The camshaft is the last component of my rebuild that I haven't decided upon yet. I'm trying to learn what I can about them so I can make an intelligent purchasing decision. Yes, I know this is a broad topic..

I don't understand the relationship between duration and lift.. what running aspects are changed as a result of increased duration? What aspects are changed as a result of increased lift? At what lift and/or duration should I worry about longevity issues?

There are so many numbers out there and the descriptions that go with each camshaft seem too vague. I can stare at the webcam page all day and still not know how different an 86 will be from an 86a or 86b.. or 494, etc etc etc. Can anyone help me or point me towards some resources that will help me translate these numbers a little better?

Thanks!

Are you an engineer ?? LOL


B

Here you go, this will either enlighten you or put you to sleep

Camshafts Explained

I recommend you read the entire page, quiz tommorow

Do you know this guy?

HD Manley

That page was helpful, thank you. I understand, in general, how camshafts work.. I guess the question I should've asked was:

Two camshafts claim the same type of performance: improved midrange and strong bottom end for street engines with carbs. One cam has a longer duration and shorter lift compared to the other that has a higher lift and shorter duration. So how are they 'different'? (or which is 'better'?) Perhaps the answer is infinitely more complex than I'm imagining it is..

I am sure someone will chime in but I think high lift short duration cams are designed for high RPM motors. Long duration shorter lift cams make power lower in the RPM band. Or is it the other way around? At any rate, Type 4 motors (unless highly modified) are not known for their High RPM ability.

Part of the equasion is going to depend upon what you are going to use the car for. If you are looking for driveabilty don't put a radical cam in.

Depending on the valve train geometry, the higher lift cam might not work with stock springs before they would bind (compress and have each coil touch)

or piston to valve clearance might be too close and the higher lift valve could hit the piston top unless the piston is machined with a relief.

The high lift/short duration cam is ideal for a turbo motor since the incomming charge could end up going out the exhaust if the duration is too long (overlap)


the longer duration cam will have a weaker vacuum signal but it will be "gentler" on the valve train since it does not have to ramp up so fast to attain the higher lift

did that help or confuse you more

Looks to me like George W's cousin

That made sense Mueller; that's what I was looking for. It reaffirmed my original thought that shorter lift/longer duration is gentler on the valve train. I can also see the reason for high lift/short duration on turbo motors.

shorter duration cams make power lower rpms but and are more fuel efficient but can't move the charge into the cylinder as efficiently at high rpms. this is your typical type 4 fuel injection cam. it has a high vacuum due to less overlap.

as you increase duration, you gain high rpm ability but sacrifice low end, which is why you get the camshaft "lumpy" idle. the longest duration cams have the worst idles, but coolest sounding, to a point.

lift is how much the valve is open, but to get more lift you almost have to increase duration to keep the "hill" the lifter climbs from getting too steep, which would wear more rapidly.

Another variable is valvetrain weight and spring rates. the lifter,pushrods, rockerarms, and valves have mass. the more mass, and the higher speed the mass is moving when it jumps the hill of the cam, the more spring pressure is needed to keep the valves from going "airborne", which is called valve float and is the clatter you hear when you rev the engine higher. more spring pressure also means more load on the components, and more wear.

more or less

So the trick is to decide what is most important to you and get the cam that supplies it, high rpms and power, low end torque, fuel efficiency and emissions, reliability, We're dealing with a lot of sh*t here.

Add to that we have to completely disassemble the engine to change the cam if we make the wrong choice, and doing your homework on the cam is pretty important.

my 2 cents.