VTEC (not really), and DOHC

I know what V-TEC is, but my question is, what are the benefits of having the valves open up only a bit, compared to having it opening up the valves all the way, Why can't I just set the VTEC to kick in at like say 1000 RPMS so I get power right when I touch the throttle? Wouldn't it make the engine "breath better"?

Or does it have something to do with the "exhaust scavenging" from that exhaust thread?

My other question is: What is DOHC? I know it stands for dual overhead cams, but how does it control the valves any better than SOHC? What exactly is "Dual Overhead Cams?" is it like 2 camshafts? Or 2 pairs of cam lobes on one shaft?

Oh and one more thing, my friend says that a 3 stage VTEC would give a civic si engine as much HP as 160. Is this true?

My other question is: What is DOHC? I know it stands for dual overhead cams, but how does it control the valves any better than SOHC? What exactly is "Dual Overhead Cams?" is it like 2 camshafts? Or 2 pairs of cam lobes on one shaft?

Oh and one more thing, my friend says that a 3 stage VTEC would give a civic si engine as much HP as 160. Is this true?




DOHC engines have two camshafts one controling the exaust valves and another controling the intake valves.


DOHC engines have 4 or sometimes 5 valves while SOHC engines regularly have only 2 valves due mainly to clearance purposes I presume (In an SOHC engine the valves have to be in line)

also in an DOHC engine you can fully control the timing of the intake and exaust valves independently from one another

for your second question "stage 3" means nothing in itself, it is just a term that some aftermarket companies use to separate their upgrade kits so without further info we cannot say if your frined is wrong or not

"for your second question "stage 3" means nothing in itself, it is just a term that some aftermarket companies use to separate their upgrade kits so without further info we cannot say if your frined is wrong or not"

No... I mean this http://www.leecao.com/honda/vtec/3stagevtec.html

"DOHC engines have two camshafts one controling the exaust valves and another controling the intake valves."

So... DOHC can handle more valves easier because the valves don't have to be inline?

"DOHC engines have two camshafts one controling the exaust valves and another controling the intake valves."

So... DOHC can handle more valves easier because the valves don't have to be inline?


well its a whole lot easier to fit in a circle a

1 2
3 4

than a

1 2 3 4

What about the SOHC engines with 4 valves per cylinder?

i dont think SOHC can have 4 valves per cylinder.

Examples, D15, C30

Depending on how far the valve open depend on how well the A/F mixture will swirl as it drawn through the opening....

So for a low RPM engine a low opening valve will keep intake velocities high and help keep the fuel atomized and distributed more evenly inside the combustion chamber, thus equaling more powerfor less usage of fuel...

But, this dosen't work and higher rpms because you would be choking the engine for air, so you have the valve open a bit more to allow the engine to breath better but still keep intake and exhaust velocities high.

VTEC has 2 sometimes 3 cam stages which improve power through each of their own RPM ranges; Intake/exhaust velocities, Valve overlap, torque, fuel efficiency.

I know what V-TEC is, but my question is, what are the benefits of having the valves open up only a bit, compared to having it opening up the valves all the way, Why can't I just set the VTEC to kick in at like say 1000 RPMS so I get power right when I touch the throttle? Wouldn't it make the engine "breath better"?

Or does it have something to do with the "exhaust scavenging" from that exhaust thread?

My other question is: What is DOHC? I know it stands for dual overhead cams, but how does it control the valves any better than SOHC? What exactly is "Dual Overhead Cams?" is it like 2 camshafts? Or 2 pairs of cam lobes on one shaft?

Oh and one more thing, my friend says that a 3 stage VTEC would give a civic si engine as much HP as 160. Is this true?


Vtec kicks in at higher rpms because you need to feed the engine more fuel in that higher range. If it kicked in at 1000, your engine would be choking with gas and oxygen making it not very efficient and not running very well.

Depending on how far the valve open depend on how well the A/F mixture will swirl as it drawn through the opening....

So for a low RPM engine a low opening valve will keep intake velocities high and help keep the fuel atomized and distributed more evenly inside the combustion chamber, thus equaling more powerfor less usage of fuel...


So are you saying that the higher the velocity, the better atomized the fuel is?


