What is VTEC and how does it work?

[crispy]

I humbly submit this "softball"...

I understand that it has something to do with variable cam timing, but how exactly is that useful and is it used in racing engines...?

[CamKing]

Originally posted by crispy:
<STRONG>I humbly submit this "softball"...

I understand that it has something to do with variable cam timing, but how exactly is that useful and is it used in racing engines...?</STRONG>

The VTEC system is not really a variable cam timing system. The VTEC has two different cam profiles and a rocker-arm system that that follows the smaller profile until a given RPM, then it follows the larger profile. Running with the smaller profile at lower RPM's allows for better milage and lower emissions, while the larger profile will give you more HP at the higher RPMs.

This system could be useful in racing, but most people that race the VTEC lock the system so it is always running on the bigger cam.

[jl]

camking pretty much hit it on the head.

Honda's VTEC system should be called a DTEC, dual timing engine control.

The racers that use it run only the high speed cam becuase they are always running at high speed, thus no need for the lower speed cam.

The whole system runs off of RPM's and a VTEC solnoid and engine oil pressure. The ecu has a set point of 4000rpms usually. When the engine hits 4000rpm's the solenoid kicks on and opens a small oil port that moves a small pin IIRC and engages the high speed cam.

Once you look at it it is actually a relatively simple system.

The great thing about Honda's and Integra's from 92-01 is that they are all the same platform. Thus ANY engine from any of these years will fit in any other car.

I have a 2000si engine (1.6 b16a2) in a 93 civic cx. 160hp and 2100lbs. Total cost ~$5,000.

how is it useful? To run well at all engine rpm's an engine's cam is actually a compromise. Ever seen a muscle car at a light just loping along? They have a big cam to get hp at the higher rpm's. The cam isn't the best for low speed driving.

A vtec system let's you get around this.

[ October 11, 2002: Message edited by: jl ]

[CamKing]

Originally posted by jl:
<STRONG>When the engine hits 4000rpm's the solenoid kicks on and opens a small oil port that moves a small pin IIRC and engages the high speed cam.

</STRONG>

What I've been wanting to do is design both profiles for racing, and then set the solinoid to an RPM that matches the cam.

Say you were going to make a 9,000rpm engine.
The larger cam profile would be designed for peak power at 8,600rpm. It would make peak torque at 6,000rpm. I would design the smaller cam profile for peak power at6,000rpm. It would make peak torque at 4,100rpm. I would set the solenoid at 6,000rpm. compared to just running the larger profile, this engine would make more power from 4,000-6,000rpm, and the same power above 6,000rpm. This would be a big advantage on road courses.

[guido]

CamKing there are several cams out their now that do that, with crossover points at around 7000-7300 rpm.The racing cams that do not use VETEC just grind the two outer lobes as regular DOHC's and get rid of the extra weight of the center rocker and lost motion valve. The problem with high revs on the 93-01 engines is a poor valve angle which limits overlap because of valve to valve interfearance.If you run large cams ( 250-260 IN @.050 & 240-250 ex @.50 ) and put the in. @ 100 centerline the ex. will have to be 107-108 to avoid hitting.No one has done an inverse type lobe yet (that I know of)

[CCFirebird]

guido said:
If you run large cams ( 250-260 IN @.050 & 240-250 ex @.50 ) and put the in. @ 100 centerline the ex. will have to be 107-108 to avoid hitting.No one has done an inverse type lobe yet (that I know of)

Just exactly what do those numbers mean? Rotational degrees? What does the spread (250-260) signify?

[CamKing]

Originally posted by CCFirebird:
<STRONG>guido said:
If you run large cams ( 250-260 IN @.050 & 240-250 ex @.50 ) and put the in. @ 100 centerline the ex. will have to be 107-108 to avoid hitting.No one has done an inverse type lobe yet (that I know of)

Just exactly what do those numbers mean? Rotational degrees? What does the spread (250-260) signify?</STRONG>

(250-260 IN @.050 & 240-250 ex @.50)
Represents the duration in crank degrees between .050"-cam lift on the opening side of the cam to .050"-cam lift on the closing side.

"centerline" is how many degrees the center of the intake or exhaust cam is from TDC.

[CCFirebird]

Thanks for the clarification, CamKing.

So if I understand this right, the total time the valve is open is about 10 rotational degrees of the crankshaft?

Why is 0.050" used as a reference?

[guido]

The .050 lift number is one that was set by US cam builders to actually give you an idea of how aggressive the cam grind is. The Japanese use .040 or 1 mm. You could have 2 cams that advertised 300 degrees duration total but one could have a more aggressive lift rate( open faster and close faster) and improve performance.the duration number (300 degrees) is the number of crank degrees the valve is open On a 4 stroke engine the whole intake-exhaust-compression cycle is 720 degrees so the valve is still closed for over half the cycle. Cams turn at half crank speed so what you see on a cam lobe is doubled at crank rotation. I hope I havn't muddled this up too much, its easier to it do than explain it.

[CCFirebird]

Thanks guido. Obviously, this is going to take some sustained thought on my part to understand what you just said. The aggressive lift rate is not too tough to understand, but 720 degrees, cam rotates half speed...

I appreciate the opportunity to learn more about the technical side of things. My work takes me into just about every kind of manufacturer there is. Although I've seen crankshaft grinding, I don't think I've ever seen a cam grinding operation.

[jl]

720 deg = 2 turns of the crankshaft.

cams run at half the speed of the crank (2:1 ratio)

Every turn of the crankshaft results in half a turn of the camshaft.

[happyscrappy]

I'm pretty sure the cam is inside the engine, and spins...like a whirley bird...pretty...birdy....