Difference between DOHC Vtec and SOHC i-vtec

+ rep for you aaronng

Tell me more about the jazz vtec. Is it even i-vtec? It just says vtec on the engine cover. I know its offtopic but want to know.

+ 1 rep point!

LOL, typing this while watching soccer.

Jazz 1.5L VTEC is not an i-VTEC because it doesn't vary the intake cam timing. Instead, it is a 12/16v VTEC system. Below 3500rpm, one of the intake valves is closed so that the intake comes in from one side and the shape of the piston head directs the charge so that it swirls around in the head for complete burning. So you get good torque at low rpm. Above 3500rpm, 1 intake valve is not enough to fill the cylinder quick enough, so both valves are opened by VTEC. So effectively, you get double the intake area above 3500rpm. So when you look at dynos of the torque chart, the torque is higher as your RPMs increases. Opposite to that of economy engines where the torque is high at the middle rpms and starts to taper down as RPM increases. The engine also has molybdenum coating on the piston skirts (it was the first non-performance honda engine to get it), while the intake manifold is divided into 2 so that the 2 farthest cylinders still get enough air without being blocked by the closest 2 cylinders.

What else... hmmm, I think the Jazz VTEC also has stainless steel headers! Someone check underneath their car to confirm this! It should be nice and shiny. Opposite to my Accord euro with steel headers that are dull and brown.

This thread makes me wish that Honda would differentiate between the different VTecs more explicitly.

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Originally Posted by JaCe

This thread makes me wish that Honda would differentiate between the different VTecs more explicitly.

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doesn't matter, we have aaronng

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Originally Posted by aaronng

SOHC VTEC = Timing and lift change in 1 step. Usually used to maximise low end torque. Because 1 cam controls both intake and exhaust valves, you cannot get good timing for high RPM power.

DOHC VTEC = Same as above, but because intake and exhaust are on separate cams, you can have very wild cams timings, duration and lift for high-cam.

VTEC-E = VTEC is used to close 1 intake valve at low RPM. So it operates at in 12 valve mode in low RPM for torque and 16 valve mode at middle to high RPM. Intake cam has 2 lobes but 3 followers. The extra follower is for the closed intake cam.

3-stage VTEC-E = Same as above, but in addition to 12v and 16v, there is a a 3rd stage where the large middle cam love for intake is used. So you have more power.

SOHC VTEC (Jazz): has 12v and 16v mode like VTEC-E.

i-VTEC (DOHC low performance, CRV, Accord): VTEC only on the intake cams, and also VTC on intake.

i-VTEC (DOHC high performance, DC5R, DC5S, Accord Euro): VTEC on intake and exhaust cams, and VTC on the intake cam.

i-VTEC SOHC (new, 2006 Civic): At low loads, the intake valve closure is delayed well into the compression stage so that part of the intake goes back out the intake valves. This reduces pumping loses for low load situation. At high load or heavy throttle, the intake valve closure becomes normal using VTEC, so it works like a normal engine. Those who know their engine types will notice that this is a Miller Cycle-type engine when the intake valve closure is delayed. Plus, VTEC can be activated at ANY RPM! A hydraulic pump increases the pressure of the engine oil at low RPM so that there is sufficient pressure to use the VTEC system. Oh, and VTC on the intake cam.

And yes.... I wrote this all from memory. :p

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So all in all, what's better and more powerful SOHC or DOHC??

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Originally Posted by Honda Enthusiast

So all in all, what's better and more powerful SOHC or DOHC??

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read again
DOHC is better

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Originally Posted by [[d a n n y]]

read again
DOHC is better

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Yeah, but Aarong said SOHC does everythign in 1 step?

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Originally Posted by Honda Enthusiast

Yeah, but Aarong said SOHC does everythign in 1 step?

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For power, the DOHC Honda engines have more aggressive cams and thus makes more power per litre. The SOHC honda engine is designed for fuel economy.

Sorry for my ignorance on this subject as I am still learning about these concepts, however, the Atkinson cycle is probably a more appropriate name for what Honda is trying to do with the i-VTEC in the 06 Civic.

The main difference between Miller cycle and Atkinson cycle is that Miller cycle always use supercharger to compress the charge. The Atkinson cycle is a Miller cycle without the supercharger, so it provides efficiency at the expense of power.

In the Miller cycle engine, a supercharger 'over-compress' the fuel-air mixture. When the piston moves upward to compress this charge, the valve is allowed to open for longer than usual, so some of the charge is pushed back out into the intake manifold. This would normally result in a loss of power (as in Atkinson engine) but in a Miller engine, the charge is already 'overfed', hence the net benefit is that less energy is required to move the piston during compression stroke, while still maintaining the same end point compression ratio, and the same power during ignition/power stroke.

The Miller cycle engine is advantageous as long as less energy is used by the supercharger to compress the fuel-air charge than what the piston would use during compression stroke. This is where VTEC is can be used to increase the valve timing when the advantages are present.

I know some part of what I said contradict aaronng's posts, I could be wrong conceptually and I don't know enough about Civic engine to comment on specifics. I am open to suggestions...

^^ You're correct. A miller cycle engine without a forced induction to bring up the filling efficiency is called an atkinson cycle engine.

Why did I refer to the miller cycle instead? Because I was trying to illustrate that where Mazda used a supercharger to overcome the drawbacks of the atkinson cycle, Honda instead used i-VTEC to do the same.

R18's SOHC i-VTEC allows the same effect of the miller cycle but without the fuel consumption drawbacks of forced induction. It allows the engine to run in the (less than 1.8L) atkinson cycle while cruising and back to the 1.8L otto cycle when under acceleration.

Thanks for the explanation, aaronng. :)