Thread: Let's Talk Valve Timing

The QR25 has great potential with in it that is waiting to be unlocked. If nobody thought so then none of us would be here and there would be no B15U.com. Team RTR, 2JR, and FI-R have proven that this engine is capable of producing amazing numbers. Many of us feel like it should be producing more power than it is especially for being a 2.5L.

With that being said i would like to take a look and have a dicussion about valve timing and cam profiles



First. Why does the Intake close 64* after BDC going into the compression stroke? I feel that this is a huge gap between BDC and TDC where that valve is open causing significant compression loss. This Air/Fuel mixture that was just sucked into the cylinder during the intake stroke is now being forced back into the intake manifold leaving 118* of the stroke for compression. Now I'm not sure if there is a reason why Nissan decided to do this but it seems excessive IMO.

Second, I would like to take about Valve Overlap. From my understanding Valve overlap is a good thing if kept within a couple degrees. The overlap helps to suck the Air/Fuel mixture into the cylinder which is especially helpful in N/A cars. According to the FSM Valve Overlap is only 3* so why is overlap a concern to some people?

Finally, I would like to talk about the exhaust. Accoring to the FSM the exhaust valves open 41* before BDC. This means that through the power stroke there is only 139* of crank revolution per cycle where the cylinder is creating power. First, is this enough time to completely burn all the fuel? Secondly, why so excessive? 41* is a lot IMO.

So lets talk about your opinions and possibilities of why some of this stuff is.

All valid reasons and discussion questions will be edited into this first post in hopes that we can all gain insight from one place.

Dont forget we have CVTC so the intake cam is always changing valve timing. I wonder if there is power to be made by taking control of the CVTC system since its prob setup for emissions (EGR) and not performance.

I would like to know what the range is for the CVTC in the degrees.

As far as the valve closing AFTER BDC:
When air is being sucked into the combustion chamber it doesnt just stop flowing as soon and the piston gets to BDC, it will naturally keep entering the cylinder ever so slightly, so in keeping the valve open a little longer (in a perfectly efficient engine) you could actually pack the cylinder with more air than the atmosphere would naturally push in. Thus creating a similar effect that putting, say, 1psi of boost at it.

Theres a lot that goes into cams, and believe it or not, the factory doesnt aim PURELY for performance, but also for emissions. It's not as simple as throwing a few ideal numbers on a sheet of paper

but doesn't 64* seem a little excessive? I feel like this engine would benefit from closing that valve a bit sooner. I am really curious what would happen if that valve closed at lets say 35-45* after BDC

i'm gonna leave that to the people who actually make cams. I'm sure JWT's already looked into it before they made cams for our cars lol

Where to start....

The cam timing shown is with the phaser off. From the factory the cvvt will advance and retard the cam from about -2 - 35* the cam is retarded at high rpms because....... At 6000 rpms each valve is opening and closing 50 times a second...... There is a delay in the time the valve opens and the air starts to move...... Once it starts moving it wants to keep moving (remember all that shit you forgot from school). Even though the piston starts moving back up before the valve closes, the inertia in the moving column of air is the greater force of the the piston moving up creates so the cylinder continues to fill.......

50 valve actions per second, 200 strokes per second at 6000 rpms

I'm on my phone so that's all you get for now

Originally Posted by Justin@2J-Racing

Where to start....

The cam timing shown is with the phaser off. From the factory the cvvt will advance and retard the cam from about -2 - 35* the cam is retarded at high rpms because....... At 6000 rpms each valve is opening and closing 50 times a second...... There is a delay in the time the valve opens and the air starts to move...... Once it starts moving it wants to keep moving (remember all that shit you forgot from school). Even though the piston starts moving back up before the valve closes, the inertia in the moving column of air is the greater force of the the piston moving up creates so the cylinder continues to fill.......

50 valve actions per second, 200 strokes per second at 6000 rpms

I'm on my phone so that's all you get for now

Are you sure its 200 rotations per second? I figured its 100 since your dividing seconds from min witch there are 60 sec to every minute so it would be 6000/60= 100. I could be wrong tho its been awhile lol

But anyway has anyone tried tuning the CVTC with the newer style computers with Zypher or whatever its called, since there pretty much a standalone it should be able to control of it? We could prob get some more bottom end with C1 cams if we could remove more overlap in the lower RPMs then whats set from the factory.