Vtec kicks in at higher rpms because you need to feed the engine more fuel in that higher range. If it kicked in at 1000, your engine would be choking with gas and oxygen making it not very efficient and not running very well.


Why would the engine be choking at 1000 RPMS when the valves are wide open?

spark ignition engines are designed to run rich during idle, lean during cruising, and rich during WOT.

i dont think SOHC can have 4 valves per cylinder.

Examples proving otherwise
99 Chevy Metro 1.3 SOHC 16 valves
95 Chevy Lumina 3.1 SOHC 24 valves
87 Porsche 944S 2.5 SOHC 16 valves
95 Dodge Neon 2.0 SOHC 16 valves
94 Mercury Sable 3.0 SOHC 24 valves

DOHC heads are more efficient cause they don't use long rocker arms to control the other set of valves. Less rotating mass = more HP. Most DOHC engines hydraulic lifters that directly engage the valve. IIRC

The last paragraph is IMO

yeah after i posted that i remember by buddy who had a dodge neone that was SOHC with 16 valves. . .

because the engine crank is rotating at a slower speed, and the air/fuel mixture let in is more than it should be for the engine speed. its designed to allow MORE fuel for HIGHER rpm.

Their are different types of VVT. The standard Honda VTEC system works with different sets of cams that allow you to change the timming and lift of the valves. Applying Cam Changing VVT (VTEC, VVL) allows the engine to breath better at higher rpm, resulting in a longer duration of work increasing your overall top end hp and rpm. This system doesn't allow a continuous degree of lift through the powerband, but there is a type of VVT that does. If you've ever read into a E46 BMW, they have a system called the "Double Vanos System". This is a type of "Cam Phasing" technique which varies the valve timming through shifting the phase angle of the camshaft. Essentially this means that it can allow an earlier/later time to open the valves. This in no way can change the duration and the lift that a Cam Changing VVT system can. What is achieved in this method is a constant improvement in torque and hp throughout the entire rpm band. However it lacks the gains in top end that the Cam Changing VVT can produce.

There are also combonations of VVT systems which incorporates both Cam Changing and Phasing. If you've heard of i-VTEC, thats one system. Also Variocam (Porsche), VVTL-i (Toyota).

DOHC heads are more efficient cause they don't use long rocker arms to control the other set of valves. Less rotating mass = more HP.


Is this the same concept as a lightened flywheel?


because the engine crank is rotating at a slower speed, and the air/fuel mixture let in is more than it should be for the engine speed. its designed to allow MORE fuel for HIGHER rpm.


If the valves were opened more, why would that affect the fuel/air mixture? How does that affect the computer? If the fuel/air mixture was more rich, how would it choke the engine? Especially since the bigger openings allow less restrictions on the intake and exhaust strokes.

Are you saying that if the RPMs are low, the ECU would allow a more rich mixture just because the valves open more?


Their are different types of VVT. The standard Honda VTEC system works with different sets of cams that allow you to change the timming and lift of the valves. Applying Cam Changing VVT (VTEC, VVL) allows the engine to breath better at higher rpm, resulting in a longer duration of work increasing your overall top end hp and rpm. This system doesn't allow a continuous degree of lift through the powerband, but there is a type of VVT that does. If you've ever read into a E46 BMW, they have a system called the "Double Vanos System". This is a type of "Cam Phasing" technique which varies the valve timming through shifting the phase angle of the camshaft. Essentially this means that it can allow an earlier/later time to open the valves. This in no way can change the duration and the lift that a Cam Changing VVT system can. What is achieved in this method is a constant improvement in torque and hp throughout the entire rpm band. However it lacks the gains in top end that the Cam Changing VVT can produce.


Sorry, I don't understand this, how does it relate to
the question "what's so bad about having the valves open more at low RPMs?"

Is this the same concept as a lightened flywheel?

Yup.

SOHC can have more than 2 valves per cylinder, in that case the use rockers which increase the friction loss and increase valvetrain mass.

DOHC is also sometimes used with two valves per cylinder even if it isn't that common.

Valve lift and timing affects cylinder filling, it hasn't anything to do with how the fuel vaporises.

