What is the Best VTEC setting?
The Honda VTEC system is a dynamic valve timing adjustment, mounted on the engine head. It dynamically adjusts the exhaust valves timing by means of oil pressure. This oil pressure is controlled at the same time by the ECM through a solenoid valve. The ECM “decides” if to activate it or not, depending on parameters like, oil pressure, oil temperature, vehicle speed, engine speed and engine coolant temperature.
This article is covering early system that use to activate or deactivate the VTEC. Not like the new “i” systems which control the VTEC with “intelligence”. It is more dynamic and more complex and not only activate or deactivate, but also controls the degrees of the activation.Back to the early models
Once the oil path is opened, the VTEC system is said to be activated and the mechanical valve timing is changed or “enhanced” for the actual condition of the engine. Two valve settings are present on the early 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 this is not true at high engine speeds. 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 case, it would be better to open the intake valves a little bit earlier than 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
Due to the facts above, engine speeds with a fixed cam, can be adjusted to low engine speeds, high engine speeds or somewhere in the middle. 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 increasingly lag a lot of power as it approaches high engine speeds. It will be great at the beginning, but will continuously loose power while the engine accelerates.
On the other hand, if valves are 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. It will start slow, but will be fast as engine speed go up. If it is set somewhere in the middle, there will be low power at launch and low power at high speeds also. The only way to make it respond the best way in the full range, would be to put a dynamic valve system which will be adjusted on demand and automatically. Here is when VTEC comes into the scene.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, power will be average at all situations. Then, why not building two engines in one? Like a fusion between a low set cam and a high set cam engines? The 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 ECM, conducting engine oil pressure to the mechanism which among other things, acts like an automatic adjustable sprocket of the camshaft.What is the best RPM setting for VTEC then?
It will be different on every engine, even if engine models are the same. There are always minor differences, for example, engines do not wear at the same rate. By default, many Honda engines 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 ECM circuitry.
Of course, a dummy inductive load will be needed when doing this, in order to fool the ECM, 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 occur.
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 from 5 – 10 ohms would be ok.
After having both VTEC and no-VTEC Dyno plots, you should compare one with another and find the point where they cross. It is exactly when the plot of the VTEC off chart starts to fall, while the other starts to raise. That will be your particular VTEC crossover point which will keep the maximum engine output through all the acceleration range when set to that 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.