supra5677
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posted December 08, 2005 01:47 PM
to board.. I am confused on the cam specs
To Board:
Thanks for the reply on the cam specs, but I have two different specs so I am confused. I dont want the duration at 50 degrees rather I want the total duration numbers.. Can somebody help?
supra
   
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dougmeyer
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posted December 08, 2005 03:31 PM
Sure supra,
You just tell me where you want to start measuring. When the dial needle moves "just a little" ? Or maybe "just a little more" ? Do you want to start measuring when the slope of the lobe sort of starts to move closer to the tappet, but does not yet touch it? Do you want to start when it DOES just touch it, but not yet moves it (although I don't know how you might be able to tell exactly where that is.....)?
Look, we have to pick a point that we can all find, that we can use as a point of comparison. Traditionally "car guys" in the US use .050" of lift at the tappet, "bike guys" (working in metric) use 1mm or .040". Doing this allows us to compare the effective duration of differently ground cams. As you might surmise, if you use a lower lift, you get a longer duration, which means nothing unless you compare that number with another cam using that same lower lift. You can expand the duration out to a "theoretical" number using the accelleration and quieting ramps on the lobe, but that isn't "effective" duration because there is no appreciable lift. You can use any number you want. The fact is that around .050-.040 gives a pretty good indication of "effective" duration, and if you choose another number none of the rest of us pay any attention.
How's that... 
D.
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Y2KZX12R
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posted December 08, 2005 05:42 PM
Yea...
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Y2KZX12R
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Y2KZX12R
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posted December 08, 2005 05:50 PM
Sorry, I should have said the 294 and 300 numbers are from the service manual. I never measured them actually. Not much use for those numbers. But the lift numbers are measured from my bike. The other numbers are...
Io46btdc
ic74abdc
eo69bbdc
ec45atdc
Hope this helps.
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Y2KZX12R
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supra5677
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posted December 08, 2005 05:58 PM
I basically want to get my cams to these specs.. .362 lift 316 duration, on the intake cam and 342 lift and 294 duration on the exhaust cam. I'm guessing these are total duration numbers.
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dougmeyer
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posted December 08, 2005 10:28 PM
 Edited By: dougmeyer on 8 Dec 2005 22:33
Again, either they are "theoretical" or measured at some specific lift. Without knowing that it's it's impossible to make a comparison. It's generally impossible to use the numbers "in the book" specifially because they are rarely given with a "measured at" lift. They are usually genreated from the original cam master, the geometry of which is determined in the basic design and the person "drawing" the lobe shape knows exactly where on the base circle the transition lies.
To further clarify, you can't "get" your cams to a different duration than that to which they are ground. BUT, you can more effectively change the way they run by altering the lobe centers.
If you decide you want some cams ground to the specs you've cited, the first thing the cam grinder will ask is "At what lift?"
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trenace
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posted December 08, 2005 10:33 PM
Supra, actually I may not be adding anything to Doug's explanation, but it's an effort to anyway.
Here's the thing... there is very little flow below 1mm lift. So it really doesn't matter how much duration there is before that point on opening, or after that point on closing. However, how much duration there is AFTER 1mm (or any other arbitrary figure around that range) is very predictive.
One cam might have very gentle, long ramps for that first or last 1mm. So, while having greater theoretical total duration, the EFFECTIVE duration -- the duration of being open at least 1mm or whatever arbitary figure around there -- might be shorter. Another cam might have longer effective duration, but due to short ramps from 0 to 1mm, have a shorter theoretical total duration.
So one just can't plan by the total duration. You could have more total duration but actually have a cam that in practice acts like less duration. Or vice-versa.
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