Why split duration cams?

[BeeMaster]

Why did porsche use a split duration cam shafts in a 911? What was there reasoning
Or what where they trying to accomplish? Thanks in advance.

[Tippy]

Although I wasn't in the minds of the engineers, they probably were shooting to meet a whole host of parameters (top end hp, low end torque, fuel consumption, etc.).

[BeeMaster]

Ok thanks I thought maybe the intake didn't flow as good so they had the intake valve
Stay open longer thanks. Does anybody know what the flow rates are for the intake and exhaust on a 2.7S?

[gtc]

42

[javadog]

This is too broad a question to answer. There's no single cam profile for a 911 engine, there are umpteen of them. Ditto for the cylinder heads, so there's no single flow number for a 911 intake or exhaust. When you have 911 engines that have been made in as many displacements and market variations as now exist, not to mention the turbocharged ones, or the race versions, you have literally dozens of answers to such a question.

This question of split duration cams gets discussed a lot on internet forums, especially on domestic V8 forums. There's no single optimum cam timing for any engine, unless you specify the usage, driver preference and a host of other things.

If you want to discuss 911 cam timing in more detail, you might narrow the question down a little further and maybe ask it on the engine forum.

JR

[BeeMaster]

I am not asking about the timing as much as the physical shape of the cams. There is
A cam book at my house about making cams that my dad has been reading so when he heard about the split cam he asked why. Usually people use a split duration cam to make up for a bad flowing port or manifold, my other thought was to keep the exhaust valve cooler they made the exhaust cam shorter. And last thought was to make up for the bad flowing Cis intake box they made the intake duration longer. I just wondered if
Someone knew the real reason why. And sorry if I posted this in the wrong spot.

[javadog]

By shorter, do you mean shorter timing duration, or less lift (shorter lobes)?

JR

[BeeMaster]

Really both but mostly the duration of the cam. How long the valve is open

[javadog]

There really isn't a simple answer to this question, as there are so many considerations when designing a camshaft that it's hard to discuss all of them without writing a book. My suggestion would be to just buy a book on basic engine tuning and read the chapters on cylinder head design and valve timing.

Plus, what was done 20 or 30 years ago might not be what is done today, exactly.

You might ask this question in the engine rebuilding forum, if you want to discuss the theory. There are a few guys there that rebuild engines for a living and one or two that design and grind camshafts. If your question is specific to the 2.7 CIS engines, you might buy a copy of Bruce Anderson's or Wayne Dempsey's books on 911 engines, as they have specific recommendations about how to modify 911 engines for more performance.

JR

[Bill Verburg]

On Porsche air cooled engines the intake always has longer duration than exhaust, the newer cars have wider lobe separation to decrease overlap and shorter duration because of smog regs., but the intake duration is always > exhaust duration. lift also varies, bigger higher performance versions tend to have more lift and more duration and more overlap(as long as smog and intake manifold designs permit)

This is normal, the exhaust doesn't need more duration for 2 reasons
1) the rising piston on the exhaust stroke pushes the burnt mixture out efficiently
2) the high pressure & temperature in the chamber efficiently force burnt mixture out

the intake process in a naturally aspirated engine isn't as efficient at getting fresh mixture into the chamber

it is common practice in poorly designed engines to reverse this to make up for poor intake flow