As you probably know the intake valve opens before top dead center in the exhaust stroke and closes after bottom dead center in the compression stroke. The exhaust valve opens before bottom dead center in the combustion stroke, it closes after top dead center in the intake stroke.
The time the intake valves are open, measured in crankshafts degrees gives us the duration for the intake valves and on the same way for the exhaust valves the exhaust duration.
This together with lift will affect how the cylinder is filled. Lift for maximum flow is usually about 40% of the inner seat diameter.

So the more we can lift the valve and the longer the valves are open the more we can fill the cylinders and get more power. However, since the intake valves closes during the compression stroke the air must have a high velocity to overcome the increased pressure in the cylinder while the piston is moving up. If the velocity is to low the air will instread flow out from the cylinder back out in the intake and we will loose power. During the overlap period, the time both intake and exhaust valve is open we can use the scavenging effect to increase cylinder filling, this do however depend on the design of the exhaust manifold and the pressure difference over the engine. As I mentioned the exhaust valve opens during the combustion stroke, in an racing engine it can open as early as 90 degrees after top dead center (half the stroke), this is done to help the exhaust flow causing a better VE while the power lost from the combustion is low since the combustion is done and the pressure has dropped. This all causes those high power high lifting long duration cams that gives extra power at high engine speeds to reduce the power at low engine speeds, so at a lower engine speed it's actually the low lift short duration cams that gives the highest power.

......If the valves were opened more, why would that affect the fuel/air mixture? ...........How does that affect the computer? ...........




Read the thread on VTEC at the top of this forum.
It will explain almost everything you need to know, and if you read ALL of it, will answer most of your questions.

However, because I feel you won't because you started this thread in the first place, clearly with out paying any attention to the clearly marked stickies at the top of the forum, Ill explain something very simply for you now.

Honda uses TWO fuel and ignition maps in its VTEC engines. One for each set of cams, with a small amount of over lap to allow for the change over.

If you set the engine to run the high speed cam at two low an RPM the high speed fuel map will be unable to provide the fuel and ignition the engine needs, the low speed cam map will have the same problem. The usual result will be either an engine that simply won't run, or worse the engine will run lean which will cause some very expensive internal damage, usualy melted pistons as combustion tempratures get to high.

SOHC can have more than 2 valves per cylinder, in that case the use rockers which increase the friction loss and increase valvetrain mass.

Don't they use rocker arms anyways?


As you probably know the intake valve opens before top dead center in the exhaust stroke and closes after bottom dead center in the compression stroke. The exhaust valve opens before bottom dead center in the combustion stroke, it closes after top dead center in the intake stroke.
The time the intake valves are open, measured in crankshafts degrees gives us the duration for the intake valves and on the same way for the exhaust valves the exhaust duration.
This together with lift will affect how the cylinder is filled. Lift for maximum flow is usually about 40% of the inner seat diameter.

So the more we can lift the valve and the longer the valves are open the more we can fill the cylinders and get more power. However, since the intake valves closes during the compression stroke the air must have a high velocity to overcome the increased pressure in the cylinder while the piston is moving up. If the velocity is to low the air will instread flow out from the cylinder back out in the intake and we will loose power. During the overlap period, the time both intake and exhaust valve is open we can use the scavenging effect to increase cylinder filling, this do however depend on the design of the exhaust manifold and the pressure difference over the engine. As I mentioned the exhaust valve opens during the combustion stroke, in an racing engine it can open as early as 90 degrees after top dead center (half the stroke), this is done to help the exhaust flow causing a better VE while the power lost from the combustion is low since the combustion is done and the pressure has dropped. This all causes those high power high lifting long duration cams that gives extra power at high engine speeds to reduce the power at low engine speeds, so at a lower engine speed it's actually the low lift short duration cams that gives the highest power.

I see, so low lift cams create more velocity in the intake stroke so that the air will not fill instantly and escape out the intake valve on the compression stroke. And on the exhaust, it creates more velocity and creates that scavenging effect that lets the exhaust constantly flow out with little chance of coming back in because of the velocity.

Read the thread on VTEC at the top of this forum.
It will explain almost everything you need to know, and if you read ALL of it, will answer most of your questions.

However, because I feel you won't because you started this thread in the first place, clearly with out paying any attention to the clearly marked stickies at the top of the forum, Ill explain something very simply for you now.

