at the same time the exhaust valves. It is achieved by setting the camshaft to do exactly so. But it is not true at high RPMs. At high RPMs, it starts to form a “discrepancy” between the high speed mechanical movements versus the slag present in the gases moving throughout the engine. In that cause it would be better to open the intake valve a little bit before the piston starts to go down in the intake stroke and leaving it opened for a longer period to allow the fulfillment of the combustion chamber in such a short time.

The Contradiction

If an engine’s valves are set to maximum power at low speeds, let’s say to have an outrageous power at launch, it will lag a lot of power as it approaches high engine speeds. It will be like a big bang, but will “die” before crossing the street.

On the other hand, if it is set to obtain maximum power at high engine speeds, it will be amazingly fast at high vehicle speeds, but will take a long time to reach it, as the launch will be like driving a Grey Hound line bus.

The Idea

If the engine is set for low RPMs will be short at high speeds and if it is set for high RPMs the contrary will happen. Also, if it is set in some point at the middle of both situations (vehicles before VTEC), power will be average at all situations, but… just average.

Why not building two engines in one? Like a fusion between a low set cam and a high set engines? And then the VTEC came to existence. This VTEC system, which is similar to today’s Toyota “VVT-i”, BMW “Vanos”, Mitsubishi “MiVEC”, and some others, uses a mechanism on the engine head which does right that; put an engine to work like two engines in one.

At low RPMs, the system is deactivated and the engine valves is at the default settings (low). When some parameters are met, which of them the engine speed is the principal one, the VTEC solenoid is energized by the ecu, conducting engine oil pressure to the mechanism which among other things, acts like an automatic adjustable sprocket at the camshaft. It employs two differently shaped cam lobes that switch over.

What is the best RPM setting for VTEC?

It will highly depend on every engine and its setup. By default, many engine are factory set to changeover around 5500 RPMs or higher if the engine load is light.

The best way of determining it for your car after making a modification is by running the car on a Dyno with the VTEC solenoid disconnected. The car should be run the whole acceleration range without VTEC and then the same with the VTEC energized the whole range. It might be done by independently energizing it with the battery voltage, outside the ecu circuit.

Of course, a dummy inductive load will be needed when doing this, in order to fool the ecu, so it doesn’t “realize” that the VTEC solenoid has been disconnected and preventing the check engine or CEL light to turn on.  So basically, the engine will be tested in two conditions through the whole RPM range; (1) VTEC solenoid manually de-activated (disconnected) with the dummy load on, and (2) manually activated by direct wiring to the battery voltage while the dummy load is still on.  Of course, all this should be done using the stock chip and stock maps.  Otherwise, wrong readings might ocurr.

A better idea should be using a chip that is programmed to leave VTEC off at all times.  That way, VTEC maps won’t come in, making the readings even more accurate.

About the dummy inductive load, it can be any spare VTEC solenoid coil for easier connection, but just any automotive coil intended for 12V (any other automotive solenoid or relay), with an approximate resistance value form 5 – 10 ohms would be ok. 

After having both VTEC and no-VTEC Dyno plots, you should interlace one with another and find the point where they cross. That will be your particular VTEC crossover point.

It should be around 4000 and 6000 RPMs anyway. So it will be more like a fine tuning of its activation point. Even if it is a fine tuning, it might raise the total horsepower output and best of all, the ETA time at the track.

Thanks for reading.

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