[beyondstockcivic]

I hate to start new threads knowing there is some information SOMEWHERE on this site but I can't find it. I'm currenty in the process of building my K20a2 motor and I'm trying to research on heads.... My original plan was the strap my built K20a2 block with a stock K20a2 head and a GT35R and call it a day. I've been reading; however, about this VTC locking and "Modding" deal and I'm lost... I understand the variable timing control advances and retards valve timing.... But I need more information and I'm hoping you all can teach me a little something on this subject of give me a link...

 

[nikos]

Stock type s VTC gears can go to 50 degrees

Stock TSX VTC gears can go to 25... but can me machined to 45 or even 50... 45 is choosen because of possible valve to piston contact on a tsx block

Locking the VTC has to do when you are using really big cams or high compression pistons with big cams... but since you are going to be using a low compression piston, it would not be much of an issue..

Normally, the check clearances, you can lock the VTC at 50 degrees and also lock the rocker in vtec mode, and check to see while turning the motor if the valves will hit... This is short of an easy way to check clearances... without getting the head out... If you really want to clay the head and find out exactly how close the valves come to the pistons, you have to remove the head and repeat the same steps using clay etc..

 

[Hybrid_hatch]

my question is, where does honda get 50 degrees from? from what I can tell the car starts at 10 degrees retard on the intake cam (or if fail safe kicks in it goes to 10 degrees retard on the intake cam) and at what honda calls a full 50 degrees is really 2 degrees advanced. I used a skunk2 and buddy club intake cam gear to spec this out and with both gears locked in a vice (outter teeth part) and a shaft running thru both to turn both inners at the same rate it came out to 10 retard 2 advance with the stock 50 degree gear being maxed both ways..

 

[signalpuke]

11-35 in the FSM.
50* comes from the difference in intake/exhaust overlap, measured in crank degrees, between fully retarded and fully advanced.

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.

 

[beyondstockcivic]

VTC is controlled by the ECU correct? What is the primary sensors that input signals to the ECU causing it to retard or advance intake timming? And in the end what is the ultimate advantage of the VTC? Why would one want to "lock" it to the extreme 50 degree?

 

[signalpuke]

Correct. Here are a couple pages from the FSM.

 

[beyondstockcivic]

So the VTC is actually a hydraulic device that is controlled by oil pressure from a VTC soleniod? I see the CMP sensor relays the ECM as to where the Camshaft is at in its rotation but is the ECM calibrated to advance/retard the system on an RPM level or by Air/Fuel ratio or what?

If anyone has a link as to more information I'm willing to read everything I can get my hands on

 

[signalpuke]

RPM and TPS (Load), here is a good picture of various cam angle maps, I got it from pencomputing.com.

The low speed cam lobes (out of VTEC)

The high speed cam lobes (VTEC)

This is a pretty good explanation from Chunky` of what the values represent:
http://www.k20a.org/forum/showthread.php?t=14642
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[signalpuke]

The links above can also be found in the FAQ/DIY section that is linked in my sig. If you find more information that you find useful elsewhere let me know, I will add it to the section. We are all here to learn, and I seem to pick up something new or forgotten when I re-read this stuff. If you have any other questions, or would like more info from the FSM just let me know, I will try to assist as best I can:up:

 

[beyondstockcivic]

Well I understand How the VTC represents 0 is actually -30 degree on the maximum lift of the cam lobe and 50 is +20 on the Maximum lift of the cam...

What are the advantages of being able to retard and advance the camshafts?

Why would you want such an overlap of intake and exhaust cam on a +20 Cam degreeing.

Would the maximum cam lobe lift on intake stroke need to be MID way through stroke therefore at BDC the intake would be shut?

 

[beyondstockcivic]

Also your charts from K-pro show cam degreeing compared to RPM and TPS (Load) what would load be?

Why is the graph showing different degreeing matrix and the color orientation is different?

 

[signalpuke]

The top picture is the low speed cam lobe, and the bottom picture is the high speed cam lobe.

The difficult part in explaining VTC is that it is always labled as degrees. It is a measurement that is relative to itself.
Remember, it is relative to TDC as well. Increasing/decreasing the overlap changes the when the intake valves will open and close. This will change the amount of unburned air/fuel that is in the cylinder.

At 0* the cam is fully retarded, the overlap period is at a minimum. The intake valve is opening closer to TDC on the exhaust stroke, perhaps even after. They are also closing later in the compression stroke, lowering the static compression. This is why we can run higher static compression pistons.
At 50* the cam angle is fully advanced, the overlap period is at a maximum. The intake valve is opening further from TDC, sooner on the exhaust stroke, and is creating a scavenging affect. This aids breathing at high RPM

 

[signalpuke]

Seperated the low speed and high speed cam tables.

 

[signalpuke]

Also your charts from K-pro show cam degreeing compared to RPM and TPS (Load) what would load be?

Why is the graph showing different degreeing matrix and the color orientation is different?

The above post shows why the degree matrix and color orintation are different. Different cam lobes.
Load is measured in the ECU. Load is increased when you accelerate. As you increase air into the engine, more fuel will be added, and the combustion pressure will increase. At low to medium load VTC helps EGR. Exhaust gasses are "frozen" and add a volume that cannot be burned. That is to say, they help stabilize load. Keep in mind that the exhaust gasses are hotter than the fresh mixture entering the engine.

 

[JasonMann]

Well I understand How the VTC represents 0 is actually -30 degree on the maximum lift of the cam lobe and 50 is +20 on the Maximum lift of the cam...

What are the advantages of being able to retard and advance the camshafts?

Why would you want such an overlap of intake and exhaust cam on a +20 Cam degreeing.

Would the maximum cam lobe lift on intake stroke need to be MID way through stroke therefore at BDC the intake would be shut?

