What is a split duration camshaft? Is it a better performer? Do you have to have speacial Springs, valves, etc?

 

I'm not sure if you mean dual pattern. The idea is to compensate for the exhaust valve being much smaller than the intake. I'm sure there are more specifics this is the nut shell version.

Chris

 

You are correct I meant the dual pattern.

 

Different duration for intake & exhaust. Not necessarily better. The car co. & aftermarket head co. can make the valves the sizes they want. Some make a relatively large intake (port &/or valve) w/ a more normal size exhaust. Can help w/ restrictive exhaust.
Also sometimes

 

Not always just the duration, the cam can also have different lift on the intake and exhaust sides. Depending on the combination of intake, valves, heads, headers forced induction etc... It may be beneficial to have more lift/duration on either the intake or exhaust side. Any combination of the two may be used depending on engine specifics.

ltlevil

 

Yes. W/ more duration there would normally be more lift, goes w/o saying.

 

If you're going to stay with the stock heads the dual pattern cam is the better choice. Just my .02

Chris

 

I don't agree with that statement.

 

Most cams, but not all, including stock factory grinds, use different lift and duration between the intake and exhaust. The exhaust is nearly always the benefactor of more lift and duration.

If you were to flowbench test most heads, you'd find that the exhaust flows 50 to 60 percent of what the intake flows. Some heads are a bit better. To have a satisfactory balance in flow for performance, most head porters shoot for at least 70 percent. The exhaust doesn't need to flow the same as the intake because exhaust gas has energy.

What a dual pattern cam does is to favor exhaust flow to get the dynamic flow closer to the intake. The dual pattern could be more lift and duration on the exhaust, or just more duration. It would be unusual to have more lift but not more duration, though, you could if that's what you wanted. The closer the exhaust flows to that magical 70 percent number, the less important dual pattern cams become. Once you get to around 80 percent, there is no benefit from increasing duration or lift on the exhaust since you are pretty much in a balanced flow situation.

You will usually find that street cars running a full exhast benefit from favoring the exhaust timing event.

The reason for all of this is that the engine can't take in what it can't exhaust out.

You may also hear asymetrical used when describing a cam. This is not the same as dual pattern. Asymetry describes a lobe. It means that the ramps for opening the valve and for closing the valve have different accelleration rates. An asymetrical cam opens the valve fast and closes it slow. Normally, the ramps are equal.