Subject: Re: Help me compare these two cams by anesthes on 2009/9/25 3:22:29
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rklessdriver wrote: Joe - I'm not following your comment about how a cam with less lift usually has the faster rate of lift.
Depending on the type of cam (flat hyd, flat solid, roller hyd, roller solid) you can only make the ramp so steep.
All cams have the same theoretical lift when cut on the same base circle. To create lift, they cut down the back side of the lobes. The more they cut, the more lift is into the cam. If you cut a lot and create a lot of lift, you have to make the ramp slower or the tappet won't follow or in some extreme conditions the side of the lifter body will crash into the lobe. So you end up with real gradual mountains.
Obviously flat hydraulic cams are the worse (flat is worse, then the hyd plunger takes all the bite out of the cam). Solid rollers can have the steepest lobes. If you can grind a cam so the lift is not high but it gets there quickly, you can make up the lift with rocker ratio.
Remember, the more lift a cam has, the farther the lifter is into the bore at seat. The closer to the cam centerline the lifter is, the harder it is to follow the lobe quickly without the lobe trying to push the lifter out of the block or the side of the lifter actually hitting the cam.
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rklessdriver wrote:
You would have to break things down with a dial indicator and a degree wheel to map out every degree of lift/duration between 2 cams but I'm positive the cam with more lift will win on rate of lift every time - provided the duration of the 2 cams your comparing (such as in this case) are close to one another.
Actually you use a cam profile machine. It would take forever to try and do it with a degree wheel and a dial indicator. Since I don't have a $50k machine, nor the time to profile a cam by hand I just go by a simple comparison with some known data to select cams.
It's almost impossible to compare cams from different manufacturers.
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rklessdriver wrote:
I'm not trying to insult you with my (following) simplified explanation but I think it may help others understand what I'm talking about.
The first cam has an Intake lobe with 264* (out of 360*) advertised duration. It has 210* (out of 360*) of duration remaining after the lifter has acheived .050 lift. In that 264* it is only capable of .318 lobe lift and back to 0 (well .020 or .006 or whatever they use for advertised duration).
The second cam has less advertised duration to begin with at only 260*... It also has the same 210* of duration remaining after .050 lift. In only 260* of duration the second cam reaches .350 lobe lift and back to 0....
You were right the first time. The second cam has the faster rate of lift on the intake lobe. Not sure if you mispoke or what in your last post.
Yes, I said the second cam has a faster intake ramp. I then said that's odd, because normally the faster cams have less total lift.
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rklessdriver wrote:
For everyone else who may be lost:
First thing you have to know is that cam lobes only have 360* of rotation to get everything done (valve closed, valve opening, valve max lift, valve closing, valve closed again).
Timing events on a cam lobe are measured in "degrees of Duration". The total amount of time a lifter is off the heel of the cam or in positive lift (be aware that this point is measured differently by different cam grinders) is refered to as "Advertised Duration".
Because .000000 lift and .0000001 lift is really hard to detect and repeat, cam grinders long ago started using .020 or .006 lift (it is a more repeatable measurement) as a place to start measuring for the Advertied Duration. It casues alot of confusion, but it's an important measurement and these days very overlooked and misunderstood.
There are a host of other cam lobe timing measuerments for people to look at. Duration at .050, .100, .200 ect. They are smiply the remaining degrees of duration (remaining from your orginal 360*) at a given amount of lift. None of them mean anything unless you correlate them to the advertised duration. Then you can compare 2 differnt cam lobes by looking at and comparing all these timing numbers.
Now I'm only talking about individual CAM LOBE timing events here. They are very simple to understand compared to...
Pretty good explanation for the new guys.
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rklessdriver wrote:
Actual entire camshaft and valve timing events, which are a product of the cam lobe timing events combined with the centerlines the cam lobes are ground on (in releation to the crankshaft)and rocker arm ratio. Valve timing events are measured by degrees in releation to the piston being at TDC or BDC of it's stroke.
You almost always see the intake lobe centerline and lobe seperation on cam cards. Lobe seperation is nothing more than the intake & exhast centerline numbers added together and divided by 2. Some people put so much emphsis on it... I have no idea why. It really dosen't tell you that much useful.
It gives a 'rough' idea what the purpose for the cam is.. If I'm looking for a blower cam, I start looking at cams with 113-115 degrees of lobe separation, then from my pool of cams I look at the rest of the specs and throw out the losers.
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rklessdriver wrote:
You sometimes you see the valve timing numbers on cam cards. These are very useful if can comprehend what is actually happening at those given points in a 4cyl engine. It's important to know how long the valve is actually off its seat when the piston is still on the up stroke and how long it continues to be open after the piston has gone past TDC and is approaching BDC. Same for the exhaust valve and for when the plug fires.