Honda uses TWO fuel and ignition maps in its VTEC engines. One for each set of cams, with a small amount of over lap to allow for the change over.

If you set the engine to run the high speed cam at two low an RPM the high speed fuel map will be unable to provide the fuel and ignition the engine needs, the low speed cam map will have the same problem. The usual result will be either an engine that simply won't run, or worse the engine will run lean which will cause some very expensive internal damage, usualy melted pistons as combustion tempratures get to high.

Didn't really understand the thread... So I made one thread that I can post whore on by asking loads and loads of questions till I actually learn something. Isn't learning what the point of this forum is? :D

Don't they use rocker arms anyways?



I see, so low lift cams create more velocity in the intake stroke so that the air will not fill instantly and escape out the intake valve on the compression stroke. And on the exhaust, it creates more velocity and creates that scavenging effect that lets the exhaust constantly flow out with little chance of coming back in because of the velocity.
No, usually cam acts directly on the inverted bucket or finger follower.

The exact explaination is quite complex that involves pressure pulses, velocities, pressures and so on. However, note that the exhaust pulse always travels with the speed of sound, like all pulses. The flow do instead travel with speeds up to the speed of sound. A low lift cam can't really solve the problems related to a low flow velocity but they can make the situation better. This is also why when you port a cylinderhead that bigger isn't always better.

Didn't really understand the thread... ....


No worries.
It is rather long and and can get rather technical.

But try reading the thread 2 or 3 times, and read with the information you have from this thread, and try the search feature to answer some of your questions as well. So far most of what you have asked has already been gone over in some detail in other threads. :sunglasse

Here's the Rep simple guide...

VTEC is the proprietary for Honda's version of variable valve and lift system. In the NSX, the VTEC can alter valve timing (ie when the cams open the valves) and can alter the amount of lift (ie how far the valves open) on the exhaust and inlet cams independently... The most basic VTEC alters valve timing on the inlet side only.

Using a DOHC vs SOHC (or for that matter a pushrod OHV)... Twin cams have been around for a long long time. They allow an angle between the valves and enable the valve openings to be better spaced at the top of the combustion chamber. This can result in bigger valves (which breathe better at higher rpm) and swirl characteristics. Also if variable timing is involved some systems allow independent variation on the timing of the inlet vs exhaust cams (which allows variation on overlap).

However, DOHC has it's downsides...
It's complex, expensive, has higher frictional losses and requires more maintenance... a snapped cambelt in a DOHC usually results in valves slamming into the top of the piston... And for various reasons DOHC engines perform better higher up the rev range and can usually offer only marginal benefit at low rpm compared to SOHC/OHV because torque tends to be more related to cubic displacementLike most things, engines are a compromise - having better breathing characteristics at high RPM yields better overall power as the torque remains at similar levels with the increased engine speed... on the downside, this usually means reduced torque at low engine speeds. The most extreme examples include F1 engines that can do nearly 20,000 rpm but have idle speeds around 5,000 and produce power in a very narrow band.

On the other hand, a large displacement SOHC or pushrod OHV engine can produce torque down low to medium engine speeds where most of us drive our engines eg the LS1 Chevy V8 is a alloy pushrod OHV which produces prodigous amounts of torque.

Variable valve timing or lift allows the engine designers (along with other tricks like variable length inlet manifolds, twin intake runners off the plenum, porting etc) to provide a greater range of usability... sometimes it works (The Accord EURO has a 2.4 NA DOHC producing a healthy 140kw standard with 90% of torque available at 2000 rpm... and sometimes it doesn't eg S2000 which doesn't do much until 7,000 rpm and then arrives with a bang like the old turbos did...

Here's the Rep simple guide...

VTEC is the proprietary for Honda's version of variable valve and lift system. In the NSX, the VTEC can alter valve timing (ie when the cams open the valves) and can alter the amount of lift (ie how far the valves open) on the exhaust and inlet cams independently... The most basic VTEC alters valve timing on the inlet side only.

Using a DOHC vs SOHC (or for that matter a pushrod OHV)... Twin cams have been around for a long long time. They allow an angle between the valves and enable the valve openings to be better spaced at the top of the combustion chamber. This can result in bigger valves (which breathe better at higher rpm) and swirl characteristics. Also if variable timing is involved some systems allow independent variation on the timing of the inlet vs exhaust cams (which allows variation on overlap).