More power and better gas mileage is what Honda was looking to do. Oh, and low emissions too.

 

[beyondstockcivic]

So I began thinkinging, If at fully advanced there is a LARGER overlap in intake valve and exhaust valve opening then at advance the intake opens earlier in the A stroke. I say this because you have Intake, Compression, Combustion, And THEN the exhaust valves ARE opened on exhaust stroke. So I was thinking that the only way an increase in valve overlap would be intake valves opening sooner or closer the DTDC.

 

[signalpuke]

TDC/TC-Top center
BDC/BC-Bottom center

The Exhaust valve will be opening towards the end of the combustion (power) stroke, this is to help "blow down". When the fuel burns it creates a lot of pressure in the cylinder, but all of this can only pass through the small exhaust ports, so the valves open early. So, the piston is going down, it has passed the half-way mark and is nearing BC, exhaust valves are opening, high pressure that has built from burning fuel is now rushing out of the ports and into the header due to the difference in pressure; high in cylinder, low behind exhaust ports/header. This is blow down.

By the time the piston reaches BDC the exhaust valve is opened up, the pressure in the exhaust and the cylinder are slowly neutralizing, the mass of the exhaust leaving has momentum. This momentum can help the piston back up (in an ideal scenario it will, and there will be no work from the crank needed to fully evacuate the cylinder of burned fuel) but this (exhaust stroke) is usually the one that causes broken parts.
If the piston is being "pulled" up then it will not have the cushion that the gasses above it provide. It will accelerate faster, and the rod can and does stretch, and at best you will have piston to valve contact, at worst you will have rod bolt failure.

So, we don't want that, we want a lil cushion. The exhaust stroke will continue as the piston rises, the exhaust valves will stay open. The piston will "push" some of the burned gas out.

Now, look at the picture from the FSM that I posted up. 12 o'clock is TC, 6 o'clock is BC. At 3 and 9 we have max piston acceleration (not shown, but food for thought).
You will see that the exhaust cam timing is depicted by the inner arrow (4 o'clock, before BC on the power stroke; to about 1 o'clock after TC on the intake stroke)
The two other arrows are the intake cam timing, at the top of the circle you will see where these overlap with the exhaust cam timing. The more overlap, the higher the VTC number. You can see that IN1 (about 1 o'clock) starts on the intake stroke, after TC, and it overlaps little with the exhaust cam. IN2 starts much earlier, infact it starts during the exhaust stroke.

Now we see that at full retard the cam overlap is minimal, the intake valve is opening after the piston reaches TC and starts it's descent on the intake stroke. But look at where it closes...well after the piston is traveling up on its compression stroke. Compression is a measure of the ratio of total volume (volume of space in the cylinder: the sum of the swept volume displaced by the piston travel and the cumbustion chamber volume in the head) and the swept volume (volume displaced by the piston travel). Where the intake valves close in relation to the piston travel decides the total volume. Notice the volume of the cumbustion chamber in the head stays constant. The swept volume is also smaller now. This lowers the static compression ratio.

When the intake cam is advanced the intake valve is opening sooner, before TC on the exhaust stroke. The exhaust valves are still open at this point, the cam overlap is maximized. At higher piston speeds there is less time for the high pressure burned gas (which has mass and momentum) to evacuate the cylinder. VTEC allows the exhaust valve to open for more time, helping the exhaust flow faster out of the cylinder. By opening the intake valves towards the end of the exhaust stroke it helps scavenging, the exhaust is exiting on one side and creates a sucking force on the intake side due to the pressure difference. This also helps clear, or "push", the burned gas out of the cylinder. This all happens very near TC on the exhaust stroke.

As the piston reaches TC and starts to travel down on the intake stroke the exhaust valves close. The burned gas has been evacuated and the intake air now has momentum from the overlap. This helps to continue filling the cylinder as the piston moves down from TC to BC on the intake stroke. Because cam timing is finite and we started earlier, it will end earlier as well. The intake valves will now close sooner relative to the crank degrees. In the picture you will see this. The IN2 arrow ends shortly after BC on the compression stroke. Why dosent it end right at BC? Remember, the air has momentum. Also, piston travel is minimal near TC and BC. By closing the intake valve earlier (closer to BC) we are creating more total volume in the cylinder by increasing the swept volume. Because the head volume remains constant we are increasing the compression ratio.

This is a VERY simplified explanation. There are many more things going on here, and temperature, atmospheric pressure, static compression, A/F ratio, cam specs, piston speed, and stroke play a very large role in all of these events. Remember, RPM is the time it takes the crank to complete ONE rotation.

 

[signalpuke]

Here is a pretty general diagram showing the pressure differences within the cylinder during operation.
Note: Pressure and load are related. Because pressure is a result of the dynamic compression, ignition timing, fresh charge air/fuel (ratio and volume), any residual gas (burned gas that is not evacuated by blowdown and overlap) and EGR (if present) low peak pressure in a healthy cylinder is throttled low RPM (idle) and high peak pressure is unthrottled high RPM (acceleration under wide open throttle)
http://www.maintenanceresources.com/referencelibrary/ezine/enginebalance.htm

 

[btf]

i can't find an answer for this anywhere, and i don't have the resources nor the expertise to clay the engine... are there p2v clearance issues running the 50 VTC with a 04 k24a2 block with a k20a head with an itr intake cam and the 04 tsx cam?

 

[signalpuke]

It has more to do with the valve relief depth of the OEM pistons. Aftermarket pistons tend to have a deeper valve relief than the OEM, but this is not always the case. The best way to know is to clay the motor.
http://k20a.org/forum/showthread.php?t=31571
http://k20a.org/forum/showthread.php?t=75424&page=2
These threads go into a brief explanation of the procedures for checking clearances. Use clay, not putty. :up: