Question about Cams...

[Mr. Land Yacht]

I don't mean to sound dumb, but what does duration actually mean?
I know what lift is, I understand that...
but I hear things like 220 degree duration and I don't know exactly what that means..
could anyone explain it for me?

thanks for any input!

-Colin

[88Vic]

I read this page and kinda figured out how camshafts work and what lift and duration mean. http://www.fordsix.com/tech/engine/cam_advice.php Maybe wikipedia or howstuffworks has a better explaination.

[Mercmarquis]

how long the valve stays open is duration, the larger the duration, the higher the power band is.

[mrltd]

Sounds like a tech question...

[Nathan in MI]

Duration is usually measured at 0.050" lift, since at any less than that there won't be much air moving. The degrees is the number of degrees of crankshaft rotation, so if a cam is rated at 220* duration, that means that the valve is open at least 0.050" for 220* rotation of the crankshaft.

[Mr. Land Yacht]

ah! I see...
I checked out howstuffworks before I posted and there's nothing in there about duration..
and yes, I tried Wikipedia as well...nothing there either.
now, Ichecked out that link and I have one more question....
What is Lobe Seperation?
I think I understand the difference between advertised duration and duration at .050".
advertised udration must be the total duration so to speak.
but what is that lobe seperation?

thanks, guys!

[DuceAnAHalf]

What is Camshaft Duration and why is it important?

Duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open. Duration (at .050" lifter rise) is the deciding factor to what the engine's basic RPM range will be. Lower duration cams produce the power in the lower RPM range. Larger duration cams operate at higher RPM, but you will lose bottom end power to gain top end power as the duration is increased. (For each ten degree change in the duration at .050", the power band moves up or down in RPM range by approximately 500 RPM.)

What is the difference in Advertised Duration and Duration at .050" Lifter Rise (Tappet Lift)?

In order for duration to have any merit as a measurement for comparing camshaft size, the method for determining the duration must be the same. There are two key components for measuring duration-- the degrees of crankshaft rotation and at what point of lifter rise the measurements were taken. Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050" lifter rise state the exact point the measurement was taken. These are the only duration figures that are consistent and can accurately be used to compare camshafts.

From http://www.cranecams.com/?show=faq&id=2

[Mr. Land Yacht]

thanks, Duce!

[Paul_]

i thit thet te wueston wsa abut cands

[turbo2256b]

Duration at .050 wass started by Crane. They began taking the duration at this point because advertized duration can very because they dont tell were it is taken. It became a industry standard. Using the at .050 lift cams from most manufactures can be compaired directly. Of coure Ford dosent use this standard in house If I remember they use .020.

Duration effects the power band of a cam. For every 10* inncrease of duration at .050 the RPM band moves up 500 RPM. What ever you gain on top end you loose at low end.

Duration is used to pack more air in the cylinder by keeping the valve open longer. Ported heads dont need as much duration because more air is supplied by porting than ever can be acheeved with a cam.

Big duration causes the engine to be very peeky hard to drive increases emissions and reduces fuel economy.

Big duration also likes higher compression due to low speedd pumping losses.

[RugerP97DC]

Moved.