Utinator

How do you determine/calculate the valve events (open/close) when all you have are the cam specs? I'm trying to figure out which boosted system will work best with my current cam. I don't want to replace the cam if I decide to boost the engine later on.

I have a TSP cam --> 224/228-114 (.600")

KCS

iTrader: (20)

Most people use the online calculators to determine what everyone calls "valve events". Cam Motion has one I believe.

JakeFusion

iTrader: (7)

That cam would be fine for a turbo system.

Utinator

Cam Motion....got it. I'll look there.

I'd really like to have a centrifugal supercharger, but I don't think my cam has enough exhaust duration for that to work efficiently. I can live with a turbo build, though.

Utinator

Found it:
http://www.cammotion.com/cam-timer

Utinator

And the machine says:

Intake:
0.0 --> Open
44.0 --> Close

Exhaust:
50.0 --> Open
-2.0 --> Close

-2.0 Overlap


Does this sounds right?

Summitracing

This is correct if it's ground or installed 2 degrees advanced. Our calculator was provided by Cam Motion and we use it All the time...very handy.

Utinator

I don't understand how the exhaust valve can close at -2. I thought these units were in degrees. There's no such thing as negative degrees, unless you are talking about the weather. The negative degree overlap makes sense, because it's a mathematical equation.

JakeFusion

iTrader: (7)

A negative value indicates before top dead center (BTDC). A positive number would be after TDC (ATDC).

So think of the 4-cycle stroke and where the valve opens in relation to the piston moving in the bore. If you're closing the exhaust before TDC, you're closing the valve a little early for max power as the piston is still traveling up.

However, because it's exhaust gas, as soon as you crack open the exhaust valve, the pressure differential is significant compared to the intake and exhaust gas is on its way out in a hurry. But as you can see, overall overlap is -2 because the intake valve is opening at 0 degrees (or at TDC) or after the exhaust valve has closed. So your overlap cycle is not helping draw more fresh air into the cylinder on the intake stroke. But you're not losing intake charge out of the exhaust either. So it makes good torque and drives with good manners.

Stampede4ever

JakeFusion, could you explain your cam specs on the exhaust? Why do you have a large split, from what I can tell most cathedral heads like a 6-8, as a rule of thumb, but you have 17. I'm still trying to learn how to spec cams based on valve events per size, I'm still lost.

Stampede.

Summitracing

Our mission is to get rid of the idea of "splits" etc. because they are so wishy-washy and focus on the events. If you're not at all used to thinking in events, the best place to start is by taking the cams you know best and plugging them into the calculator. Then you start seeing what events have changed between the two and it helps explain their behavior. Since the introduction of the Pro LS cams on Independence day, we've written quite a few articles and produced quite a few videos to help explain. Here are links in order of importance for what you are trying to learn about.

Valve event article
Valve event video
Cam Timer article
Cam Timer video

Finally, you can watch the Pro LS cams video and you'll see how the events between Stages 1 to 4 are set up incrementally. It's a longer video, but we think you'll get a lot out of it.
Pro LS Cam article

When you have had a chance to look at these, please feel free to report back with questions and we'll do our best to answer them.

Utinator

Ok, now that makes sense. So, the valve event numbers are more of a relationship than an actual measurement.

Well, I wanted good driving manners, and a cam that *could* be used for boost. It might have been better to close the exhaust valve a little later, but at least the negative overlap will help to keep the boost from going out the tail pipe.

JakeFusion

iTrader: (7)

And my cam is setup for a Procharger. Opening the exhaust valve early to give a crutch for the forcefed air in the intake.

For NA... it hurts power down low... but creates a lot of juice uptop.

Summitracing

Valve events are actually the real measurement, and each is responsible for a behavior. Duration and lobe separation and advance are derivative of the valve events. It's important to think like the engine and understand the exhaust side of things is moving before the intake. So E/O first, I/O (Naturally aspirated) second, E/C third, and I/C fourth. If you closed the exhaust valve a little later, you would be increasing overlap. The more backpressure you have between the turbo and the combustion chamber, the more burnt air will re-enter the cylinder and even the intake runner depending on how much the valve is open at TDC. That's reversion and it's worst at part throttle when there's vacuum in the intake runners trying to make flow go backwards. Supercharging changes the balance the opposite way as the exhaust side will be atmospheric pressure or a 1.5 depending on how poorly the exhaust system flows.