However, DOHC has it's downsides...
It's complex, expensive, has higher frictional losses and requires more maintenance... a snapped cambelt in a DOHC usually results in valves slamming into the top of the piston... And for various reasons DOHC engines perform better higher up the rev range and can usually offer only marginal benefit at low rpm compared to SOHC/OHV because torque tends to be more related to cubic displacementLike most things, engines are a compromise - having better breathing characteristics at high RPM yields better overall power as the torque remains at similar levels with the increased engine speed... on the downside, this usually means reduced torque at low engine speeds. The most extreme examples include F1 engines that can do nearly 20,000 rpm but have idle speeds around 5,000 and produce power in a very narrow band.

On the other hand, a large displacement SOHC or pushrod OHV engine can produce torque down low to medium engine speeds where most of us drive our engines eg the LS1 Chevy V8 is a alloy pushrod OHV which produces prodigous amounts of torque.

Variable valve timing or lift allows the engine designers (along with other tricks like variable length inlet manifolds, twin intake runners off the plenum, porting etc) to provide a greater range of usability... sometimes it works (The Accord EURO has a 2.4 NA DOHC producing a healthy 140kw standard with 90% of torque available at 2000 rpm... and sometimes it doesn't eg S2000 which doesn't do much until 7,000 rpm and then arrives with a bang like the old turbos did...

Actually DOHC are less complex (the cam acts direct on the bucket to the valve) and cause in fact less friction since it doesn't need any rockers and the lower interia means lower spring rates and the springs is what causes most of the friction in a valvetrain system. DOHC also improve the power over the whole powerband, this with low exhaust emissions. Furthermore, the system doesn't need anymore maintenance than any other system and when it's needed everything is placed in the head which means that it's easy to work with. What usualyl cause problems is the hydraulic lifters that most manufacturers use, but they aren't more different than other hydraulic lifters, today they have also begun to go back to mechanical systems which today need almost no maintenance.

Valves slamming into the piston is basicly a clearence problem, with chains or gears it's not a problem because they don't break without any warning signs. A belt will hold for a long time today and if they are replaced like they should they won't give any problems.

Actually DOHC are less complex (the cam acts direct on the bucket to the valve) ........


Honda's DOHC VTEC engines use rockers, 3 per valve with a lost motion device on the middle rocker. ;)

Honda's DOHC VTEC engines use rockers, 3 per valve with a lost motion device on the middle rocker. ;)
On the images I've seen it looks like they use finger followers; they should not be confused with rockers.

On the images I've seen it looks like they use finger followers; they should not be confused with rockers.



:eek7:

Please explain the differnce? You just enlightened me yet again.

I really would like an explanation of the difference as well but until then I'm going to answer this question with a little less of a technical explanation.If the valves were opened more, why would that affect the fuel/air mixture?......... If the fuel/air mixture was more rich, how would it choke the engine? Especially since the bigger openings allow less restrictions on the intake and exhaust strokes.From this quote I'm going to assume that you don't understand the meaning of rich and lean and that would explain why you don't understand how you can choke the engine. If you open the inlets wider and allow more gas and air than the engine is going to be able to use then you will end up exspelling fuel into your exaust (thus wasted fuel). There is such a thing as too much fuel. You can reach a point that there is so much fuel and not enough air in the chamber that you won't create an explosion. This is how you can "choke your engine" (also know as flooding). By controlling the amount of fuel and air that is allowed into the chamber and at what RMPs, you can create a balance that will allow you efficently use your fuel.

The technical aspect of this comes when you take into account that your engine has to use some of the power created by the explosion to expell what is left in the chamber. The more fuel that you put in the chamber and don't use is hp lost because you have more to expell. This is also why changing the diameter of your exaust pipes can increase HP. The less resistance to pushing the expended fuel and air out of the chamber, the less power required to move it. The less power you use to exspell your exaust, the more power that goes to your wheels (or the many other aspects of your engine that require part of that power).

Rich means that the engine is recieving more fuel than can be used while lean is just the opposite (not enough fuel).