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Cam timing (overlap) question

Old 09-23-2011, 04:17 PM
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Cam timing (overlap) question

I realise this may sound silly but I am having difficulty figuring out what happens to cam overlap when you alter the lobe center measurement (have a slightly dyslexic side at times it seems, lol). As an example, if one went from a timing of 106 (in)/108 (ex) to, say, 104/108, has the overlap increased or decreased by 2 degrees?

Thanks
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Old 09-23-2011, 05:48 PM
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A decrease in LC values translates into more overlap. And vice versa. As long as you are talking about the same cam, or at least ones with the same amount of duration. LC values and their relation to overlap will not necesarily be the same for two different sets of cams.

I struggled with this a little at first too. I just degreed my stock cams but was looking at a lot of data from aftermarket cams beforehand, which really threw me off. I took more than a few glances to realise that the after market cams can have have more overlap while having higher LC values than stock. This is of course due to their longer duration.
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Old 09-23-2011, 06:03 PM
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I always find these useful:

Check Your Lobe Centers, Ma'am?



"Very generally speaking, the effect of moving lobe centers is as follows:
Advancing the intake and retarding the exhaust (“closing up the centers”) increases overlap and should move the power up in the RPM range, usually at the sacrifice of bottom end power. The result would be lower numerical values on both intake and exhaust lobe centers.
Retarding the intake and advancing the exhaust (“spreading the centers”) decreases overlap and should result in a wider power band at the sacrifice of some top end power. This condition would be indicated by higher numerical values on both intake and exhaust lobe centers. By moving only one cam the results are less predictable, but usually it is the intake that is moved to change power characteristics since small changes here seem to have a greater effect. With twin cam engines we have the luxury of moving the cams independently."
Attached Thumbnails Cam timing (overlap) question-800-615-valvetimingillustration-002.jpg  
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Old 09-23-2011, 06:29 PM
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Thanks Bill.

I had read that bit before, and indeed seen that graph. but was unable to truly understand it. Now I have
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Old 09-23-2011, 06:34 PM
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Yeah that graph always confuses the snot out of me. And because I haven't have any direct use for it, I never bothered to understand it till now...
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Old 09-23-2011, 06:44 PM
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It seems to me that to understand it (the graph), one must look at the exhaust LC number as being a negative number. Then it all makes sense (to me anyhow)....

Last edited by mikstr; 09-23-2011 at 06:53 PM.
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Old 09-23-2011, 07:19 PM
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Yeah because.one is measured in relation to bdc and the other tdc the way the numbers make the lobes move is not intuitive.

Last edited by cliby; 09-23-2011 at 07:33 PM.
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Old 09-23-2011, 08:32 PM
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I like to think as "zero" as a sort of center. Then I think of the two LC numbers as being one on each side of zero (now that you mention it, using a negative number make sense here). So the bigger the numbers, the more you are spread away from that center. Smaller numbers, closer to center.
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Old 05-31-2012, 10:38 PM
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whats the degree on the cam i wanna make my intake 110 and ex `110 can u guys tell me how and how many tooth do i needs to move it ?
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Old 06-01-2012, 10:59 AM
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You don't move it by teeth on the cam sproket. You move it by slotting the cam bolt holes and slightly rotating the cam gears on the cams. To do this properly you need a degree wheel on the crank with 0 set to TDC, and a dial indicator on the lifter to see when you are at a given valve lift.

Has anyone had any problems with the cam gears moving when doing this? I've just been curious if slotting the gears if they would be prone to move when they are not coming up against the side of the hole or if the clamping force of the two bolts is enough to hold them.

Also has anyone messed with degreeing them in with more or less overlap or tried adjusting the timing at all to see if you get any better performance one way or the other?
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Old 06-01-2012, 11:50 AM
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I've currently got mine set with just a touch more overlap than SPEC. Not from what it actually was, which was slightly off spec. IIRC, I went a degree and a half on each cam.

Worthwhile?.... idk. Runs about the same as it did previously. But it was an intersting little project. Maybe I'll screw with it some more over one of these winters. I just like to tinker with it mainly, and learn new things.
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Old 06-01-2012, 11:52 AM
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And yes, the clamping force is all that is needed. Slotted gears aren't unusual. Torque and loctite them and it'll be fine.

I even just slotted mine with a dremel, so they are certainly not perfect. I have no worries about them ever slipping.
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Old 06-01-2012, 02:51 PM
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I'm curious if anyone else has found any real difference with doing this. I'm with you I like to tinker on stuff like that and get it all dialed in. If anyone has noticed a difference on it I'd take the time to do it. But if it's a bunch of work for not really anything I may not. But who knows, I may do it anyway.

I remember making those graphs in performance clas when I went to MMI. It's always fun to do that stuff and see what it looks like.
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Old 06-02-2012, 01:19 PM
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what would be if i advance the intake 1 tooth and retard the ex one tooth olso
i have been reading for ever how to degree the dam cams and i cant fegur it out
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Old 06-03-2012, 12:06 AM
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If you moved them one whole tooth you'd likely be WAY to far out. Only bike I've known of that you could go that far with is on the YZF450s and WR450s. You retard the exhaust cam one tooth and it matches the YZ450 timing. This gives you more power on top but you loose some on the bottom. Try it with the SH and you'll likely just throw them way to far out.
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