Vtec Explained

VTEC Information

Read the definitions first!
Volumetric Efficiency, Torque, Power, The Camshaft, Engine Breathing, ECU

VTEC
VTEC uses two camshaft profiles, one will lower duration for good low speed torque, and one with longer duration and valve lift for good high speed torque. The computer switches camshafts at about half engine speed to combine the best features of each camshaft. Sounds simple! The resulting torque curve is M shaped - it has a torque peak for the low speed camshaft (at about 3500 rpm in my car) and a torque peak for the high speed camshaft (at about 7000 for my engine). The part of the torque curve in between the low and high speed camshaft peaks, has a torque dip because the low speed camshaft torque is dropping off and the high speed camshaft torque is picking up. When the camshafts switch, you are actually at the lowest point of engine torque from about 2000 - 8000 rpm! I avoid this engine speed and try to keep the engine at the low speed camshaft torque peak (for normal driving) or the high speed camshaft torque peak (for getting somewhere fast).

The Non-VTEC Arrangement
The DOHC (non-VTEC) engine camshafts have one cam lobe (the oval shaped part that opens the valves) per valve. The cam lobe is above a short rocker arm, which is fixed at one end and sits on top of the valve at the other end. Some engines have the cam lobe directly in contact with the valve head, but Honda did not do it this way so that they could get more valve lift, and open the valve quicker. Using a rocker made the valve train heavier, which uses more power and limits engine speed, so Honda hollowed out the cam lobes (as well as the camshaft) to save weight.

The VTEC Arrangement
The VTEC head looks similar to the DOHC head. There is a small rocker arm for each valve, and the camshaft is positioned above this about half way along it. The difference is that there are three cam lobes for each set of two valves (two intake or exhaust for each cylinder). When using the low speed camshaft, the outer two cam lobes press on the rockers and open the valves in much the same way as the DOHC head. The third cam lobe (which is in the middle) just follows the cam lobe profile without doing anything else.

Switching Camshafts
When the computer decides to switch camshafts, it closes a valve that forces oil along passageways through the camshaft into the third rocker. It has little pistons which are forced outwards (I’m a bit fuzzy here, but I think this is right) into the outer two rockers. All three rockers are then locked together and operate as one. The middle cam lobe has more lift than the outer two so it then controls the lift and duration of the set of valves. When switching back to the low speed cam the ECU just opens the valve, lets the oil out of the rockers, the pistons unlock the rockers and everything operates as before.

When to Switch Camshafts
The ECU is constantly comparing the torque curves of the low and high speed camshafts. It calculates the expected volumetric efficiency of the engine based on the current environmental conditions (air temperature and pressure) and the engine conditions (temperature, engine load, throttle position), and then derives the expected torque from the volumetric efficiency for each camshaft. Most of this has to be done anyhow in order to determine how much fuel to inject.

When conditions are right (the revs are over about 4500 rpm, the engine is warm, there is enough oil pressure to activate the pistons and the car is moving) the ECU will switch from the low to high speed camshaft when the expected torque of the low speed camshaft equals the torque of the high speed camshaft. The ECU closes a solenoid valve that then forces engine oil, under pressure, along the camshafts to active the third rocker arm.

VTEC Controllers
A few people have asked what VTEC controllers are, and how they affect the engine. A VTEC controller is basically just a RPM activated switch that connects to the VTEC control valve and switches cams at a pre-determined engine speed, rather than letting the ECU figure things out. I have reverse engineered a commercial VTEC controller to see how one works, and found that they also look at the oil pressure and water temperature sensors like the ECU normally does, so that the cams are not switched if something is wrong. I have heard of people using an off the shaft rpm switch as a VTEC controller.

VTEC controllers are useful if the engine has been modified, and the ECU switches cams too early/too late, and for certain engines where Honda has got the cam switch point wrong. The only example of this that I know is the VTEC prelude, which has a huge jump in the torque curve because the cams are switched too late. Rumour has it that Honda did this deliberately to get a good EPA gas mileage, but there definitely are benefits from getting the prelude to switch cams earlier.

With the stock B16 engine this is little to be gained from changing the cam switch point - the ECU does a much better job than a VTEC controller because it can compare the torque curves of each cam and switch where they overlap. If you need a VTEC controller then it will be evident from a jump (up or down) in the torque curve when the cams switch. This may be difficult to judge even from a dyno because the cams should switch at different speeds with different engine loads, but a dyno print out would be the way to check.

Definitions:

Volumetric Efficiency
The engine produces a certain force from every power stroke as a result of burning air/fuel expanding. This force generally gets less for every power stroke as the engine revolves faster, as the air/fuel mixture has less time to get sucked into the cylinder. The volumetric efficiency of a engine at a certain speed is the pressure of air/fuel mixture inside the cylinder when the piston has finished sucking in the mixture, as a percentage of the atmospheric pressure. Thus an engine with 80% volumetric efficiency at a certain speed will have a mixture pressure of 80% of atmospheric pressure when the piston is at bottom dead centre after the intake stroke.

Torque
The torque of an engine is the total force the engine produces at a certain speed. This is a rotating force, but the easiest way to think of torque is to imagine an engine with a drum attached to it, winching up a weight vertically. The torque of the engine is the force that raises the weight.
The torque of an engine will increase as the engine rotates faster, because the number of power strokes per time period increases. However, the volumetric efficiency of an engine will drop after a certain speed, so each power stroke has less force. The point where the increase in force (from the increased number of power strokes) is equal to the drop in force (because of less efficiency) is the point of peak torque. This occurs anywhere from 2000 - 7000 rpm, depending on the engine.