No. He's right. The cam spins at half the speed of the crank so for every 720 degrees the cams only spin 360. In 720 degrees there is four strokes. Intake compression power and exhaust. A valve opens once in 720 degrees of crank

Why is it a longer duration cam make more power higher up in the rev range if you think it should close earlier to make more power?? Counter intuitive.

The reason the cam advances is partly because there is less inertia in the charge and to close the valve earlier. This will make more TQ at lower RPMs.

You have to talk about static compression when you start to talk about cams.

aaaahhhhhh. I never went to school for autos. or at least not yet. this is all good information on how and why the valve train works the way it does.

now time to find some info on static and dynamic CR

Good read about DCR and SCR


The first thing to understand is that "compression ratio" (CR) as it is usually talked about is best termed "static compression ratio". This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). For example, if a hypothetical cylinder had a displacement of 450cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. If we were to mill the head so that the volume above the piston crown was decreased to 40cc, the CR would now be 490/40, or 12.25:1. Conversely, if we hogged the chamber out to 60cc, the CR would now be 510/60, or 8.5:1.

Everyone knows that high performance engines typically have higher compression ratios. Simply put, higher compression makes more hp. Higher CR also improves fuel efficiency and throttle response. So why not bump up the CR even further? Once CR exceeds a certain point, detonation will occur. Detonation kills power and it kills engine. The amount of compression a given engine can handle is determined by many factors. These include combustion chamber design, head material, use of combustion chamber coatings, etc. Once these mechanical aspects of the engine have been fixed, the main variable is fuel octane. Higher octane = more resistance to detonation and the ability to tolerate more compression.

The above brings up the question that is often on the mind of performance enthusiasts and engine builders: how high should my CR be? Even if you know all about your engine and have decided what fuel you are going to use, the question cannot be answered as phrased. Why? Because without reference to the camshaft specs, talking about (static) CR is next to meaningless!

How is this so? Well, think about the Otto cycle and how a four stroke engine works. The power stroke has been completed and the piston is heading up in the bore. The intake valve is closed and the exhaust valve is open. As the piston rises it is helping to push the spent combustion gasses out the exhaust port. The piston reaches TDC and starts back down. The exhaust valve closes and the intake valve opens. Fresh fuel and air are drawn into the cylinder. The piston reaches bottom dead enter (BDC) and starts back up. This is the critical point as far as understanding DCR. At BDC. the intake valve is still open. Consequently, even though the piston is rising up the bore, there is no compression actually occurring because of the open intake valve. Compression does not begin until the intake valve closes (IVC). Once IVC is reached, the air fuel mixture starts to compress. The ratio of the cylinder volume at IVC over the volume above the piston at TDC represents the dynamic compression ratio. The DCR is what the air fuel mixture actually "sees" and is what "counts", not the static CR. Because DCR is dependent upon IVC, cam specs have as much effect on DCR as does the mechanical specifications of the motor.

DCR is much lower than static CR. Most performance street and street/track motors have DCR in the range of 8-8.5:1. With typical cams, this translates into static CR in the 10.0-12.0:1 range. Higher than this, there may be detonation problems with pump gas. Engines with "small" cams will need a lower static CR to avoid detonation. Engines with "big" cams have a later IVC point and can tolerate a higher static CR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Of course, race motors also have much larger camshafts which is another reason they can get away with such high static CR, often in the 13-15:1 range.

Note: there is some confusion about use of the term "Dynamic Compression Ratio". Some people use it to refer to the characteristics of an engine combo running at high speed. In that case, the engines volumetric efficiency will have a major effect on cylinder pressure. In this case, a larger cam will increase cylinder pressure when within its' rev range. Thus, more power and more cylinder pressure will be created. We prefer to think of this concept as "cylinder pressure" to avoid confusion.

taken from:Kennedys DynoTune- Dynamic Compression Ratio Explained

I think we stumbled on some cam magic........ time to go to the dyno

do explain

Originally Posted by Justin@2J-Racing

I think we stumbled on some cam magic........ time to go to the dyno

2 x Intake cams? i would like to know what the dyno looks like for this one.... additional 20* duration and 0.07" lift..... can only imagine how that can help but want to seez it.

Originally Posted by BudgetPhoenix

do explain

we'll be on the dyno soon with this setup.

Magic cams. Lol

How goes those magical cams? Thinking of getting some C1 cams but if the ones you guys are testing are better ill wait it out.

Wish you guys could test the c1s , grand am, and rtrs against each other!

All that technical stuff will hopefully start to make sense to me after I take Calculus II and physics I and II next year.