If two cams have identical SAE advertised intake and exhaust durations, but different durations at .050, what would that tell us about the cam lobes, power production and how the cam would behave in an engine?

Assume the same max lift for both cams.

 

Im gonna take a guess and say that the one with more duration at .050 will have a faster ramp rate. Maybe I woirded that wrong. You know it would start the opening of valves sooner. It seems this would make the power band broader, as your valves would be open longer. Im sure a guru will inform us

 

OK, what about the inverse, equal .050 durations but different advertised?

 

White 1 is correct. As a short answer to your second question, the wider the advertised duration the more overlap. Optimum cam would be the most amount of advertised duration as you or your engine's needs can stand and the maximum amount of duration @.050. The ramp rates are typically limited to the strenth of the spring. One way they can get around some on the weak spring issues is by making and agressive ramp on the leading side but a more conservative ramp on the closing side. Basically so the spring can hold the follower(lifter) on the cam and it doesn't try to close the valve faster than the spring can. Next you may ask, why don't you just run a stiffer spring? Spring tension is limited on a hydraulic lifter. Too stiff of a spring and the oil will be pushed out of the lifter aka. lifter collapse. You would not get optimum lift. Solid roller cams have much more agressive ramp rates because you can run a much stiffer spring. The valve ends up staying open wider for more degrees of rotation with the solid roller thus you get more air in and out.
An performance hydraulic spring may have a seat pressure of 115-130 and an open pressure around 300lbs. A solid roller will typically have a spring with a seat pressure of at least 200lbs and open pressure of 700lbs.

 

What does increased overlap do to power production?

 

Overlap is the time in which the intake and exhaust valves are open at the same time during transition from the exhaust stroke to the intake stroke. more overlap causes a decrease in low end power but improvement in upper rpm efficiency. Overlap is also what causes the choppy idle. Basically the intake valve opens when there is still some exhaust pressure in the cyl and causes some exhaust gases to creep up into the intake runner. This pulsing of the intake charge is what causes the choppy idle. When people talk about a wide lobe seperation(114+) that means that the centerline of the intake and exhaust is further apart, thus less of the lobes, at low lift, overlap.
Anyone can feel free to throw a BS flag at anytime.

 

For a turbo overlap is bad all the time. As MJ said above, overlap is the time in which the intake and exhaust valves are open at the same time. Since turbos have tremendous amounts of pressure between the turbo and the exhaust valve, overlap allows exhaust to bleed back into the cylinder and possibly into the intake (the exhaust pressure is higher than the MAP) fouling the charge. Since S/Cs do not build up these tremendous exhaust pressures, overlap is not as problematic (which is why s/c, n2o, and n/a share common cams, but turbos have their own variety).

Good posts MJ

 

so i geuss this would mean my cam would be backwards for a turbo cam

 

You'd probably get more performance from a reverse split or dual pattern cam. If you wanna change, look at a 224/220 111+ lsa or 224/224 112+ lsa or 220/220 110+ lsa

Do you need to change cams? Nah, should be okay because of the lsa. Rule of thumb for turbo cams is int. dur + exh. dur / 4 = lsa target. For your cam, that comes out to 112 lsa, and you're beyond that. Note that this is just a hack to estimate overlap, but a good hack

 

sweet thanx i'd rather not change if its not a must.