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Old 06-07-2006, 02:34 PM   #1
 
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Default Explanation of VTC

There is a VERY good thread going on on k20a.org about how VTC works. IMO there is a lot of people that just dont know how it works, why it works, and what exactly it does. the following resposne was from CHUNKY, and it made ALOT of things clear about our new K series engines.

Just thought it was usefull info that was needed to be cross posted. thought you guys might find this interesting, and maybe start a discussion on this board.

Quote:
Originally Posted by CHUNKY
Cam phase is a tricky topic. For a fixed cam gear, the "degree" of a cam relates a reference point on the cam to a reference point on the crankshaft. With VTC, things get sticky. A chosen reference point on the cam is matched to a reference point on the cam gear which is matched to a reference point on the crank. The VTC system then allows you to vary the correlation of these reference points on the fly.

So what does "zero degrees" really mean? Not much for a fixed cam gear, and it means even less for a VTC system. The zero degrees that everyone knows is based on the manufacturer chosen location of the dowel pin or keyway on the cam. It means that when installed according to mfg specs, the dowel pin will point up when the number one cylinder is at TDC. That's it. Nothing else. It tells you nothing specific about where the valve begins to open, where peak lift occurs, or where the valve closes. Now keep in mind that when building fixed cam gear engines, companies like Honda would set the dowel pin such that the cam is oriented to the crank in a way that makes peak power. An aftermarket cam would need to be oriented to the crank in a different way in order to make peak power, thus the concept of degreeing cams enters.

It's ALL relative. That "zero" means nothing in an absolute sense. There is an optimum relative orientation between the cam and crank. That is, you want to match a cam reference point with a particular crankshaft degree on the combustion stroke for the intake and on the exhaust stroke for the exhaust. The industry standard cam reference point is the centerline, or the point where peak lift occurs. What does the centerline have to do with the zero degree marking? Nothing. You can specify the centerline as being "XXX degrees after zero" assuming that the dowel pin is zero. So the goal is to match the lobe centerline with a specific crank angle.

Now because the k20 motors have VTC on the intake, when the cam is engineered, they have to determine how to set the dowel pin such that the VTC is useful. Typically they will set the VTC such that the lobe centerline can be varied about the optimum crank angle. If the VTC system allows 50degrees of phase adjustment relative to the crank, they will set the dowel pin such that the lobe centerline can be varied +/- 25 crank degrees about the optimum crank angle. Honda decided to use a 0 to 50 scale instead of a -25 to 25 deg scale. This also means that Honda most likely used 30deg as the reference point for where the lobe centerline would correlate with the optimum crank angle given a stock setup. This is confirmed by the fact that peak power almost always occurs at 30deg with stock cams. The reasoning for this is presumably because Honda wanted to minimize the interpolation that would result from using a number like 25 degrees. Assuming that 30 deg is where the lobe centerline occurs, that means when k-pro shows zero deg advance, the lobe centerline occurs 30 degrees to late. When k-pro shows 30 degrees advance, the lobe centerline is optimized. I hate to anaolgize this notion, but people seem to have a better time understanding that 30deg advance orients the cam and crank in the way that zero deg would orient the cam and crank on a fixed cam gear motor. That is, on a stock k20a2, 30deg advance is analagous to 0deg, 0deg is analgous to -30deg, and 50deg is analgous to +20deg. So the range of adjustment is -30deg to +20deg.

It's much simpler to gain an understanding of how changing cam phase affects the correlation between lobe centerline and crank angle using a fixed cam gear.

Here is an example.

Assume that the lobe centerline is at 110 deg from the dowel pin on the cam. Assume also that you want the lobe centerline to correlate to 170deg from TDC.

When you model the movement of the cam and crank, you will see that after 170 degrees of crank rotation, the cam will have rotated only 85 degrees (cam moves at 1/2 crank speed). This means that the lobe centerline will occur 25deg (cam) or 12.5deg (crank) after the "optimum" point. That means you want to adjust the cam phase such that the centerline is shifted 25deg advance. So then you model cam/crank movement again. You find that at TDC, the dowel pin is shifted forward 25deg. When the crank has moved 170deg, the cam will still have moved 85deg. However, because of the 25deg advance imparted by the cam gear, the cam will be oriented such that the point on the lobe 85+25deg from dowel pin, or 110deg from dowel pin will be correlated to the crank angle of 170deg. Thus the cam has been degreed

So how do you know at what crank angle the lobe centerline should occur? This orientation is determined via experience for most people. Some companies, like IPS, have done us the favor of engineering their cams such that they can be installed using the stock markings to produce optimized correlation between the lobe centerline and crank angle for common builds. That is to say, if they engineered their cams for k20a2 motors with an AEM intake, PRC manifold, Comptech race header with no exhaust, then customers with similar setups will find that the lobe centerline is already optimized relative to crank angle. I suspect that most companies select a certain type of engine build, and then engineer the cam centerline around that. So if the company engineered the cam for a high compression, all out engine, the degree correlation will be quite different when used on a stock compression motor. From what my contacts tell me, the Skunk2 cams are engineered for such builds. If used on a motor without a lot of compression, you stand to gain a decent amount of HP from degreeing the exhaust because it is not adjustable via VTC. So cams from different companies will require different degree settings for the intake and exhaust.

So where does VTC fit in? Well VTC allows the intake cam centerline to be optimized for a range of situations, not just peak power. With a fixed cam gear, you can only optimize the centerline for one situation, usually peak power. You can make the exact same peak power with a fixed cam gear as you can with VTC. VTC just allows you to have a broader powerband. That's it.
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Old 06-08-2006, 08:51 AM   #2
 
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Default Re: Explanation of VTC (VtecKiDD)

i guess no one found it as interesting as me lol
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Old 06-08-2006, 08:55 AM   #3
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Default Re: Explanation of VTC (VtecKiDD)

i read the same thing, but what the number 50, 40, 30, 20, etc. in the map represents im still trying to picture it in my head. I can understand the position of the intake cam in relation to crank position. A 3D diagram would be nice to show what happens to the intake cam when you change from 30 to 20 and so on. Im surprised not one Kseries owner didn't explain what it means. I guess tuning isn't important for most B and K owners...they just want dyno charts so they can copy...sad.

Im curious to find out. Click the image to open in full size.
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