A higher performance engine will generally have a higher efficiency and maintain this longer, so will have peak torque at higher revs. In the case of my B16A VTEC engine, the torque peak is at about 7000 rpm, which is one of the highest of any mass produced vehicle engine.

Power
The gearbox modifies torque from the engine to torque at the wheels. If one engine produces the same torque as another, but at a higher engine speed, then force at the wheels will be higher for the first engine one the engine speed is converted by the gearbox to the same wheel speed. The power of an engine is the measurement of the torque of an engine at different engine speeds. Going back to our engine winching analogy, it is easy to see that if the engine is geared down so that the drum rotates half as fast, then weight will be raised slower be more weight can be lifted.

The peak power point for an engine is the point where, ideally geared, the most force will be available at the wheels. The peak power point will always be above the peak torque point. In my B16A engine, the peak power occurs at about 7800 rpm.

More Volumetric Efficiency
The volumetric efficiency of an engine is largely determined by the engine’s ability to suck in fuel/air mixture and expel the exhaust gas. An engine with small openings, tight corners and constricting passages either in the inlet or outlet flow paths will not be able to suck in mixture or expel exhaust as well as an engine with larger openings, and so will have less volumetric efficiency and therefore less torque.

The mixture being sucked into the engine has mass and therefore momentum. Once the inlet valves shut, the mixture will keep moving for a while and compress the mixture in front in it, eventually stopping. If the inlet value opens again just as the mixture has stopped moving, then the mixture will be forced into the cylinder. This will increase the volumetric efficiency of the engine (more mixture = more power from the power stroke = more torque etc.). Some engines can achieve over 100% efficiency using this effect.

The same applies to the exhaust. The gas will leave the cylinder under pressure, move into the exhaust system and expand. Once the exhaust valve closes, then the gas will keep moving and cause a slight vacuum next the exhaust value. Next time the exhaust value opens, the exhaust will be sucked out of the cylinder. With four exhausts going into the same pipe, a further effect is created where the moving exhaust gas from the last power stroke will suck out the exhaust gas from a different cylinder.

Why not make all these openings as big as possible? If (say) the inlet path into the cylinder is made bigger, then the gas velocity will be lower for the same gas mass . Lower velocity = less momentum = less pressure forcing the mixture in = lower efficiency = less torque = less power. Same applies to the exhaust, but it is not as sensitive as the intake path. With only one intake or exhaust valve per cylinder there is only so much mixture/exhaust that can flow through the opening, so two valves doing the same job allow more mixture/exhaust to flow and therefore increase efficiency. This is most noticeable at high engine speeds which is why four valve heads have a reputation for having more power at higher engine speeds.

The Camshaft
The camshaft has a very big influence on engine breathing. The camshaft controls how long the intake and exhaust valves are open, and how high they open. The intake valves always open before the piston is at the top of the cylinder (and started sucking) and close after the piston is at the bottom of the cylinder (and stopped sucking). The shape of the cam lobes limits the valve opening and closing to a gradual opening from closed to fully open, then a gradual closing to fully shut. (Otherwise the value train will destroy itself at high speeds) So while the value opens before the cylinder is sucking, it is not open that much.

There is a trade off in terms of efficiency with the camshaft. It is possible to open the values earlier, and have the valve open further for a longer period while the engine is sucking in mixture (it works the same for the exhaust). The valve will be open before the piston has reached the top of the cylinder, and some of the mixture will be pushed out of the cylinder but the piston. Because of the momentum effect of the intake mixture, this loss is less at higher revs, and more at lower speeds, when the intake mixture has not much momentum to overcome mixture being forced out of the cylinder.

A camshaft that opens the values early and closes them late (called long duration, or ‘wild’ or ‘lumpy’) will be more efficient at higher engine speeds and less efficient at lower engine speeds. A camshaft that opens later and closes earlier (called short duration, or ‘mild’) will be more efficient at lower engine speeds and less efficient at higher engine speeds.

Engine Breathing
To get more power you can sacrifice low speed torque (and have an engine that is difficult to drive around town in) for high speed torque (and more power) by altering one or more of the components that affect engine breathing. The trick is to know what will give the best high speed gain for the least low speed loss. Honda has tuned the size of all the components and the camshaft profile to get the best possible compromise between low speed torque and high speed torque of the engine (I think that they do a pretty good job of this).

There are many other factors that influence the power that an engine produces, such as internal friction, rotating and reciprocating mass, arrangement of various components, which I have skipped over for simplicity.

ECU
The ECU (electronic control unit = the fuel injection computer) is the heart of the engine. Basically the purpose of the ECU is to control fuel injection and ignition for the engine, for all the conditions which the engine can be expected to run under. This is a fairly complicated job considering the number of external factors that can influence the amount of fuel that needs to be injected into the engine, and the rate at which events happen. At 8500 rpm the ECU has to control 280 injector openings/closing per second and 280 ignition signals per second, while coping with 2400 signals from the distributor per second. Plus there are another 16-odd signals and sensor reading from the engine and outside world that ECU needs to know about. It is expected to do this flawlessly, under conditions that the designers may not have anticipated, for the lifetime of the car without servicing. It does this fairly well, the only common problems are from external components failing (e.g. the distributor bearing failing and destroying the sensors) or from ‘user mis-use’ (e.g. getting an air pocket in the cooling system and feeding the water temperate sensor an incorrect reading).

http://members.lycos.nl/hendrik/honda/vtecinfo.htm

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it’s archivethis :noob: :smiley:

great info Schu, a must for the archives:bow: :read:

Good stuff man. Just the other day I was thinking to myself, it would be nice to have an article explaining how exactly VTEC works (I swear to gawd!), this did the job nicely. Great article!

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